Critical Fields Determining Magnetic Coercivity in Microparticles of SmCo5 and LaCo5

Magnetic hysteresis curves measured by a specially designed magnetometer on a single particle taken from a ground SmCo5 powder show that the coercivity is determined by pinning of domain walls. Heat treatment at 100°C in air reduces the pinning force. Magnetizing the particle in a strong field increases the pinning force. A wall-pinning model is proposed to explain this effect. The coercivity of a particle taken from the powder after electrolytic etching seems to be determined by nucleation of domain walls. Heat treatment at 100°C in air does not significantly influence the field at which nucleation occurs. The coercivity of an LaCo5 particle is determined by pinning. The pinning force is reduced by hydrogen absorption.