Demagnetizing effects in granular hard magnetic bodies

Hard magnetic materials are usually measured and used in an open magnetic circuit, for which demagnetizing effects are present. The applied field H a has been conventionally corrected to the volume averaged internal field H by a formula H = Ha - NmM, where M is volume averaged magnetization and Nm is magnetometric demagnetizing factor calculated for the case of uniform magnetization as a function of body shape. Since Nm ≥ 0, such a correction always increases the slope of the descending M (Ha) curve without influencing the value of the coercive field with respect to M. However, we show by numerical calculations that if the hard magnetic material is granular and modeled as a rectangular prism, the magnetostatic interactions among the grains will result in a formula H = H a - Nghm M + δHc, where the demagnetizing factor for granular hard magnets N ghm can be either positive or negative and the usually positive coercivity increment δHc is correlated to collective magnetic reversal of longitudinal grain chains. © 2011 American Institute of Physics.