Fengyuan Yang

Despite numerous reports of an insulating ferromagnetic state in epitaxial LaCoO₃ thin films, no consensus has been reached on the details of ferromagnetism in these films. To better understand the origins of magnetic order in such films, stoichiometric LaCoO₃ films have been deposited on SrTiO₃(001) and LaAlO₃(001) substrates using off-axis sputtering. This technique allows growth to occur in conditions that minimize deviations from the ideal stoichiometry. SQUID magnetometry shows that ferromagnetism is stabilized only in films grown under tensile strain on SrTiO₃. The magnetic properties of these films (T _C ≈ 70 K, M _sat ≈ 0.3 μ_B/Co, and H _C ≈ 5 kOe) are essentially independent of thickness, consistent with nearly uniform magnetization. At room temperature, strain induced by the SrTiO₃ substrate breaks the rhombohedral symmetry of the bulk structure, leading to a ^– a ^– c ⁰ octahedral tilting and an anisotropic distortion of the Co-centered octahedra. Low-temperature (T = 36 K) X-ray absorption spectroscopy reveals that tensile strain inherent to the SrTiO₃ substrate stabilizes a substantial fraction of high- or intermediate-spin Co³⁺ ions, facilitating magnetic order, whereas films grown on LaAlO₃ are made up nearly entirely of low-spin Co³⁺ ions.

To investigate the scope of ferroelectric behavior in La-substituted BiFeO3 films, LaxBi1−xFeO3 epitaxial films were synthesized using off-axis co-sputtering on SrTiO3(001) and DyScO3(110) substrates with a SrRuO3 bottom electrode layer. A digital-doping deposition method was used to enable precise control and continuous tuning of La concentration in high-quality LaxBi1−xFeO3 films across a wide range of x = 0.05–0.60, which was systematically investigated using piezoresponse force microscopy. Robust and reversible out-of-plane ferroelectric switching has been observed up to x = 0.35, while films with x ≥ 0.37 exhibit no measurable ferroelectric behavior, indicating a sharp ferroelectric-to-paraelectric phase transition between x = 0.35 and 0.37. This represents the highest reported La concentration in LaxBi1−xFeO3 films that retains ferroelectric ordering, highlighting opportunities to engineer ferroelectric and multiferroic properties in complex oxide heterostructures.

Electrical control of antiferromagnets is essential for the field of antiferromagnetic spintronics, which promises ultrafast speed and terahertz dynamics. However, the large family of complex oxide antiferromagnets such as perovskites has been largely unexplored. Here, we demonstrate electrical switching and detection of Néel vector in LaFeO₃ epitaxial thin films with an epitaxy-enabled biaxial anisotropy, of which the switching behavior is understood by comparing to magnetic field-controlled spin Hall magnetoresistance measurements. The electrical switching is corroborated by imaging of antiferromagnetic domains by using X-ray magnetic linear dichroism photoemission electron microscopy. Electrical switching of 3 and 10 nm LaFeO₃ films was achieved over a wide temperature range of 25-300 K. This work offers an attractive platform for exploration of antiferromagnetic spintronics and other emerging phenomena such as potential insulating altermagnetism.

Magnetic bilayers consisting of an epitaxially grown ferrimagnetic insulator and a heavy metal layer are attractive for spintronic application because of the opportunity for electric control and readout of spin textures via spin–orbit torque. Here, we investigate ultrathin thulium iron garnet (TmIG)/Pt bilayers for TmIG layer thicknesses of 3, 2.7, and 2.3 nm using a sensitive Sagnac magneto-optical Kerr effect technique. We compare the hysteresis loops from out-of-plane and in-plane applied magnetic fields. The preferred magnetization orientation evolves with the TmIG thickness and the presence of the Pt overlayer. We quantify the evolution of the magnetic anisotropy in these ultrathin films and find a significant change even when the TmIG thickness is varied by less than 1 nm. In these ultrathin films, the presence of a Pt overlayer changes the effective anisotropy field by more than a factor of 2, suggesting that the interfacial anisotropy at the Pt/TmIG interface plays a critical role in this regime.

We present a thorough study of spin-to-charge current interconversion in bulk and thin films of (0001) α-Fe_{2}O_{3}/Pt heterostructures by means of all-optical polarization-controlled microwave excitation at subterahertz frequencies. Our results demonstrate that coherent spin pumping is generated through excitations of both the quasiferromagnetic and quasiantiferromagnetic (q-AFM) modes of antiferromagnetic resonance, provided that the corresponding selection rules are met for the relative orientation between the microwave magnetic field h[over →]_{ac} and the magnetic moment m[over →]_{0} of the hematite. In particular, our results unambiguously show that while a microwave field with h[over →]_{ac}⊥m[over →]_{0} pumps a net spin angular momentum from the quasiferromagnetic mode, spin pumping from the q-AFM mode is only enabled when h[over →]_{ac}||m[over →]_{0}, as expected from the selection rules imposed by the Néel vector dynamics. Our findings significantly advance current understanding of spin pumping in easy-plane antiferromagnets not only by claiming a substantial contribution from the Néel-vector dynamics of the q-AFM mode but also by experimentally determining the relative strength of the intra- and cross-sublattice components of the spin-mixing conductance, challenging recent reports where the absence of spin pumping from the q-AFM mode in hematite was interpreted as a complete cancellation between these two components. We also provide an explanation for the previously reported observations and show how the q-AFM spin pumping actually vanishes for thin films, which we speculate being either due to an increased level of inhomogeneities or to insufficient film thickness for the q-AFM mode to fully realize.

In this work, we report the growth of 300 nm c-axis oriented barium titanate (BTO) films on (001)-strontium titanate (STO) substrates via off-axis radio frequency (RF) sputtering. The as-grown films exhibit exceptional crystallinity, with a rocking curve full width at half maximum of ∼0.03°, and atomically smooth surface morphology with a root mean square roughness of 0.23 nm, ensuring minimal optical losses. Transmission ellipsometric characterization of the film reveals a record-high electro-optic (EO) coefficient, r ₅₁ ∼ 550 pm V⁻¹ that exceeds by a factor of 3 compared to the previously reported values for RF-sputtered BTO. This high-quality, industry-compatible material paves the way for the development of efficient and compact EO modulators based on hybrid BTO-on-silicon waveguides.

Magnetic bilayers consisting of an epitaxially grown ferrimagnetic insulator and a heavy metal layer are attractive for spintronic application because of the opportunity for electric control and read-out of spin textures via spin orbit torque. Here, we investigate ultrathin thulium iron garnet (TmIG)/Pt bilayers when the TmIG layer thickness is 3 nm and below using a sensitive Sagnac magneto-optical Kerr effect technique. We compare the hysteresis loops from out-of-plane and in-plane applied magnetic fields. The preferred magnetization orientation evolves with the TmIG thickness and the presence of the Pt overlayer. We quantify the evolution of the magnetic anisotropy in these ultrathin films and find a significant change even when the TmIG thickness is varied by less than 1 nm. In these ultrathin films, the presence of a Pt overlayer changes the effective anisotropy field by more than a factor of 2, suggesting that the interfacial anisotropy at the Pt/TmIG interface plays a critical role in this regime.

Antiferromagnets with highly stable but switchable Néel vectors are desired for antiferromagnetic spintronics with ultrafast speed and terahertz frequencies. Electrical switching of antiferromagnetic insulators has been demonstrated using binary antiferromagnets, while large families of complex antiferromagnets such as perovskites are largely unexplored. Here, we show that epitaxial LaFeO₃ thin films on SrTiO₃(001) exhibit clear, robust biaxial anisotropy with a spin-flop field of a few tesla. Angular-dependent spin-Hall magnetoresistance (SMR) characterizations of Pt/LaFeO₃ bilayers with the current channel along SrTiO₃ [100] and [110] reveal distinct, intriguing shapes and field dependence. Simulations using a macrospin model accurately describe the main behavior and fine features of the SMR data from which key antiferromagnetic parameters are extracted. Furthermore, remanent SMR measurement confirms the high fidelity of the Néel vector along either easy axis of the biaxial anisotropy, indicating that epitaxial films of LaFeO₃ and potentially other perovskite antiferromagnets offer an attractive platform for antiferromagnetic spintronics.