MAGNETIC-PROPERTIES AND ROTATIONAL HYSTERESIS OF FE3O4 AND GAMMA-FE2O3 PARTICLES SIMILAR-TO 250 NM IN DIAMETER

We report the magnetic properties for dispersed, spherical Fe3O4 and g amma-Fe2O3 particles similar to 250 nm in diameter in the temperature range T = 4.2-294 K. For Fe3O4, hysteresis properties show anomalies a t 130 K, where the magnetocrystalline anisotropy constant K-1 vanishes ; the ratio H-c/I-s (H-c = coercive force, I-s = saturation magnetizat ion) follows a linear relationship with K-1/I-s(2) between 130 and 294 K. Below the Verwey transition temperature T-V (similar to 120 K), th e hysteresis properties depend on whether the sample is cooled in a st rong magnetic field or in zero field; no magnetic anomalies were obser ved for gamma-Fe2O3 particles, generated by oxidizing the Fe3O4 partic les under study. The rotational hysteresis integral at 294 K is simila r to 0.7 for both compounds, pointing to a coherent-type magnetization reversal for assumed uniaxial single-domain particles (SDPs); however , the reduced saturation remanence is only similar to 0.2, far below t hat for SDPs. Possible models for the particle magnetization modes are discussed.