Magnetic glass ceramics and Stoner-Wohlfarth systems with dipolar interaction

The quasi-static magnetization curve of the magnetic Ba-ferrite (BaFe12O19) 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, di sk-shaped, uniaxial, single-domain particles. An extension of the classical Stoner-Wohlfarth theory is derived where a relatively weak dipolar interac tion appears as an effective stochastic magnetic field which becomes, for l arge volume packing fractions eta of the magnetic particles, strongly depen dent on the particle orientation and details of the short-range order. Thre e parameters are sufficient to characterize the structural details of the a ssembly: eta, the mean aspect ratio <d > of the particles, and a new one de scribing the short-range order. An analytic approximant of the theoretical magnetization curve for eta = 0.64 is outlined. Both the shape anisotropy o f the particles and the fluctuating part of the effective stochastic magnet ic field diminish the theoretical H-c compared with the Stoner-Wohlfarth va lue. The stable branch of the experimental magnetization curve (first quadr ant) of a dense glass ceramic (eta approximate to 0.6) is well fitted by th e theory thus allowing the determination of structural parameters, There re main significant deviations between the (fitted) theoretical and experiment al magnetization curve near H-c. In particular, the experimental H-c is sma ller than the predicted one. This is explained by an unsuitable particle si ze distribution with a significant contribution of large particles which tu rn into a multi-domain behavior approaching H-c. (C) 2001 Elsevier Science BN. All rights reserved.