Nucleation and stability of ferromagnetic states

Each ferromagnetic state, such as the single-domain (SD) state, has critica l fields and grain sized at which it becomes unstable. To determine the pro perties of these critical points, a three-dimensional numerical micromagnet ic model is combined with nucleation theory. Isothermal hysteresis and grai n growth are simulated for cuboids with no internal stress or magnetocrysta lline anisotropy. Most jumps in hysteresis loops are turning points at whic h the susceptibility goes to infinity. The SD state becomes unstable at a p itchfork bifurcation, which has a jump in susceptibility but a continuous c hange in magnetic moment. This is a generalization of curling mode nucleati on. As the grain size increases, there is an increasing gap in field betwee n the curling mode nucleation and the first irreversible jump in magnetizat ion. A similar gap is often seen experimentally between the formation of a small spike domain and the appearance of a full size body domain. For the f irst time in a micromagnetic simulation, minor branches are traced from the main hysteresis loop. When they occur, the main loop becomes wasp waisted. At any given grain size the lowest-energy state has SD-like stability in r esponse to changes in magnetic field. A high-stability component of remanen ce is commonly observed in pseudo-single-domain grains. It has previously b een assumed that the high stability must be due to SD-like regions in large r grains, but the micromagnetic simulations demonstrate that SD-like stabil ity does not require a SD-like mechanism.