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In this course the general introduction to Raman spectroscopy and microscopy will be provided and practical tips as well as examples will be given. The capability of Raman spectroscopy for the analysis of real-life samples (paint components, clays, coating materials, etc.) taken from historical and archaeological objects will be discussed.

1. Principles of Raman spectroscopy

Raman spectroscopy is widely used in the investigation of cultural heritage materials due to its high spatial resolution (typically in the range of 1 to 10 µm), large amount of obtainable information, non-destructivity and ability to perform in-situ analysis.^1,2 With Raman spectroscopy it’s possible to analyse various materials: minerals, inorganic and organic pigments, binding media, varnishes, ceramics, plastics, textile fibres etc.²

The following video explains the principles and instrumentation of Raman spectroscopy.https://www.uttv.ee/embed?id=29055

Similarly to infrared spectroscopy, Raman spectroscopy is classified as vibrational spectroscopy.³ Raman spectroscopy is based on Raman scattering (or Raman effect) that reveals the vibrational, rotational and other low frequency modes of molecules⁴. In this technique, the sample is exposed to an intense beam of monochromatic light (typically a laser beam) in the frequency range of visible, near-infrared or near-ultraviolet region.⁵ The electromagnetic radiation, interacting with a substance, can be transmitted, absorbed, or scattered⁶. When the monochromatic radiation is scattered by molecules, the majority of the radiation undergoes the common Rayleigh scattering (radiation’s  frequency/wavelength is unchanged). However, a small fraction of the scattered radiation is observed to have a slightly different frequency from that of the incident radiation. This is known as the Raman effect⁷. The Raman lines show up pairwise. The dominant Stokes lines have a lower frequency (longer wavelength) than the initial radiation, whereas the weaker (often nondetectable) anti-Stokes lines have a higher frequency (shorter wavelength).^4,5 The frequency shifts are virtually independent of the excitation wavelength and are characteristic of the particular substance/molecule. Usually one only records the relatively strong Stokes lines, which therefore are attributed a positive frequency shift. Such spectral coordinate is called the Raman shift and measured in wavenumbers (in cm^-1).⁴ See scheme in Figure 1.

Figure 1. Scheme of Raman scattering.

In Raman spectroscopy, as it is a scattering technique, samples are simply placed in the laser beam and the scattered radiation is collected and analysed⁸. Raman spectrometer measures the wavelength-dependent intensity of the inelastically scattered light.

The obtained Raman spectra are essentially vibrational spectra. Hence, if presented in the Raman shift scale, they are directly comparable to corresponding infrared absorption spectra (see Figure 2). However, Raman spectrum arises in a different manner and the rules, which vibrations are Raman-active (and thus produce signals in the spectrum), are different. It turns out that a vibration is Raman-active (i.e. revealed as a spectral line in the Raman spectrum), if the polarizability of the molecule changes during the vibration.⁷ It often happens that vibrations that are active (or give high-intensity signals) in Raman scattering are inactive (or give low-intensity signals) in the infrared, and vice versa.⁷Therefore, Raman spectra often provide complementary information to IR spectra.

Figure 2. Raman (laser 514.5 nm) and IR spectra of benzene.

1.1. Instrumentation

There are two types of Raman spectrometers: dispersive spectrometers (based on the use of diffraction grating) and interferometer containing Fourier-transform Raman spectrometers (FT-Raman)⁹.

In general the main components of Raman spectrometers are presented on the following scheme:

In Raman spectroscopy, the choice of excitation wavelength and intensity is very important. Different wavelengths are suitable for the analysis of different types of material. The wavelength will affect the Raman intensity, spatial resolution, background fluorescence, and potential damage to the sample. Almost exclusively lasers are used as excitation sources, because they are highly monochromatic, give high-intensity radiation and can be efficiently focused due to their high coherence. Only continuous wave (CW) lasers are used, as pulsed lasers easily damage the sample. Some popular CW lasers are presented in Table 1. Traditionally, laser wavelengths up to 830 nm have been used in dispersive instruments while the 1064 nm laser line has been employed in FT-Raman setups. With the availability of sensitive InGaAs array detectors, it has become meaningful to use also the 1064 nm lasers with dispersive Raman instruments.

Table 1. Laser sources for Raman spectroscopy.

Raman scattering efficiency decreases with increasing excitation wavelength as λ⁻⁴. However, short-wavelength lasers more easily induce fluorescence, absorb in the sample or cause other undesirable effects due to their high photon energy. Hence, most common laser wavelengths in Raman spectroscopy are in the visible and NIR region (such as 633 or 785 nm) which offer low fluorescence whilst retaining relatively high Raman intensity. For samples which exhibit fluorescence even under red excitation (for example dyes), the 1064 nm laser may be needed. While near-infrared lasers have a smaller photon energy, compared to visible lasers, they are usually more powerful, in order to compensate for the reduced Raman scattering efficiency. Therefore, they may still damage the sample. It is especially important for strongly absorbing (black) samples, in which case the UV/visible lasers (operating at lower intensities) may yield a stronger Raman signal.

Dispersive Raman spectrometers

A dispersive spectrometer utilizes a diffraction grating to angularly disperse the light. As a result, at the detector plane, different wavelengths become spatially separated. Nevertheless, prior to entering the spectrometer, the incoming light should go through a special edge or notch filter to suppress the primary (Raman-scattered) light and thereby reduce the scattering inside the spectrometer. A matrix detector is used to record the dispersed spectrum. Typically, a silicon-based cooled CCD is used, which is very sensitive in the visible and NIR region (up to 1100 nm).

FT-Raman spectrometers

Commercial FT-Raman spectrometers were introduced in the late 1980s¹⁰. Their operating principle is similar to that of FTIR spectrometers and is based on an interferometer. As the Raman-scattered light enters the instrument, the interferometer selectively modulates the individual spectral components by systematically changing an optical path length difference. The resulting beam of light is recorded by a point detector. FT-Raman is superior to a dispersive instrument in the near-IR region beyond 1000 nm. Commonly, the 1064 nm laser excitation along with germanium or indium gallium arsenide (InGaAs) detector is used. They also offer excellent wavelength accuracy and can potentially combine IR absorption and Raman measurement capacity in single instrument. However, FT-Raman frequently needs to use high laser intensities due to the reduced Raman scattering efficiency at longer wavelengths, which may damage the sample.

Different types of Raman Spectroscopy

A variety of Raman instruments and special techniques are used for the analysis of cultural heritage materials. The choice of instrument determines the sensitivity, spectral range and resolution, spatial resolution, availability of different excitation sources, and convenience of operation. 

• Micro-Raman spectrometer (or Raman microscope) is the most common bench-top Raman instrument. A high-resolution spectrometer (either dispersive or FT) and one or several laser sources are coupled through an optical microscope. The excitation beam is focused and the secondary emission is collected simultaneously by the microscope objective in backscattering geometry. A high-numerical aperture (NA) objective yields both a high spatial resolution and a high collection efficiency.
• Surface-enhanced Raman spectroscopy (SERS) involves inelastic light scattering by molecules placed close to nanometal surfaces, which amplify the scattering by plasmonic resonance. One approach is to study molecules adsorbed onto corrugated metal surfaces such as silver or gold nanoparticles¹¹. Another approach is to stimulate the molecules by a sharp metal tip. Such tip-enhanced Raman spectroscopy is typically implemented by combining a confocal microscope and a scanning probe microscope. 
• In Resonance Raman spectroscopy (RRS) the incident photon energy is close in energy to an electronic transition of a compound or material under examination. 
• In a portable Raman spectrometer, a miniature dispersive spectrometer and a small laser source are integrated into a portable, hand-held device. Hence, the instrument can be used to perform in situ analysis in museums, archives, also outdoors on archaeological sites for the analysis of mural or cave paintings. Such portable devices frequently employ a fiber-optic probes. 

1.2. Problems with Raman spectroscopy

Compared to IR absorption, the primary disadvantage of Raman spectroscopy is the fluorescent background (see Figure 3). As Raman scattering is inherently weak, one has to use an intense laser beam for excitation, and for many materials, this results in a strong fluorescence – either due to the material itself of impurities. Sometimes even trace impurities – if they are strongly fluorescent – can lead to disturbing fluorescence background. Fortunately, Raman lines are spectrally close to the laser beam whereas fluorescence has typically a large Stokes shift. 

Figure 3. Example of the fluorescence in the Raman spectrum of red lead containing paint.

Relative to the Raman signal, the fluorescent background can be highly intense and even the tail of the fluorescence band may obscure the Raman spectrum. Although the problem can be partially resolved by careful sample preparation, time resolved spectroscopy or coherent anti-Stokes Raman spectroscopy (CARS), there will always be experiments that remain difficult to perform.⁷

In addition to fluorescence, intense focused laser irradiation can cause heating and degradation of the sample. The problems are typical for organic, soft, photosensitive or dark/coloured materials whereas transparent inorganic materials have usually quite high damage threshold.

2. Analysis with Raman spectroscopy

In the following video Senior Research Fellow Dr. Valter Kiisk demonstrates and explains how to perform measurements with a typical micro-Raman spectrometer.https://www.uttv.ee/embed?id=29396

Identification of the composition of the studied material is often based on the comparison of its Raman spectrum with a spectral library of reference materials.¹² Different papers and books have been published from where Raman spectra or information about excitation wavelengths and list of wavenumbers in the Raman spectra are available ^5,13,14. Also a very valuable on-line database is made available by the Infrared & Raman Users Group (IRUG)– http://irug.org/ – from where different Raman (and also IR) spectra of cultural heritage materials can be obtained free of charge.

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Vibrating Sample Magnetometry (VSM) is a measurement technique which allows to
determine the magnetic moment of a sample with very high precision. The aim of this
lab course M106 is to enlarge upon the use of this widespread technique introduced in
the lab course B512, where different ferromagnetic samples were characterized
concerning magnetic hysteresis and demagnetization. Here, we will gain a deeper
understanding of the behavior of magnetic materials and its measurement.

In order to
lay the foundations, first the measurement principle and the properties of ferromagnetic
materials will be summarized (magnetic domains, magnetic hysteresis,
demagnetization) and then we will elaborate on the magnetic anisotropy of
ferromagnetic materials.

A vibrating sample magnetometer (VSM) systems are used to measure the magnetic properties of materials. The vibrating component causes a change in the magnetic field of the sample, which generates an electrical field in a coil based on Faraday’s Law of Induction.

If the sample is placed within a uniform magnetic field H, a magnetization M will be induced in the sample. In a VSM, the sample is placed within suitably placed sensing coils, also held at the desired angle.

And the vibrating sample component is made to undergo sinusoidal motion, i.e., mechanically vibrated.

The hysteresis loop shows the “history dependent” nature of magnetization of a ferromagnetic material. Once the material has been driven to saturation, the magnetizing field can then be dropped to zero and the material will retain most of its magnetization (it remembers its history).

Procedure
Before using the VSM, you must carry out a series of configuration steps.
• Insert the Ni standard into the VSM. The standard is ball-shaped, therefore
magnetically isotropic, and has a magnetic moment of 6.92 emu at 5000 Oe.
• Find the exact position of the standard in respect to the center of the pickup coils. The
vibrating rod can be adjusted by three screws on top of the VSM for x, y and z
direction. The pickup coils are connected in a way that, the sample being in the center
of the coils, there will be a signal minimum along x-, a maximum along y- and a
maximum again along z-direction.
• Run Calibrations → Moment gain to calibrate the instrument, i.e. to convert the
measured voltage signal into the correct value of the magnetic moment.
After calibration of the VSM, the following measurements aim to address two topics. The
first part covers basic magnetic characterization and the information that can be
deduced from magnetization curves. The second part cope with demagnetization.

1. Magnetocrystaline anisotropy energy:
   Fix the Fe single crystal to the sample holder. Set H0 to 3500 G and record the
   magnetic moment of the crystal during a 360° rotation of the sample.
   Find the angles corresponding to the different crystallographhic / magnetic axes and
   record the magnetization curves of the easy axis and the hard axis.
2. Stress induced magnetic anisotropy
   Mount a sheet sample clamped in a sample holder provided by the supervisor into the
   VSM. Record the magnetization curves of the sample with and without applied stress
   along and perpendicular to the stress direction.
   Determine the volume of each sample that you have measured

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Operation

A sample is made to oscillate using a vibrational unit extended on a rod. The sample is placed between two electromagnetic pieces which are used as the applied field for this this experiment. With the sample oscillating induces a voltage between the search coils which creates a signal to determine the magnetic properties of the sample. Reference coils are used to create a reference signal such that noise generated from the signal can be filtered using a lock-in amplifier [1]. Because the signal and the reference signal are directly related through its voltage and amplitude means that precise measurements can be recorded using a voltmeter. Calibration methods are important to determine the relation between the voltages induced by the magnetic field and the sample and their magnetic properties.

Calibrating the applied field is done by increasing the voltage in steps measuring the field until reaching a maximum. The system is calibrated using a nickel standard normally as a number of volts per unit of magnetic moment. Many materials such as types of barium ferrite or alnico materials can be placed inside to determine properties. These properties include remanence, coercivity, intrinsic coercivity and operating points once the system has been calibrated.

Advantages and Disadvantages in terms of experimental facets

The key advantage is the precision and accuracy of VSMs. Taking measurements at a range of angles once detection arrangements for the coils have been devised can be done. The advantage of sample vibration perpendicularly to the applied field can be found once the detection coils have been arranged appropriately. This means that there is the ability to test the sample at different angles. The positioning of the coils are done in a way to reduce the effects of sample position variation and external field variation- essentially deep into the applied field shown in figure 1. Disadvantages are that they are not well suited for determining the magnetisation loop or the hysteresis curve due to the demagnetising effects of the sample. Another problem is that, particularly for the VSM used in the third year laboratory is that temperature dependence cannot be controlled.

Figure 1. A schematic layout of the VSM

2. B-H Hysteresis Loop Tracer

Operation

The B-H hysteresis loop tracer is essentially two coils, one with a sample and the other which is empty for comparison. The insertion of a sample into the pickup coils causes a voltage proportional to the rate of change of the vector field to occur across the difference amplifier. After passing through an integrator, a voltage proportional to the intrinsic induction is passed to the Y-amp of the oscilloscope. This voltage combined with an X-voltage representing the magnetising field generated from the solenoid without the sample results in the generation of a hysteresis loop on the oscilloscope. Calibration is through a balance and phase adjustment to establish a trace on the oscilloscope. They are done to make sure that the magnetising field is linear and that every vector corresponds to the applied field. Measurements for the magnetic properties can then be made.

Advantages and disadvantages in terms of experimental facets

The coils have the ability to heat the sample such that temperature variance can be observed in the way that the material behaves when influenced by a magnetic field. On the other hand, this could cause overheating of the system which could result in a failure. Using a BH-looper can give the user a more improved visualisation compared to a VSM of the way a material behaves. The values plotted on the scope are only proportional to the absolute values, therefore display yields qualitative not quantitative information about a material magnetic properties. The precision is generally low compared to a VSM. Because a hysteresis loop is viewed using an oscilloscope means that observations of whether the material is a soft or hard magnetic material. And this is why it is used in quality control testing industries like the control of ferromagnetic oxides in a magnetic tape factory.

Figure 2. A schematic layout of a BH loop tracer [2].

3(I) Difference between concepts of Vector Field B, Magnetisation M and the magnetising field H

The vector field B represents the magnetic induction. Magnetisation M is the magnetic moment per unit volume of a solid. Magnetising H field is the magnetic field strength. These three quantities are related by the equation.

With μ₀ being the permittivity of free space. To show the difference between these quantities, hysteresis loops for a magnetic material shown in figure 4 are used. One of the key differences shown is that the magnetisation saturates whereas the B field increases at a constant rate for certain values for H. The magnetisation is generated by the spin and the orbital angular momentum of electrons in the solid. H is generated outside the material by electrical currents[3]. Therefore, from the equation above, the B field is the combination of H and M which shows the difference between the quantities with the inclusion of the permittivity of free space.Find out how UKEssays.com can help you!

A way to show the difference between the 3 parameters is through the representation of a bar magnet in a magnetic field shown in figure 3. Unfortunately, due to the age of the diagram, the labels are a bit old. Hence the ‘True’ field denotes the vector field B and the Applied field represents the magnetisation M. However, the arrows represent the direction and strength of each parameter. It is clear from figure 3 that the Magnetisation is much stronger than the demagnetising field.

Figure 3 An example of a magnet being demagnetised in an applied field

From figure 4, the two sketches representing of B and M against H can give an understanding of other magnetic properties of the material. The curve on the left can show the saturation of the magnetic material as well as the remanence M_r– the remaining magnetisation after the applied field has been turned off. The right hand diagram can show the remanent induction B_r and the saturation point of the applied field. In terms of the difference between the parameters, M, B and H, they yield different properties of the material in question.

Figure 4 Hysteresis loops showing (a) M and (b) B field against H

3(II) The difference between the susceptibility and relative permeability

The relative permeability μ_r and susceptibility χ are very closely related as shown by the equation below:

The relative permeability represents a characterisation of magnetic materials. Paramagnetic or diamagnetic materials have permeabilities close to the permeability of free space. However for ferromagnetic materials, the permeability is large in comparison. It represents a multiplication factor. For example, the use of an iron core with a relative permeability is 200 times greater than just an air coil used. So this is a measure of the actual magnetic field within a ferromagnetic material. Susceptibility is a measure to an extent to which a material may be magnetised in a magnetic field. It represents a ratio of how much a material is magnetised compared to the applied field on that material [4]. So the susceptibility specifies how much the relative permeability differs from one as shown in the equation above.

References

[1] Foner S 1959 Versatile and Sensitive Vibrating-Sample Magnetometer* Rev. Sci. Instrum. 30 548–57

[2] Howling D H 1956 Simple 60-cps Hysteresis Loop Tracer for Magnetic Materials of High or Low Permeability Rev. Sci. Instrum. 27 952

[3] Jiles D 1990 Introduction to Magnetism and Magnetic Materials (Chapman and Hall)

[4] Magnetic Susceptibilty http://www.britannica.com/EBchecked/topic/357313/magnetic-susceptibility

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Increasing media storage density continues to be a very active area of research. Magnetic media may be divided into particulate and continuous media.

Particulate media are comprised of small magnetic particles bonded on a plastic tape or disk, the thickness of the magnetic overcoat is typically on the order of 10,000 Å. Since these are single domain particles, the information is stored by inverting the magnetization of some of the particles. Continuous media are metallic thin films, typically a few hundred angstroms in thickness. Particulate media are advantageous in that they are relatively simple to prepare and are chemically stable, however their recording density is relatively low.

Continuous media on the other hand have higher storage densities and the shapes of their hysteresis loops (and hence recording characteristics) may be varied in a controlled way.

Hard and Soft Magnetic Materials

Magnetic materials are classified into two broad categories, soft or hard. Soft magnetic materials are characterized by large permeabilities and very small coercivities, typically less than 1 Oe. Hard magnetic materials are most often used in permanent magnet applications, and are characterized by large saturation magnetizations, large coercivities, typically greater than 10 kOe, and also by large energy products (i.e., BHmax). Intermediate magnetic materials are generally characterized by coercivities on the order of 1 kOe, and these materials are usually used in magnetic media.

Intermediate magnetic materials include; Gamma-Fe₂O₃, Co₈₀Cr₂₀, Co₇₇Ni₁₀O₁₃, and thin films. The characteristics of any magnetic material, whether it is hard, soft, or intermediate, are best described in terms of their hysteresis loop. The most common measurement method employed for hysteresis loop determinations at ambient temperature is the Vibrating Sample Magnetometer (VSM).

This paper will discuss the utility of the VSM in the characterization of magnetic media materials. We will limit our discussion to longitudinal recording media, i.e., where the magnetization is parallel to the plane defined by the substrate/film. Perpendicular media, where the magnetization is perpendicular to the plane defined by the substrate/film, and magneto-optical materials are currently enjoying considerable research effort because of their potential for increasing areal storage densities.

Vibrating Sample Magnetometer (VSM) Systems

Vibrating Sample Magnetometer (VSM) systems are used to measure the magnetic properties of materials as a function of magnetic field, temperature, and time. They are ideally suited for research and development, production testing, quality and process control. Powders, solids, liquids, single crystals, and thin films are all readily accommodated in a VSM.

Contemporary commercial VSM’s feature virtually automated operation via data acquisition/control and analysis software that runs on a personal computer, thus making the VSM accessible to the non-specialist. This has dramatically increased the utility of this measurement technique in a broad range of measurement applications.

Theory of Operation of Vibrating Sample Magnetometer Systems

If a material is placed within a uniform magnetic field H, a magnetic moment m will be induced in the sample. In a VSM, a sample is placed within suitably placed sensing coils, and is made to undergo sinusoidal motion, i.e., mechanically vibrated. The resulting magnetic flux changes induce a voltage in the sensing coils that is proportional to the magnetic moment of the sample.

The magnetic field may be generated by an electromagnet, or a superconducting magnet. Variable temperatures may be achieved using either cryostats or furnace assemblies. In the context of the current discussion, we will consider electromagnet based systems only, as magnetic media are usually characterized at ambient temperature, and for only moderate field strengths. Tape and thin film samples to 1 inch in diameter may be characterized in the Lake Shore VSM.

The Hysteresis Loop

In the case of a typical recording medium the hysteresis loop gives the relation between the magnetization M and the applied field H. A hysteresis loop of a magnetic recording medium is illustrated schematically in Figure 1.

The parameters extracted from the hysteresis loop that are most often used to characterize the magnetic properties of magnetic media include; the saturation magnetization Ms, the remanence Mr, the coercivity Hc, the squareness ratio SQR, S* which is related to the slope at Hc , and the switching field distribution SFD. The loop illustrated in Figure 1 shows the behavior for the easy axis of magnetization (i.e., in the anisotropy direction). The loop has a rectangular shape and exhibits irreversible changes of the magnetization.

The hard axis loop, where the hard axis is at right angles to the easy axis, is more or less linear and generally hysteresis free, i.e., the magnetization is reversible. Magnetic materials that show a preferential direction for the alignment of magnetization are said to be magnetically anisotropic. When a material has a single easy and hard axis, the material is said to be uniaxially anisotropic.

The intrinsic saturation is approached at high H, and at zero-field the remanence is reached. The squareness ratio is given by the ratio of (M_r/M_s) and is essentially a measure of how square the hysteresis loop is.

In general, large SQR values are desired for recording medium. The formal definition of the coercivity Hc is the field required to reduce the magnetization to zero after saturation. The physical meaning of Hc is dependent on the magnetization process, and may be the nucleation field, domain wall coercive field, or anisotropy field.

H_c is a very complicated parameter for magnetic films and is related to the reversal mechanism and the magnetic microstructure, i.e., shape and dimensions of the crystallites, nature of the boundaries, and also the surface and initial layer properties, etc.

Parameters of Importance to Magnetic Media

S* and SFD are of particular importance in characterizing the magnetic properties of magnetic media. S* is related to the slope of the hysteresis loop at Hc, i.e., dM/dH|H_c = M_r/(H_c(1 – S*)). This is known as the Williams-Comstock construction. For longitudinal recording media there are two important parameters associated with the recording process that are intimately related to S*.

Namely, the maximum output signal depends on Mr, Hc, and S*, and the optimal bias current also depends on S*. The SFD = ΔH/Hc where ΔH is the full width at half maximum of the differentiated curve dM/dH (as illustrated in Figure 1) can be thought of as a distribution function of the number of units reversing at a certain field. For a particulate medium without collective behavior, the SFD has a close relation to particle size distribution because differently sized and shaped particles will reverse at different field strengths.

For longitudinal media the SFD is related to recording parameters such as noise, optimal bias current, and time dependent behavior. Media with high Hc and small SFD are desirable for high density recording.

Remanence Curves

In addition to the full hysteresis loop properties of magnetic media, there has been increased interest in the measurement of remanence curves. Measurement of remanence determines only the irreversible component of magnetization and thus enables the phenomena of switching to be deconvoluted from the hysteresis measurement, which generally includes a reversible component.

There are two principle remanence curves; the isothermal remanence (IRM) and the DC demagnetization curve (DCD). The IRM is measured after the application and removal of a field with the sample initially demagnetized. The DCD is measured from the saturated state by application of increasing demagnetizing fields. Both are illustrated schematically in Figure 2. These remanence curves are of importance because they yield the true SFD for the material. The VSM may also be used to measure the IRM and DCD remanence curves.

The remainder of this paper will present magnetic data for thin film magnetic media, thus demonstrating the utility of the Lake Shore VSM for measuring media magnetic properties.

Magnetic Measurements Using the Lake Shore VSM

The Lake Shore VSM features variable-gap electromagnets providing field strengths to over 2 tesla. Experimental flexibility, both in terms of achievable field strengths, and in terms of allowable sample sizes are provided since the gap spacing may be adjusted to maximize either.

Auto-rotation and Vector options facilitate investigations of anisotropy in magnetic media. With the auto-rotation option the sample may be rotated such that the applied field is oriented parallel to either the easy or hard axis of magnetization, or at any angle in between. The Vector option, which includes 2-axis or 3-axis coil sets placed at right angles to one another, permits simultaneous measurement of both easy and hard axis magnetization for fields oriented parallel to either axis. This option also permits the derivation of torque since torque is equal to the cross product of the field and magnetization vectors (i.e., t = M x H).

Data collection is fully automated with Windows based data acquisition/control and analysis software. Broad application versatility is maintained since measurement parameters may be easily defined and controlled. The software automatically extracts any of a number of parameters, e.g., Ms, Mr, Hc, SQR, S*, SFD, etc., directly from the measured hysteresis loop. And, extensive data analysis capabilities are also provided, e.g., derivative (SFD) curves, substrate and paramagnetic background corrections, etc.

Measurement Results

Hysteresis Loops for a Thick Film Disk Material

Figure 3 shows the initial, minor, and major hysteresis loop for a thin film disk material. In the context of the present discussion, the minor loops of magnetic media are sometimes of interest as they relate to modeling of the write process. Taken together with the major loop, the minimum head field strength required to ensure saturation and hence maximum remanence are determined.

Magnetization Curve, Major Hysteresis Loop and Remanence Curve for a Flexible Magnetic Media Material

Figure 4 shows the initial magnetization curve, major hysteresis loop, and also the DCD demagnetization or remanence curve for a flexible magnetic media material.

Major Hysteresis Loop for a Flexible Media Material

Figure 5 shows the major hysteresis loop for a flexible media material, and the derivative curves are also illustrated. These derivative curves are directly related to S* and the SFD. Since small SFD’s are desirable, the sharpness and width of these derivative curves are of interest. A narrow and stable switching transition produces a small SFD, and hence the derivative curves yield useful information concerning the magnetic structure of the media, which in turn is related to the microstructure and chemical inhomogeneities in the layer. These parameters are principally related to the deposition process itself.

Isothermal Remanence (IRM) and DC Demagnetization (DCD) Remanence Curves for a Flexible Media Material

Figure 6 shows the isothermal remanence (IRM) and DC demagnetization (DCD) remanence curves for a flexible media material. Interaction effects may be investigated by analyzing these curves. If the particulate media is characterized by non-interacting particles then a Henkel plot, i.e. IRM(H) vs. DCD(H), will be linear, and the forward and reverse SFD’s will be identical. Deviations from linearity are attributable to the effects of interactions in the system.

Figure 7 shows the Henkel plot corresponding to Figure 6 and Figure 8 illustrates the forward and reverse SFD’s obtained from differentiation of the IRM and DCD curves. Clearly the SFDs are not identical. The extent to which interactions exist in the system are revealed by these types of ΔM vs H curves. A larger interaction yields a larger ΔM peak. The use of this type of analysis is becoming increasingly common in the investigation of interaction effects in particulate and thin film media. A strong correlation exists between the form of these interaction effects, and the degree of dispersion of the particles.

Hysteresis Loop for Hard Disk Magnetic Film

Figure 9 shows a hysteresis loop for hard disk CoPt magnetic film deposited on a rigid disk substrate. Critical M(H) loop parameters are indicated in the figure.

Figures 10 and 11 show the hysteresis loop and derivative curve, respectively, for a hard disk film sample.

Summary

Selecting a VSM and Future Requirements

There are a number of considerations that come into play when selecting an appropriate VSM. These include; the types of materials that are to be measured, i.e., both intrinsic magnetic characteristics and physical properties and dimensions are important, required magnetic field strength, accessible temperature range, available measurement options, ease-of- use which is largely dictated by the software interface, etc.

Current research trends in magnetic media include the development of perpendicular recording media, magneto-optical materials, the development of pseudo-contact recording techniques, the use of magnetoresistive (i.e., GMR and CMR) multi-layer films for read heads, the use of alternative substrate materials (e.g., glass), and patterned media. Additionally, the superparamagnetic limit is being approached as magnetic film thicknesses are decreased. This trend will force VSM manufacturers to enhance the sensitivity characteristics of their VSM’s.

This paper has discussed some of the more important magnetic properties of magnetic media, their relation to the recording process, and their determination utilizing a Vibrating Sample Magnetometer measurement methodology. The wide spread use of magnetic media materials for audio, video, and data storage systems results in a continual research effort to increase storage densities, and decrease access time.

Advances made possible by materials science, combined with the development of commercially available computer automated characterization tools, such as the VSM will certainly result in significant advances in this area.

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It is the shift in wavelength of the inelastically scattered radiation that provides the chemical and structural information. Raman shifted photons can be of either higher or lower energy, depending upon the vibrational state of the molecule under study. A simplified energy diagram that illustrates these concepts is shown below.

Stokes radiation occurs at lower energy (longer wavelength) than the Rayleigh radiation, and anti-Stokes radiation has greater energy. The energy increase or decrease is related to the vibrational energy levels in the ground electronic state of the molecule, and as such, the observed Raman shift of the Stokes and anti-Stokes features are a direct measure of the vibrational energies of the molecule. A schematic Raman spectrum may appear as shown below.

The energy of the scattered radiation is less than the incident radiation for the Stokes line and the energy of the scattered radiation is more than the incident radiation for the anti-Stokes line. The energy increase or decrease from the excitation is related to the vibrational energy spacing in the ground electronic state of the molecule and therefore the wavenumber of the Stokes and anti-Stokes lines are a direct measure of the vibrational energies of the molecule.

In the example spectrum, notice that the Stokes and anti-Stokes lines are equally displaced from the Rayleigh line. This occurs because in either case one vibrational quantum of energy is gained or lost. Also, note that the anti-Stokes line is much less intense than the Stokes line. This occurs because only molecules that are vibrationally excited prior to irradiation can give rise to the anti-Stokes line. Hence, in Raman spectroscopy, only the more intense Stokes line is normally measured – Raman scattering is a relatively weak process. The number of photons Raman scattered is quite small. However, there are several processes which can be used to enhance the sensitivity of a Raman measurement.

Simplified energy diagram

If the wavelength of the exciting laser coincides with an electronic absorption of a molecule, the intensity of Raman-active vibrations associated with the absorbing chromophore are enhanced by a factor of 102 to 104. This resonance enhancement or resonance Raman effect can be extremely useful, not just in significantly lowering the detection limits, but also in introducing electronic selectivety. Thus the resonance Raman technique is used for providing both structural and electronic insight into species of interest.

Metalloporphyrins, carotenoids and several other classes of biologically important molecules have strongly allowed electronic transitions in the visible, making them ideal candidates for resonance Raman spectroscopy. Resonance selectivity has a further practical use, in that spectrum of the chromophoric moiety is resonance enhanced and that of the surrounding environment is not. For biological chromophores, this means that absorbing active centres can be specifically probed by visible excitation wavelengths, and not the surrounding protein matrix (which would require UV lasers to bring into resonance).

Resonance Raman spectroscopy is also an important probe of the chemistry of metal centred complexes, fullerenes, polydiacetylenes and other “exotic” molecules which strongly absorb in the visible. Although many more molecules absorb in the ultraviolet, the high cost of lasers and optics for this spectral region have limited ultraviolet (UV) resonance Raman spectroscopy to a small number of specialist groups.

Schematic Raman spectrum

Vibrations which are resonantly enhanced fall into two or three general mechanistic classes. The most common case is Franck-Condon enhancement, in which a component of the normal coordinate of the vibration occurs in a direction in which the molecule expands during an electronic excitation. The more the molecule expands along this axis when it absorbs light, the larger the enhancement factor. The easily visualized ring breathing (in-plane expansion) modes of porphyrins fall into this class. Vibrations which couple two electronic excited states are also resonantly enhanced, through a mechanism called vibronic enhancement. In both cases, enhancement factors roughly follow the intensities of the absorption spectrum. The fuller theory of resonance enhancement is beyond the scope of this section.

Resonance enhancement does not begin at a sharply defined wavelength. In fact, enhancement of 5x to 10x is observed if the exciting laser is within even a few 100 wavenumbers below the electronic transition of a molecule. This “pre-resonance” enhancement can be experimentally useful.

The Raman scattering from a compound (or ion) adsorbed on or even within a few Angstroms of a structured metal surface can be 103 to 106x greater than in solution. This surface-enhanced Raman scattering is strongest on silver, but is observable on gold and copper as well. At practical excitation wavelengths, enhancement on other metals is unimportant.

SERS arises from two mechanisms:

1. The first is an enhanced electromagnetic field produced at the surface of the metal. When the wavelength of the incident light is close to the plasma wavelength of the metal, conduction electrons in the metal surface are excited into an extended surface electronic excited state called a surface plasmon resonance. Molecules adsorbed or in close proximity to the surface experience an exceptionally large electromagnetic field. Vibrational modes normal to the surface are most strongly enhanced.
2. The second mode of enhancement is by the formation of a charge-transfer complex between the surface and analyte molecule. The electronic transitions of many charge transfer complexes are in the visible, so that resonance enhancement occurs. Molecules with lone pair electrons or pi clouds show the strongest SERS. The effect was first discovered with pyridine.

Other aromatic nitrogen or oxygen containing compounds, such as aromatic amines or phenols, are strongly SERS active. The effect can also be seen with other electron-rich functionalities such as carboxylic acids. The intensity of the surface plasmon resonance is dependent on many factors including the wavelength of the incident light and the morphology of the metal surface. The wavelength should match the plasma wavelength of the metal. This is about 382 nm for a 5μm silver particle, but can be as high as 600nm for larger ellipsoidal silver particles. The plasma wavelength is to the red of 650nm for copper and gold, the other two metals which show SERS at wavelengths in the 350-1000 nm region. The best morphology for surface plasmon resonance excitation is a small (<100nm) particle or an atomically rough surface. SERS is commonly employed to study monolayers of materials adsorbed on metals, including electrodes.

Other popular surfaces include colloids, metal films on dielectric substrates and, recently, arrays of metal particles bound to metal or dielectric colloids through short linkages. Although SERS allows easy observation of Raman spectra from solution concentrations in the micromolar (10x-6) range,non-reproducability of quantitative measurements has in the past marred its utility for analytical purposes. However, standardization in production of SERS active media is steadily improving its potential in this area also.

UVRRS is a powerful tool in the molecular analysis of complex biological systems. Most biological systems absorb UV radiation and hence have the ability to offer resonance with UV Raman excitation. This results in the highly selective resonance Raman effect enabling enhancement of important biological targets such as protein or DNA. For example, excitation around 200nm enhances the Raman peaks from vibrations of amide groups; excitation around 220nm enhances peaks from certain aromatic residues. The Raman scatter from water is weak, allowing for analysis of very weak aqueous systems.

Fiber optic UVRRS configuration

Due to the selective nature of UVRRS, a tunable laser is typically required as the excitation source. Since truly tunable continuous-wave lasers are not yet available, a Nd:YAG-pumped dye laser with frequency-doubled output is one suitable UVRRS system. Depending on the dyes used, this laser setup can give almost any required UV wavelength. Intensified CCDs (ICCDs) with UV photocathodes, back-illuminated CCDs or CCDs with UV enhancing (BASF lumogen)coatings can be used as detectors for UVRRS. These detectors are used on account of their high detection efficiency and multichannel capabilities. The primary obstacle to the merging of the worlds of UVRRS and fiber-optic spectroscopy is solarization, the process by which UV radiation causes opacity of fiber-optics (even quite pure silica fibers). This opacity impairs transmission, rendering standard fiber-optics useless for UVRRS.

Species of Interest

Pulsed lasers are typically utilized in the study of short-lived species. A laser pulse can be supplied to a molecular system with enough energy to redistribute the electrons in a molecule causing the formation of an excited state as illustrated on the right. The Raman spectrum of this excited state molecule can be studied either using the same laser pulse or a different pulse from a second laser (single color and two-color pulsed Raman). Excited states of interest can have lifetimes, from picoseconds to milliseconds, but the majority can be studied using gating in the order of 5ns. As the majority of excited states are generated using UV and visible lasers, photocathodes with high UV and visible Quantum Efficiencies (QEs) are typically suitable.

Schematic of pump-probe (two color) Raman

The simplest pulsed laser experiments are so-called single-color experiments where high irradiance laser pulses are used both to initiate the photoreaction, and then to Raman probe the transient species created within the pulse width. By opening the intensifier tube as shown on the right, only the Raman spectrum of the excited state will be recorded. This pulse/ICCD gate combination will be repeated and accumulated hundreds to thousands of times in order to achieve a good overall signal-to-noise ratio with high dynamic range.

In Time Resolved Resonance Raman (TR3) spectroscopy, pairs of laser pulses of different wavelength are used to photolyse (optically “pump”) and then to Raman probe the transient species of interest. The spectral window of the spectrograph/detector is chosen so that it corresponds to the frequency range of the Raman scattering from the probe laser.

Pulsed two color Raman layout with delays under the control of a delay generator

In Time Resolved Resonance Raman (TR3) spectroscopy, pairs of laser pulses of different wavelength are used to photolyse (optically “pump”) and then to Raman probe the transient species of interest. The spectral window of the spectrograph/detector is chosen so that it corresponds to the frequency range of the Raman scattering from the probe laser.

The time evolution of the transient signal is monitored by recording a series of spectra at different delays after the photolysis event, i.e. at a series of time delays between the excitation and probe pulses. The ICCD camera or either of the lasers can supply the trigger. A delay generator is used to control the delays.

In Raman microscopy, a research grade optical microscope is coupled to the excitation laser and the spectrometer, thus producing a platform capable of obtaining both conventional images and in addition generating Raman Spectra from sample areas approaching the diffraction limit (~1 micron). Imaging and spectroscopy can be combined to generate “Raman cubes”, 3- dimensional data sets, yielding spectral information at every pixel of the 2D image.

A motorized xyz microscope stage can be used to automatically record spectral files, which will constitute the basis of Raman images, Raman maps or a set of Raman spectra recorded from preselected points. Specific software routines will allow the quick and easy reconstruction of these maps. The possibility of generating two-dimensional and three-dimensional images of a sample, using various special features, is an evident advantage over either traditional spectroscopy or microscopy.

Time delay sequences

The first ever Raman “instrument” was constructed in 1928. This instrument used monochromatized sunlight as a light source and a human eye as a detector. Raman instrumentation was developed (based around arc lamps and photographic plates) and soon became very popular up until the 1950s. Since these early days, Raman instrumentation has evolved markedly. Modern instrumentation typically consists of a laser, Rayleigh filter, a few lenses, a spectrograph and a detector (typically a CCD or ICCD).

Typical Continuous Wave (CW) Raman layout

One of the major advantages of dispersive Raman is that it offers the possibility to select the optimal laser excitation wavelength to permit the recording of the best Raman information. For example, wavelengths can be selected to offer the best resonance with the sample under investigation.

One might also need to tune wavelength to avoid fluorescence and thermal emission backgrounds. Nowadays, it is possible to use laser lines from UV, (down to 200nm) up to the infrared, (1.06μm Nd:YAG laser line), from microWatts up to several Watts.

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Light interacts with matter in different ways, transmitting through some materials, while reflecting or scattering off others. Both the material and the colour (wavelength) of the light affect this interaction. We call the study of this light ‘spectroscopy’. Which parts of the visible spectrum enter our eyes determines which colours we perceive.

A substance might appear blue, for example, if it absorbs the red parts of the spectrum of light falling upon it, only reflecting (or scattering) the blue parts into our eyes.

Raman spectroscopy looks at the scattered light

If you were to shine blue light—from just one part of the spectrum—onto the material, you might expect to just see blue light reflected from it, or no light at all if it is completely absorbed (i.e. a black material).

However, by using a Raman spectrometer, you can see that often a very tiny fraction of the scattered light has a different colour. It has changed frequency because, during the scattering process, its energy changed by interacting with molecular vibrations. This is the Raman scattering process, named after its discoverer, the famous Indian physicist C.V. Raman. He was awarded the 1930 physics Nobel Prize for this great discovery.

By studying the vibration of the atoms we can discover the chemical composition and other useful information about the material.

The Raman effect is very weak; only about 1 part in 10 million of the scattered light has a shifted colour. This is too weak to see with the naked eye, so we analyse the light with a highly sensitive spectrometer.

Raman spectrometers

These systems consist of:

• one or more single coloured light sources (lasers)
• lenses (both to focus the light onto the sample and to collect the scattered light)
• filters (to purify the reflected and scattered light so that only the Raman light is collected)
• a means of splitting the light into its constituent colours (normally a diffraction grating or prism)
• a very sensitive detector (to detect the weak light)
• a device such as a computer to control the whole system, display the spectrum and enable this information to be analysed

Raman scattering offers significant advantages for the investigation of materials over other analytical techniques, such as x-raying them or seeing how they absorb light (e.g. infrared absorption or ultraviolet absorption).

aman spectroscopy reveals the chemical and structural composition of samples. Generally, all materials produce Raman spectra, with the exception of pure metals.

Raman scattering

Raman scattering occurs when light interacts with molecular vibrations. This is similar to the more widely known infrared absorption spectroscopy, but different rules apply. A change in molecular polarisability is required during the vibration for the Raman effect to occur.

You will see some vibrations in the Raman spectrum that are not visible in the infrared spectrum, and vice-versa, because of the different selection rules. For example, Raman spectroscopy is superb for studying the carbon atoms that make up the structure of diamond, unlike infrared absorption spectroscopy.

Scattered light

The first step in producing a Raman spectrum is to illuminate your sample with a monochromatic light source, such as a laser.

Most of the light that scatters off is unchanged in energy (‘Rayleigh scattered’). A minute fraction—perhaps 1 part in 10 million—has lost or gained energy (‘Raman scattered’). This Raman shift occurs because photons (particles of light) exchange part of their energy with molecular vibrations in the material.

Where energy is lost the Raman scattering is designated as ‘Stokes’; where energy is gained the Raman scattering is designated as ‘anti-Stokes’. We rarely use anti-Stokes Raman light as it is less intense than the Stokes, however it does represent equivalent vibrational information of the molecule.

Vibrating atoms

The change in energy depends on the frequency of vibration of the molecule. If it is very fast (high frequency)—light atoms held together with strong bonds—the energy change is significant. If it is very slow (low frequency)—heavy atoms held together with weak bonds—the energy change is small.

Raman spectrometers

Renishaw inVia systems consist of:

• single or multiple lasers, from UV (244 nm) to IR (1064 nm) – switch with a single click
• high quality objective lenses, from high confocal 100× to long working distance and immersion options
• custom designed motorised spectrometer lenses­ – automatically align for each configuration
• laser-line-specific Rayleigh filters with a dual filter arrangement to optimise sensitivity
• highest quality master diffraction gratings provide exceptional dispersion and longevity
• thermoelectrically cooled (- 70 ºC) CCD detector – stable and sensitive
• high specification multi-core PC for data collection and analysis

Raman spectra

We graphically depict the results of our measurements as Raman spectra. We plot the intensity of the scattered light (y-axis) for each energy (frequency) of light (x-axis). The frequency is traditionally measured in a unit called the wavenumber (number of waves per cm, cm^-1).

We plot the x-axis frequencies relative to that of the laser as it is the shift in energy of the light that is of particular interest.

How do I get the information I want from my spectrum?

You can tell a great deal about a material from its Raman spectrum, with different features relating to different aspects of the material.

The key features are:

The Raman shifts and relative intensities of all of the Raman bands of the material
With this, we can identify the material.

Individual band changes
A band may shift, narrow or broaden, or vary in intensity. These changes can reveal information about stresses in the sample, variations in crystallinity, and the amount of material respectively.

Variations in spectra with position on the sample
This will reveal changes in the uniformity (homogeneity) of the material. You can analyse at several arbitrary points, or systematically measure an array of points (enabling the production of images of composition, stress, crystallinity, etc.)

What do the Raman bands represent?

It is easy to understand the Raman spectrum of crystals with a regular array of identical atoms, all in the same configuration (such as the carbon atoms in diamond). In these cases, you often see just one dominant Raman band (because there is just one molecular environment of the crystal).

The Raman spectrum of polystyrene, however, is much more complex because the molecule is less symmetric and has hydrogen atoms in addition to carbon atoms. There are also different bond types connecting the atoms.

Vibration frequencies

The frequencies of vibration depend on the masses of the atoms involved and the strength of the bonds between them. Heavy atoms and weak bonds have low Raman shifts. Light atoms and strong bonds have high Raman shifts.

We see the high frequency carbon-hydrogen (C-H) vibrations in the polystyrene spectrum at about 3000 cm^-1. The low frequency carbon-carbon (C-C) vibrations are at around 800 cm^-1. The C-H vibrations have a higher frequency than the C-C vibrations because hydrogen is lighter than carbon.

We see the vibrations of two carbon atoms linked by strong double bonds (C=C) at around 1600 cm^-1. This is at a higher frequency than two carbon atoms lined by a weaker single bond (C-C, 800 cm^-1).

You can use these simple rules to explain many of the features of Raman spectra.

Vibrations in detail

You can see more subtle effects if you inspect spectra closely. The strength of bonds also affects their vibration rates. For example, the C-H vibrations of polystyrene appear in two bands, at approximately 2900 cm^-1 and 3050 cm^-1. The carbons in the former are part of carbon chains (‘aliphatic’), whereas the carbons in the latter form part of carbon rings (‘aromatic’).

You can view the vibrations of a complex molecule as partly consisting of many simple diatomic vibrations. However the full richness of the Raman spectrum can only be understood by considering the vibrations of larger groups of atoms (such as the expanding/contracting ‘breathing mode’ of the aromatic carbon ring that appears at 1000 cm^-1 in polystyrene).

Low frequency vibrations

You can also study Raman bands with low Raman shifts, below 100 cm^-1. These originate from very heavy atoms or very large-scale vibrations, such as the whole crystal lattice vibrating. Renishaw’s Raman instruments enable you to study these modes and explore a wide range of materials and crystals, and distinguish between different crystalline forms (polymorphs).

The big picture

A Raman spectrum therefore consists of a range of features, each associated with a vibrational mode. The spectrum is unique to the material and enables you to identify it. It is important to note that, although a full understanding of the vibrational modes is of interest, you rarely need this as you can use a reference database for identification.

When a sample is illuminated by a laser, both Raman scattering and photoluminescence (PL) can occur. The latter can be many times stronger than the former and can prevent successful Raman analysis.

PL comprises both fluorescence and phosphorescence processes and originates from an absorption/emission process between different electronic energy levels in the material. The amount and type of PL depends on which material you are studying and which laser wavelength you are using. Unwanted fluorescence interference can normally be avoided by choosing an appropriate laser wavelength.

• Energy diagram showing absorption of light and the processes involved in the emission of light as fluorescence and phosphorescence.

What PL can tell us

In many cases photoluminescence carries useful information that can facilitate sample analysis and augment the Raman data. inVia confocal Raman microscopes are suited to the analysis of both Raman scattering and PL.

Fluorescence imaging (a type of PL) is often employed in the biological sciences, where fluorescent tags are used to reveal the presence and distribution of molecular species. However, this approach is more invasive than Raman analysis, which is typically tag-free. Renishaw’s inVia confocal Raman microscope can be used to generate images of fluorescent tags, but more commonly provides valuable tag-free chemical information.

You can also use PL to study crystal defects, such as atomic vacancies and substitutions. This is of particular importance for materials such as diamond and silicon carbide (SiC). Not only can you identify the defect, but you can also tell if the crystal has internal stresses.

• Stress image generated from the ruby R2 PL band position

How to avoid PL backgrounds

Occasionally PL bands are strong and broad, masking Raman information. You can counter this by using a different laser wavelength. This can move the Raman bands away from the peak emission of the PL band and may even avoid generation of the PL entirely.

Ideally, a Raman instrument should be able to switch rapidly and easily between different laser wavelengths, so that you can select or avoid PL features, depending on your requirements.

Raman images (sometimes referred to as maps) depict a variation in spectral information from different points on, or in your sample. They can take the form of one-dimensional profiles, two-dimensional images, or three-dimensional rendered volumes. With them, you can rapidly see how a Raman parameter alters with position.

The parameter could be as simple as the intensity of a particular Raman band, or you could derive it from a more complicated analysis of the whole Raman spectrum.

The two main methods of collecting the spectral data to generate these images are Raman mapping and Raman imaging.

• White light and Raman images of washing powder

Raman mapping

Raman mapping collects a spectral hypercube (a Raman spectrum from each position on the sample in a single file), rather than a simple intensity image. The hypercube is analysed to produce Raman images.

There are several Raman mapping methods, such as:

• Point-by-point mapping
  The laser is focused to a spot. A motorised stage moves the sample under the laser. Spectra are sequentially acquired from an array of sample points spanning the defined region of interest. Fast versions of this are Renishaw’s StreamHR™ and StreamHR Rapide.
• Line focus mapping
  This is similar to point-by-point mapping, but the laser illuminates a line on the sample, rather than a spot. This enables you to simultaneously collect spectra from multiple positions on the sample, saving time. With this method you can use higher laser powers without damaging the sample (reducing exposure times). Renishaw’s StreamLine™ is a sophisticated modern implementation of this concept.

It is important to consider the potentially undesirable effects of undersampling when mapping. This is most clearly illustrated when point-by-point mapping: parts of the sample will be ‘missed’ if the laser spot is smaller than the spacing between acquisition points. Renishaw has solved this problem through the use of the StreamLine™ Slalom mode.

Generating Raman images from map data

Once all the Raman spectra are collected from the mapping experiment, they can be analysed to produce profiles, images or rendered volumes. Analysis options in Renishaw’s WiRE software include:

• Intensity at one frequency in the spectrum
  This produces an equivalent image to that from Raman imaging. These are quick to generate but may be misleading because it is not possible to differentiate between intensities arising from a Raman band of interest and those associated with a broad background fluorescence.
• Curve fit parameters
  All the spectra in the set have a theoretical curve fitted to one of the Raman bands. Images are then made based on the theoretical curve parameters for each spectrum. Images are often made using the centre frequency of the curve (band), or the full width at half maximum (FWHM), as this is sensitive to stresses and crystallinity within the sample respectively.
• Multivariate parameters
  Images can be generated using chemometric tools, such as generic principal component analysis (PCA), or Renishaw’s Empty Modelling™, which is optimised for Raman data. The Empty Modelling method reveals systematic variations between the Raman spectra, and highlights the distribution of these variations across the sample as an image. This is achieved without the need for prior knowledge of what is present within the sample, which greatly simplifies the analysis process. Multivariate analysis is very powerful because it uses information from the entire spectrum, not just one part of it (intensity at one frequency) or one curve-fitted band. This typically results in higher quality Raman images.

Raman imaging

Raman imaging is analogous to taking a photograph; spectral intensity values are collected simultaneously from the entire area of interest. The laser illuminates a square or circular region on the sample. The light is filtered so that the intensity of just one narrow part of the spectrum is recorded on the detector.

The single image collected contains limited information, just the intensity of the light at that frequency. However, these images can be acquired rapidly. This is especially true if you have a high power laser; because the light is spread over an area, you can use all the power without damaging your samples, with correspondingly short exposure times.

Two-dimensional images are typically produced using this method. Renishaw’s True Raman Imaging is an example of Raman imaging.

Note that it is possible to collect intensity values covering multiple points of the spectrum by using multiple and/or tuneable filters.

Spatial resolution

Point-by-point Raman mapping

Spatial resolution is determined by a combination of the laser spot size and the spacing between acquisition points on the sample.

• Laser spot size
  This is a function of the objective magnification and the laser wavelength (higher magnification and shorter wavelengths produce smaller spot sizes)
• Spacing between acquisition points on the sample (sampling)
  This is a function of the sample stage (ideally stages should have a large travel range while still enabling a step size down to 100 nm, smaller than the smallest spot size)

Raman imaging

Spatial resolution is determined by the magnification of the optics in the system and the size of the elements in the detector. Ultimately this is limited, by the inherent wavelike nature of light, to a little under a micrometre.