Magnetic Annealing Effects in Silicon-Doped Garnets

Abstract

Many properties of polycrystalline yttrium iron garnet (YIG) and reduced single-crystal YIG have been attributed to the presence of Fe2+ ions which provide mobile electrons that can diffuse locally through the lattice. It was the purpose of this research to study the modification to the low-temperature anisotropy caused by the presence of Fe2+ ions in silicon-doped YIG, Y3Fe5−δSiδO12 with 0<δ<0.18. Silicon-doped YIG demonstrates a time-dependent induced anisotropy proportional to doping when magnetically annealed to low temperatures. The kinetics and temperature dependence of this induced anisotropy have been studied. It is further found that these doped garnets show a constant, high-field component of rotational hysteresis which is both frequency- and temperature-dependent. The intrinsic, unannealed anisotropy behaves in a novel way. At room temperature the anisotropy coincides with normal YIG, while between 150°K and 300°K | K1 | increases above that of pure YIG Below about 150°K the curves develop asymmetry and appear to be no longer purely cubic. At 4.2°K the easy axis has shifted to the [100] direction (>3% doping) indicative of a change in sign in K1. Silicon-doped lutetium iron garnet and polycrystalline lutetium iron garnet were found to behave in a similar manner. The results are interpreted in terms of an electron-diffusion model which enables the Fe2+ ion to effectively wander among four nonequivalent octahedral sites. A simple kinetic rate theory is at least partially successful in explaining: (1) the time dependence of the torque curves, (2) the rotational hysteresis, and (3) some of the apparent low-temperature deviations from cubic symmetry.