Magnetized materials Ferromagnetism

moving domain walls in grain of silicon steel caused increasing external magnetic field in downward direction, observed in kerr microscope. white areas domains magnetization directed up, dark areas domains magnetization directed down.

thus, piece of iron in lowest energy state ( unmagnetized ) has little or no net magnetic field. however, magnetic domains in material not fixed in place; regions spins of electrons have aligned spontaneously due magnetic fields, , can altered external magnetic field. if strong enough external magnetic field applied material, domain walls move process of spins of electrons in atoms near wall in 1 domain turning under influence of external field face in same direction electrons in other domain, reorienting domains more of dipoles aligned external field. domains remain aligned when external field removed, creating magnetic field of own extending space around material, creating permanent magnet. domains not go original minimum energy configuration when field removed because domain walls tend become pinned or snagged on defects in crystal lattice, preserving parallel orientation. shown barkhausen effect: magnetizing field changed, magnetization changes in thousands of tiny discontinuous jumps domain walls snap past defects.

this magnetization function of external field described hysteresis curve. although state of aligned domains found in piece of magnetized ferromagnetic material not minimal-energy configuration, metastable, , can persist long periods, shown samples of magnetite sea floor have maintained magnetization millions of years.

heating , cooling (annealing) magnetized material, subjecting vibration hammering it, or applying rapidly oscillating magnetic field degaussing coil tends release domain walls pinned state, , domain boundaries tend move lower energy configuration less external magnetic field, demagnetizing material.

commercial magnets made of hard ferromagnetic or ferrimagnetic materials large magnetic anisotropy such alnico , ferrites, have strong tendency magnetization pointed along 1 axis of crystal, easy axis . during manufacture materials subjected various metallurgical processes in powerful magnetic field, aligns crystal grains easy axes of magnetization point in same direction. magnetization, , resulting magnetic field, built in crystal structure of material, making difficult demagnetize.