Session R37: Antiferromagnetism and Emergent Magnetism in Oxide Thin Films

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Recently manipulation of antiferromagnetic (AFM) order has been gaining the attention of the spintronics community. Magnetic switching has been reported in AFM thin films based on electronic transport methods, which provide only spatially averaged information on AFM states. Other techniques, such as x-ray magnetic linear dichroism (XMLD), can provide local information on AFM domains. In this study, we report direct observation of spin reorientation in response to current pulses in (0001) α-Fe₂O₃/Pt heterostructures. Our experiment combines the application of current pulses and spatially resolved photoemission electron microscopy (PEEM) with X-ray magnetic linear dichroism to detect antiferromagnetic contrast. Pulses were applied in two different configurations, and XMLD images were interleaved with current pulse sequences to reveal changes in the AFM domains. Our polarization dependent analysis reveals that Neel vector of our α-Fe₂O₃ films have an out-of-plane component in their equilibrium state, contrary to what previous work indicate. Finally, we show clear evidence of AFM domains reorientation outside of the current path, which leads us to the conclusion that spin-orbit torque may not be the only driven effect in the electrical switching process.   

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Strain-induced magnetic anisotropy has long been studied to control magnetic properties of ferro- and ferri-magnetic thin films. However, in antiferromagnetic (AFM) thin films, the effect of epitaxial strain on magnetic anisotropy has not yet been experimentally investigated. Here we perform spin Seeback effect (SSE) measurements on epitaxial Cr₂O₃ thin films grown on Al₂O₃ substrates to demonstrate the presence of the strain-enhanced magnetic anisotropy. Different tensile strains produced by varying the film thickness and annealing temperatures are characterized by X-ray diffraction, which reveals a correlation between the epitaxial strain and resulting spin flop (SF) transition. From the SSE results of these films with different strains, we observe significantly enhanced SF fields compared to the reported bulk result. The large contrast in the SF fields between thin film and bulk Cr₂O₃ indicates the existence of strong strain-induced magnetic anisotropy due to the magnetoelastic effect. Therefore, the epitaxial strain can potentially tune the magnetic anisotropy and consequently spin dynamics in Cr₂O₃ and other AFM thin films for AFM spintronic applications.   

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Engineering magnetic anisotropy in two-dimensional systems has enormous scientific and technological implications. The uniaxial anisotropy universally exhibited by two-dimensional magnets has only two stable spin directions, demanding 180° spin switching between states. We demonstrate a previously unobserved eightfold anisotropy in magnetic SrRuO₃ monolayers by inducing a spin reorientation in (SrRuO3)₁/(SrTiO3)_N superlattices, in which the magnetic easy axis of Ru spins is transformed from uniaxial 〈001〉 direction (N < 3) to eightfold 〈111〉 directions (N ≥ 3). This eightfold anisotropy enables 71° and 109° spin switching in SrRuO3 monolayers, analogous to 71° and 109° polarization switching in ferroelectric BiFeO₃. Our first-principle calculations reveal that increasing the SrTiO₃ layer thickness induces an emergent correlation-driven orbital ordering, tuning spin-orbit interactions and reorienting the SrRuO₃ monolayer easy axis. Our work demonstrates that correlation effects can be exploited to substantially change spin-orbit interactions, stabilizing unprecedented properties in two-dimensional magnets and opening rich opportunities for low-power, multistate device applications.   

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Complex oxides offer possibilities to engineer spin textures via their many degrees of freedom, and have been proposed for use in energy efficient magneto-electric spin-orbit logic architectures.¹ Recent antiferromagnetic (AFM) spintronics work has focused on ultrafast magnetization dynamics geometries with no stray field.² However, lack of stray field makes AFM domain structure imaging difficult though conventional imaging techniques. I will discuss the use of X-ray photoemission electron microscopy (PEEM) with both x-ray linear and circular dichroism to spatially and chemically resolve shape- and voltage-controlled ferroic order in heterostructures of AFM, ferroelectric(FE), and ferromagnetic (FM) layers including (La,Sr)FeO₃^3,4, and CoFe/(La,Bi)FeO₃⁵.
I will show that direct control over spin textures is possible through many routes. Lateral confinement tailors the easy axis orientation in (La,Sr)FeO₃ wires as edge-induced anisotropy in microstructures can induce nearly-single domain states,³ and in the limit of few unit-cell thick layers, the AFM spin axis lies in the plane of the film in contrast to thicker layers whose AFM spin axis cants out of plane.⁴ Furthermore, magneto-electric switching energy density can be scaled through careful engineering in a BiFeO₃-based system towards 10 µJ cm^-2 at a switching field of 200 mV.⁵ Coupling at these interfaces allows for mutual control of the domain structure via a variety of pathways, including shape anisotropy and magneto-electric interactions, and thus offers additional degrees of freedom towards low-energy AFM-based spintronics.

1. Manipatruni et al, Nature 565 35 (2018).
2. Baltz et al, Rev. Mod. Phys 90, 015005 (2018).
3. Bang, RVC et al, Appl. Phys. Lett 115 112403 (2019).
4. Lee, RVC et al, J. Appl. Phys 127, 203901 (2020).
6. Prasad, RVC et al, Adv. Mat. 2001943 (2020).   

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Due to the strong interaction between the charge, spin, lattice, and orbital degrees of freedom, complex oxides possess a wide range of technologically relevant properties such as ferromagnetism, ferroelectricity, and superconductivity. Interfacial interactions provide additional sources of emergent behavior which enable the engineering of spin textures when patterned down to nanoscale dimensions. When patterned using a local structural modification from an ion implantation process, a complex 3D strain state develops within the magnetic islands from the combined effects of the underlying substrate and the surrounding amorphous matrix. As a result, distinctive and coupled ferromagnetic (FM) and antiferromagnetic (AFM) spin textures are observed in heterostructures composed of FM La_0.7Sr_0.3MnO₃ and AFM La_1-xSr_xFeO₃ sublayers. These spin textures can be manipulated using magnetic parameters such as exchange interactions, as well as shape and magnetocrystalline anisotropy energies. These studies demonstrate that complex oxide heterostructures provide a unique platform for engineering spin textures for future memory and spintronic device applications.   

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LaNiO₃ (LNO) is unique among rare-earth nickelates because it is a paramagnetic metal at all temperatures, while others undergo magnetic and metal-insulator transitions. Recently, we have found that the unique strain state at the (111) LaNiO₃/LaAlO₃ interface stabilizes a ferromagnetic, insulating phase in LNO. The constraints on NiO₆ octahedra at the (111)-oriented perovskite interface limit the tilts and rotations that usually relax strain in (001)-oriented films. X-ray diffraction and dynamical x-ray simulations show this results in an elongation of the out-of-plane lattice parameter in the first 5 unit cells of LNO(111) films. Transport measurements show the anomalous Hall effect and negative, hysteretic magnetoresistance in films up to 26 u.c.. This ferromagnetism can originate from Ni²⁺-Ni³⁺ superexchange interactions in the distorted, interfacial LNO(111). We also discuss the magnetic depth profile deduced from polarized neutron reflectometry. The emergence of ferromagnetism indicates a potential for increased functionality in other (111)-oriented perovskite films.   

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Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO₃)_m/(LuFe₂O₄)₁ superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In a significant conceptual advance, interface spectra establish how Lu-layer distortion selectively enhances the Fe²⁺ to Fe³⁺ charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature. Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe₂O₄ layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement.   

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The different magnetic behaviors of LaCoO₃ films grown on LaAlO₃ and SrTiO₃ are related to the Co-O-Co bond angles and the constraints imposed on the Co-O bond lengths by the substrate geometries. Long-range magnetic order occurs in the films grown on SrTiO₃ below T = 90 K because the Co-O-Co bond angle exceeds the critical value of 163° due to the biaxial tensile strain imposed by the substrate. A LaAlO₃ substrate, on the other hand, prevents magnetic-long-range order at low temperatures. Here, we will present a common basis for these strikingly different magnetic behaviors under different strain conditions using x-ray diffraction and magnetization measurements on PLD grown LaCoO₃ thin films. The fundamental antiferromagnetic interaction in the films was characterized using Curie’s law above the critical temperature, and the ensuing ferromagnetic moment was analyzed in view of the strain induced by the substrate, and the results were correlated with the purported Co-O-Co bond angle determined from x-ray diffraction.   

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The development of next generation memory, logic, and energy conversion devices motivates the search for materials where their physical properties can be controlled through the migration of charged ions. Among the candidate materials, perovskite oxides ABO₃ have received considerable interests because of their multitude of related phases such as the Grenier (ABO_2.7), brownmillerite (BM, ABO_2.5), square planar (ABO₂), and Ruddlesden-Popper (RP, A_n+1B_nO_3n+1 where n = integer) phases. In this work, a series of topotactic transformations between these perovskite-related phases has been observed in La_0.7Sr_0.3CoO_3-d thin films upon annealing under highly reducing conditions. At intermediate annealing conditions, a composite phase consisting of ferromagnetic Co nanoparticles embedded in a RP matrix was observed. We show that each structure possesses its own distinct set of physical properties including ferromagnetic/metallic, anti-ferromagnetic/insulating, and ferromagnetic/insulating phases. These results highlight the potential for harnessing these topotactic transformations in next-generation spintronic, neuromorphic, and energy conversion devices based on the movement of oxygen anions.   

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We report experimental and theoretical investigations of emergent properties of heterostructures with one- or two-unit cells of SrRuO₃ (SRO) between two materials, SrTiO₃ (STO, insulating, diamagnetic) or CaRuO₃ (CRO, metallic, paramagnetic). In STO⁵/SRO¹/STO⁵, where the RuO₆ octahedra network is not continuous across the interface, SRO is cubic and the film is nonmagnetic and insulating, but in CRO⁵/SRO¹/CRO⁵, the RuO₆ octahedra network is continuous, the structure is orthorhombic and the film is ferromagnetic (FM) and metallic. Transmission electron microscopy/spectroscopy show no octahedral tilts in STO⁵/SRO¹/STO⁵ while in the CRO⁵/SRO¹/CRO⁵ the tilt angle is larger than in bulk SRO. STO⁵/SRO²/STO⁵ is strongly FM [T_C~120K] replicating a Curie-Weiss behavior. CRO⁵/SRO²/CRO⁵ is FM with slightly larger , exhibiting magnetism similar to a Griffiths phase. Density-functional-theory (DFT) calculations reproduce and provide insights into the structural, electronic and magnetic properties of the heterostructures. The highly localized Ru 4d orbitals are responsible for both the insulating and nonmagnetic behavior in STO⁵/SRO¹/STO⁵. Our study shows that in the ultrathin limit, a continuous RuO₆ octahedra network is key to the magnetic behavior of ruthenates.   

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The coupling between ferroelectric and magnetic orders in multiferroic materials and the nature of magnetoelectric (ME) effects are enduring experimental challenges. In this work, we have studied the response of magnetization to ferroelectric switching in thin-film hexagonal YbFeO₃, a paradigmatic improper multiferroic. The bulk ME decoupling and potential domain-wall ME coupling were revealed using X-ray magnetic circular dichroism (XMCD) measurements with in-situ ferroelectric polarization switching. Our Landau theory analysis suggests that the bulk ME-coupled ferroelectric switching path has a higher energy barrier than that of the ME-decoupled path; this extra barrier energy is also too high to be reduced by the magneto-static energy in the process of breaking single magnetic domains into multi-domains. In addition, the reduction of magnetization around the ferroelectric domain walls predicted by the Landau theory may induce the domain-wall ME coupling in which the magnetization is correlated with the density of ferroelectric domain walls.   

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Antiferromagnetic insulators are a ubiquitous class of magnetic materials, holding the promise of low-dissipation spin-based computing devices that can display ultra-fast switching and are robust against stray fields. However, their imperviousness to magnetic fields also makes them difficult to control in a reversible and scalable manner. Here we demonstrate a novel proof-of-principle ionic approach to drive a 90° spin-reorientation reversibly in the earth-abundant antiferromagnetic insulator α-Fe₂O₃ (hematite) – now an emerging spintronic material that hosts exotic topological antiferromagnetic spin-textures and long magnon-diffusion lengths. We use a low-temperature catalytic-spillover process involving the incorporation (or removal) of hydrogen from α-Fe₂O₃, post-growth, driving pronounced changes in its magnetic anisotropy, Néel-vector orientation and canted magnetism via electron injection and localized strains. We explain these effects with a detailed magnetic anisotropy model and first-principles calculations. Tailoring our work for future applications, we demonstrate control of the room-temperature spin-state by reversibly doping hydrogen in Rh-substituted α-Fe₂O₃.   

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NiO and NiO solid solutions are typically the first passivating oxides to form during the corrosion of Ni-rich alloys. Ni diffusion from the alloy interface, through the oxide, to the exposed surface is the rate-limiting kinetic process for continued oxidation. From first principles, we calculate the effects of magnetic order and electronic doping on the minimum energy diffusion pathways. We elucidate the origin of the diffusion barrier dependencies by assessing changes in the defect states and chemical bonding along the diffusion paths. Our work highlights the complexity of diffusion in even structural simply but strongly correlated oxides with practical recommendations for corrosion resistance that may be transferable to diffusion-mediated redox processes in energy storage materials.