Magnetic glass ceramics and Stoner–Wohlfarth systems with dipolar interaction

The quasi-static magnetization curve of the magnetic Ba-ferrite (BaFe 12O 19) glass ceramics is analyzed. This recently presented material is isotropic with a high coercivity H c. From its structural peculiarities, the magnetic properties are explained as an assembly property of mutually separated, disk-shaped, uniaxial, single-domain particles. An extension of the classical Stoner–Wohlfarth theory is derived where a relatively weak dipolar interaction appears as an effective stochastic magnetic field which becomes, for large volume packing fractions η of the magnetic particles, strongly dependent on the particle orientation and details of the short-range order. Three parameters are sufficient to characterize the structural details of the assembly: η, the mean aspect ratio 〈 d〉 of the particles, and a new one describing the short-range order. An analytic approximant of the theoretical magnetization curve for η=0.64 is outlined. Both the shape anisotropy of the particles and the fluctuating part of the effective stochastic magnetic field diminish the theoretical H c compared with the Stoner–Wohlfarth value. The stable branch of the experimental magnetization curve (first quadrant) of a dense glass ceramic ( η≈0.6) is well fitted by the theory thus allowing the determination of structural parameters. There remain significant deviations between the (fitted) theoretical and experimental magnetization curve near H c. In particular, the experimental H c is smaller than the predicted one. This is explained by an unsuitable particle size distribution with a significant contribution of large particles which turn into a multi-domain behavior approaching H c.