Journal of Physical Studies 5(3/4), 293–303 (2001)
DOI: https://doi.org/10.30970/jps.05.293

A ‟SOFT” SUPERPARAMAGNETIC STATE AND BOUNDARIES OF ITS STABILITY IN AN ISING MODEL OF A DISORDERED GRANULAR MAGNET

E. E. Kokorina, M. V. Medvedev1

Institute of Electrophysics, the Urals Division, Russian Academy of Sciences
106 Amundsen Str., Ekaterinburg, 620016, Russia
1E-mail: medvedev@ief.uran.ru

An Ising model of a disordered granular magnet is proposed. In this model, identical grains with approximately $N≈ 10^4-10^5$ magnetic atoms are placed at random into a nonmagnetic matrix, Ising spins inside each grain are coupled with each other by a strong ferromagnetic exchange, and the spins of different granules coupled by a weak random exchange interaction which is described by a nonsymmetric Gaussian distribution. The model exhibits two critical temperatures of magnetic ordering: first, local ferromagnetic ordering appears inside grains in the absence of a long-range correlation between spontaneous magnetic moments of different grains; then, at a lower temperature either a long-range superferromagnetic state or a macrospin glass one is established over the whole sample. Above the upper critical temperature the usual paramagnetic behaviour of Ising spins is observed, and a ‟soft" superparamagnetic state (with temperature-dependent absolute values of spontaneous magnetic moments of grains) is established between the upper and lower critical temperatures. The partition function of the model is calculated in the molecular-field approximation, and the temperature dependence of the linear and nonlinear magnetic susceptibilities in the ‟soft" superparamagnetic state is analyzed. As well the de Almeida-Thouless instability line at finite magnetic fields is calculated for the case when the system shows a transition from the superparamagnetic state into a low-temperature macrospin glass one.

PACS number(s): 64.60.Cn, 05.50.+q, 75.10.Nr

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