Perpendicular magnetic anisotropy in conducting NiCo\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e films from spin-lattice coupling

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/m3 in modestly strained (–0.3%) epitaxial NiCo2O4 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 x2-y2 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

Perspective on Epitaxial NiCo2O4 Film as an Emergent Spintronic Material: Magnetism and Transport Properties

The ferrimagnetic inverse spinel NiCo2O4 has attracted extensive research interests 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

Interfacial and Surface Magnetism in Epitaxial NiCo2O4(001)/MgAl2O4 Films

NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry, x-ray magnetic circular dichroism (XMCD) based on x-ray absorption spectroscopy, and spin-polarized inverse photoemission spectroscopy (SPIPES) with various thickness down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion

Interfacial and surface magnetism in epitaxial NiCo\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e(001)/MgAl\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e films

NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry and x-ray magnetic circular dichroism based on x-ray absorption spectroscopy and spin-polarized inverse photoemission spectroscopy with various thicknesses down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2 ± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion

Domain‑wall magnetoelectric coupling in multiferroic hexagonal YbFeO\u3csub\u3e3\u3c/sub\u3e films

Electrical modulation of magnetic states in single-phase multiferroic materials, using domain-wall magnetoelectric (ME) coupling, can be enhanced substantially by controlling the population density of the ferroelectric (FE) domain walls during polarization switching. In this work, we investigate the domain-wall ME coupling in multiferroic h-YbFeO3 thin films, in which the FE domain walls induce clamped antiferromagnetic (AFM) domain walls with reduced magnetization magnitude. Simulation according to the phenomenological theory indicates that the domain-wall ME effect is dramatically enhanced when the separation between the FE domain walls shrinks below the characteristic width of the clamped AFM domain walls during the ferroelectric switching. Experimentally, we show that while the magnetization magnitude remains same for both the positive and the negative saturation polarization states, there is evidence of magnetization reduction at the coercive voltages. These results suggest that the domain-wall ME effect is viable for electrical control of magnetization

Strain and Surface Effects on the Magnetism of Epitaxial Nickel Cobaltate Thin Films

Research into the behavior of NiCo2O4 (NCO) has accelerated in the last two decades, especially due to its promising applications in catalysis and in supercapacitive energy storage. The transport properties of NCO have been shown to be tied to its magnetic behavior, suggesting research into the magnetism of the material, especially in thin film form, may help inform other aspects of its properties. While NCO is known to exhibit perpendicular magnetic anisotropy (PMA), no work has analyzed the size of the effect, nor the mechanism of the anisotropy. This dissertation presents the observations of strong magnetic anisotropy for epitaxial films grown via pulsed laser deposition onto MgAl2O4 substrates. By fabricating a variety of sample geometries, the size of the anisotropy is determined and is close to that of similar oxides & similar thin-film materials. Using both phenomenological and microscopic studies, we then demonstrate how the modest strain induces the tetragonal distortion leading to the PMA. The second part of the dissertation presents the observation of surface magnetization which is not observed in conventional SQUID magnetometry. Through use of surface-sensitive x-ray magnetic circular dichroism (XMCD) measurements, the surface magnetization is observed to be confined to approximately the top one nanometer of the film, with magnetic transition temperature well below that of the bulk film. Evidence for the reduced magnetism coming from vacancies within the spinel structure are also presented

Magnetic field perturbations to a soft x-ray-activated Fe (II) molecular spin state transition

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

Domain-wall magnetoelectric coupling in multiferroic hexagonal YbFeO3 films

Electrical modulation of magnetic states in single-phase multiferroic materials, using domain-wall magnetoelectric (ME) coupling, can be enhanced substantially by controlling the population density of the ferroelectric (FE) domain walls during polarization switching. In this work, we investigate the domain-wall ME coupling in multiferroic h-YbFeO3 thin films, in which the FE domain walls induce clamped antiferromagnetic (AFM) domain walls with reduced magnetization magnitude. Simulation according to the phenomenological theory indicates that the domain-wall ME effect is dramatically enhanced when the separation between the FE domain walls shrinks below the characteristic width of the clamped AFM domain walls during the ferroelectric switching. Experimentally, we show that while the magnetization magnitude remains same for both the positive and the negative saturation polarization states, there is evidence of magnetization reduction at the coercive voltages. These results suggest that the domain-wall ME effect is viable for electrical control of magnetization