Evaluation of Heat Treatment of Mu-Metal Based on Permeability Under Very-Low-Frequency Micromagnetic Fields

Mu-metal is used to shield highly sensitive machines against magnetic noises. Magnetoencephalography is an application that uses mu-metal to construct a magnetically shielded room (MSR). Each layer of a multi-layer MSR is exposed to a different strength of magnetic noise. The outer layers are exposed to higher noise levels, whereas the inner layers are exposed to lower noise levels. The mu-metal in each layer of the MSR is required to have different magnetic properties depending on the magnetic noise level to which it is exposed. In this study, we propose the use of the index of permeability μ under a very-low-frequency micromagnetic field to evaluate the magnetic properties of mu-metals used in the innermost layer of the MSR. A measurement system for μ is outlined in this article. Measurements were conducted using different heat treatments. Three test pieces (TPs) of the mu-metal from the same lot were heat-treated in a hydrogen atmosphere and subjected to different isothermal conditions and cooling rates. The measurement results demonstrated that the TPs exhibited different magnetic properties at very slow micromagnetic fields, and the properties of the mu-metal could be controlled by heat treatment.