Nature of the Ferromagnetic Griffiths Phase

A simple model of a disordered cluster ferromagnetic phase is proposed, in which magnetic disorder is determined by random local magnetic fields Hl with a power-law distribution function w ~ |Hl |–ξ (ξ < 1), which allows analytically describing the experimentally known magnetic properties of Griffiths ferromagnetic phases from a unified point of view, including the transition from the Curie–Weiss law for magnetic susceptibility χ ~ 1/(T–TC) to an anomalous power-law dependence χ ~ 1/(T–TC)ξ in the range of temperature T exceeding the Curie temperature TC. The developed approach makes it possible for the first time to explain the appearance of a power-law dependence of magnetization M on the magnetic field H, M ~ H1– ξ, in the ferromagnetic region T < TC and to propose a method for experimental determination of the order parameter. Comparison with experimental data leads to conclusion that it is precisely the disorder of this type that plays the main role in ferromagnetic Griffiths systems, and the division into magnetic clusters should be valid for temperatures both above and below the Curie point. It is shown that the peculiarities of the magnetic properties of the Griffiths cluster system are not associated with the anomalous behavior of the magnetization of an individual cluster, but arise as a result of the disorder-induced modification of the integral characteristics of a disordered ferromagnet.