Demagnetizing field in single crystal ferromagnetic shape memory alloys

Abstract

Ferromagnetic materials, including ferromagnetic shape memory alloys (FSMAs or MSMAs), are subject to demagnetization. Because of this demagnetization, the MSMA literature is inconsistent with regards to where the applied magnetic field should be measured experimentally. In addition, many researchers assume a constant demagnetization factor when modeling MSMAs, but there has been little analysis to determine the accuracy of that assumption. In this work, we use finite element (FE) simulations to determine (1) where the applied magnetic field should be measured experimentally and (2) how accurately the assumption of a constant demagnetization factor captures the magnetic field experienced by MSMAs. To determine the correct location for measuring the applied magnetic field experimentally, FE simulations and a constant demagnetization factor were used to calculate an average applied magnetic field with ellipsoidal specimens. This value was then compared to the magnetic field measured in the FE simulations at various locations. Simulations showed that the average magnetic field in the air volume where the MSMA would be placed, but without the MSMA present, was the correct method for measuring the applied magnetic field. Measuring the applied field at the center point of where the MSMA would be placed, but without the MSMA present, provides an easier to measure location, which is nearly as accurate. Additional FE simulations were conducted to test the validity of using a volume average demagnetization factor in calculating an MSMA’s internal magnetic field for prismatic specimens. Comparing the internal magnetic field determined using FE analysis to calculated values of the internal magnetic field using a constant demagnetization factor showed that these two values were similar when the FE calculated internal field was spatially averaged, however, significant variation of the internal magnetic field was present within the volume of prismatic specimens.