1:30PM, R1 *Invited
"Electrical Conductivity and Magnetoresistance in Perovskite Structures:" SHUFENG ZHANG, 4 Washington Place, Physics Department, New York University, New York, NY 10003
Doped perovskite lanthanum maganites La~ xA,~Mn O3 (A=Ca, Ba, Sr) display two characteristic transport phenomena. The first is the crossover from metallic conduction at low temperatures to semiconducting at high temperatures. A natural understanding of this phenomenon is to introduce electron- phonon or electron-magnon interactions to the system. The interaction leads to decrease of the bandwidth of mobile carriers as the temperature increases. At certain temperature, a gap opens and hopping conduction dominates. We show that the standard polaron and spin-polaron theories indeed reproduce the gross features of the resistivity observed experimentally. The second phenomenon is the colossal magnetoresistive effect near the transition temperature. If one assumes that the role of the magnetic field is to change the energy scale by a Zeeman energy, the magnetoresistance will be an order of magnitude smaller than observed. We propose a model that assumes a non-uniform distribution of spin interactions. We show that the thermally activated domains (in contrast with ordinary static domains) appear; their structures depend on temperature, magnetic field and details of the distribution of interactions. Magnetoresistance is discussed within this framework.
This work is supported in part by Office of Naval Research and Hewlett- Packard Laboratories.
2:10PM, R2
"Theory of the Colossal-Magnetoresistance Manganites:" JUN ZANG, H. Röder, A. R. Bishop, Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM 87545
We present an overview of various aspects of theoretical studies of the colossal-magnetoresistance manganites: the combined effects of double exchange and Jahn-Teller coupling, Berry phase effects on the local spin ferromagnetism and charge transport, and polaron and multi-polaron structure and ordering. Within the double exchange model, we find that the local Berry phases can affect the spin dynamics and render the ferromagnetic phase unstable. At high temperature, the electrons will be scattered by the random geometric gauge field due to the Berry phases. The Jahn-Teller effects are studied using a variational Lang-Firsov approximation. Various properties of small polarons in the CMR are discussed.
2:30PM, R3+
Colossal Magnetoresistance in Sol-Gel Derived La-Ca-Mn-Co-O Epitaxial Thin Film: SEUNG-YOUNG BAE, Shan Xiang Wang, Department of Materials Science and Engineering, Center for Research on Information Storage Materials; G. Jeff Snyder, Theodore H. Geballe, Department of Applied Physics, Stanford University, Stanford, CA 94305-4055
Colossal magnetoresistance (CMR) oxides based on perovskite- type manganites have become a mainstream research subject by virtue of their unusual properties which are not well explained within the existing theoretical framework, as well as for their potential in magnetic sensors and magnetic recording devices. However, a rapid implementation of CMR transducer is not likely to be realized in a foreseeable future, mainly due to (1) a low field sensitivity and (2) relatively low room temperature MR ratio. In an effort to overcome such obstacles and to gain useful clues to solve the theoretical conundrum, we report in this paper a sol-gel epitaxial deposition of La0.67Ca0.33Mn1-y CoyO3 and their electrical, magnetic and magnetotransport properties.
Stock solution was prepared from metal salts (nitrates and acetates) dissolved in 2-Methoxyethanol. Thin film with a thickness of 1000Å was deposited on LaAlO3(100) substrate through repeated spin casting and pyrolysis at 400deg.C and subsequent annealing at 800deg.C in 02. Crystal structures were examined using an X-ray diffractometer. Surface morpholgy was observed using AFM, and XPS was employed for compositional analysis. Electrical, magnetic and magnetotransport was characterized using SQUID and customized Hall apparatus.
XRD symmetric and [[phi]] scan showed nearly perfect crystalline orientation both in-plane and out of plane of the thin film. The samples with Co replacement possessed the same cubic perovskite structure as La0.66 Ca0.33 MnO3 and the epitaxiality and homogeneity (single phase) was preserved in the composition range of y=0-0.3. Rocking scan on (002) peak of the film showed good epitaxiality with Full-Width-Half-Maximum values falling in the range of 0.3deg. ~ 0.5deg.. Grain size of the film was typically 500 A and surface roughness was 30 Å in RMS value. No residual carbon and nitrogen were detected by XPS. Compared with similarly prepared La0.66 Ca0.33 MnO3 (y=0), the resistivity of LaO.67CaO33Ml- llyCOy O3 (y=0-1) increased by an order of magnitude and Tc dropped from 230 K to 195 K. The peak MR ratio (MRpeak), defined as (Ro -RH)/RH, increased from 900% to 1900%.
Electron hopping model based on small polaron is generally accepted to be responsible for the electrotransport in semiconductor region (above Tc). The activation energy (Ea) for electron hopping can be calculated from the slope in an Arrehnius plot of log p/T vs 1/T which is based on Mott's small polaron model:.
P = poT exp(E a/KT)
where Ea includes energy required for carrier generation and for thermal- assisted hopping from one ion to another. For y = 0, activation energy was 0.130 eV under no field and dropped to 0.115 eV under H = 7 T ([[Delta]]Ea = Ea(0) - Ea(7 T) = 0.015 eV). In constrast, for y=0.1, Ea(0) was 0.120 eV while Ea(7 T) was 0.075 eV ([[Delta]]Ea=0.045 eV). The larger Mrpeak in y=0.1 than in y=0 seems to occur due to a greater drop of activation energy induced by an applied magnetic field. Similar relation between MRpeak and [[Delta]]Ea could be established in the thin films with y=0.2 and y=0.3.
2:30PM, R3+
"Colossal Magnetoresistance in Sol-Gel Derived La-Ca-Mn-Co-O Epitaxial Thin Film:" SEUNG-YOUNG BAE, Shan Xiang Wang, Department of Materials Science and Engineering, Center for Research on Information Storage Materials; G. Jeff Snyder, Theodore H. Geballe, Department of Applied Physics, Stanford University, Stanford, CA 94305-4055
Colossal magnetoresistance (CMR) oxides based on perovskite- type manganites have become a mainstream research subject by virtue of their unusual properties which are not well explained within the existing theoretical framework, as well as for their potential in magnetic sensors and magnetic recording devices. However, a rapid implementation of CMR transducer is not likely to be realized in a foreseeable future, mainly due to (1) a low field sensitivity and (2) relatively low room temperature MR ratio. In an effort to overcome such obstacles and to gain useful clues to solve the theoretical conundrum, we report in this paper a sol- gel epitaxial deposition of La0.67Ca0.33Mn1-y CoyO3 and their electrical, magnetic and magnetotransport properties.
Stock solution was prepared from metal salts (nitrates and acetates) dissolved in 2- Methoxyethanol. Thin film with a thickness of 1000Å was deposited on LaAlO3(100) substrate through repeated spin casting and pyrolysis at 400deg.C and subsequent annealing at 800deg.C in 02. Crystal structures were examined using an X-ray diffractometer. Surface morpholgy was observed using AFM, and XPS was employed for compositional analysis. Electrical, magnetic and magnetotransport was characterized using SQUID and customized Hall apparatus.
XRD symmetric and [[phi]] scan showed nearly perfect crystalline orientation both in-plane and out of plane of the thin film. The samples with Co replacement possessed the same cubic perovskite structure as La0.66 Ca0.33 MnO3 and the epitaxiality and homogeneity (single phase) was preserved in the composition range of y=0- 0.3. Rocking scan on (002) peak of the film showed good epitaxiality with Full- Width-Half-Maximum values falling in the range of 0.3deg. ~ 0.5deg.. Grain size of the film was typically 500 A and surface roughness was 30 Å in RMS value. No residual carbon and nitrogen were detected by XPS. Compared with similarly prepared La0.66 Ca0.33 MnO3 (y=0), the resistivity of LaO.67CaO33Ml- ll yCOy O3 (y=0- 1) increased by an order of magnitude and Tc dropped from 230 K to 195 K. The peak MR ratio (MRpeak), defined as (Ro - RH)/RH, increased from 900% to 1900%.
Electron hopping model based on small polaron is generally accepted to be responsible for the electrotransport in semiconductor region (above Tc). The activation energy (Ea) for electron hopping can be calculated from the slope in an Arrehnius plot of log p/T vs 1/T which is based on Mott's small polaron model:.
P = poT exp(E a/KT)
where Ea includes energy required for carrier generation and for thermal- assisted hopping from one ion to another. For y = 0, activation energy was 0.130 eV under no field and dropped to 0.115 eV under H = 7 T ([[Delta]]Ea = Ea(0) - Ea(7 T) = 0.015 eV). In constrast, for y=0.1, Ea(0) was 0.120 eV while Ea(7 T) was 0.075 eV ([[Delta]]Ea=0.045 eV). The larger Mrpeak in y=0.1 than in y=0 seems to occur due to a greater drop of activation energy induced by an applied magnetic field. Similar relation between MRpeak and [[Delta]]Ea could be established in the thin films with y=0.2 and y=0.3.
2:50PM, R4
"Magneto-Transport, Optical, and Microwave Spectroscopy Measurements on the Lanthanum-Manganate CMR Materials:" D. C. VIER, D. Schurig, S. Schultz, Center for Magnetic Recording Research, University of California, San Diego, Dept. of Physics, 0319, 9500 Gilman Dr., La Jolla, CA 92093-0319; R. Ramesh, University of Maryland, Center for Superconductivity Research, College Park, MD 20740; S. B. Oseroff, C. Rettori, San Diego State University, Physics Department, San Diego, CA 92182; J. Sanjurjo, Universidade Estadual de Campinas, Caixa Postal 6165, Campinas, Sao Paulo, Brasil 13083-970; J. Neumeier, Los Alamos National Laboratory, Los Alamos, NM 87545; S.-W Cheong, AT&T Bell Labs, 600 Mountain Avenue, Murray Hill, NJ 07974
We have made measurements on single crystals, ceramic powder, and pulsed laser deposition (PLD) films of La(A)MnO3, (A = Ca, Sr, or Ba). We investigate the dc magneto-resistance, dc magnetization, microwave EPR and FMR, non- resonant microwave absorption, and Raman optical spectroscopy, as a function of sample preparation conditions and subsequent oxygen and argon annealing. In particular, we find FMR to be the best indicator of inhomogeneity in the epitaxial PLD thin films and ceramics. We will report on our measurements in single crystals of the linewidth and Dysonian lineshapes as a function of temperature with focus on T < Tc.
This work supported by the Center for Magnetic Recording Research, and NSF grants DMR-93-02913, DMR- 94-00439,and DMR-91-17212.
3:30PM, R5 *Invited
"The Combinatorial Synthesis of Functional Solid State Materials:" X. -D. XIANG, Molecular Design Institute, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720
I will review the development and applications of combinatorial synthesis/screening of novel solid state materials, a non- conventional approach to the discovery and optimization of functional materials. I will address the technology development and scientific issues in the materials library fabrication, processing, and screening. The application of this approach to the magnetoresistive oxides materials and a novel microwave scanning detection system for screening magnetoresistivity will be discussed in details.
4:10PM, R6
"Anneal Tunable Tc and Electrical Transport of La0.67Ca0.33MnO3:" D.C.. WORLEDGE, G.J. Snyder, M.R. Beasley, T. H. Geballe, Stanford University, Dept. of Applied Physics, Ginzton Laboratory, Stanford, CA 94305-4090; R. Hiskes, S. Dicarolis, Hewlett-Packard, 3500 Deer Creek Rd., Palo Alto, CA 94304
Resistivity measurements on a La0.67Ca0.33MnO3 film are reported for a series of Ar anneals at successively higher temperatures. Tc, the resistivity maximum, increases uniformly with increasing anneal temperature and anneal time. Hence a wide range of Tc's can be achieved on a given sample by appropriate annealing. A full anneal can be achieved with only Ar anneals. Data from Tc up to 1200K show activated conduction with p = BT exp(Ea/KT), in support of the view that the conductivity is due to adiabatic polaron hopping. This model fits both data from the partially annealed and fully annealed sample. This data can not be fitted to a variable range hopping or semiconductor model. The activation energy Ea decreases with increasing maximum anneal temperature. Furthermore, B is observed to decrease after each anneal. A creep type equation is followed for the resistivity as a function of time during an anneal at a fix temperature: p =-a ln(t-b) +c.
4:30PM, R7
"Preparation of La0.67Ca0.33MnO3 Particles Via Sol-Gel Technology:" C. V[[exclamdown]]ZQUEZ- V[[exclamdown]]ZQUEZ, M.A. López-Quintela, R. D. Sánchez, Departments of Physical Chemistry, J. Rivas, Department of Applied Physics, University of Santiago de Compostela, E-15706 Santiago de Compostela, Spain; S. B. Oseroff, Department of Physics, College of Sciences, San Diego State University, San Diego, CA 92182
The study of submicrometric particles in already known systems has resulted in a renew interest due to the large differences found in their properties when the particle size is reduced, and because of possible new technological applications.
We report the preparation of La0.67Ca0.33MnO3 particles via sol- gel technology starting from an aqueous solution of the metallic nitrates and using urea as gelificant agent. After the solution is concentrated by heating evaporation on a hot plate, the mixture leads to a gel when cooling down. Gels were decomposed at 250deg.C and, after, calcined 3 hours at temperatures ranging 300 to 1000deg.C. The powders were structurally characterized by X- ray diffraction and we have observed an incipient crystallization of the perovskite about 500deg.C. Complete crystallization takes place about 600deg.C. The particle size was determined from the X-ray diffraction peak-broadening and by transmission electron microscopy (TEM). For crystallized samples, the average size increases from 20 to 140 nm as the calcination temperature, Tcal., increases as follows from X-rays. TEM micrographs show elongated particles which polar (long) axis size increases from 40 to 300 nm as Tcal increases.
First magnetization and magnetoresistance studies has been reported previously. New work is in progress and we are trying to modify the oxygen content via electrochemical oxidation.
4:50PM, R8
"Electron Paramagnetic and Ferromagnetic Resonances and Zero Field Microwave Absorption Studies of Nanoparticles of La0.67Ca0.33N=MnO3:" M. T. CAUSA, A. Butera, M. Tover, B. Alascio, Centro Atómico Bariloche, Argentina; S. Oseroff, San Diego State University, San Diego, CA 92182; R. D. Sánchez, C. Vázquez-Vázquez, M. A. López-Quintela, J. Rivas, Universidad de Santiago de Compostela, E-15706 Spain
We have synthesized La0.67Ca0.33MnO3 by the sol- gel method. Nanoparticles of diameter d between 30 and 150 nm were obtained by annealing at different temperatures. In all the cases ferromagnetic materials with GMR properties were obtained [R.D. Sanchez et al Appl. Phys. Lett. 68,134, 1996.]. We present measurements of spin resonance at 1.5 GHz, 9.5 GHz, and 35 GHz as a function of T and d, performed in the paramagnetic (PM) and ferromagnetic (FM) phases. The results are analysed taken into account magnetization and resistivity measurements made on the same samples.
The EPR room temperature g-factorvalues spread around g = 1.989. No g-
shift is observed when the temperature is varied from the PM so FM phases. The
integrated line intensity I decreases with d, being as 300 K in the PM phase
I(d=150 nm)/I(d=30nm) =-2. A similar value is found for the ratio between the FM saturation magnetizations (Ms) at the lowest temperatures. The peak-to-peak
linewidth has a minimum ([[Delta]]H pp = 200 Gauss) showing the cross-
over of two relaxation mechanisms as temperatures near Tc . Both the minimum value
of the linewidth, and the temperature where this value is reached, depend on
the operating frequency (or magnetic field) and on the particle size. In the PM
region the linewidth has the characteristic behaviour found in magnetic
insulators (D.L.Huber and M.S. Seehra. J. Phys. Chem. Sol. 36,723 (1975).]:
[[Delta]]H pp (T) [[proportional]] [[Delta]]H pp(T->*)/([[chi]](T) T).
Here [[chi]](T) is the magnetic susceptibility measured on the same sample.
[[Delta]]H pp (T->*) is the high-temperature linewidth value which is approximately constant for all the samples. In the FM region [[Delta]]H pp (T) is roughly proportional to the measured Ms(T) in all the cases. Therefore the low temperature limit of the linewidth depends on the particle size. We obtain the function F(T) = [[Delta]]H pp (T) -
Ms(T). F(T) shows characteristic peaks [M.Dominguez et al Sol. State Comm.
97,193 (1996)] which vary for different samples. We analyse F(T) in terms
of sample inhomogenities not detected with other experimental methods.
Magnetically modulated zero field absorption as function of T was observed at T
= Tc only in samples where d > 50 nm. We discuss this result in terms of
domain structure and relate these experiments with magnetization measurements
on the same samples.
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