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 gamma-Fe2O3 particles similar to 250 nm in diameter in the temperature range T = 4.2-294 K. For Fe3O4, hysteresis properties show anomalies at 130 K, where the magnetocrystalline anisotropy constant K-1 vanishes; the ratio H-c/I-s (H-c = coercive force, I-s = saturation magnetization) 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), the hysteresis properties depend on whether the sample is cooled in a strong magnetic field or in zero field; no magnetic anomalies were observed for gamma-Fe2O3 particles, generated by oxidizing the Fe3O4 particles under study. The rotational hysteresis integral at 294 K is similar 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 that for SDPs. Possible models for the particle magnetization modes are discussed.