Size dependence of the nucleation field of rectangular ferromagnetic parallelepipeds

Abstract

A numerical micromagnetic model based on the finite difference method was used to calculate the nucleation field H N of a rectangular ferromagnetic parallelepiped as a function of the edge length d . The inhomogeneous equilibrium states of the magnetization M s and the magnetic stray field H s were calculated in the range 0.01 μm < d < 100 μm. The particle was assumed to have uniaxial crystalline anisotropy, the easy axis being perpendicular to the lateral edge. The nucleation field H N was found to decrease with increasing particle size d . For d = 100 μm approximately half of Brown's ideal value 2 K 1 / M s was obtained. The reversal process can be divided into two parts: For 0 < H ext < H N reversible inhomogeneous rotation processes occur. At the corners of the particle the stray field H s leads to partial demagnetization even before H N is approached. For H ext = H N an irreversible reversal process occurs. The metastable state transforms into the stable one. The hysteresis curve is nearly rectangular. In the vicinity of the corners the magnitude of H s is less than 5.6π M s .