Placed in a steady magnetic field, the flux density inside a permalloy-shielded volume decreases over hours and days

Benedict Feinberg,Harvey Gould

doi:10.1063/1.5009926

Abstract

Following the application of an external magnetic field to a thin-walled demagnetized Permalloy cylinder, the magnetic flux density at the center of the shielded volume decreases by roughly 20% over periods of hours to days. We measured this effect for applied magnetic fields from 0.48 A/m to 16 A/m, the latter being comparable to the Earths magnetic field at its weakest point. Delayed changes in magnetic flux density are also observed following alternating current demagnetization. We attribute these effects to delayed changes in magnetization, which have previously been observed in thin Permalloy films and small bulk samples of ferromagnetic materials. Phenomenological models of thermal activation are discussed. Some possible effects on experiments that rely on static shielding are noted.

Highlights

The magnetic field strength H and the magnetization M are in units of ampere per meter (A/m), and μ0 = 4π × 107 henry per meter is the permeability of free space
For a Permalloy (Carpenter HyMu “80”TM) magnetic shield in a region of low initial magnetic field, the increase in magnetic flux density in the shielded volume, from the application of an external magnetic field, is followed by a decrease in the magnetic flux density over time, totaling about 20% of the increase. This is seen in every one of our measurements, done at six different applied magnetic field strengths, from 0.48 A/m to 16 A/m, and spanning times of over thousands of minutes. This is a change in the magnetic flux density, B, over time with no change in the applied magnetic field strength, H, and may be viewed as the equivalent of the effect of producing a larger shielding factor
Following a prompt change in applied magnetic field, delayed changes in magnetic flux density of about 20% and spanning hours or days are observed for Permalloy shields

Summary

Introduction

A. Magnetization changes due to an applied magnetic field. Following the application of an external magnetic field H to a ferromagnetic shield, the magnetic flux density B inside the shielded region does not immediately reach a steady state. This stems from a delay in reaching the full applied field due to shield eddy current effects, and from a delay in the change in magnetization M of the shield.[1] The magnetization is the net magnetic dipole moment per unit volume. The effect of eddy currents, from turning on or off the applied magnetic field, on the magnetic field penetrating a cylindrical magnetic shield is to delay the magnetic field changes. The method of Laplace Transforms is used by Jaeger[55] to solve the problem for both the transverse and axial field cases

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Conclusion