Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G53: Magnetic Thin Films: Novel Synthesis/MultiferroicsFocus Recordings Available
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Sponsoring Units: GMAG Chair: Margaret Kane, Stanford Room: McCormick Place W-475B |
Tuesday, March 15, 2022 11:30AM - 11:42AM |
G53.00001: Reversible ionic control of antiferromagnetic anisotropy Hariom K Jani Antiferromagnets (AFMs) are a ubiquitous class of magnetic materials, holding the promise of low-dissipation spintronic computing devices that can display ultra-fast switching, density scaling and robustness against stray fields. However, magnetic sublattice compensation makes it difficult to detect and control AFM textures in a reversible and scalable manner via standard techniques. We overcame this limitation by developing a novel ionic approach to reversibly tailor AFM anisotropy. We focussed on the earth-abundant AFM α-Fe2O3, which exhibits a spin-reorientation (Morin) transition between in-plane and out-of-plane configurations. Developing reversible control of AFM anisotropy in α-Fe2O3 is important for prospects in topological spintronics and magnonics. Regarding the former, I will discuss our recent results where the Morin transition was exploited to stabilize a wide family of exotic AFM topological textures - half-skyrmions and bimerons - at room temperature. These topological textures have core sizes (of ≈ 100 nm) and can be scaled further with anisotropy tuning. In this context, I will discuss our findings on ionic control of antiferromagnetism in epitaxial α-Fe2O3 films. The catalytic-spillover process employs Pt nano-structures to hydrogenate the AFM films, thereby, driving pronounced changes in the anisotropy, Néel vector orientation and canted magnetism via local charge-doping. As H ions are very small and light, they can be added/removed from the host lattice, without significantly disturbing the overall structure. This allows our approach to be stable yet reversible. Tailoring our work for future applications, we demonstrated reversible control of the room-temperature AFM-state by doping/expelling H ions in Rh-substituted α-Fe2O3. I will conclude by presenting the wider implications of our work, such as how AFM-state control could eventually be realized electrically and translated to a wider variety of AFMs (e.g. orthoferrites, orthochromites). |
Tuesday, March 15, 2022 11:42AM - 11:54AM |
G53.00002: Novel method of thin-film vanadium dioxide growth on (100)-oriented YSZ substrate using RF magnetron sputtering Adam S Christensen, Agham Posadas, A. Alec Talin, Alexander A Demkov We describe a method of depositing highly epitaxial thin films of the monoclinic phase of vanadium dioxide (VO2-M1) on a (100)-oriented yttria-stabilized zirconia (YSZ) substrate using RF magnetron sputtering. We first deposit a thin film of vanadium metal onto the YSZ in a pure Ar atmosphere. After which, the sample is annealed in a pure oxygen environment, yielding the desired VO2 phase. X-ray photoelectron spectroscopy (XPS), performed in situ, confirmed the stoichiometry to be nearly pure VO2-M1 with some higher oxides also present. Out-of-plane x-ray diffraction measurements, performed ex-situ, indicate that the VO2-M1 thin film has a (020) orientation, and rocking curves of the corresponding (020) peak, have a FWHM of approximately 0.07°, indicating a highly epitaxial structure. Preliminary conductivity measurements, performed ex situ, appear to demonstrate the presence of the insulator-to-metal transition. |
Tuesday, March 15, 2022 11:54AM - 12:06PM |
G53.00003: Epitaxial growth and properties of perovskite vanadate thin films Jason D Hoffman, Jenny E Hoffman Perovskite vanadates have recently attracted significant attention for their wide range of functional properties. Below around 140 K, LaVO3 undergoes a structural transition, and becomes an orbitally-ordered antiferromagnetic insulator. SrVO3, on the other hand, remains a strongly correlated paramagnetic metal down the lowest measured temperature of 1.8 K. In this work, we use oxygen-plasma assisted molecular beam epitaxy (MBE) to deposit epitaxial LaVO3 and SrVO3 thin films on insulating SrTiO3 substrates. We use ex-situ x-ray diffraction, magnetotransport measurements, and SQUID magnetometry to compare the properties of (001) and (111)-oriented samples. |
Tuesday, March 15, 2022 12:06PM - 12:42PM |
G53.00004: Hidden magnetoelectric multipoles in complex oxides Invited Speaker: Nicola A Spaldin Most magnetic materials, phenomena and devices are well described in terms of magnetic dipoles of either spin or orbital origin. There is mounting evidence, however, that higher-than-dipolar order multipoles both exist (often in hiding) and lead to intriguing magnetic behaviors. In this talk I will discuss the relevance of the so-called magnetoelectric multipoles, which form the next-order term, after the magnetic dipole, in the multipolar expansion of the magnetization density in a magnetic field. First, I will describe how magnetoelectric multipoles underlie multiferroic behavior and in particular how they determine the magnetic response to applied electric fields. Then I will discuss signatures of hidden magnetoelectric multipolar order, how it can be unearthed using electronic-structure calculations and possibilities for its direct measurement. |
Tuesday, March 15, 2022 12:42PM - 12:54PM |
G53.00005: Topotactic reduction of Brownmillerite cobalt oxide Woo Jin Kim, Michelle Smeaton, Berit H Goodge, Kyuho Lee, Motoki Osada, Lena F Kourkoutis, Harold Y Hwang The soft chemistry topotactic reduction technique has been used for various transition metal oxide compounds to investigate low oxidation states and their resultant physical properties.1 Among recent studies, the discovery of superconducting infinite-layer nickelate thin films highlights the synthetic opportunities provided by topochemistry.2 Here, we adopt this low-temperature reduction method to explore the reduction of brownmillerite cobaltate into a lower oxidation state, using epitaxial thin films. |
Tuesday, March 15, 2022 12:54PM - 1:06PM |
G53.00006: Surface stability and H adsorption on the magnetic properties of SrCoO2.5 Yupu Wang, Gaofeng Teng, Junyi Zhu Strontium cobalt oxide attracts great research attention because of its potential applications in electronic, magnetic, and energy devices. However, the surface reconstructions, electronic and spintronic properties are largely unknown because of its complexed crystal and spintronic structure that contains oxygen vacancy channels. Whether the electron counting model is applicable is also unknown. In this work, we compute the phase diagram and analyze surface reconstructions by a passivation scheme using a new approach to count the electrons based on each layer, which is verified by DFT calculations. Based on the most stable reconstructions, we discovered a coupling mechanism between the surface states and the thin film magnetic states. The surface magnetic properties can be tuned and both FM and AFM order can be obtained with different concentrations of H adsorption on the top surface. Our finding sheds light to the understanding of interplay between surface states and bulk magnetism in transition metal oxides. |
Tuesday, March 15, 2022 1:06PM - 1:18PM |
G53.00007: Determining the oxygen stoichiometry of cobaltite thin films Shenli Zhang, I-Ting Chiu, Minhan Lee, Brandon Gunn, Mingzhen Feng, Tae Joon Park, Padraic Shafer, Alpha T N'Diaye, Fanny Rodolakis, Shriram Ramanathan, Alex Frano, Ivan K Schuller, Yayoi Takamura, Giulia Galli Transition metal oxides (TMOs) are promising materials to realize low-power neuromorphic devices. Their physical properties critically depend on their oxygen vacancy concentrations, whose experimental determination remains a challenging task. Here we focus on cobaltites, in particular La1-xSrxCoO3-δ (LSCO) and present a strategy to identify fingerprints of oxygen vacancies in X-ray absorption (XA) spectra. Using a combination of experiment and theory, we show that the variation of the oxygen vacancy concentration in the perovskite phase of LSCO is correlated with the change of the relative peak positions of the O K-edge XA spectra. We also identify an additional geometrical fingerprint that captures both the changes of the Co-O bond length and Co-O-Co bond angle in the material due to the presence of oxygen vacancies. Finally, we predict the oxygen vacancy concentration of experimental samples and show how the resistivity of the oxide material may be tuned as a function of the defect concentration, in the absence of any structural transformation. Our study [1] shows that, in order to predict the complex transport properties of Mott materials, it is crucial to gain a detailed understanding of their oxygen defect density. |
Tuesday, March 15, 2022 1:18PM - 1:30PM |
G53.00008: Effects of Boron diffusion on the surface NĂ©el temperature revealed by magnetotransport and cold neutron depth profiling in B-doped Cr2O3 films Syed Qamar Abbas Shah, Ather Mahmood, Will Echtenkamp, Jamie L Weaver, Jeffrey W Lynn, Christian Binek Multi-functional thin films of boron (B) doped Cr2O3 grown by pulsed laser deposition exhibit voltage-controlled and nonvolatile Néel vector reorientation in the absence of a magnetic field. Isothermal toggling of antiferromagnetic states is demonstrated in prototype device structures at CMOS compatible temperatures between 300 and 400 K. Although isothermal switching is achieved, selecting a single domain state via a magnetoelectric annealing protocol is hampered most likely by a thermally activated runaway effect of the Néel temperature. This behavior can be understood by considering B diffusion within the thin Cr2O3 film. Cold Neutron Depth Profiling (cNDP), performed at National Institute of Standards and Technology, points at progressing depletion of B atoms in the bulk with temperature. At the same time the B-concentration increases near the surface. The Spin Hall measurements, sensitive to the surface magnetic state, indicate a shift in TN towards higher values associated with the increase in B-concentration near the film surface. |
Tuesday, March 15, 2022 1:30PM - 2:06PM |
G53.00009: Novel Solid-Source Metal-Organic MBE for "Stubborn" Metal Oxides Invited Speaker: Bharat Jalan A fascinating consequence of electron-electron correlation and spin-orbit coupling is the emergence of a wide range of quantum phases in complex oxides such as iridates and ruthenates. However, it has been notoriously difficult to grow these materials in an atomically precise manner using molecular beam epitaxy (MBE) due to ultra-low vapor pressures and difficulty of oxidation for Ru and Ir. In this talk, we will present our group’s effort to address these challenges using a novel solid-source metal-organic MBE approach. We show, for the first time, controlled synthesis of metal and metal oxides of these “stubborn” elements with the same ease and control as afforded by III-V MBE. We will present detailed growth study with the focus to understand and control electronic and magnetic ground states in defect-managed complex oxide films. |
Tuesday, March 15, 2022 2:06PM - 2:18PM |
G53.00010: Epitaxial realization of magnetically frustrated TbInO3 thin films by oxide molecular beam epitaxy Johanna Nordlander, Margaret A Anderson, Ismail El Baggari, Charles M Brooks, Julia A Mundy Magnetically frustrated materials offer a playground for realizing exotic magnetic ground states such as quantum spin ices and spin liquids. Synthesizing such quantum magnets in thin-film form allows further tuning of the magnetic ground state with dimensionality and epitaxial strain as well as the possibility to integrate these states with other functional materials. That said, thin film quantum spin liquid materials remain scarce. We use reactive oxide molecular beam epitaxy to synthesize thin films of hexagonal TbInO3, a magnetically frustrated rare-earth system that was recently proposed as a quantum spin liquid candidate. Due to a lattice distortion accompanied by an improper ferroelectric polarization at high temperatures, the Tb3+ sublattice exhibits a stuffed honeycomb geometry suggested to influence its magnetic ground state. Here, we investigate this ferroelectric distortion in TbInO3 thin films using in-situ RHEED and post-deposition HAADF-STEM and use SQUID magnetometry to investigate the low-temperature magnetic behavior. |
Tuesday, March 15, 2022 2:18PM - 2:30PM |
G53.00011: Double spirals in multiferroic h-ScFeO3 film Xin Li, Xiaoshan Xu, Yu Yun Double-spiral-like screw dislocations, were investigated in epitaxially grown multiferroic h-ScFeO3 films, and the formation was attributed to the coalescence of the antiphase boundaries. The change of critical thickness for the formation of screw dislocations on different substrates indicates that competition between interfacial energy and surface energy dominates the coalescence process. Based on graph theory analysis, the evolution of the screw dislocation networks during annealing follows the scaling law for the annihilation of paired screws with opposite chirality. Combining in-situ RHEED and kinematic simulations, the weakly distorted surface and dependence of double spirals with hexagonal phase were investigated. In the context of improper multiferroic hexagonal ferrites, the screw dislocations are expected to pin down magnetic and ferroelectric vortices simultaneously because both the spontaneous polarization and the magnetization are along the dislocation line. |
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