Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P40: Emergent Magnetism in Oxide Films and HeterostructuresFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Jian Liu, University of Tennessee Room: BCEC 208 |
Wednesday, March 6, 2019 2:30PM - 2:42PM |
P40.00001: Decoupling the Strain and Doping Effects on Magnetic Anisotropy in LaxSr1-xMnO3 via ferroelectric Polarization Control Yifei Hao, Anil K Rajapitamahuni, Lingling Tao, Xiaoshan Xu, Evgeny Y Tsymbal, Xia Hong In this study, we use epitaxial Pb(Zr,Ti)O3 (PZT)/ La0.8Sr0.2MnO3 (LSMO) heterostructures as a model system to decouple the effects of epitaxial strain and charge doping on the magnetocrystalline anisotropy (MCA) in LSMO. By switching the polarization of PZT, we have achieved nonvolatile modulation of the biaxial in-plane magnetic anisotropy in LSMO, as revealed by planar Hall effect studies. The magnetic anisotropy energy (MAE) in the hole accumulation state is enhanced by about 22% compared to the depletion state, in quantitative agreement with the DFT calculations. In contrast, the MAE extracted in chemically doped LSMO films is significantly lower than the theoretical prediction, clearly illustrating the difference between the electric field effect doping and chemical substitution. The discrepancy has been attributed to the different lattice parameters for bulk LSMO at these compositions, where the higher doping sample is subjected to a larger tensile strain on SrTiO3 substrates, concomitantly lowering the MCA. Our result provides the first combined experimental and theoretical study that unambiguously identifies the role of charge doping in determining the MCA in LSMO. |
Wednesday, March 6, 2019 2:42PM - 2:54PM |
P40.00002: Ultra-low-power magnetization rotation by orbital selection at a La0.67Sr0.33MnO3/SrTiO3 interface Le Duc Anh, Takashi Yamashita, Hiroki Yamasaki, Daisei Araki, Munetoshi Seki, Hitoshi Tabata, Masaaki Tanaka, Shinobu Ohya Reducing the power consumption for magnetization switching is essential for the realization of energy-saving spin devices. Here, using a magnetic tunnel junction consisting of La0.67Sr0.33MnO3 (LSMO) / SrTiO3 (STO) / LSMO grown on STO (001), we demonstrate a deterministic and magnetic-field-free 90°-magnetization switching solely by applying an extremely small electric field of 0.05 V/nm on the tunnel barrier and an infinitesimal current density of ~ 10–2 A/cm2, which is ~8 orders of magnitude smaller than that in the present magnetic random access memory. We reveal that this magnetization rotation is induced by a sharp change in the magnetic anisotropy (MA), which occurs at the bias voltage V of ~ ±0.1 V. By measuring the angular dependence of the density of states on the magnetization direction for the same device, we show that the change of MA occurs when the quasi-Fermi level (EF) moves from eg to t2g bands at the LSMO/STO interface with increasing |V| [1]. These findings suggest that highly efficient magnetization control can be realized by designing materials so that the EF lies close to the band edges of different-symmetry orbitals. |
Wednesday, March 6, 2019 2:54PM - 3:06PM |
P40.00003: Oxygen Vacancies Control Interface Magnetism in La0.7Sr0.3MnO3/SrTiO3 Heterostructures Juan I. Beltran, Javier Grandal, Javier Tornos, Carlos Leon, Jacobo Santamaría, Maria Varela, M. Carmen Muñoz O vacancies strongly influence the properties of complex oxides and give rise to novel and unexpected phenomena. Here we combine first-principles density-functional theory with high spatial resolution electron energy-loss spectroscopy and energy-loss magnetic chiral dichroism (EMCD) to show that O vacancies control interface magnetism in La0.7Sr0.3MnO3/SrTiO3 heterostructures grown by high-pressure O2 sputtering. EMCD measurements obtained at low temperature (<100K) are sensitive to the local magnetization with atomic resolution, revealing the formation of Ti and the increase of Mn magnetic moments near the interface. Comparison to the calculated electronic properties unambiguously demonstrates the O vacancy origin of the enhanced interface magnetism. The ideal interface - cleavage of bulk components - |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P40.00004: Local Control of Magneto-crystalline Anisotropy of La0.7Sr0.3MnO3 Thin Films through Ion Implantation Michael Steven Lee, Rajesh V Chopdekar, Elke Arenholz, Yayoi Takamura Control of the functional properties of thin films, including the magneto-crystalline anisotropy, is critical for the development of future memory and spintronic devices. Perovskite oxides offer stimulus-sensitive properties due to the strong coupling between their spin, orbital, lattice, and charge degrees of freedom. In this work, we found that ion implantation of La0.7Sr0.3MnO3 films induces changes in the magnetic anisotropy by causing an out-of-plane lattice expansion while the film remains coherently strained to the substrate. However, the ion implantation also leads to a reduction in Curie temperature and saturation magnetization. X-ray absorption spectroscopy shows this degradation in magnetic properties is in part due to the reduced Mn oxidation state, in addition to crystalline damage. With the proper substrate selection and optimization of ion implantation conditions, the easy axis of the film can be tuned from in-plane to out-of-plane while minimizing the reduction in magnetization. These results may enable the design of unique magnetic spin textures such as magnetic skyrmions in a single layer of material containing adjacent regions of in-plane and out-of-plane magnetization. |
Wednesday, March 6, 2019 3:18PM - 3:30PM |
P40.00005: Controlling magnetic properties of LaMnO3/SrTiO3 heterostructures by electronic reconstruction and stoichiometry: atomic scale evidence Changjian LI, Mengsha Li, Chunhua Tang, Stephen J Pennycook Interface driven magnetic effects and phenomena associated with spin-orbital coupling and intrinsic symmetry breaking are of importance for fundamental physics and device applications. How interfaces affect the interplay between charge, spin, orbital and lattice degrees of freedom is the key to boost the device performances. In LaMnO3 (LMO)/SrTiO3 polar-nonpolar heterostructures, electronic reconstruction leads to an antiferromagnetic to ferromagnetic transition, making them viable for spin filter applications. The detailed electronic structure across the interfaces plays a critical role in understanding the microscopic origins of the observed magnetic phase transition, from antiferromagnetic at 5 unit cells (ucs) of LMO or below to ferromagnetic at 6 ucs or above. Here, we offer an atomic resolution picture of electronic reconstruction by quantifying the charge distribution across this abrupt magnetic transition using STEM-EELS. We find that the electronic reconstruction is confined within the first 2 ucs of LMO from the interface, and more importantly, it is robust against oxygen non-stoichiometry. When restoring stoichiometry, we achieve a ferromagnetic insulating state in LMO films with a thickness as thin as 2 nm, making LMO readily applicable as barriers in spin filters. |
Wednesday, March 6, 2019 3:30PM - 4:06PM |
P40.00006: Emergent Spin State Ordering Driven Ferromagnetism in Strained LaCoO3 Invited Speaker: George Sterbinsky The various paramagnetic and metal-insulator transitions in LaCoO3 make it among the most intriguing of the perovskite oxides, and the discovery of ferromagnetism in epitaxial LaCoO3 further intensified the drive to understand the nature of its magnetic and electronic properties [1]. Key to achieving this goal is the elucidation of the atomic and electronic structural responses of LaCoO3 to epitaxial strain. X-ray diffraction and extended x-ray absorption fine structure (EXAFS) spectroscopy reveal that tensile strain lengthens the in-plane Co-O bonds, thereby distorting the cobalt containing oxygen octahedra, and lowers the symmetry from rhombohedral in bulk to monoclinic [2]. These structural distortions reduce the Co-O orbital hybridization as observed in the x-ray absorption near edge structure (XANES) and stabilize an ordered arrangement of Co sites. In turn, concurrent order of Co charge and spin states arises, as shown by a combination of resonant x-ray scattering and computational density functional theory (DFT) calculations [3]. In essence, spin state order is frozen by epitaxial strain, precipitating the unique transition from paramagnet to ferromagnet found in LaCoO3 films. |
Wednesday, March 6, 2019 4:06PM - 4:18PM |
P40.00007: Size restricted magnetotransport in the non-magnetic delafossite metals PdCoO2 and PtCoO2 Nabhanila Nandi, Thomas Scaffidi, Seunghyun Khim, Pallavi Kushwaha, Joel Moore, Andrew Mackenzie Ultra-pure delafossite metals can present bulk mean free paths as long as tens of microns, long enough such that the momentum conservation of the electron fluid might play a role in electrical transport [1]. As the signatures of this are most prominent in the mesoscopic limit, we use focussed ion beam (FIB) techniques to thin bulk samples down to widths as small as submicron, much smaller than the bulk mean free path. In this talk I will present magnetotransport measurements in these highly-conducting delafossites, specifically PdCoO2 and PdCoO2, at the mesoscopic limit. Magnetic field introduces a variable length-scale, the cyclotron radius, to the system which can be used to tune through different transport regimes. I will discuss the ballistic and hydrodynamic signatures in the transport that in principle become accessible through magnetic field tuning in such mesoscopic samples. |
Wednesday, March 6, 2019 4:18PM - 4:30PM |
P40.00008: Emergent Ferromagnetism in (111)-oriented CaRuO3/CaMnO3 superlattices Margaret Kane, Charles Flint, Arturas Vailionis, Alexander Grutter, Yuri Suzuki Emergent ferromagnetism has been observed at the interface of CaRuO3 (CRO), a paramagnetic metal, and CaMnO3 (CMO), an antiferromagnetic insulator. The leakage of itinerant CRO electrons into the CMO results in a double exchange interaction among interfacial Mn. Since CMO is a G-type antiferromagnet, films grown in (111) direction should have fully uncompensated surfaces and higher interfacial moments than compensated (001) surfaces. We demonstrate the synthesis and characterization of (111) CRO/CMO superlattices with 3 layers of CRO and 3-19 layers of CMO. To grow smooth (111) superlattices, we developed an interval pulsed laser deposition process. SQUID magnetometry measurements indicate larger interfacial Mn moments in our (111) superlattices compared to (001) superlattices. Coercive fields of the superlattice are similar, regardless of CMO thickness, and with increasing CMO thickness, the exchange bias field in the magnetization loops increases. Since CMO and CRO are isovalent, CRO/CMO systems do not exhibit a polarity mismatch across the interface, providing a model system to explore emergent ferromagnetism. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P40.00009: Strain induced topological Hall effect of SrRuO3 single-layered thin films Ludi Miao, Hari Nair, Nathaniel Schreiber, Jacob P Ruf, Yingfei Li, Cyrus Zeledon, Shengwei Jiang, Berit Goodge, Ismail El Baggari, Kin Fai Mak, Jie Shan, Lena Kourkoutis, Darrell G. Schlom, Kyle M Shen Topology in condensed matter physics has led to fruitful areas such as topological insulators, Majorana fermions and valleytronics. Among them, the topological Hall effect (THE) of magnetic materials has attracted much attention due to the discovery of magnetic skyrmions, a topologically protected spin texture that is promising for future magnetic/electronic devices. Recently, a THE has been observed at the interface between a ferromagnetic SrRuO3 (SRO) and a heavy metal SrIrO3 (SIO), although the spin texture of both bulk SRO and bulk SIO are topologically trivial. Here, we will demonstrate the discovery of an unexpected THE on perovskite SRO single-layered films stabilized by molecular beam epitaxy. We will show that the epitaxial strain in SRO films stabilizes a non-coplanar spin texture and leads to the topologically non-trivial phenomenon. SRO is a widely-used material in complex oxide heterostructures. Our results can potentially lead to major progresses in novel phenomena in SRO-based heterostructures as well as the applications of next generation all-oxide electronic and magnetic devices. |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P40.00010: Anisotropic magnetoresistance in Nb:SrTiO3 thin films under epitaxial strain Lucía Iglesias, Francisco Rivadulla We report the effect of epitaxial strain and cationic vacancies on the anisotropic magnetoresistance (AMR) of SrTiO3 (STO) thin films. Cationic vacancies affect the structural properties of the STO thin films, determining the rotation pattern of the TiO6 octahedra [1], and produce a characteristic minimum at T* in the temperature dependence of the electrical resistivity of this material. Our results show a change of sign of the AMR, from negative above T*, to positive below this temperature. Seebeck coefficient measurements do not support an interpretation based on the Kondo effect. We will discuss the possible existence and the nature of a magnetic phase below T*. As the effect is only observed in thin films <20 nm, and it is lost in thicker films (»35 nm), this suggests a contribution from accumulated defects close to the interface with the substrate. |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P40.00011: Magnetic and Electrical Transport Properties of YbFe2O4 Ram Rai, J Hinz, Michelle K Pascolini, James Pawlak, M DeMarco
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