Closed-Circuit Versus Open-Circuit Characterization of Hard Magnets

Two effects must be considered for the purpose of comparing hysteresis loops of hard magnetic materials (e.g., Nd-Fe-B magnets), measured in open circuit (e.g., vibration sample magnetometer or extraction magnetometer) to those measured in closed circuit (i.e., in a hysteresigraph). The first of these is related to the influence of magnetic viscosity during magnetization reversal, which reduces coercivity. Due to longer measurement times with magnetometers compared to hysteresigraphs (typically 100 s versus 1 s), the coercive field is reduced more in open-circuit measurements than in closed-circuit ones, by typically 20 kA/m. The second effect to be considered is due to differences in various contributions to the demagnetizing field. The slope of the magnetization variation in open-circuit loops is different to that due to the usually considered demagnetizing effects (applicable to soft materials). A reduction in the open-circuit coercive field value compared to the closed-circuit one is found, which is of the order of 25 kA/m for the systems discussed here. This is attributed to the fact that the reversal process itself is affected by how the measurement is made. Reversal has less collective character during open-circuit than during closed-circuit measurements.