Flux reversal in rare-earth-substituted yttrium iron garnets

Abstract

The switching properties of polycrystalline toroids of pure YIG (yttrium iron garnet) and YIG containing 1% Dy, 3% Dy, and 4% Sm have been measured over the temperature range 4.2°to 300°K. In each of the rare-earth- substituted materials, the field required to switch 50% of the flux in a given time shows a peak at a temperature which is approximately 0.4 times the temperature at which a peak is observed in the ferrimagnetic resonance linewidth. It is assumed that at low switching speeds flux reversal occurs by domain wall motion, and an explanation of the temperature dependence of the switching field is sought in a consideration of the effect of the loss mechanisms which determine the linewidth on the domain wall mobility. The field above threshold H - H0 required for complete dissipation of magnetostatic energy in the moving wall at a given domain wall velocity V, and thus a given switching speed, should be proportional to the linewidth at the effective precession frequency applicable to domain wall motion. For 180°domain walls this frequency is approximately Vπ/d, where d is the domain wall thickness. Measurements of the frequency dependence of the linewidth of rare-earth-substituted YIG have indicated that the loss mechanisms in these materials are "slow" relaxation processes. For slow relaxation processes the linewidth is given by the expression ΔH (T, ωτ) = F (T)ωτ/1+ω2τ2, where ω is the experimental frequency and τ is the relaxation time. H - H0 (T, Vπτ/d) should depend in a similar way on Vπτ/d. This requires that Vπτ/d be a double-valued function of H - H0, the higher value of which is inaccessible. The increase of ωτ beyond unity contributes to the decrease of the linewidth at low temperature, but the decrease in the switching field at low temperature cannot be explained in this way. If the switching time chosen for reference were sufficiently short that at low temperatures Vπτ/d increases beyond unity, the required field would not be reduced but a discontinuous increase in switching speed, which is not observed, would occur at the field H - H0 (T, Vπτ/d=1). However, the behavior of the switching field is understandable if the explicit temperature dependence of H - H0 (T, Vπτ/d) is such as to produce the observed decrease at low temperature. A report of this work and related topics has been submitted for publication. © 1964 The American Institute of Physics.