Critical properties of the models of small magnetic particles of the antiferromagnet MnF 2
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Critical Properties of the Models of Small Magnetic Particles of the Antiferromagnet MnF2 V. A. Mutailamova,*, A. K. Murtazaeva,b, and M. A. Magomedova,b a
Institute of Physics, Dagestan Scientific Center, Russian Academy of Sciences, ul. Yaragskogo 94, Makhachkala, 367003 Russia b Dagestan State University, ul. M. Gadzhieva 43a, Makhachkala, 367025 Russia *email: [email protected] Received December 5, 2013
Abstract—The static critical behavior of the models of small magnetic particles of the real twosublattice antiferromagnet MnF2 is investigated by the Monte Carlo method taking into account the interaction of the second nearest neighbors. Systems with open boundaries are considered to estimate the influence of the sizes of particles on the pattern of their critical behavior. The behavior of thermodynamic functions in the phase transition region is investigated. The data on the maxima of the temperature dependences of heat capacity and magnetic susceptibility are shown to be insufficient to unambiguously determine the effective tempera ture of the phase transition in the models of small magnetic particles. This requires an additional investigation of the spatial orientation of the sublattice (sublattices) magnetization vector for the models under study. DOI: 10.1134/S1063776114050069
1. INTRODUCTION In modern condensed matter physics, special attention is given to studying the phase transitions and critical properties of small particles of magnetic mate rials containing from hundreds to tens of thousands of atoms (spins, ions). Such systems possess a number of interesting properties that macroscopic objects do not possess. Studying the influence of finite sizes of small particles on particular physical properties of a material in the phase transition region is of great interest. Greatly simplified models are generally considered when small particles are studied theoretically, although they are fairly diverse. In many cases, despite the model simplicity, the analytical theories encounter great difficulties. In addition, the passage to the ther modynamic limit is impossible for such systems. An experimental study of the properties of small particles also runs into considerable difficulties: the number of interacting elements, the particle shape, the presence of impurities and defects, the appearance of an oxide shell, and the effects attributable to interparticle inter actions often cannot be controlled directly. The numerical simulation methods are devoid of these shortcomings. Developing the models of small mag netic structures and the algorithms for their investiga tion is an important task of modern computational physics. The results obtained by the methods of com putational physics are not only competitive in accu racy with other theoretical methods but often surpass them [1, 2].
Until now, the systems that can be described by simple classical models of the first approximation, such as the Ising model, the Heisenberg model, the XY model, etc., hav
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