Training of Barkhausen emission in nickel and iron

Abstract

Barkhausen emission from annealed nickel and iron was measured with a SQUID gradiometer that could detect magnetization jumps above a noise level of 3×10−8 emu. The equipment was installed in a three-axis 12.7-m-diam Braunbeck coil system at NASA’s Goddard Space Flight Center. This arrangement provided an overall cancellation of the geomagnetic field to ±10−6 Oe, and a field homogeneity of 0.001% over a spherical volume of 24.4 m3. Auxiliary windings in the Braunbeck coils were used to generate uniform magnetizing fields at sweep rates as slow as 0.81 μOe/s. The nickel measurements were carried out in field ranges that interpolated between those used in previous training experiments with iron and gadolinium samples. In all cases, repeated field sweeps eliminated or diminished the Barkhausen signals in a systematic way paralleling the Kaiser effect in acoustic emission. These results can be related to the training behavior predicted by a general hysteresis theory.