Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G32: Focus Session: Manganite Thin Films |
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Sponsoring Units: GMAG DMP Chair: Jason Hoffman, Argonne National Laboratory Room: 207B |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G32.00001: Strain driven anisotropic magnetoresistance in antiferromagnetic La$_{0.4}$Sr$_{0.6}$MnO$_{3}$ thin films T. Zac Ward, A.T. Wong, Yayoi Takamura, Andreas Herklotz Antiferromagnets (AFM) are a promising alternative to ferromagnets (FM) in spintronic applications. The reason stems from the fact that at high data storage densities stray fields could destroy FM set states while AFMs would be relatively insensitive to this data corruption. This work presents the first ever example of antiferromagnetic La$_{0.4}$Sr$_{0.6}$MnO$_{3}$ thin films stabilized in different strain states. Strain is found to drive different types of AFM ordering, and these variations in ordering type are shown to have a profound impact on both the magnitude and character of the materials' resistive response to magnetic field direction, or anisotropic magnetoresistance (AMR) behavior (one standard of spintronic suitability). The compressively strained film shows the highest recorded AMR response in an ohmic AFM device of 63{\%}, while the tensile strained film shows a typical AFM AMR of 0.6{\%}. These findings demonstrate the necessity of understanding electron ordering in AFM spintronic applications and provide a new benchmark for AMR response. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G32.00002: Magnetic anisotropy and anisotropic damping in LSMO/STO(001) Hankyu Lee, Igor Barsukov, Liu Yang, Adrian Swartz, Bongju Kim, Harold Hwang, Ilya Krivorotov La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) is a promising material for spintronics applications due to its half-metallic nature. To successively exploit LSMO, both the magnetic anisotropy (MA) and damping need to be well understood and, ultimately, controlled. Here, we study 30 nm epitaxial LSMO thin films grown by pulsed laser deposition on TiO$_{2}$ terminated (001)SrTiO$_{3}$. By means of angle- and frequency dependent ferromagnetic resonance (FMR) at room temperature, we separate various contributions to the in-plane MA: i) The four-fold magnetocrystalline anisotropy is present but negligibly small. ii) The strongest contribution Buni = 4.2 mT is uniaxial with EA along [010]. While uniaxial MA in LSMO systems is commonly related to terrace formation from the substrate miscut, we find that the terrace direction and the MA symmetry axes do not correlate, indicating a different origin of the MA. By evaluating the FMR linewidth, three nonlinear magnetic damping channels due to the two-magnon scattering are found: j) The dominant four-fold contribution with maxima along $<100>$ axes emerges due to the crystalline defects. jj) A two-fold contribution with the maximum along [010] and jjj) a small two-fold contribution with maximum perpendicular to the terraces are identified. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G32.00003: Anisotropic magnetotransport behavior in electronic phase-separated La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) films under anisotropic strain Longqian Hu, Liuqi Yu, Stephan von Molnar, Peng Xiong, Lingfei Wang, Wenbin Wu Anisotropic transport measurements have been performed on LCMO films grown on NdGaO$_{3\, }$(001) substrates. Three samples from a film 48 nm thick were post-annealed for 1.5h, 5h and 20h to produce increasing degrees of anisotropic strain, which promotes electronic phase separation (PS). As demonstrated previously, the presence and growth of antiferromagnetic insulating (AFI) regions in the samples can be controlled by the strain, resulting in a state of coexisting ferromagnetic metallic (FMM) and AFI domains. To study the effects of the strain anisotropy on the PS and formation of the AFI states, we carried out simultaneous magnetotransport measurements along the two orthogonal in-plane directions using an L-bar geometry. Substantial anisotropy in the temperature and magnetic field dependent resistivity between the two directions was observed, implying the formation of the AFI states has an orientation preference under the anisotropic strain. These differences are dramatically enhanced with increasing strain. Furthermore, accompanying the emergence of the AFI states, a glass-like behavior signified by time relaxation was observed in the field-dependent resistivity, which provides new insight into the dynamics of the phase-separated AFI and FMM domains. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G32.00004: Critical thickness for ferromagnetism in LaMnO3 films Invited Speaker: Hans Hilgenkamp We studied LaMnO3 (001) films, deposited by pulsed laser deposition, with thicknesses ranging from 1 unit cell to 24 unit cells. Using Scanning SQUID Magnetic Microscopy, the local magnetization was mapped for these films with micrometer resolution. We find that the magnetic ground state of the films switches sharply from antiferromagnetic to ferromagnetic state when the film thickness exceeds a critical value of 5 unit cells. The films remain electrically insulating. The magnetic transition at the critical thickness is qualitatively explained in terms of an internal electronic reconstruction in the polar LaMnO3 thin films, driven by a polar catastrophe.\\[4pt] [1] X. Renshaw Wang et al., ArXiv 1409.0520 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G32.00005: Tunneling Anisotropic Magnetoresistance with Half-Metallic Electrodes J.D. Burton, Evgeny Y. Tsymbal Tunneling anisotropic magnetoresistance (TAMR) is the difference in resistance of a magnetic tunnel junction due to a change in direction of the magnetization of one or both of the magnetic electrodes with respect to the flow of current, i.e. tunnel conductance for magnetization in the plane differs from magnetization out of the plane. The origin of the effect is spin-orbit coupling (SOC). We will present results of first-principles density functional calculations of the TAMR effect in a half-metallic material, i.e. a metal that has free carriers only in one spin channel. In particular we explore the TAMR effect in magnetic tunnel junctions with La0.7Sr0.3MnO3 (LSMO) electrodes and a SrTiO3 (STO) tunneling barrier. We find $\sim$ 500{\%} difference in resistance between magnetization in the plane and out of the plane. This large TAMR effect originates from the half-metallic nature of LSMO: when magnetization is out-of-plane SOC contributions to the transmission comes only from spin-flip scattering, which is still inherently small due to the half-metallicity. For in-plane magnetization, however, there is a large non-spin-flip SOC contribution to the conductance. The spin-flip vs. non-spin-flip dichotomy along with the orbital character of the states on the Fermi surface of LSMO leads to the large TAMR effect. This effect should be a general feature of half-metallic or highly spin-polarized magnetic electrodes and could open the door to enhanced spintronic device functionalities. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G32.00006: Metal-insulator transition at a surface of a ferromagnetic-metal La$_{0.75}$Ca$_{0.25}$MnO$_3$/SrTiO$_3$(100) thin film Ryota Shimizu, Shunya Nakamura, Yasunobu Ando, Emi Minamitani, Katsuya Iwaya, Takeo Ohsawa, Satoshi Watanabe, Taro Hitosugi We have performed low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on a ferromagnetic-metal La$_{0.75}$Ca$_{0.25}$MnO$_3$/SrTiO$_3$(100) thin film surface. Our topographic images show two-domain zigzag patterns with ($\sqrt{2}\times\sqrt{2}$) periodicities in the perovskite structure. In addition, we measured an energy gap at the Fermi level in our STS spectra, in contrast to the ferromagnetic-metal properties obtained by ensemble measurements. First-principle calculations suggest that the topmost zigzag structure is caused by the structural relaxation based on the orthorhombic nature in bulk, suppressing the carrier itinerancy. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G32.00007: Magneto-thermoelectric transport in ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ thin films Cong Tinh Bui, Francisco Rivadulla La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) thin films have been widely used in spintronics because of its nearly full spin-polarization and high Curie temperature. However, the spin-dependent thermoelectric effects in this material such as spin Seebeck effect (SSE), anomalous Nernst effect (ANE), planar Nernst effect (PNE), which might either interfere the performance of the spintronic devices or be utilized for other potential applications, have not been attended adequately. Herein, we present the observation of PNE and ANE in thin films of LSMO grown epitaxially on SrTiO$_{3}$ substrate. Through a careful control of the thermal gradients, the ANE can be suppressed and separated from the symmetric PNE response. The observation of sign change of ANE over the temperature, which can be understood in terms of Mott relation between anomalous Nernst and Hall coefficients, leads to the intrinsic scattering mechanism of anomalous Hall effect. We also observe a perfect correspondence between the magneto-thermal effects and their electrical counterparts. Finally, the absolutely comparable ANE and PNE signals using either Pt or Au exclude any contribution from SSE within our resolution limit. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G32.00008: Bismuth Manganite Thin Film Characterization Daniel Pajerowski, Lisa Krayer, Bruce Ravel, Julie Borchers, Hyoung Jeen Jeen, Amlan Biswas We have performed detailed characterization of bismuth manganite (BMO) deposited via pulsed laser deposition onto crystalline strontium titanate. BMO is a ferromagnet that has a pseudo-perovskite structure. Actually, the structure deviates from perovskite depending upon growth conditions, and one important parameter is the oxygen content. Thin films of BMO are interesting because they can show ferroelectricity as well as ferromagnetism, but it is not entirely clear why. Another open question is why BMO films are reported to have much less magnetism than expected, based upon comparative measurements of BMO powders. Neutron reflectometry is useful to interrogate these issues because neutrons are sensitive to oxygen content and polarized neutrons can probe the depth dependence of magnetism. These neutron data are supported by high-resolution transmission electron microscopy, X-ray diffraction, and temperature dependent X-ray absorption studies. By considering these techniques in concert, a consistent model of the film is presented. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G32.00009: Epitaxial growth of ultra thin films of electron doped manganites Srimanta Middey, M. Kareev, D. Meyers, X. Liu, Y. Cao, S. Tripathi, D. Yazici, M.E. Maple, P.J. Ryan, J.W. Freeland, J. Chakhalian We report on the fabrication of ultra-thin films of the electron doped manganite in a layer-by-layer growth mode on SrTiO$_3$ (001) substrates by pulsed laser interval deposition. A combination of RHEED (reflection high energy electron diffraction), synchrotron based x-ray diffraction and x-ray absorption spectroscopy measurement confirmed the excellent structural, chemical, and electronic quality. All grown films show a ferromagnetic ground state as revealed by both dc magnetization and x-ray magnetic circular dichroism (XMCD) measurements and remain insulating. The present study opens exciting possibilities of merging this electron doped material with the other hole doped systems to fabricate oxide based spintronic junction. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G32.00010: Enhancement of Magnetic Anisotropy in Ultrathin Epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ Thin Films via Nanostructure Engineering Anil Rajapitamahuni, Le Zhang, John Burton, Vijay Singh, Evgeny Tsymbal, Xia Hong We report a more than ten-fold enhancement of magnetic anisotropy in nanostructured La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) thin films grown epitaxially on (001) SrTiO$_{3}$ substrates. We have etched periodic linear trenches in 6 nm LSMO films, and investigated magnetic anisotropy in these nanostructured thin films via the planar Hall effect (PHE). These trenches have depth of 2 nm and periodicities of 200 - 400 nm. The PHE resistance of the un-patterned LSMO films exhibits sinusoidal angular dependence in an in-plane magnetic field, and shows four-fold sharp resistance switching below a critical magnetic field of 400 Oe, corresponding to a biaxial magnetic anisotropy of $\sim$ 1x10$^{5}$ erg/cm$^{3}$ along \textless 110\textgreater directions. In the nanostructured samples, we observe an additional two-fold resistance switching feature, which persists in magnetic fields higher than 4000 Oe, corresponding to a uniaxial magnetic anisotropy \textgreater 1x10$^{6}$ erg/cm$^{3}$ along one of the biaxial magnetic easy axes. This significant enhancement of magnetic anisotropy cannot be accounted for by shape anisotropy or a uniform strain modulation. We also discuss the effects of the orientation and periodicity of the nano-trenches on the anisotropy enhancement. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G32.00011: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G32.00012: Control of the magnetic properties of LaMnO$_{3}$ epitaxial thin films grown by Pulsed Laser Deposition Benjamin Martinez, Jaume Roqueta, Alberto Pomar, Lluis Balcells, Carlos Frontera, Zorica Konstantinovic, Felip Sandiumenge, Jose Santiso LaMnO$_{3}$ (LMO), the parent compound of colossal magnetoresistance based manganites has gained renewed attention as a building block in heterostructures with unexpected properties. In its bulk phase, stoichiometric LMO is an A-type antiferromagnetic (AFM) insulator (T$_{N}=$140K) with orthorhombic structure that easily accommodate an oxygen excess by generating cationic (La or Mn) vacancies. As a result, a fraction of Mn $^{3+}$ changes to Mn $^{4+}$ leading to a double-exchange mediated ferromagnetic (FM) behavior. In thin films the AFM phase has been elusive up to now and thin films with FM ordering are usually reported. In this work, we have systematically studied the growth process of LaMnO$_{3}$ thin films by pulsed laser deposition on SrTiO$_{3}$ (001) substrates under different oxygen partial pressures (PO$_{2})$. A close correlation between the structure (explored by XRD) and the magnetic properties (SQUID measurements) of the films with PO$_{2}$ has been identified. At high PO$_{2}$ FM behavior is observed. In contrast, at very low PO$_{2}$, the results obtained for unit cell volume (close to stoichiometric bulk values) and magnetic moment (0.2 $\mu_{B}$/Mn) strongly indicate antiferromagnetic ordering. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G32.00013: Electrical transport and magnetic properties of epitaxial LSMO films grown on STO substrates Wei Yuan, Yuelei Zhao, Tang Su, Qi Song, Wei Han, Jing Shi La0.7Sr0.3MnO3 (LSMO) is a very attractive material for spintronics due to its half-metallic ferromagnetic properties. The LSMO films are epitaxially grown on STO (100) substrates using pulsed laser deposition. The effects of substrate temperature, laser power, oxygen pressure, and annealing on the LSMO growth are systematically investigated by the reflection high energy electron diffraction and atomic force microscopy. Under the optimized growth condition, we have achieved atomically flat LSMO thin films with a wide terrace width of more than 5 micro-meters. The electrical transport properties of LSMO thin films of various thicknesses ranging from 8 to 20 monolayers are studied by measuring the resistivity as a function of temperature. We find that the growth condition plays an important role in the critical film thickness for the metal-insulator transition and the Curie temperature. [Preview Abstract] |
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