Raman Spectroscopy of
Colossal Magnetoresistive
Perovskite Material
By
Kendal Clark
Central Methodist College
Mentor:
Dr. Sooryakumar
The Ohio State University Physics Department
REU Summer 2001
National Science Foundation
August 16, 2001
Abstract
The popularity of studying Colossal Magnetoresistive (CMR) materials has grown
in the last few years with the thought of using these materials for making larger capacity;
smaller size hard drives for computers. Along with applications of these materials
scientists want to learn more about what causes this colossal magnetoresistive effect. This
paper shows one way Raman spectroscopy can be used to study these materials. The
paper also gives background information on Raman scattering and CMR material along
with a description of the experimental setup that was used. The paper concludes with data
and results that I have taken from some specific CMR perovskite materials.
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Raman Spectroscopy
Raman spectroscopy can simply be described as the inelastic interaction of light
with a material. When light, in this experimental case laser light, strikes an object some
of the light is transmitted, some is absorbed, but a majority of the light is scattered. Most
of the light that is scattered is elastically scattered. This scattering is called Rayleigh
scattering and the scattered light has the same energy and frequency as that of the
incident light. The smaller portion of the scattered light is inelastic scattered having a
higher or lower frequency and therefore a different energy than the incident light. This
inelastic scattered light makes up the Raman spectrum. This is a very small portion of the
total scattered light and precise instruments are needed to detect it. The shift in frequency
of the light is due to the interactions of the photons of light with the vibration and rotation
of the molecules in the material being studied. If this interaction shifts the frequency of
the scattered photon to a higher frequency, corresponding to a lower final energy (an
energy equal to (ωincident - ωscattered) given to the scattering medium) then it is an antiStokes process if it is shifted to a lower frequency or higher final energy it is called a
Stokes process [1]. Figure 1 shows the energy of the vibrational levels of the sample
material. The anti-Stokes process does not occur as often as the Stokes process, because
the initial vibrational level of the molecule in the anti-Stokes process is in an excited state
and not the ground state like it is in the case of the Stokes process.
Figure 1
(http://www.kosi.com/raman/applications/ramantutorial/)
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Since its discovery in 1928 Raman spectroscopy has been very useful for finding
molecular vibration information of materials. In recent years Raman spectroscopy has
become even more accurate and easier to use thanks to advancements in optics, laser and
computer technology. Chargecoupled Device (CCD) detectors have enormously helped
the use of Raman spectroscopy by allowing scientist to take data quicker and with more
precision that they were able to with the older photo multiplier tubes. The CCD has an
array of detectors that can look at a range of wavelengths at one time greatly reducing the
collection time [2]. In older spectrometers with photo multiplier tubes the grating of the
spectrometer would physically move in small increments over a period of time to take a
scan of the spectrum which is a very time consuming process.
Raman spectroscopy can be used on liquids, solids and gases making it very
versatile for studying various materials. Because of the distinct spectra that certain
classes of materials give off, due to their structural arrangement, Raman spectroscopy can
be used to determine the composition of unknown substances. This also makes Raman
spectroscopy ideal for qualitative analysis of materials. In Raman spectroscopy no probe
physically touches the material the laser light is the only thing to disturb the sample, this
means that the material is not disturbed by the probe physically touching it and in some
cases is the only way to accurately study a material.
Surface enhanced Raman spectroscopy (SERS) and resonance Raman effect
(RRE) are different types of Raman spectroscopy. The goal of these two processes is to
enhance the weak signal of the Raman spectra. Micro Raman spectroscopy (MRS) is
another type of Raman spectroscopy this process reduces the spot size of the light source
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on the sample, which is helpful if a small area of the sample is to be observed. It is also
used to reduce damage or heating of the sample by the laser light [3].
Colossal Magnetoresistive Perovskite Material
Colossal Magnetoresistive
(CMR) materials exhibit a very
large change in resistance with
the application of a magnetic
field. The reason they are called
colossal is to distinguish them
from the giant magnetoresistive
materials that do not exhibit as
big of change in resistance as the
CMR materials do. Along with
Figure 2.
Resistively verses temperature at 0 and 6 T magnetic fields.
(http://www-dmg.msm.cam.ac.uk/dmg/research/cmr/whatcmr1.html)
the magnetic field dependence this change in resistance is also very temperature
dependent. Figure 2 shows the change in resistance of a CMR material at various
temperatures in the presents of and with no magnetic field. This material in figure 2 has
very similar properties to the material that I
studied.
Perovskite structures are a simple
RE1-xAEx
cubic structure (shown in figure 3) having
O2-
the general formula RE1-xAExMnO3,
Mn
where RE is a trivalent rare earth element
like La, Pr, Nd, etc. AE is a divalent alkaline
Figure 3.
Perovskite Structure
(http://www.u.arizona.edu/~wbetush/min/basic.html)
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earth ion such as Sr, Ca, or Ba [4]. The structure of this cube can be shifted by the
application of magnetic fields or with a change in temperature. This distortion of the
perovskite of form ABO3 (A = RE1-xAEx and B = Mn) is determined by the tolerance
factor f defined as f = (rB + rO)/(2(rA + rO))1/2. Where ri (i = A, B or O) is the averaged
ionic size of each element. The tolerance factor measures the lattice-matching of the
sequential AO and BO2 planes. If f is close to 1
then the structure is cubic but if 0.96 < f < 1 it
forms a rhombohedral structure and if f < 0.96
then a orthorhombic structure is present. Figure 4
shows the orthorhombic and rhombohedral
structures, note the distortion from the cubic
Figure 4
(http://www.u.arizona.edu/~wbetush/min/temp.html)
structure. These transitions to different structures
will give different Raman spectra and correspond to the peaks that show up on the graphs
[4].
Applications of these CMR materials, which drive the scientists to study them, are
in the possibilities of using them for magnetic read heads in computer hard drives. With
these CMR perovskite materials in computer hard drives the storage capacity would
continue to increase, with hard drive storage well above anything we have today. This is
possible because of the areal density that these read heads would have, they could have
around 30 to 60 gigabytes per in2 or even more. With this increase in density comes
smaller and faster hard drives. Many companies are working very hard to develop this
type of hard drive and understanding the properties of this material is very important to
their success.
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Experimental Setup
Figure 5.
Figure 5 is a schematic of the Raman setup I was using. In this experiment three
different wavelengths of light could be used to study the sample; blue (488nm), green
(514.5nm) or red (674.1nm) laser light. This helps with many aspects of the experiment
one of which is that some samples give better spectrums with different wavelengths of
light so we could change the light to accommodate this. Also by changing the laser light
you can confirm if a peak is a true Raman peak and not a peak just associated with the
wavelength of the laser light that was used. The sample was placed into the cryostat
chamber were the temperature could be varied from 300 to 20 K. The low temperature
was achieved by the use of liquid helium that cooled the cryostat. The cryostat was kept
at a vacuum of around 20 to 30 militorr by a roughing vacuum pump, this was to make
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sure that no laser light scattering was coming from the particles of the air in the chamber;
it was also necessary because at low temperatures water vapor in the chamber would
deposit on the cryostat window and no measurement could be made. Before every
measurement the scattered light would have to be aligned in the spectrometer so that
maximum signal would be hitting the detector. This was achieved by moving the sample
to different positions so the laser was focused on it and also fine tuning the mirrors and
lenses on the optical table. Calibration of the spectrometer was done periodically with a
silicon sample. The main peak of silicon is know so we would place silicon in the
cryostat to make sure the spectrometer was reading the correct wave number. Silicon was
also used to maximize the signal in the spectrometer by adjusting and focusing the
mirrors and lenses. Using the CCD detector an almost real-time graph could be viewed
on the computer screen. Another advantage to the CCD detector was the ability to scan a
range of wave numbers at one time allowing for more data to be taken in a given time.
Experimental Data
Data taken from the Raman setup are shown in figures 6 and 7. Figure 6 is a graph
of a thin film of (La0.6 Pr0.4)0.87 Ca0.33 MnO3 grown on a LAO (LaAlO3) substrate. The
graphs have been stacked to help to see the emergence of the peaks. The data has been
modified by a derivative filter to remove the cosmic ray peaks that hit the detector during
the experiment. Figure 7 is a scan of only the LAO substrate and the same derivative
filter was used to remove cosmic peaks. In both graphs the sample was studied with the
read laser at 647.1nm and the power ranged from 30 to 40 mW. Care was taken to ensure
that the laser was focused on the film and not on the substrate in figure 6. Both graphs are
of the Stokes process because of the greater intensity of that shift as discussed earlier.
(La0.6 Pr0.4)0.87 Ca0.33 MnO3 Thin Film
Figure 6
LAO (LaAlO3) bulk substrate
Figure 7
8
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Results
The results that I can draw from the data that I have taken is that the peak that we
are seeing at a Raman shift of about 460 cm-1 (in figure 6) is not coming from the
substrate but is coming from the sample itself. If this peak was coming from the substrate
it would not be as broad and it would be closer to the 490 cm-1 wave number. It is
important to determine that the spectrum is not coming from the substrate to make sure
you are seeing properties of the material. I also studied this same thin film on a NGO
(NdGaO3) substrate and the graph of the sample (not shown) was nearly identical to the
LAO substrate sample.
Figure 8 shows a graph taken from the book Raman Scattering in Materials
Science by Weber and Merlin page 473.
This graph is showing a spectrum from a
La0.7Ca0.3MnO3 sample that is similar to
the sample I studied. Note the broad peak
that is at the 460 cm-1 position, this is
similar to the peak at the same spot in the
data I collected in figure 6. Its behavior as
the temperature decreases is also similar to
Raman shift (cm-1)
that in figure 6 with the one broad peak
becoming two sharper peaks as
Figure 8
La0.7Ca0.3MnO3 Film. Temperature from top to bottom 300,
250, 220, 180, 70, 7 K, respectively. [1]
temperature decreases. By this similarity I would conclude that the peaks are coming
from the same vibrational mode, but more analysis is needed to directly determine this
correlation. I would like to talk more about the graphs of the sample but at this point we
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cannot assign any of the peaks to vibrational modes because we do not have sufficient
data or knowledge of the structure itself, as more information is gathered the structural
changes in this material will be able to be determined.
Acknowledgements
I would first like to thank Dr. Sooryakumar for helping with any questions I had
and for getting me started with information about what I was doing. I would also like to
thank Jared Gump for answering all my questions in the lab. I greatly thank Rudra
Bandhu for taking time to teach me how to use the lab equipment and allowing me to
work with him in the lab. Finally I would like to thank Tom Lemberger, William Palmer,
and Shirley Royer for all the help in making this a great REU program, and also the
National Science Foundation for sponsoring this group.
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References
[1] Raman Scattering in Materials Science W.H. Weber and R. Merlin. Springer 2000
[2] Analytical Applications of Raman Spectroscopy Michael J. Pelletier. Blackwell
Science 1999
[3] Lucazeau, G. Abello, L. 1995 Raman spectroscopy in sold state physics and
material science. Theory, techniques and applications. 23, 301-3111
[4] Tokura, Y. Tomioka, Y. 1999 Colossal magnetorestive manganites. Journal of
Magnetism and Magnetic Materials. 200 1-23