Investigating Magnetic Properties of Double Perovskites

Transcription

1 Investigating Magnetic Properties of Double Perovskites Ian Gilbert 2009 NSF/REU Program Physics Department, University of Notre Dame Advisors: Dr. Howard A. Blackstead Matthew P. Smylie

2 Abstract We synthesized the double perovskites La 2 LiNbO 6 and La 2 CuSnO 6 to investigate their electronic and magnetic properties. La 2 LiNbO 6 proved to be very difficult to prepare properly, likely due to the volatility of the lithium ions; the samples prepared were nonmagnetic insulators. Our study of La 2 CuSnO 6 is ongoing; however, initial results suggest that it is antiferromagnetic when cooled in the absence of a magnetic field and ferromagnetic when cooled in a field. X-ray diffraction suggests we are able to synthesize this material with greater phase purity than previous researchers. Introduction High temperature superconductivity remains one of the great mysteries of physics. Superconductivity was first discovered in 1911 by Heike Kamerlingh Onnes, a Dutch physicist. He had recently succeeded at liquefying helium, which allowed him to study metals at temperatures lower than ever before. While he was investigating the resistance of mercury at very low temperatures, he discovered that the resistance abruptly dropped to zero around 4 K. This phenomenon came to be known as superconductivity. It was not until the 1950 s that a theoretical explanation was given for superconductivity, by three now-famous physicists, Bardeen, Cooper, and Schrieffer. At sufficiently low temperatures, the distortions in the crystal lattice that an electron creates as it moves through a metal attract the second electron, effectively allowing conduction with exactly zero resistance. These pairs of electrons are known as Cooper pairs. Bardeen, Cooper, and Schrieffer were awarded the 1972 Nobel Prize in physics, and their explanation is known as BCS Theory in their honor.

3 BCS Theory predicts that no superconductors can have transition temperatures greater than about 30 K (the transition temperature of a material is the highest temperature at which the material can superconduct). In 1986, however, Bednorz and Müller discovered a ceramic material that superconducted at temperatures as high as 35 K. This discovery was so astonishing that they were given the Nobel Prize in physics the following year. Over the next few years, a number of other so-called high temperature superconductors were discovered with transition temperatures as high as 138 K. Condensed matter physicists continue to search for superconductors with ever-higher transition temperatures. If a superconductor with a transition temperature above room temperature is found, it would revolutionize electronic technology, as well as many other industrial applications of condensed matter physics. One of the hallmarks of superconductivity is the expulsion of all magnetic field lines from the superconductor. Known as the Meissner Effect, this phenomenon is one of the easiest ways to detect superconductivity. Thus the study of superconductivity and other magnetic behaviors of materials often go hand in hand. This summer we studied La 2 LiNbO 6 and La 2 CuSnO 6, two materials having interesting magnetic and possibly superconducting properties. X-ray diffraction (XRD), electron spin resonance (ESR) spectroscopy, and a superconducting quantum interference device (SQUID) magnetometer were used to study these materials. The resistance as a function of temperature was also measured for several of the samples. The first compound we studied was La 2 LiNbO 6. This was investigated because it is a homologue of other high temperature superconductors, and because it does not

4 contain any magnetic ions. In general, magnetic ions tend to prevent superconductivity because they break up Cooper pairs. We also studied the compound La 2 CuSnO 6. Our interest in this material is due to the layered structure of the crystal. Instead of forming planes of like ions along the body diagonal of the unit cell like other well-known O6 materials, this compound forms horizontal sheets of copper and tin ions. Figure 1 presents a simplified unit cell of this material and that of a more typical O6 material, Ba 2 YRuO 6. As many high temperature superconductors have planes of copper oxide, La2CuSnO 6 is a candidate for superconductivity. Previous studies of this material by Anderson and Poeppelmeier [1] failed to reveal superconductivity. However, our processing techniques are superior to those used by Anderson and Poeppelmeier; in particular, our high temperature furnaces allow for hotter, better sintering of samples, which leads to higher phase-purity. Figure 1. Simplified Unit Cell Diagram for La 2 CuSnO 6 and a Typical O6 Material, Ba 2 YRuO 6. Note the horizontal planes of CuO 2 in the La 2 CuSnO 6 sample compared with diagonal planes of CuO 2 in Ba 2 YRuO 6 (ex. Cu ions at bottom back right, top front left, and center of the Ba 2 YRuO 6 unit cell).

5 Experiment The La 2 LiNbO 6 sample was prepared by the following reaction: La 2 O Li 2 CO Nb 2 O 5 La 2 LiNbO C + O The three powder reactants (La 2 O 3, Li 2 CO 3, and Nb 2 O 5 ) were weighed out in stoichiometric proportions and ground together by hand in an agate mortar and pestle for 35 minutes. The resulting powder was then baked at 900 ºC for 24 hours (a process know as calcination) and ground again for 30 minutes. The powder was then pressed into three 1 g pellets using a hydraulic press to provide 10,000 lbs of force for three minutes per pellet. The pellets were then stacked in an aluminum oxide boat and fired in a high temperature furnace. The extra calcined powder was spread underneath the pellets in the boat to prevent the boat (or residue from previous samples left in the boat) from reacting with the pellets. The pellets were then sintered for twelve hours at 1300 ºC, with a four hour ramp up from 500 ºC and a similar 4 hour ramp down to 500 ºC. The sintering was done under a 70/30 mixed argon/oxygen atmosphere. Following sintering, the magnetic susceptibility of the sample was measured using a Quantum Design MSPS SQUID magnetometer. A sample for SQUID magnetometry was taken from the center pellet from the sintering stack, as the center pellet has the least interaction with the atmosphere and the boat. The susceptibility was measured in a field of 10 Oe. A powder XRD pattern was also taken. The peak positions agreed well with those predicted by the lattice parameters given by Demazeau, et al. [2]. However, a few unexplained peaks were seen around 2θ = 28º, suggesting some impurity phases were present.

6 Suspecting that the impurities in the XRD pattern were at least in part due to vaporization of lithium ions during sintering (lithium is quite volatile at sintering temperatures), another sample was prepared with 20% (molar) extra lithium carbonate. Processing was otherwise similar to the previous sample. However, the XRD pattern for this sample following sintering was no cleaner than that of the previous sample of La 2 LiNbO 6, and mechanically, the material was much less sturdy, indicating different reactions had taken place. The La 2 CuSnO 6 samples were prepared from oxides of the constituent metals via the following reaction: La 2 O 3 + CuO + SnO 2 La 2 CuSnO 6 The processing was otherwise very similar to that used to synthesize the La 2 LiNbO 6 samples. The reactant oxides were ground together by hand in an agate mortar and pestle for 60 minutes and then calcined at 900 ºC for 24 hours. After calcination, the powder was ground again for 30+ minutes. A total of 16 g of powder were made, and twelve 1 g pellets were pressed. Four sets of three pellets each were sintered for twelve hours at 1100, 1200, 1300, and 1400 ºC under a 70/30 argon/oxygen atmosphere to examine the effect of sintering temperature on phase formation. The magnetic susceptibility of each of the four La 2 CuSnO 6 samples was measured as a function of temperature using the SQUID magnetometer with fields of 10 and 1000 Oe. Powder XRD was also performed on all the samples, and resistivity was measured as function of temperature for the sample sintered at 1200 ºC.

7 Results and Discussion The sintered La 2 LiNbO 6 pellets, especially the ones made with the extra lithium carbonate, proved to be extremely brittle. Cutting bars from the pellets for resistivity and SQUID measurements only resulted in completely shattering the sample into fragments and powder. The interior of the pellets was white, but the exterior was a yellowish color, which, together with the XRD patterns, suggests the presence of one or more impurity phases. Magnetic susceptibility data from the SQUID magnetometer are presented in Figure 2. The magnetic susceptibility is very small and inversely proportional to temperature, which is indicative of weak paramagnetism. Due to the difficulties of processing and characterizing the material brought about by its fragile nature, and due to our inability to produce phase-pure samples, we subsequently focused our efforts on the La 2 CuSnO 6 samples. 2.5e-05 #765A La2LiNbO C H = 10 Oe ZFC 2e-05 (emu/g/oe) 1.5e-05 1e-05 5e T (K) Figure 2. Magnetic Susceptibility versus Temperature for La 2 LiNbO 6.

8 XRD Pattern for La 2 CuSnO 6 Various Sintering Temperatures Intensity (counts 10 4 ) Sintered at 1400 C Sintered at 1300 C Sintered at 1200 C Sintered at 1100 C Calcined Powder (degrees) Figure 3. X-ray Diffraction Patterns for La 2 CuSnO 6 Sintered at Various Temperatures. The XRD patterns for 1400 ºC sintered La 2 CuSnO 6 look significantly cleaner and more well-defined than for 1200 or 1100 ºC material, suggesting that this higher sintering temperature leads to better primary-phase formation (see Figure 3). This also indicates that the lower 980 ºC sintering used by Anderson and Poeppelmeier probably did not result in the formation of pure samples of La 2 CuSnO 6. Comparison of our XRD data to Anderson and Poeppelmeier s seems to confirm this suspicion. The resistivity versus temperature curve for the 1200 ºC sintered La 2 CuSnO 6 sample is exponential, indicating that the material is a semiconductor (see Figure 4). Stoichiometry implies that the material should be a charge-balanced insulator. Either the oxides used to form the material were not weighed out precisely enough, leading to a charge imbalance, or some other impurity exists in the sample to dope it. It is possible that Al 3+ ions from the boat leached into the sample during sintering. This idea is

9 supported by the fact that at higher sintering temperatures, this material severely stained the boat, indicating that reactions between sample and boat occurred during sintering. Resistivity vs. Temperature #766B La 2 CuSnO 6 Sintered at 1200 C 1 ( m) T (K) Figure 4. Semilog Plot of Resistivity versus Temperature for La 2 CuSnO 6 Sintered at 1200 ºC. 8e-05 #767B La 2 CuSnO 6 Sintered at 1400 C H = 10 Oe 6e-05 ZFC FC FCW (emu/g/oe) 4e-05 2e T (K) Figure 5. Magnetic Susceptibility versus Temperature for La 2 CuSnO 6 Sintered at 1400 ºC.

10 Magnetic susceptibility measurements of La 2 CuSnO 6 with the SQUID magnetometer indicate the material is antiferromagnetic when cooled without a magnetic field and ferromagnetic when cooled in a magnetic field. The data are presented in Figure 5. The zero field cooled (ZFC) measurements were made after the material was cooled in zero applied field, and the field cooled (FC) and field cooled warming (FCW) data were made with the material cooled in an applied field of 10 Oe. For some of the samples, additional measurements were made with an applied field of 1000 Oe. Similar results were seen at 10 Oe and 1000 Oe. Conclusion Our investigation of La 2 LiNbO 6 determined that it is very weakly paramagnetic (as is to be expected, as none of the constituent ions are magnetic) and that it is very difficult to properly synthesize, probably because of the volatility of the lithium ions. We are still experimenting with La 2 CuSnO 6 to determine the optimum sintering temperature. However, our initial results suggest that high temperatures ( ºC) create more phase-pure material. The magnetic properties of this material are rather interesting: when cooled without the presence of a magnetic field, it appears antiferromagnetic; when cooled in an applied field, it appears ferromagnetic. There remains much to be done studying La 2 CuSnO 6. Up to now our investigation of both materials has failed to reveal credible evidence of superconductivity.

11 Acknowledgements I would like to thank Dr. Howard A. Blackstead for his guidance and advice on my work this summer. I also thank Matthew Smylie for his guidance and for much practical training on lab equipment. I thank Dr. Umesh Garg for directing the University of Notre Dame s physics REU program this summer, and also Shari Herman for all she did to keep the program running smoothly.

12 References: [1] Anderson, Mark T. and Poeppelmeier, Kenneth R., La 2 CuSnO 6 : A New Perovskite- Related Compound with an Unusual Arrangement of B Cations. Chem. Mater. 1991, 3, [2] Demazeau, Gérard, et al. A Vanadate (V) Oxide with Perovskite Structure: La 2 LiVO 6, Comparison with Homologous La 2 LiMO 6 Phases (M = Fe, Nb, Mo, Ru, Ta, Re, Os, Ir). Mat. Res. Bul. 1987, 22, [3] Doss, James D., Engineer s Guide to High Temperature Superconductivity. New York: John Wiley & Sons, [4] Hook, J.R., and Hall, H.E., Solid State Physics, 2E. New York: John Wiley & Sons, [5] Kittel, Charles, Introduction to Solid State Physics, 8E. New York: John Wiley & Sons, 2005.