Low temperature magnetic properties of Metglas 2714A and its potential use as core material for EMI filters

Abstract

Inductive filters to reduce electromagnetic interference (EMI) are commonly made by winding coils around a magnetic material with high permeability. This permeability should remain high at high frequencies, so that the impedance of the inductor made from it can be large to block off any high frequency EMI signals. Room temperature magnetic material like ferrite loses most of its permeability at low temperatures. Some low temperature materials like Cryoperm-10 are known to have high magnetic permeability. However, the permeability of all of them decreases to a low value at frequencies higher than a few hundred hertz due to eddy current shielding of the magnetic field. We have measured the complex relative permeability of Metglas 2714A (also known as Magnaperm) at 4.2 K, and found that its magnitude is larger than 10,000 at frequencies up to 100 kHz. We also measured the magnetization noise density from this material using a SQUID magnetometer. We find that the noise density agrees well with the predictions of the fluctuation dissipation theorem. This implies that low temperature inductors and transformers with predictable noise characteristics can be designed for applications where the lowest noise is not a limiting factor of performance. For very low noise applications, a common mode filter can be made with this material, where the two input leads to a pre-amplifier are wound as a pair around the material, so that any magnetization noise is cancelled out.