Temperature dependent resonant microwave absorption in perpendicular magnetic anisotropy epitaxial films of a spinel ferrite

Abstract

We report on the temperature (T), magnetic field (μ0H), and angle (Θ, φ) dependent resonant absorption of X-band microwaves in spinel ferrite epitaxial films subjected to two distinct states of growth strain. The polar angle (Θ) dependence of the resonance field (Hres) in films with ∼0.5% ab-plane expanded unit cell establishes a distinct perpendicular magnetic anisotropy (PMA). The anisotropy field (Han⊥) for Θ = 0° increases monotonically on lowering the temperature from 300 K to 90 K following the behavior of the saturation magnetization (Ms) keeping Han⊥/Ms ≈ 1. The narrow resonance linewidth (μ0ΔHres⊥ = 3.7 mT at 300 K) and its negligible (±0.3 mT) variation with temperature establish the magnetic softness of these PMA films. The dependence of Hres on Θ, φ, and T in films subjected to compressive stress shows in-plane cubic anisotropy whose strength is nonmonotonic in temperature. The ∼2.0% compression of the unit cell basal plane also appears to accentuate noncollinearity of sublattice magnetization of such films as indicated by the T-dependence of ΔHres. The thicker films with PMA display spin wave resonances whose position allows determination of the spin wave stiffness constant together with independent determination of Han⊥. The resonance characteristics of the PMA films qualify them as potential candidates for frequency agile microwave devices and magnonic circuit elements.