High perpendicular magnetic anisotropy (PMA), a property needed for nanoscale spintronic applications, is rare in oxide conductors. We report the observation of a PMA up to 0.23 MJ/m 3 in modestly strained (-0.3%) epitaxial NiCo2O4 (NCO) films which are room-temperature ferrimagnetic conductors. Spin-lattice coupling manifested as magnetoelastic effect was found as the origin of the PMA. The in-plane x 2 -y 2 states of Co on tetrahedral sites play crucial role in the magnetic anisotropy and spin-lattice coupling with an energy scale of 1 meV/f.u. The elucidation of the microscopic origin paves a way for engineering oxide conductors for PMA using metal/oxygen hybridizations. sin( ) cos( ) = [sin 2 ( ) − sin( ) cos( )], with 127, the spherical angles of magnetization, and the angle between magnetization and anisotropy axis. As 128 the anisotropy scale K increases, the magnetization direction of minimum energy tilts further from the field 129 direction and more towards the anisotropy axis. 131A consequence of the relative orientations of the laser incidence and magnetization is that a large enough 132 misalignment of the directions leads to a flipping of the sign of the magneto-optic effect. Explicitly, 133̂′ ⋅̂= 0 when the angle of incidence satisfies tan( ) = , and on either side of this value, the effect of 134 the magnetization switches signs. This is displayed visually in the supplementary Fig S3(a)-(d). The severe 135 misalignment of the laser k'-vector and magnetization direction seen in the NCO/MAO(111) measurements 136 leads directly to the emergence of inverted hysteresis loops, and the three-fold rotational symmetry of the 137 angle-dependent MOKE measurement. 139We can further analyze the effect of magnetization direction relative to incidence direction modeling the 140 Magneto-optic Kerr Effect in terms of the Lorentz force experienced by charges in the magnetized 141 material. In our case of s-polarized light (i.e. polarization perpendicular to the plane of reflection), the 142 electric field forces an oscillation of the charges, which then experience a Lorentz force. With light 143 polarization along the x-axis (i.e. s-polarized light), charges will be forced to oscillate along this same 144 axis. Given the most general magnetization direction ̂= ( , , ), the force experienced will be
The ferrimagnetic inverse spinel NiCo2O4 has attracted extensive research interest for its versatile electrochemical properties, robust magnetic order, high conductivity, and fast spin dynamics, as well as its highly tunable nature due to the closely coupled charge, spin, orbital, lattice, and defect effects. Single-crystalline epitaxial thin films of NiCo2O4 present a model system for elucidating the intrinsic physical properties and strong tunability, which are not viable in bulk single crystals. In this Perspective, we discuss the recent advances in epitaxial NiCo2O4 thin films, focusing on understanding its unusual magnetic and transport properties in light of crystal structure and electronic structure. The perpendicular magnetic anisotropy in compressively strained NiCo2O4 films is explained by considering the strong spin–lattice coupling, particularly on Co ions. The prominent effect of growth conditions reveals the complex interplay between the crystal structure, cation stoichiometry, valence state, and site occupancy. NiCo2O4 thin films also exhibit various magnetotransport anomalies, including linear magnetoresistance and sign change in anomalous Hall effect, which illustrate the competing effects of band-intrinsic Berry phase and impurity scattering. The fundamental understanding of these phenomena will facilitate the functional design of NiCo2O4 thin films for nanoscale spintronic applications.
The X-ray-induced spin crossover transition of an Fe (II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior which is measured in the field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.
Compact domain features have been observed in spin crossover [Fe{H2B(pz)2}2(bipy)] molecular thin film systems via soft X-ray absorption spectroscopy and photoemission electron microscopy. The domains are in a mixed spin state that in average corresponds to roughly 2/3 the high spin occupation of the pure high spin state. Monte Carlo simulations support the presence of intermolecular interactions that can be described in terms of an Ising model in which interactions beyond nearest-neighbors contributing to the formation of domain structures cannot be neglected. This suggests the presence of short-range order to permit interaction between molecules and would explain the formation of a spin state domain structure as the result of cooperative effects.
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