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The threshold of Barkhausen emission and hysteresis in a polycrystalline sample of iron has been measured using a SQUID (superconducting quantum-interference device) magnetic gradiometer in a residual field of less than 10^-6 Oe. In the range 10^-610^-4 Oe, the magnetization was found to be proportional to the applied field and varied reversibly without any discernible Barkhausen emission greater than 10^-8 emu. At higher field levels, the appearance of isolated Barkhausen jumps coincided with the onset of magnetic hysteresis. Repeated field cycles in this threshold region removed the Barkhausen signals and extended the range of reversible magnetization (equivalent to a magnetic Kaiser effect). At still higher field levels, a second threshold of nonvanishing Barkhausen emission and hysteresis appeared. Direct summation of the individual Barkhausen energy losses indicates that the total energy dissipated during the onset of hysteresis varies as (H-Hₓ) + (H-Hₓ) ^2, where Hₓ is the field at which the first Barkhausen jump occurs. Measurements also were made on a single-crystal iron whisker. At higher field levels, occasional Barkhausen emission was observed, but in most instances the data showed irreversible behavior without observable Barkhausen discontinuities. The observations for both the multicrystalline and single-crystal specimen are found to be consistent with the consequences of a general phenomenology of hysteresis.
Weinstock et al. (Fri,) studied this question.