Understanding on coercivity behavior of M-type strontium hexaferrite through thin-film experiment and micromagnetic modeling

We report on the magnetization reversal and coercivity behavior of M-type strontium hexaferrite (SrM) granular structure through thin-film experiment and micromagnetic simulation. We found that with decreasing film thickness (t), the coercivity increases gradually to its maximum of 5.2 kOe at t = 200 nm, turns to decrease at t > 100 nm, and then it decreases rather rapidly at t < 100 nm. In this work, we have determined the origin of this phenomenon and the reason of reduction in coercivity at t < 100 nm, accompanied by a sudden film morphology change from granular to plate or rod shapes, is attributed to the surface crystalline defects of the film, which is fully confirmed by micromagnetic modeling. The relationship between coercivity and film thickness should be correlated with that between coercivity and grain size in bulk SrM.