On the effect of the Magnetic Images on Torque Magnetometer Measurements

Anisotropic magnets are composed of materials that exhibit magnetic anisotropy (MA). Understanding MA is essential for improving the performance of permanent magnetic materials that are widely used in electric motors, wind turbines, MRI machines, and speakers. MA is normally determined from torque curves measured directly by a true Torque Magnetometer (TM) or indirectly by a Vector Magnetometer. However, when a magnetic sample is placed between the soft ferromagnetic pole pieces of an electromagnet, magnetic images are induced in the pole pieces at low fields (before saturation occurs) which introduces additional torque. In this study, the image effects influence on the observed torque in a TM was investigated. A theoretical model is proposed which demonstrates how the magnetic images exert a torque on a magnetic sample centered in between the pole pieces. A laboratory model was realized to model how the sample prefers to align itself with its magnetic dipole moment perpendicular to the pole pieces. Using a MicroSense true TM, measurements were performed on a cylindrical NdFeB magnet in 0 Oe field which showed that the torque curve in zero field contains a significant 2θ contribution.