Domain Wall and Magnetoelastic Resonances in Hematite

Abstract

The rf susceptibility of synthetic hematite (α‐Fe₂O₃) has been recorded as a function of applied field for frequencies between 80 kHz and 80 MHz at 300°K. Above 25 MHz, a notch about 11‐Oe wide appears in the susceptibility near zero field, with a maximum depth at 42 MHz. Analysis of the data in terms of domain wall resonance dispersion leads to the following parameters for mobile walls: resonance frequency ν₀=25 MHz, effective wall mass μ=3.0×10⁻¹³ g/cm², wall stiffness constant α=7.4×10³ g/cm² sec², and damping constant β≈8.5×10⁻⁵ g/cm² sec. The presence of a large number of strongly pinned domain walls is also confirmed by the data. In the 80–500‐kHz region strong narrow resonance dispersions have been found at fields up to the experimental limit of ±500 Oe. These resonances are magnetically tuneable over a 2:1 frequency range, and have a magnetoelastic nature; they are also dependent upon the quality of the crystal's annealed state. A preliminary interpretation is based on the coupling of the crystal's acoustic resonance modes with low‐lying spin‐wave modes excited near local crystal strains, resulting in magnetoelastic standing waves.