Magnetic hysteresis properties and rotational hysteresis losses of synthetic stress-controlled titanomagnetite (Fe2.4Ti0.6O4) particles-II. Rotational hysteresis losses

Abstract

SUMMARY Hysteresis parameters as a function of temperature were measured for milled and etched synthetic 2.4, 12.5 and 165 mm titanomagnetite (TM60) particles. The results show characteristics that appear to be dictated by microstresses, generated by a high density of dislocations and other lattice defects, that were introduced in the spinel lattice by the milling procedure. The following features were found. (1) 2.4 mm particles are single domain (SD) with a saturation remanent magnetization ratio s rs /s s (20°C)#0.5 and coercive force H c (20°C)#75 kA m’1; the latter value can only be interpreted in terms of stress-induced anisotropy because shape and magnetocrystalline anisotropies are too low by far. The nearly temperature-independent coercivity ratio H cr /H c #1.5 up to ~200°C is higher than expected for Stoner‐Wohlfarth SD particles with uniaxial anisotropy; this behaviour is attributed to irregularly directed microstresses as a consequence of lattice defects. By contrast, a Henkel plot shows almost the behaviour expected for Stoner‐Wohlfarth particles. Due to fracturing processes during milling, partial low-temperature oxidation has taken place, resulting in an enhanced Curie temperature T c #230‐260°C, as extrapolated from data of saturation magnetization. (2) 12.5 mm particles (T c =205°C) show typical pseudo-single-domain (PSD) behaviour, with s rs /s s (20°C)#0.18, H c (20°C)#11 kA m’1, both values being much higher than for multidomain (MD) particles, and H cr /H c (20°C)#1.9. Magnetization processes appear to be dictated largely by reversible and irreversible domain rotations; that is, domain wall displacements play a minor role because of strong pinning forces for domain walls at lattice defects. As the temperature rises, a comparatively weak decrease in s rs /s s takes place, whence it follows that the PSD character persists up to the Curie point. H c falls with temperature steadily to a minimum of H c (160°C)# 4k A m’1; a further increase towards 200°C may be associated with an inhomogeneous magnetic behaviour throughout a particle because of the formation of some sort of SD regions, related in part to the microcrystalline particle structure; furthermore, thermal fluctuations may play a role. (3) 165 mm particles (T c =200°C) exhibit MD behaviour with s rs /s s (20°C)#0.025, H c (20°C)# 1k A m’1 and H cr /H c (20°C)#8. While H c shows a smooth variation with temperature up to 160°C, H cr /H c falls with temperature to a minimum H cr /H c (160°C)#2 and an increase occurs again to H cr /H c (200°C)#4.5.