Searching for new materials and phenomena to enable voltage control of magnetism and magnetic properties holds compelling interest for the development of low-power nonvolatile memory devices. In particular, reversible and nonvolatile ON/OFF controls of magnetism above room temperature are highly desirable yet still elusive. Here, we report on a nonvolatile voltage control of magnetism in epitaxial SrCo1–x Fe x O3−δ (SCFO). The substitution of Co with Fe significantly changes the magnetic properties of SCFO. In particular, for the Co/Fe ratio of ∼1:1, a switch between nonmagnetic (OFF) and ferromagnetic (ON) states with a Curie temperature above room temperature is accomplished by ionic liquid gating at ambient conditions with voltages as low as ±2 V, even for films with thickness up to 150 nm. Tuning the oxygen stoichiometry via the polarity and duration of gating enables reversible and continuous control of the magnetization between 0 and 100 emu/cm3 (0.61 μB/f.u.) at room temperature. In addition, SCFO was successfully incorporated into self-assembled two-phase vertically aligned nanocomposites, in which the reversible voltage control of magnetism above room temperature is also attained. The notable structural response of SCFO to ionic liquid gating allows large strain couplings between the two oxides in these nanocomposites, with potential for voltage-controlled and strain-mediated functionality based on couplings between structure, composition, and physical properties.

Single-phase multiferroic materials that allow the coexistence of ferroelectric and magnetic ordering above room temperature are highly desirable, motivating an ongoing search for mechanisms for unconventional ferroelectricity in magnetic oxides. Here, we report an antisite defect mechanism for room temperature ferroelectricity in epitaxial thin films of yttrium orthoferrite, YFeO3, a perovskite-structured canted antiferromagnet. A combination of piezoresponse force microscopy, atomically resolved elemental mapping with aberration corrected scanning transmission electron microscopy and density functional theory calculations reveals that the presence of YFe antisite defects facilitates a non-centrosymmetric distortion promoting ferroelectricity. This mechanism is predicted to work analogously for other rare earth orthoferrites, with a dependence of the polarization on the radius of the rare earth cation. Our work uncovers the distinctive role of antisite defects in providing a mechanism for ferroelectricity in a range of magnetic orthoferrites and further augments the functionality of this family of complex oxides for multiferroic applications.

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According to the Boltzmann transport theory of phonons, the lattice thermal conductivity (κ L ) depends mainly on structure-related parameters including the specific heat, the velocity, and the scattering of phonons. One common strategy for the manipulation of phonon transport has been to introduce lattice imperfections in materials which alters the mean free path of phonons, particularly for applications in thermoelectric energy conversion. [1] Based on the well-established Abeles theory of interactions between phonons and defects in alloys or semiconductors, [2] point defects, including substitutionals, [3] interstitials, [4] and vacancies, [5] are considered to cause additional scattering and therefore lower κ L due to the mass differences and local strain that they introduce. However, in addition to localized mass and strain effects, point defects can also yield significant structural changes, particularly in complex oxides. [6][7][8][9][10] In this case, the role that defects play in the phonon transport could be more subtle, which merits re-evaluation of the typical expectation that defects lower the thermal conductivity.In this work, we focus on epitaxial tungsten trioxide (WO 3 ), which exhibits an ABO 3 perovskite structure with the A-sites vacant and can accommodate a range of lattice distortions via octahedral tilts and rotations. [11][12][13] We find that the roomtemperature (RT) thermal conductivity of the WO 3 thin films evolves significantly upon electrolyte gating, accompanied by reversible structure changes caused by the intercalation or deintercalation of hydrogen (H i ), enabling a reversible control of thermal conductivity. The modulation in thermal conductivity is comparable with that reported in LiCoO 2 [14] and MoS 2 [15] by electrochemically tuning the degree of lithiation, but works in a much simpler configuration. What is more, the variation of thermal conductivity is highly dependent on the substrate on which the epitaxial WO 3 thin films are grown. An unusual increase of κ L is observed after gating with the intercalation of hydrogen, which contradicts the expectation that point defects cause additional thermal resistance and therefore reduce κ L . [16] We further investigate how another common type of point defect, the oxygen vacancy (V O ), influences the lattice structure and κ L in WO 3 thin films. The results collectively indicate that the dominant factor determining κ L is the lattice dimension, i.e., the unit-cell volume. κ L increases as the lattice contracts and Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO 3 thin films. Depending on the substrate, the lattice of epitaxial WO 3 expands or contracts as protons are intercalated by electrolyte gating or oxygen vacancies are introduced by adjusting growth conditions. Surprisingly, the observed lattice volume, ins...