Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V32: Focus Session: Magnetic Semiconductors and Novel Magnetic Materials |
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Sponsoring Units: GMAG DMP Chair: Kenneth Burch, Los Alamos National Laboratory Room: Morial Convention Center 225 |
Thursday, March 13, 2008 11:15AM - 11:27AM |
V32.00001: Structural and chemical ordering and spin polarization in epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{1-x-y}$ and Co$_{x}$Mn$_{y}$Si$_{1-x-y}$ B.A. Collins, L. He, F. Tsui, Y.S. Chu, Y. Zhong The Heusler alloys of Co$_{2}$MnGe and Co$_{2}$MnSi have been predicted to be half-metallic. However, half-metallicity has not been realized owing to its sensitivity to disorders associated with off-stoichiometry and to epitaxial constraints. Combinatorial epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{1-x-y}$ and Co$_{x}$Mn$_{y}$Si$_{1-x-y}$ have been grown on Ge (111) substrates in and around the Heusler stoichiometry using molecular beam epitaxy techniques. The structural and chemical ordering of the films has been examined using x-ray microbeam techniques, including energy dependent anomalous diffraction and diffraction anomalous fine structure. These experiments are shown to be sensitive to various disorders, including stacking faults, and chemical dependent vacancies and site swapping. They reveal that the ordering is very sensitive to the atomic ratio between Co and Mn. Magnetic and spin dependent properties have been examined by magnetoopitc effects and in-situ point contact Andreev reflection spectroscopy, and they exhibit strong correlation with the structural and chemical ordering. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V32.00002: Control of selforganized magnetic nanocrystals aggregation in (Ga,Fe)N by co-doping with shallow donors and acceptors A. Bonanni, A. Navarro-Quezada, T. Li, M. Kiecana, M. Sawicki, T. Dietl A number of possible room temperature functionalities has recently been proposed for magnetically doped semiconductors, in which spinodal decomposition leads to the self-organized formation of coherent ferromagnetic nanodots or nanocolumns [1]. It has also been suggested that the decomposition can be controlled in a wide range by growth conditions and co-doping [2]. We have extended our previous structural and magnetic studies of (Ga,Fe)N [3] by examining the effects of Si and Mg co-doping. As before, we have found the magnetic response to consist of a paramagnetic signal from substitutional Fe and of a ferromagnetic component due to Fe1-xN nanocrystals. Our results demonstrate that the co-doping reduces the fractional concentration of Fe contributing to the nanocrystals. This shows that tuning of the Fermi energy by changing the charge state of the transition metal ions affects their aggregation, as proposed recently [2].\\ 1. H.Katayama-Yosida et al., phys.stat. sol. (a) 204, 15 (2007); T.Dietl, arXiv:0711.0343. 2. S.Kuroda et al., Nature Mat. 6, 440 (2007). 3. A.Bonanni et al., Phys. Rev. B 75, 125210 (2007). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V32.00003: Giant anisotropic magnetoresistance in ultra thin (Ga,Mn)As Rashid Gareev, Markus Schlapps, Janusz Sadowski, Werner Wegscheider, Dieter Weiss We describe the effect of giant anisotropic magnetoresistance (GAMR) for epitaxial ultra thin (Ga,Mn)As below the metal-insulator transition (MIT). The GAMR is observed for 5 nm-thick Ga$_{0.95}$Mn$_{0.05}$As films after annealing in optimized conditions in planar geometry for patterned Hall bars at T$<$10K, where longitudinal resistance R$_{o}\sim $h/e$^{2}$. The GAMR manifests itself in magnetization-dependent high-resistance (HR) and low-resistance (LR) states along different crystallographic directions. We demonstrate that holes are strongly localized in HR states and localization depends on orientation of magnetization, amplitude of current and magnetic field. The decrease of current amplitude is accompanied by an enhancement of the GAMR for both in-plane and orthogonal-to-plane orientations of magnetic field. Changes of R$_{xx}$ between HR and LR states reach$\sim $100{\%} at T=1.7K. In the Hall geometry the changes of transverse component R$_{xy}$ in magnetic field exceed 1000{\%}. The behaviour of GAMR we ascribe to anisotropic spin-orbit scattering, strong localization below MIT and localization-delocalization effects in magnetic and electric fields. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V32.00004: Atom-byatom substitution of Mn in GaAs and visualization of their hole-mediated interactions Invited Speaker: |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V32.00005: Ferromagnetism in CuO-ZnO multilayers Sudakar Chandran, B.J. Kirby, K. Padmanabhan, G. Lawes, R. Naik, Sanjiv Kumar, V.M. Naik The magnetic properties of CuO-ZnO heterostructures are examined to elucidate the origin of the ferromagnetic signature in Cu doped ZnO. The CuO and ZnO layer thickness varied from 15 nm to 350 nm, and we observed no significant diffusion of either Cu$^{2+}$ in the ZnO layers or of Zn$^{2+}$ in the CuO layers using Rutherford backscattering spectrometry. Bulk magnetization measurements established that the multilayers exhibit a ferromagnetic moment at room temperature, with a saturation magnetization ($\sim $2-5 emu/cc of CuO) that depends on the CuO size, but not the CuO-ZnO interfacial area. Polarized neutron reflection studies suggest that the ferromagnetism arises from the CuO layers, and not from the interdiffusion of CuO and ZnO. These results indicate that the ferromagnetism in these multicomponent structures arises from the uncompensated surface spins of CuO nanoparticles in the layer rather than from regions of interdiffusing ZnO and CuO. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V32.00006: Role of Donor defects in stabilizing room temperature ferromagnetism in Co doped ZnO Lubna Shah, Yuan-Qiang Song, Huai-Wu Zhang, Weigang Wang, Zhu Hao, John Xiao Extensive experimental and theoretical work has been done on transition metal doped ZnO diluted magnetic semiconductor, in which defects play a vital role in promoting carrier-mediated ferromagnetism. We explored the influence of interstitial Zinc on physical properties of Co doped ZnO by exposing Co0.05Zn0.95O in a Zn vapor. Both X-ray diffraction and X-ray photoelectron spectroscopy indicate the substitution of Co2+ in the ZnO lattice. Observed room temperature ferromagnetism in bulk samples shows a decreasing trend with decreasing temperature. This can be explained by taking into the reduction of the carrier density, induced by the interstitial Zn ions, at low temperature. Carrier density at room temperature is about 8.5 x 10$^{19}$/cc, which is consistent with predicted value for polaron percolation threshold to induce long range ferromagnetism. The carrier density fell below this threshold at low temperature, results in the disappearance of the ferromagnetism. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V32.00007: Ferromagnetism in Rutile Structure Cr doped VO$_{2}$ Thin Films prepared by Reactive Bias Target Ion Beam Deposition Kevin G. West, Jiwei Lu, Li He, David M. Kirkwood, Wei Chen, T. Paul Adl, Michael S. Osofsky, Syed B. Qadri, Robert Hull, Stuart A. Wolf First generation spintronics has entered the mainstream of information technology through its utilization of the Magnetic Tunnel Junction (MTJ) in applicable devices such as read head sensors for hard disk drives and Magnetic Random Access Memory (MRAM). The future of spintronic devices requires next generation spintronic materials. Here we report on the novel structural, transport, and magnetic characteristics of V$_{1-x}$Cr$_{x}$O$_{2}$ (0.1 $\le $ x $\le $ 0.2) thin films deposited on (001) Al$_{2}$O$_{3}$ substrates. We show that the metal-insulator transition (MIT) of VO$_{2}$ is suppressed and the rutile structure is stable down to 100 K. The films are remarkably smooth having a root-mean squared (RMS) surface roughness of 0.3 nm. Films are conductive at room temperature and appear to follow a variable-range-hopping conduction mechanism below that. Ferromagnetism is observed at room temperature and is dependent on Cr concentration. The combination of these characteristics makes V$_{1-x}$Cr$_{x}$O$_{2}$ a viable candidate material for next generation spintronic multilayer devices. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V32.00008: Phase transition temperatures and magnetic entropy change in Ni-Mn-In-B based Heusler alloys Arjun Pathak, Bhoj Gautam, Igor Dubenko, Naushad Ali One of the aspects of great attention of Heusler alloys is the large value of magnetic entropy change ($\Delta $S$_{M})$ and their possible application as a working material in magnetocaloric effect based magnetic refrigerators. It was reported earlier that Ni$_{50}$Mn$_{34.8}$In$_{15.2}$ has first order martensitic transition temperature T$_{M}\approx $ 212K, Curie temperature of austenitic phase T$_{C}\approx $ 328K and $\Delta $S$_{M}$ value associated with T$_{M}$ and T$_{C}$ are respectively 13 and -7 J/kg K [1]. In the present study, we are reporting the effect of partial substitution of In by B in Ni$_{50}$Mn$_{34.8}$In$_{15.2}$ by AC susceptibility, thermal expansion, and magnetization measurements. We observed that substitution of boron sharply increase T$_{M,}$ and significantly enhance the $\Delta $S$_{M}$ peak value higher than 30 J/kg K at T$_{M}\approx $ 296K; however the $\Delta $S$_{M}$ value remains almost same at T$_{C}$. Therefore, the Ni-Mn-In-B based Heusler alloys will be potential material for the study of room temperature magnetic refrigerator materials. Reference: [1] A. K. Pathak, M. Khan, I. Dubenko, S. Stadler, and N. Ali, Appl. Phys. Lett. \textbf{90}, 262504 (2007). [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V32.00009: Magnetic excitations in a quantum spin dimer system Ba$_{3}$Cr$_{2}$O$_{8}$ Maiko Kofu, Jung-Hwa Kim, Seung-Hun Lee, Bella Lake, Yiming Qiu, Hiroaki Ueda, Yutaka Ueda We report our neutron scattering measurements on a powder sample of a new quantum spin system Ba3Cr2O8 in which Cr5+ (3d1, S = 1/2) ions form a double-layered triangular lattice. The system does not undergo any magnetic ordering down to 50 mK. Instead, bulk susceptibility data exhibit a broad peak around 16K. Our inelastic neutron scattering data at temperatures lower than 30 K show a prominent excitation at hw = 2.2 meV. Q- and temperature dependences of the integrated intensity of the excitation can be well accounted for by a model of weakly coupled quantum dimers. When an external magnetic field is applied, the excitation splits into three energies, which confirms the dimer model. We have investigated how the singlet-to-triplet excitations evolve with increasing the magnetic field up to 14.5 Tesla. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V32.00010: Understanding Magnetic Frustration on the Diamond Lattice of Transition Metal Oxide Spinels Brent Melot, Katharine Page, Ram Seshadri, Doron Bergman, Thomas Proffen We present structural and magnetic measurements on the solid solution CoAl$_{2-x}$Ga$_{x}$O$_4$ and demonstrate how frustration on the diamond sublattice can be modified through chemical substitution. The effect of substitution is two-fold. Increasing values of $x$ are accompanied by an increase in the lattice parameter resulting in an elongated and consequentially weakened exchange pathway. Our data indicates that the amount of site mixing also increases across the substitution series weakening the A-O-B-O-A superexchange pathway. Monte Carlo simulations were used to compute the exchange coupling constants between nearest and next-nearest neighbor Co atoms. Density functional calculations were also performed to approximate the nearest neighbor coupling constant. For most values of $x$, we find that the frustration parameter, $\Theta_{CW}/T_N$, decreases which we attribute to the weakening competition between nearest and next-nearest-neighbor Co exchange interactions. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V32.00011: Half-,metallic Silicon Nanowires Engin Durgun, Deniz Cakir, Nurten Akman, Salim Ciraci This study investigates the atomic structure, mechanical, electronic and magnetic properties of silicon nanowires using first-principles plane wave calculations within density functional theory. We considered bare, hydrogen terminated and transition-metal adsorbed silicon nanowires oriented along [001] direction. Nanowires of different sizes are initially cut from the bulk Si crystal in rod-like forms and subsequently their atomic structures are relaxed. We first presented an extensive analysis of the atomic structure, stability, elastic and electronic properties of bare and hydrogen terminated Si nanowires. The energetics of adsorption and resulting electronic and magnetic properties are examined for different level of transition metal atom coverage. Adsorption of transition metal atoms resulted in magnetic ground state. The net magnetic moment increases with increasing coverage. While specific Si nanowires acquire half-metallic behavior at low coverage, at high coverage ferromagnetic nanowires become metallic for both spin-direction and some of them have very high spin polarization at the Fermi level. Present results are not only of scientific interest, but can also initiate new research on spintronic applications of silicon nanowires. [1] \newline \newline [1] E.Durgun, D. Cakir, N. Akman and S. Ciraci, Phys. Rev. Lett (2007) (in press). [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V32.00012: Spin dynamics in single-molecule magnets combining surface acoustic waves and high frequency electron paramagnetic resonance Stephen Hill, Jonathan Lawrence, Ferran Macia, Joan Manel Hernandez, Javier Tejada, Paulo Santos, Christos Lampropoulos, George Christou We report a new experimental technique that integrates high frequency surface acoustic waves (SAWs) with high frequency electron paramagnetic resonance (HFEPR) spectroscopy in order to measure spin dynamics on fast time scales in single-molecule magnets. After driving the system out of equilibrium by triggering magnetic avalanches, or simply by heating with short SAW pulses, the evolution of the spin populations within fixed energy levels is measured using HFEPR spectroscopy. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V32.00013: Crystal Growth and High Field Magnetization of the Spin Dimer Compound Ba$_{3}$Cr$_{2}$O$_{8}$ Adam Aczel, Hanna Dabkowska, Graeme Luke, Eun-Sang Choi Spin dimer compounds provide means to access exotic magnetically ordered states. The ground state of these systems is a product of singlets as a result of antiferromagnetic intradimer exchange. However, an applied magnetic field can be used to close the spin gap to excited triplet states, resulting in a state characterized by long range magnetic order at low temperatures. Ba$_{3}$Cr$_{2}$O$_{8}$ is an example of a system that shows the behaviour discussed above. To investigate the field-tuned phase transition in this system, we grew single crystals of this material for the first time by the traveling solvent floating zone method. We then proceeded to measure the magnetization of the resulting crystals by the torque magnetometry technique at the National High Magnetic Field Lab. The magnetization is flat until $\sim $ 12.5 T, then shows a rapid, nearly linear increase with field and saturates at $\sim $ 23 T.$_{ }$ This behaviour is reminiscent of that seen in the spin dimer compounds BaCuSi$_{2}$O$_{6}$ and Ba$_{3}$Mn$_{2}$O$_{8}$ and has been attributed to Bose-Einstein condensation of triplet excitations. [Preview Abstract] |
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