Irreversible relaxation behaviour of a general class of magnetic systems

It is shown that magnetic systems after magnetization in a weak external field for a finite time t(mag) exhibit a universal time-dependent relaxation behaviour. The normalized magnetization decay after switching off an external field does not depend on any sample parameters and follows a universal law m(t) similar to log(1 + t(mag)/t). This universal time dependence is confirmed by magnetic relaxation measurements performed on different powders of small barium hexaferrite magnetic particles at room temperature. The measurements were performed using the PTB SQUID magnetometer in the Berlin Magnetically Shielded Room.