Field dependence of the switching field for nonellipsoidal single domain particles

Experimental data on a model system of a two-dimensional array of single domain garnet particles, switching by incoherent rotation, are presented to show that the switching field of individual particles, Hsw, and the coercivity of the major hysteresis loop for ∼1000 particles, Hc, depend on the previously applied saturating field. For the system measured the asymptotic, “true” value of Hc in large fields is 321 Oe, in contrast with Hc=225 Oe, measured in an applied field of Hsat=188 Oe, i.e., the smallest field adequate to close the major loop. Statistical data were collected on switching of a single particle, with an asymptotic value of Hsw=150 Oe. After the application of Hsat=160 Oe Hsw decreased to 111 Oe. Due to the nonellipsoidal shape of the particles, a significant canting of the magnetization near corners and edges persists up to very high fields. The torque, due to these canted magnetic moments, facilitates premature switching in lower fields. It is proposed that defects are responsible for the irreversibility of the process.