Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X42: Magnetic Oxides |
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Sponsoring Units: GMAG Chair: Sashi Satpathy, University of Missouri-Columbia Room: LACC 150B |
Friday, March 25, 2005 8:00AM - 8:12AM |
X42.00001: Correlation between AMR and PHE in LSMO thin films Jeng-Bang Yau, Y. Bason, L. Klein, X. Hong, C.H. Ahn In magnetic conductors, the dependence of resistivity on the angle $\theta$ between the current and magnetization, a phenomenon known as anisotropic magnetoresistance (AMR), is given by $E_{x} = \Delta \rho_{\perp} j_{x} + (\Delta \rho_ {\parallel} - \Delta \rho_{\perp}) j_{x} \cos^{2} \theta$, where $\Delta \rho_{\parallel}$ and $\Delta \rho_{\perp}$ are the resistivities parallel and perpendicular to the magnetization, respectively. In addition, a transverse voltage, known as planar Hall effect (PHE), is generated with the angular dependence given by $E_{y} = (\Delta \rho_{\parallel} - \Delta \rho_{\perp}) j_{x} \sin \theta \cos \theta$. Doped manganite La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) thin films have been demonstrated to exhibit both AMR and PHE with the expected angular dependencies\footnote{Y. Bason, L. Klein, J.-B. Yau, X. Hong, and C. H. Ahn, Appl. Phys. Lett. \textbf{84}, 2593 (2004)}. However, while $\Delta \rho_{AMR}$ and $\Delta \rho_ {PHE}$, the resistivities extracted from AMR and PHE with $\Delta \rho = (\rho_{\parallel} - \rho_{\perp})$, are expected to be identical, we find that $\Delta \rho_{AMR} \geq \Delta \rho_{PHE}$ throughout our measurements. Further investigation of this apparent discrepancy reveals that for a fixed magnetic field, $\Delta \rho_{AMR}$ and $\Delta \rho_{PHE}$ become nearly equal at temperatures around (below) $T_{c}$. Moreover, the discrepancy between $\Delta \rho_{AMR}$ and $\Delta \rho_ {PHE}$ becomes more substantial with increasing doping concentrations. We will discuss possible mechanisms responsible for this behavior. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X42.00002: First-principles study of the electronic and magnetic properties of La$_{1-x}$Sr$_x$MnO$_3$ and La$_{1-x}$Ca$_x$MnO$_3$ M.-H. Tsai, Y.-H. Tang, H. Chou Using spin-polarized first-principles calculations, we find that the observed insulator behavior of $LaMnO_3$ is due to the existence of a trough in the total density of states, D(E), just above the Fermi level, $E_F$. D(E) in this trough, though not zero, is very small, so that $LaMnO_3$ has a high resistivity at low temperature. The trough is similar to an energy gap, so that the resistivity decreased with temperature. The observed optical energy gap is due to that the states immediately above $E_F$ have the same orbital symmetry as those immediately below $E_F$. $LaMnO_3$ has a deficiency of majority-spin $e_g$ states immediately above $E_F$, so that the O mediated super-exchange coupling dominates and the material is antiferromagnetic. The calculated spin-polarized partial densities of states of $La_{1-x}Sr_xMnO_3$ and $La_{1-x}Ca_xMnO_3$ show that Sr and Ca induce delocalization of majority-spin $e_g$ states, which render these materials semimetallic. The empty majority-spin $e_g$ states immediately above $E_F$ enhance delocalized-state mediated Mn-Mn spin couplings, so that these materials are ferromagnetic. Another effect of Sr and Ca doping is the lowering of the minority-spin $e_g$ band down to $E_F$, which may explain colossal magnetoresistance. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X42.00003: Manipulation of the magnetic nanostructure of a hole-doped manganite using an electric field Tara Dhakal, Jacob Tosado, Yun Sung-Hee, Amlan Biswas The observation of colossal magnetoresistance (CMR) and phase co-existence in hole-doped manganese oxides (manganites) have sustained the interest in these materials for over a decade. We have studied the phase co-existence in the manganite (La$_{1-y}$Pr$_y$)$_{1-x}$Ca$_x$MnO$_3$ (where x = 0.34 and y = 0.5) using magnetotransport and scanning tunneling microscopy (STM) measurements. The temperature dependence of the resistivity of this sample while cooling and warming shows a hysteresis which is a signature of first order phase transition and phase coexistence. We will present our results showing the effect of an applied electric field in this hysteretic region. The voltage current (V-I) characteristics in this temperature range shows a hysteresis around a certain current range. This hysteresis is pushed towards higher current by the application of magnetic field. Our data suggest that the applied electric field reorients the metallic regions of the material. Using this electric field driven reorientation we will suggest methods to manipulate the magnetism of manganites using electric fields. We will also present preliminary STM images showing direct evidence of the effect of an electric field on the magnetic nanostructure of the material. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X42.00004: Long range spin tunnelling in [La$_{0.67}$Sr$_{0.33}$MnO$_{3}$]$_{1-x}$ / [La$_{0.67}$Sr$_{0.33}$MnO$_3$SrO]$_{x}$ composite Hsiung Chou, W.T. Wu, B.Y. Liao, S.J. Sun The composites of [La$_{0.67}$Sr$_{0.33}$MnO$_{3}$]$_{1-x}$/[ La$_{0.67}$Sr$_{0.33}$MnO$_3$SrO]$_{x}$ have been prepared by co-sintering the pure 3D ferromagnetic colossal magnetoresistance La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (113) and the 2D spin-glass magnetic insulator La$_{0.67}$Sr$_{0.33}$MnO$_{3}$SrO (214) with an ordered 2D magnetic-insulator superstructure. Part of the 113 phase reacted with 214 phase during the 1400$^{circ}$C annealing process and formed the antiferromagnetic La$_{2-2y}$Sr$_{1+2y}$Mn$_{2}$O$_{7}$ (327) phase with the unknown y content. The microstructure and the compositional analysis indicates that the 113 grains are bridged by the large, few tens of micrometers, 327 grains in a sponge-like morphology for x $\ge $ 0.1. The low-field magnetoresistance is enhanced dramatically at all temperature range starting below the Curie temperature, 368K, of the pure 113. This enhancement can be attributed to the spin polarization tunneling via hopping or flipping through the periodically altered spin lattice in 327 grains and to the strong-spin fluctuation at the vicinity of 113/327 grain boundaries. [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X42.00005: Relaxation times of charges and spins of La$_{0.7}$Ca$_{0.3}$MnO$_3$ Cheng-Chung Chi, Hsin-Chia Ho, Thiam Hong Goh, Sheng-Fu Horng The time-resolved photoreflectance and MOKE measurements are used to measure the charge and spin dynamics of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$, a typical colossal magnetoresistance material (CMR). We have directly observed the transient behaviors of charges and spins at temperatures above and below its ferromagnetic transition temperature T$_{C}$ (about 250 K) and in the presence of an external magnetic field ranging from 0 to 6 T. It is surprising that the relaxation time of spins created by a circularly polarized laser pulse is extremely fast at T $>$ T$_{C}$. It is about 130 fs and much faster than the charge carrier relaxation times inferred from the transient reflectance measurements. At T $<$ T$_{C}$, relaxation of spins becomes more complicated. The relaxation starts with a fast time similar to that of above T$_{C}$, then follows with a much slower relaxation time of 64 ps towards a level lower than its original level. The original equilibrium is finally reached about 5 ns later. This complicated relaxation processes of the spins can be explained by a strong spin-spin interactions and the supercooling effect induced by the circularly polarized pump laser. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X42.00006: Antiferromagnetic fluctuations and polaron correlation in colossal magnetoresistive layered manganites T. Perring, D. Adroja, J. Champion, G. Aeppli, T. Kimura, Y. Tokura We report results of a study of the antiferromagnetic (AF) fluctuations and polaron correlations in the bilayer manganite La$_{2{\-}2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, x=0.30 and x=0.35. The AF fluctuations, seen earlier in zero field for x=0.40$^{ }$[1], similarly appear on warming through the magnetic ordering temperatures (90K, 121K respectively), coexist with ferromagnetic critical scattering, and persist to at least 300K. The intensity tracks that of the resistivity both as a function of magnetic field and temperature. Polaron correlations were seen at wavevectors ($\pm \delta $,0,$\pm $1), similar to those reported for x=0.40 [2], with magnetic field and temperature dependency of the intensity that also tracks the resistivity. The results indicate that the AF and polaron correlations are closely related. [1] T.G.Perring et al, Phys Rev Lett. \textbf{78} 3197 (1997) [2] L.Vasiliu-Doloc et al, Phys Rev Lett \textbf{83} 4393 (1999) [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X42.00007: Large magnetoresis\-tance in ferromagnet / super\-con\-ductor La$_{0.7}$Ca$_{0.3}$MnO$_3$ / YBa$_2$Cu$_3$O$_7$ trilayers Jacobo Santamaria, V. Pe\~na, Z. Sefrioui, D. Arias, C. Leon, J.L. Martinez, S. te Velthuis, A. Hoffmann We show magnetoresistance in excess of 1000{\%} in trilayers containing highly spin polarized La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ manganite and high Tc superconducting YBa$_{2}$Cu$_{3}$O$_{7}$. This large magnetoresistance is reminiscent of the giant magnetoresistance (GMR) in metallic superlattices but with much larger values, and originates at spin imbalance due to the injection of spin polarized carriers. This result, aside from its fundamental importance, may be of interest for the design of novel spintronic devices based on F/S structures. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X42.00008: Observation of Strain Induced Crystal Field Changes in La0.8MnO3 Single Crystal Manganite Films Qing Qian, Trevor Tyson, M DeLeon, J Bai, C.-C. Kao We have studied La$_{0.8}$MnO$_{3}$ single crystal manganite films with varying thickness (from 4000 {\AA} to 50 {\AA}) on LaAlO$_{3}$ substrates. We measured the Mn K$_{\beta }$ x-ray emission spectrum, which is sensitive to the crystal field about the Mn sites. Our high-resolution x-ray emission results showed there exists a strong crystal field in manganite films near the substrate due to substrate induced compression of the films. The strong crystal field leads to a partial Mn 3d high spin to low spin transformation. This observation provides a clue to the origin of the reduction in the bulk magnetization in ultrathin manganite films. [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X42.00009: Is Electron Doping Possible in Colossal Magnetoresistance Manganites? W.J. Chang, J.-Y. Lin, J.Y. Tsai, J.Y. Juang, K.H. Wu, T.M. Uen, Y.S. Gou, J.M. Lee, J.M. Chen, H.-T. Jeng X-ray absorption near edge spectroscopy (XANES) was used to investigate the hole states in La$_{0.7}$Ce$_{0.3}$MnO$_{3}$ (LCeMO) thin films prepared by pulsed laser deposition. A substantial decrease in the spectral weight of the $e_{g}$ orbital was observed in LCeMO compared to that in the hole-doped La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCaMO) when both are in low- temperature ferromagnetic states. The results of XANES are consistent with those from LDA+U calculations, in that the doping of Ce has shifted up the Fermi level significantly which, in turn, results in marked reduction of hole pockets originally existing in LCaMO. In addition, the calculations also show significant effects originating from the $f$-electrons of the Ce element. The Hall measurements also indicate that in LCeMO the carriers are indeed displaying the characteristics of holes. This work was supported by the National Science Council of Taiwan, under grants: NSC 93-2112-M-009-015 {\&} NSC 93-2112-M-009-016 [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X42.00010: Magnetic properties of Sr$_{8}$CaRe$_{3}$Cu$_{4}$O$_{24}$ Masanori Kohno, Xiangang Wan, Xiao Hu Magnetic properties of Sr$_{8}$CaRe$_{3}$Cu$_{4}$O$_{24}$ are investigated by numerical simulations. This material has anomalously high Curie-temperature ($T_{c}$=440K) among ferromagnetic cuprates. The magnetic effective model of this compound is derived based on the LSDA+U result as a spin-alternating Heisenberg model in three dimensions. We apply quantum Monte Carlo methods (the loop algorithm and the directed-loop algorithm) to the effective model. The temperature-dependence of the magnetization is consistent with experimental results. The effective coupling constant $J$ is estimated as about 700K. We further predict magnetic properties of this compound through investigation of the effective model. Numerical results of susceptibilities near the transition point indicate that the transition is second order and belongs to the universality class of the three-dimensional Heisenberg model. Behaviors of the specific heat at finite temperatures and the magnetization process in the low-temperature regime are also investigated. Some of the numerical results on the effective model may be accessible experimentally. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X42.00011: Spin polarization measurements on $SrRu_{0.92}O_{3}$ and $SrRu_{0.8}Ti_{0.2}O_{3}$ using point contact Andreev Reflection J. Sanders, G. T. Woods, H. Srikanth, B. Dabrowski, S. Kolesnik Recently, highly spin polarized ferromagnetic materials have been investigated for their potential use in the next generation electronics devices. Point contact Andreev reflection (PCAR) has been a useful tool in the study of ferromagnetic metallic oxides in determining the magnitude of the transport spin polarization P. The ferromagnetic metallic 4d oxide SrRuO$_{3}$, with Tc $\sim $ 163 K and magnetic moment m $\approx $ 1.6 $\mu _{B}$/Ru, has been measured by PCAR with a result of P $\sim $ 60 {\%}. This result is particularly interesting due to the fact that the density of states of the majority and minority spins are approximately the same but whose Fermi velocities are dramatically different. We will present our results from a study on the polycrystalline samples SrRu$_{0.92}$O$_{3}$ and SrRu$_{0.8}$Ti$_{0.2}$O$_{3}$. Resistive and magnetic results show that the inclusion of Ti substitution and Ru vacancies lowers Tc and renders materials less metallic. Our PCAR results show that for the Ti substitution the value of P compared to the parent compound is not affected, remaining around 60 {\%}, but the sample with Ru vacancies show less coherent Andreev reflection processes rendering a difficult determination of P. * J. Sanders is an NSF IGERT fellow and is supported by NSF-DGE-0221681 ** Work at NIU was supported by the NSF-DMR-0302617 [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X42.00012: Testing the Berry phase model for extraordinary Hall effect in SrRuO$_{3}$ Yevgeny Kats, Isaschar Genish, Lior Klein, James W. Reiner, M. R. Beasley Recently it has been suggested that the complicated temperature dependence of the extraordinary Hall effect (EHE) in the itinerant ferromagnet SrRuO$_{3}$ could be attributed to the existence of Berry phase monopoles in the crystal momentum space [Z. Fang \textit{et al.}, Science \textbf{302}, 92 (2003)]. We test this model by measurements of EHE as a function of an applied magnetic field at a constant temperature. This provides a supplementary degree of freedom for exploring the nature of the EHE, in addition to the typical temperature-dependent measurements. We show that when temperature-dependent and field-dependent measurements are combined, the results for SrRuO$_{3}$ disagree with the Berry phase model. [Y. Kats \textit{et al.}, Phys. Rev. B \textbf{70}, 180407(R) (2004)] [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X42.00013: New Structure Observed in a High Resolution RIXS Study of Magnetite Timothy Learmonth, Per-Anders Glans, Jinghua Guo, Kevin E. Smith Despite being one of the earliest transition-metal oxides discovered, dating back as far as 4000 B.C.E., Fe3O4 electronic structure is still the subject of debate. Recently, x-ray emission spectroscopy (XES) has been used extensively to investigate the electronic structure of many transition-metal oxides. Because XES can effectively measure energy splittings caused by both a ligand field and electron correlation, the technique can be used to probe the nature of electronic structure near the Fermi level in these materials. Here we report resonant inelastic x-ray scattering (RIXS) measurements of the transition-metal oxide Fe3O4 at both the O K edge and the Fe L edge. Although taken with higher resolution, the O K edge spectra are substantially the same as measurements previously published with low resolution. The Fe L edge emission, however, reveals structure that was not observed in previous studies. Specifically, a sharp elastic feature near the Fermi level and a 3.5eV excitation at slightly higher excitation energies are observed. These are interpreted in light of symmetry considerations and ligand field splitting. Supported in part by U.S. DOE under DE-FG02-98ER45680 [Preview Abstract] |
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X42.00014: Exchange coupling induced antiferromagnetic-ferromagnetic transition in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$/La$_{0.5}$Ca$_{0.5}$MnO$_3$ superlattices. W. Prellier, P. Padhan Superlattices built from two antiferromagnetic (AFM) charge/orbital order compounds, Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$, have been studied as the thickness of La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (LCMO) varied. High structural quality thin films were obtained on LaAlO$_{3}$ substrates using the pulsed laser deposition technique. An antiferromagnetic-to-ferromagnetic transition, in addition to an enhancement of the coercivity, are observed as the LCMO layer thickness increases. The small shift in the origin of the field-cooled hysteresis loop along the field axis indicates the presence of ferromagnetic and antiferromagnetic phases in the superlattices. We attribute these features to the AFM spin fluctuations at the Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ / La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ interfaces resulting from the strain effects. [Preview Abstract] |
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