Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Interface Effects |
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Sponsoring Units: DMP GMAG Chair: Jason Hoffman, National Institute of Standards and Technology Room: 108 |
Thursday, March 6, 2014 8:00AM - 8:36AM |
S6.00001: An Emergent Spin-Filter at the interface between Ferromagnetic and Insulating Layered Oxides Invited Speaker: Yaohua Liu Complex oxide heterostructures are of keen interest because modified bonding at the interfaces can give rise to fundamentally new phenomena and valuable functionalities. Particularly, an induced magnetization is widely observed at epitaxial interfaces between layered transition-metal oxides; however, much less effort has been spent on investigating how it affects the charge transport properties. To this end, we have studied magnetic tunneling junctions consisting of ferromagnetic manganite La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO) and insulating cuprate PrBa$_{2}$Cu$_{3}$O$_{7}$ (PBCO). Contrary to the typically observed steady increase of the tunnel magnetoresistance with decreasing temperature, this system exhibits an anomalous decrease at low temperatures. Polarized neutron reflectometry (PNR) and x-ray magnetic circular dichroism (XMCD) studies on LCMO/PBCO/LCMO trilayers show that the saturation magnetization of the LCMO contacts increase as the temperature decreases. In other words, degradation of the ferromagnetic contacts is ruled out as a cause. Interestingly, there exists induced net Cu moments, which indicates that the spin degeneracy of the conduction band of the PBCO barrier is lifted and thus the barrier becomes spin selective. Our calculations, within the Wentzel-Kramers-Brillouin approximation, show that the complex temperature dependence can arise from a competition between the high positive spin polarization of the manganite electrodes and a negative spin-filter effect from the interfacial Cu magnetization [1]. This work illustrates that the interface-induced magnetization in layered oxide heterostructures can have non-trivial effects on the macroscopic transport properties. Work performed in collaboration with FA Cuellar, Z Sefrioui, C Leon, J Santamaria (Universidad Complutense de Madrid), JW Freeland, SGE te Velthuis (ANL) and MR Fitzsimmons (LANL). \\[4pt] [1] Yaohua Liu, FA Cuellar, Z Sefrioui, JW Freeland, MR Fitzsimmons, C Leon, J Santamaria, SGE te Velthuis, ``An emergent spin-filter at the interface between ferromagnetic and insulating layered oxides,'' Phys. Rev. Lett. \textit{in press} (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S6.00002: Charge transfer at YBa$_{2}$Cu$_{3}$O$_{7}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ interface Jiunn-Yuan Lin, Vu-Thanh Tra, Ying-Hao Chu In this paper, the ferromagnetic (F) /La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/superconducting (S) YBa$_{2}$Cu$_{3}$O$_{7}$ heterostructures of two distinct interfaces with atomically precise interface control have been fabricated to explore the coupling between these two functional layers. A new mechanism of charge transfer in these heterostructures was identified and confirmed by the results of the first principle calculations. This charge transfer, in addition to the previously considered F/S proximate effect, is critical to to the superconductivity and magnetism in these heterostructures. Direct observation of the charge transfer by x-ray absorption spectroscopy is presented. The results from resonant x-ray scattering is likely to be discussed for both type of interfaces. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S6.00003: Non-Collinear Spin Structures in LaNiO$_3$/La$_{2/3}$Sr$_{1/3}$MnO$_3$ Superlattices Jason Hoffman, Brian Kirby, Anand Bhattacharya The exchange coupling between magnetic layers separated by non-magnetic spacers can give rise to spin structures that are distinct from those observed in the bulk constituents. In this work, we investigate a non-collinear spin arrangement in superlattices containing paramagnetic LaNiO$_3$ and ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_3$. We use molecular beam epitaxy, to fabricate a series of (LaNiO$_3$)$_n$/(La$_{2/3}$Sr$_{1/3}$MnO$_3$)$_9$ superlattices on (001) SrTiO$_3$ and LSAT substrates, where $n$ is varied between 1 and 9 unit cells. The total thickness of the superlattices is kept constant at 60 nm by varying the number of superlattice repetitions. The magnetic structure of the superlattices was investigated as a function of temperature and in-plane magnetic field using polarized neutron reflectometry. We find the magnetization of neighboring La$_{2/3}$Sr$_{1/3}$MnO$_3$ layers to be non-collinear at low fields due to an antiferromagnetic interlayer exchange coupling, which persists to temperatures above 250 K. We discuss underlying mechanisms for the observed behavior and possible applications to oxide-based magnetoresistive devices. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:36AM |
S6.00004: Spin structure in an interfacially-coupled epitaxial ferromagnetic oxide heterostructure Invited Speaker: Xianglin Ke We report the spin structure of an exchange-biased ferromagnetic oxide heterostructure, La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ / SrRuO$_{3}$, through magnetization and polarized neutron reflectometry measurements. We reveal that the magnetization reversal process of the La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ biased layer critically depends on the frozen-in spin structure of the SrRuO$_{3}$ biasing layer during the cooling process. Furthermore, we observe unexpected double-shifted hysteresis loops of the biased layer that originates from the formation of lateral 180$^{\circ}$ magnetic domains within the biasing layer, a new mechanism not found in conventional exchange-bias systems [1]. The effects of the thus-formed spin structure on the magnetotransport properties will be presented as well. This work was done in collaboration with L. J. Belenky, V. Lauter, H. Ambaye, C. W. Bark, C. B. Eom, M. S. Rzchowski, J. Smith, and M. Zhu. \\[4pt] [1] X. Ke et al, Phys. Rev. Lett. \textbf{110}. 237201 (2013). [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S6.00005: Magnetic ordering temperatures at oxide interface LaAlO$_3$/SrTiO$_3$ Tomoya Asaba, Gang Li, Ben Lawson, Fan Yu, Ziji Xiang, Colin Tinsman, Harold Hwang, Jochen Mannhart, Lu Li A number of recent experiments demonstrate the existence of magnetic ordering at the conductive oxide interface LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO). Understanding the origin of this magnetism requires determination of the magnetic state at elevated temperature. In this study we carried out torque magnetometry measurements to track the magnetic transition temperatures in the interface samples with different LAO thickness. The magnetic ordering temperature is found to vary greatly as the thickness of LAO changes. . Our results suggest that the growth condition such as LAO thickness affects the magnetic coupling of the interface magnetic moments. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S6.00006: Electronic and Magnetic Properties of Ultrathin SrRuO3 (111) Film on SrTiO3 Bongjae Kim, B.I. Min We have investigated electronic and magnetic properties of ultrathin SrRuO$_{3}$ (SRO) film grown on (111) SrTiO$_{3}$ substrate using the {\it ab initio} electronic structure calculations. Ru-terminated SRO (111) film suffers from strong surface atomic relaxations, while SrO$_{3}$-terminated one preserves the surface structure of ideal perovskites. Both Ru- and SrO$_{3}$-terminated SRO (111) film show unexpected interlayer antiferromagnetic (AFM) structure at the surface, but with different characters and mechanisms. The AFM structure for the former results from the large surface atomic relaxation, whereas that for the latter results from the truncated film effect. Interestingly, for the SrO$_{3}$-termination case, the half-metallic nature emerges despite the interlayer AFM structure. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S6.00007: Control of electrical and magnetic properties of Mn thin film on BaTiO3 Anh Tuan Duong, Yooleemi Shin, Van Quang Nguyen, Duc Dung Dang, Sunglae Cho Bulk Mn material is one of transition metals that has been well known as an antiferromagnetic material due to an anti-parallel spin alignment with negative exchange integral. However, theory predicted that the magnetic properties of Mn can be transited to ferromagnetic with the expansion in volume following Hund's rule. A current active research topic is electric field controlled magnetism. To accomplish this goal, a way is to use multiferroic material. Epitaxial Mn thin film has successfully been grown on BaTiO3 substrate by using molecular beam epitaxy (MBE). We could control the degree of a structural deformation of Mn thin film using unique four different crystal structures of BaTiO3 below 400 K. We observed three jumps at 185, 290, and 390 K in temperature dependent electrical resistivity, corresponding to the temperatures of structural phase transitions in BaTiO3. The modification of magnetism from antiferromagnetism to ferrimagnetism in Mn film was observed. We also observed two jumps at 290 and 365K in the temperature dependent magnetization. The calculated magnetic moment was 0.66 $\mu$B/Mn at 320K. These results indicate the possibility of the tuning of electrical and magnetic properties from antiferromagnetic to ferrimagnetic or vice versa in Mn film by modulating strain. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S6.00008: Ferroelectric and Magnetic SrTiCoO3 films on Silicon and Niobium-doped SrTiO3 substrates Mehmet Onbasli, Andy Cruz, T. Goto, Caroline Ross Perovskites hold great potential for fundamental studies of structure-multiferroicity relationship as well as technological applications such as multi-level memories. We demonstrate multiferroic behavior of Cobalt-substituted SrTiO3 (STCo) films on Silicon and on Niobium-doped SrTiO3 substrates (Nb:STO). STCo films were grown on Si, silicon-on-insulator, Nb:STO, 3 $\mu $m thick SiO2 coated Si, and pure STO substrates using pulsed laser deposition under different oxygen pressures (1, 3, 6 $\mu $Torr, 1.6 mTorr). The film composition is SrTi0.70Co0.30O3-$\delta $, as confirmed by $\omega $-2$\theta $ scans of x-ray difractometer. Magnetic hysteresis loops indicate that the films have out-of-plane easy axis with anisotropy field of several kOe, which is attributed to magnetoelastic anisotropy. Saturation magnetizations of 0.9, 0.3, 0.5 and 0.2 $\mu $B/Co ion were obtained for samples grown on Nb:STO under oxygen pressures 1, 3, 6 $\mu $Torr, 1.6 mTorr, respectively. Ferroelectric saturation polarizations of 67 to 118 $\mu $C/cm2 and resistivities between 1e6 to 1e9 $\Omega \cdot$ cm were obtained for STCo on Nb:STO and on Silicon. The origin of the magnetic and ferroelectric properties will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S6.00009: Interface states in CoFe$_{2}$O$_{4}$ spin-filter tunnel junctions Pavel Lukashev, J.D. Burton, Alexander Smogunov, Julian Velev, Evgeny Tsymbal Spin-filter tunneling is a promising way to generate highly spin-polarized current, a key component for spintronics applications. In this work we explore the tunneling conductance across the spin-filter material CoFe$_{2}$O$_{4}$ interfaced with Au electrodes, a geometry which provides nearly perfect lattice matching at the CoFe$_{2}$O$_{4}$/Au(001) interface.\footnote{P. Lukashev, et al., Phys. Rev. B, \textbf{88}, 134430 (2013).} Using density functional theory calculations we demonstrate that interface states play a decisive role in controlling the transport spin polarization in this tunnel junction. For a realistic CoFe$_{2}$O$_{4}$ barrier thickness, we predict a tunneling spin polarization of about $-$60{\%}. We show that this value is lower than what is expected based solely on considerations of the spin-polarized band structure of CoFe$_{2}$O$_{4}$, and therefore that these interface states can play a detrimental role. We argue that this is a rather general feature of ferrimagnetic ferrites and could make an important impact on spin-filter tunneling applications. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S6.00010: Spin-orbit coupling and the ultimate limit for spin-polarized tunneling from half-metallic electrodes J.D. Burton, Evgeny Y. Tsymbal Half-metallic materials, i.e. metals that have free carriers only in one spin channel, should act as ideal materials for spin-polarized transport applications. In magnetic tunnel junctions with identical half-metallic electrodes, for example, there would in principle be zero tunneling transmission (infinite resistance) when the magnetization of the electrodes are aligned anti-parallel, making the tunneling magnetoresistance (TMR) ratio infinite. In practice, however, it is thought that this idealized case can only hope to work at zero temperature and when the electrodes are in a truly mono-domain configuration: effects which are generally very difficult to minimize. Also, however, one factor that can never be suppressed is the mixing of the spin-polarized carriers induced by spin-orbit-coupling (SOC). We will present results of density functional calculations on idealized magnetic tunnel junctions with La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ (LSMO) electrodes and SrTiO$_{\mathrm{3}}$ (STO) tunneling barrier. In the absence of SOC, LSMO is predicted to be a half-metal having Fermi-level density of states only for majority spins, and an electronic gap for the minority spin-channel. Indeed, transport calculations based on a generalized scattering approach predict an infinite TMR effect in LSMO/STO/LSMO junctions. The inclusion of SOC into the calculations, however, opens a channel for transmission through the barrier in the anti-parallel magnetic configuration leading to a large, yet finite, TMR ratio. With all other spin-flip mechanisms suppressed, this represents the ultimate limit for TMR in idealized junctions. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S6.00011: Spin polarization at the interface of LaMnO3 and Si heterostructure Huiping Zhu, Guoping Zhang, Xiaoshan Wu Spin injection at a ferromagnet- semiconductor interface is one of the promising ways to add new functionality to the conventional devices. Here we study the spin injection from ferromagnetic LaMnO3 to semiconductor Si using first-principles calculations. We use two different methods: (1) Changing the distance between Si and LaMnO3 layers and (2) introducing the dimerization at the first Si layer. We find that when we reduce the distance between Si and LaMnO3 layer, both the total spin moment and the spin polarization at the Fermi-level change. There is a general trend that a stronger spin moment corresponds to a weaker spin polarization, but they do not follow exactly this trend at each distance. For the Si dimerization case, when the distance between the two Si atoms is reduced, the total spin moment increases whereas the spin polarization at the Fermi-level is not. Our results indicate that Si atoms with a smaller total spin moment may have a stronger spin polarization at the Fermi-level, and vice versa. Since the electrons at the Fermi-level play a key role in transport, the spin polarization at the Fermi-level, therefore, is more important than the total spin moment for spin injection. [Preview Abstract] |
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