Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W17: Focus Session: Magnetic Oxide Thin Films - Cobaltate and Ferrous Oxide Thin Films |
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Sponsoring Units: GMAG DMP Chair: Chris Leighton, University of Minnesota Room: D174 |
Thursday, March 24, 2011 11:15AM - 11:51AM |
W17.00001: Fist-principles design of magnetic oxides Invited Speaker: First-principles design of magnetic oxides is one of most ambitious challenges in modern computational physics. The electronic and magnetic properties of these materials are substantially affected by strongly localized electrons, complex crystalline structures, and mixed valencies of magnetic ions. In my talk I shall present an ab-initio Green function method within density functional theory which provides an accurate description of the electronic structure of these materials. In particular, the treatment of strongly localized electrons is improved considerably by applying a self-interaction correction (SIC), thereby removing the spurious interaction of an electron with itself. By localizing a particular electronic configuration using the SIC, we simulate various valencies of magnetic ions. A mixed-valency state can then be efficiently treated within the coherent potential approximation which is implemented in our multiple-scattering Green function code that can be used as well for simulations of any kind of substitutional disorder. In my talk I shall demonstrate the power of our approach on complex magnetic oxidic surfaces and interfaces. In particular, I shall discuss effective exchange interactions in systems with mixed valency and the influence of structural imperfections, such as defects and relaxations, on the electronic and magnetic properties of these materials. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W17.00002: Cobalt spin state and hyperfine interaction in ferromagnetic insulating LaCoO$_3$ thin films Renata Wentzcovitch, Han Hsu, Peter Blaha, Chris Leighton At low temperatures, bulk LaCoO$_3$ is a non-metallic diamagnet. In contrast, thin-film LaCoO$_3$ experiencing a tensile epitaxial strain is a ferromagnetic insulator in the same temperature range. It is difficult to properly describe this phenomenon with density functional theory (DFT) calculations, even with the Hubbard $U$ correction. Previous calculations have found ferromagnetic conducting thin-film LaCoO$_3$ with all Co ions in the intermediate-spin (IS) state, incompatible with experimental data. In this work, using the DFT+$U$ approach, we show that a strained LaCoO$_3$ thin film can be stabilized in a ferromagnetic insulating state. We also predict the electric field gradient (EFG) at the Co nucleus in the magnetic thin film, which can be helpful in identifying Co spin states via nuclear magnetic resonance (NMR) spectroscopy. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W17.00003: First-principles study of strain-induced ferromagnetism in LaCoO$_{3}$ Hosung Seo, Alexander Demkov We study theoretically the effect of biaxial strain on magnetic properties of LaCoO$_{3}$ (LCO) using density functional theory combined with the Hubbard U method. LCO is normally a non-magnetic insulator with trivalent cobalt ions in low-spin state (t$_{2g}^{6})$. Owing to close interplay between orbital, spin, and lattice degrees of freedom, it shows rich magnetic behavior such as temperature-induced spin state transition. Recently, the ferromagnetic tensile-strained LCO films have been reported. The underlying physics of the ferromagnetic state is, however, unclear. Using a large tetragonal cell we calculate full structural response of the system to applied strain for non-magnetic and magnetic solutions. We show that beyond tensile strain of 3.8{\%} the ferromagnetic solution with Co ions in intermediate-spin state (t$_{2g}^{5}$ e$_{g}^{1})$ is stabilized accompanied by partial untilting of CoO$_{6}$ octahedral network. We also perform the calculation for compressive-strained structures and the difference between these and the tensile strained structures will be presented. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W17.00004: Spin-polarized scanning tunneling spectroscopic (SP-STS) studies of the intrinsic electronic heterogeneity in ferromagnetic (FM) cobaltites and manganites Jing Shi, Li Zhang, Wei-Hsun Lin, Hao Chu, Cameron Hughes, Nai-Chang Yeh The perovskite manganites Ln$_{1-x}$A$_{x}$MnO$_{3}$ and cobaltites Ln$_{1-x}$A$_{x}$CoO$_{3}$ (Ln: trivalent rare earth ions; A: divalent alkaline earth ions) exhibit interesting magnetotransport properties in their FM state: the former show colossal magnetoresistance (CMR) and the latter display giant anomalous Hall effect (AHE), where the AHE coefficient peaks near the Curie temperature ($T_{C})$. These novel phenomena are associated with the intrinsic electronic heterogeneity resulting from strongly correlated multi-valence multi-spin electronic configurations. We perform SP-STS studies on epitaxial films of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ and La$_{1-x}$A$_{x}$CoO$_{3}$ (A = Sr, Ca; 0.4 $\le $ x $\le $ 0.6). In the manganites electronic heterogeneity on the scale of $\sim $10$^{2}$ nm is found to develop below $T_{C}$ and diminish with increasing magnetic field. A surface FM insulating phase is manifested by the spin filtering effect and is attributed to the MnO$_{2}$ surface layer. Similar studies are conducted on the cobaltites to reveal possible correlation between magnetic clustering effects and the Berry phase. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W17.00005: Epitaxial growth and characterization of strained LaCoO$_{3}$ on Si (100) Agham Posadas, Morgann Berg, Hosung Seo, David Smith, Alexander Kirk, Dmitry Zhernokletov, Robert Wallace, Alex de Lozanne, Alexander Demkov LaCoO$_{3}$ is a correlated oxide that normally has a diamagnetic ground state in bulk. The material undergoes a spin-state transition and becomes paramagnetic at higher temperatures. In this work, we report the epitaxial growth of strained LaCoO$_{3}$ on silicon via a fully-relaxed SrTiO$_{3}$ buffer layer using molecular beam epitaxy (MBE). We confirm that the strained LaCoO$_{3}$ becomes ferromagnetic with a Curie temperature of ~85 K, similar to recent reports for films grown by pulsed laser deposition. We will discuss the issues related to the MBE growth of LaCoO$_{3}$ and show results of x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, and SQUID magnetometry measurements on LaCoO$_{3}$ films grown on silicon. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W17.00006: Control of the Octahedral Tilts in Lanthanum Cobaltite and the Impact on Magnetic Properties Michael Biegalski, Haile Ambaye, Valeria Lauter, Hans Christen Strain can be accommodated in two ways in perovskite materials, either via the extension of bond lengths or the rotation of the relatively rigid BO$_{6}$ octahedra. To explore the effects of octahedral tilts on magnitism, we have used epitaxy to control the octahedral tilting in the unit cell of La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ by growth on cubic La$_{0.3}$Sr$_{0.7}$Al$_{0.65}$Ta$_{0.35}$O$_{3}$ (LSAT) and orthorhombic NdGaO$_{3}$. From X-ray diffraction, the films grown on LSAT were shown to have a cubic structure whereas the films grown on NdGaO$_{3}$ show an orthorhombic distortion. Due to the large paramagnetic response of the NdGaO$_{3}$ polarized neutron reflectometery was used to probe the magnetic structure of the films. The polarized neutron reflectometry demonstrates that changes in the crystal structure due to the epitaxially imposed symmetries alter the magnetism in these materials. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W17.00007: Magnetic Phase Separation in Oxygen Doped SrCoO$_{3-y}$ F.J. Rueckert, C.K. Xie, Y.F. Nie, B.O. Wells, J.I. Budnick, W.A. Hines, B. Dabrowski SrCoO$_{3-y}$ forms the perovskite structure with oxygen vacancies and is ferromagnetic for y$<$0.25. We have performed a study on polycrystalline samples, controlling the oxidation state using electrochemistry. Under these conditions we have found that magnetically the system segregates into separate, stable phases that correspond to SrCoO$_{2.75}$(T$_{C}$ = 165 K), SrCoO$_{2.875 }$(T$_{C}$ = 220 K), and SrCoO$_{3}$ (T$_{C}$ = 280 K), with two phase behavior for intermediate oxygen concentrations. Surprisingly, these same samples show only a single structural phase that evolves smoothly. We have recently learned to grow high quality epitaxial films of SrCoO$_{y}$, allowing for more typical single crystal diffraction experiments. Our initial results indicate that magnetic phase separation is suppressed in the films. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W17.00008: Examining the Magnetic Properties of LaCoO$_3$ Thin Films Using Magnetic Force Microscopy Morgann Berg, Agham Posadas, Alex de Lozanne, Alexander Demkov In contrast to the non-magnetic ground state of bulk LaCoO$_3$ (LCO) at low temperatures, ferromagnetism has been observed in elastically strained thin film specimens. The origins of ferromagnetism in strained LCO thin films have been obscured by conflicting experimental results. Pulsed laser deposition (PLD) is the current standard of preparation techniques used to grow thin films of LCO, but results from thin film LCO samples prepared by PLD have been questioned on the basis of chemical inhomogeneity and film defects. Using magnetic force microscopy, we investigate the microscale magnetic properties of strained thin films of LCO prepared by molecular beam epitaxy and deposited on lanthanum aluminate and strontium titanate substrates. We observe these properties across a temperature range surrounding the Curie temperature (T$_c \sim$ 80K) and compare our results to global magnetic characteristics of these films as measured by a SQUID magnetometer. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W17.00009: Magnetic resonance at the Verwey transition in epitaxial $Fe_3O_4$ M. Pechan, B. Kaster, J. Dou, P. Jayathilaka, C. Bauer, C. Miller $Fe_3O_4$ is of interest due to its potential applications in the field of spintronics. Previous studies on magnetite films and $Fe_3O_4$/Cr/NiFe spin valves indicate the presence of a uniaxial anisotropy when the magnetite is grown in a magnetic field. Two 170 nm thick films of $Fe_3O_4$ were reactively sputtered simultaneously onto (100) MgO substrates without and with an applied field (100 Oe). Epitaxy was confirmed by in-plane x-ray diffraction and optimal oxygen stoichiometry is confirmed by the 119 K Verwey transition temperature ($T_V$). Ferromagnetic resonance (FMR) measurements at 35 GHz in the plane of the film revealed four-fold anisotropy confirming high quality (100) epitaxy, with an additional uniaxial contribution present in the field-grown sample. Temperature dependent FMR on the sample grown without field clearly reflects $T_V$ in the linewidth and in-plane and out-of-plane anisotropies. The in-plane uniaxial anisotropy for the sample grown with field exhibits an even stronger temperature response at $T_V$. Detailed discussions of thermal variations of magnetization, anisotropy and relaxation processes will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W17.00010: LuFe2O4 nanostructures on MgO(111) substrate Xiaoshan Xu, Wenbin Wang, Gai Zheng, Paul Snijder, Thomaz Ward, Jian Shen LuFe$_2$O$_4$ nanostructures have been deposited on MgO(111) substrate using pulsed laser deposition. Substrate temperature and gas pressure are found to be very critical to form the LuFe$_2$O$_4$ phase. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) show very well crystalized morphology with triangular symmetry. The common orientations of the nanosctructurals are consistent with epitaxial growth. X-ray diffraction data show that (001) face of LuFe$_2$O$_4$ is parrallel to the substrate face MgO(111). [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W17.00011: Edge-Imposed Domain Ordering in Antiferromagnetic LaFeO3 Nanostructures J.K. Grepstad, E. Folven, T. Tybell, A. Scholl, A. Young, S.T. Retterer, Y. Takamura The antiferromagnetic (AFM) domain structure of submicron-sized LaFeO$_{3}$ nanostructures was imaged with photoemission electron microscopy in combination with x-ray magnetic linear dichroism. These nanostructures were defined in epitaxial LaFeO$_{3}$ thin films using e-beam lithography and Ar$^{+}$ ion implantation to locally destroy the magnetic order in the surrounding matrix. Extended domains were found to form along the perimeter of rectangular-shaped islands, when their edges were aligned with the in-plane ${<}$100${>}$ axes of the cubic SrTiO$_{3}$ substrate. The AFM spin axis of these domains was confined to lie within the film plane, aligned with the edges of the nanostructures. This domain configuration predominated for nanoislands scaled down to 500x500 nm$^{2}$. However, no edge-imposed domain ordering was observed for rectangular islands rotated by 45$^{\circ}$ with respect to the in-plane crystalline axes, suggesting a magnetocrystalline origin of the extended edge-bound AFM domains. These findings may prove important to spintronic devices relying on exchange-biased nanostructures, where domain engineering in antiferromagnets remains relatively unexplored and has the potential to provide new device opportunities. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W17.00012: Fe$_3$O$_4$/ZnO: a high-quality magnetic oxide-semiconductor heterostructure Andreas Mueller, Markus Paul, Dominik Kufer, Sebastian Brueck, Eberhard Goering, Martin Kamp, Jo Verbeeck, He Tian, Michael Sing, Ralph Claessen Magnetite (Fe$_3$O$_4$) is ranked among the most promising materials to use as a spin injector into a semiconducting host. We demonstrate epitaxial growth of Fe$_3$O$_4$ films on ZnO which presents a further step towards incorporation of magnetic materials into semiconductor technology. X-ray spectroscopy results evidence that the iron-oxide is phase-pure and nearly stoichiometric magnetite. Diffraction measurements indicate highly oriented epitaxy and almost complete structural relaxation. The microstructure consists of domains separated by anti-phase boundaries or twin boundaries as a result of island-like growth. The magnetic behavior shows a rather slow approach to saturation at high fields in comparison with bulk crystals, which is likely due to antiferromagnetic coupling at the anti-phase boundaries. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W17.00013: The control of Morin transition temperature on hematite $\alpha$-Fe$_2$O$_3$(0001) thin film SeongHun Park, J.-H. Park, B.-G. Park, J.-Y. Kim The Morin transition of $\alpha$-Fe$_2$O$_3$(0001) thin film was investigated by using soft x-ray absorption spectroscopy (XAS).The epitaxial thin films were grown by cycles of evaporation and post-oxydation method on Al$_2$O$_3$(0001) substrate. The x-ray diffraction (XRD) revealed that the strain is changed with variation of the thickness and the buffer layer. The Morin temperature were measured by x-ray magnetic linear dichroism (XMLD).Interestingly, the Morin transition temperature increased up to room temperature in hematite thin film. In addition, the Co metal overlayer suppressed the Morin transition temperature. Finally, we discuss the magnetic anisotropy including the strain and the interlayer exchange interaction. [Preview Abstract] |
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