Raman Spectroscopy Of

Raman Spectroscopy of Colossal Magnetoresistive Perovskite Material

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. 2

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 anti-

Stokes 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/) 3

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 4 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 Figure 2. Resistively verses temperature at 0 and 6 T magnetic fields. CMR materials do. Along with the (http://www-dmg.msm.cam.ac.uk/dmg/research/cmr/whatcmr1.html) 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 cubic structure (shown in figure 3) having RE1-xAEx the general formula RE1-xAExMnO3, O2- where RE is a trivalent rare earth element Mn like La, Pr, Nd, etc. AE is a divalent alkaline Figure 3. earth ion such as Sr, Ca, or Ba [4]. The Perovskite Structure (http://www.u.arizona.edu/~wbetush/min/basic.html) 5 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 =

1/2 Mn) is determined by the tolerance factor f defined as f = (rB + rO)/(2(rA + rO)) . 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 structure. These Figure 4 (http://www.u.arizona.edu/~wbetush/min/temp.html) 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. 6

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 7 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. (La Pr ) Ca MnO Thin Film 8 0.6 0.4 0.87 0.33 3

Figure 6

LAO (LaAlO ) bulk substrate 3

Figure 7 9

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 Figure 8 La Ca MnO Film. Temperature from top to bottom 300, 250, 0.7 0.3 3 becoming two sharper peaks as 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 10 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. 11

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