Incremental permeability of mu-metal in low magnetic fields for design of multi-layer-type of magnetically-shielded rooms

In order to improve the design efficiency of multilayer-type magnetically shielded rooms (MSRs), the magnetic property of mu-metal in low magnetic fields was investigated. The shielding effects (SEs) of multilayered cylinders composed of mu-metal were measured by using a superconducting quantum interference device (SQUID) magnetometer. Then, the incremental permeability of mu-metal was estimated by numerical analysis. It is shown that the incremental permeability becomes constant against the biasing flux density, and is a function of only the magnetic fluctuations, when the biasing flux density is under 0.1 T, which is less than that of the typical MSR set in the geomagnetism.