Verification by finite element modeling for the origin of the apparent image effect in closed-circuit magnetic measurements

Closed-circuit magnetic measurements using a hysteresisgraph have generally been considered free from errors associated with the demagnetizing factor, as well as from the image effect, both of which can occur in open-circuit measurements. However, measurements on magnetic samples clamped between the pole pieces of an electromagnet may show an apparent drop in magnetization with increasing applied field, similar to the image effect found in open-circuit measurements. We have shown that as the saturation magnetization of the sample increases, the drop in apparent magnetization becomes greater and appears at lower fields; the effect also increases as the length-to-diameter ratio L/D of the sample decreases. This behavior has been attributed to distortion of the magnetic field distribution around the sample resulting from localized saturation of the electromagnet pole pieces. This paper presents the results of computer modeling using finite element method, FEM, which confirm this explanation and show that the field acting on the sample is highly non-uniform when the sample L/D is small. The modeling results are in good agreement with experimental data, and show that the apparent drop in magnetization is caused by measurement of the applied field that does not accurately reflect the field acting on the sample.