Modeling of the Magnetocaloric Effect in Heusler Ni-Mn-X (X = In, Sn, Sb) Alloys Using Antiferromagnetic Five-State Pott
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Modeling of the Magnetocaloric Effect in Heusler Ni-Mn-X (X = In, Sn, Sb) Alloys Using Antiferromagnetic Five-State Potts Model with Competing Interactions Vladimir V. Sokolovskiy1,Vasiliy D. Buchelnikov1, Konstantin I. Kostromitin1, Sergey V. Taskaev1 and Peter Entel2 1 Chelyabinsk State University, Br. Kashirinykh str. 129, Chelyabinsk, 454001, Russia. 2 University of Duisburg-Essen, Lotharstr. 65, Duisburg, 470848, Germany. ABSTRACT A simple theoretical five-state Potts model for the investigation of magnetocaloric effect in systems with competing ferromagnetic and antiferromagnetic interactions has been proposed. It is shown that this simple model can be applied to the description of the origin of the negative and positive magnetocaloric effect in systems with competing interactions, for example, Heusler alloys. INTRODUCTION In recent years, theoretical and experimental investigations of the magnetocaloric effect (MCE) in magnetically ordered materials are very important, because materials with large MCE values can be use as refrigerants in a magnetic cooling technology. The MCE is an intrinsic property of any magnetic materials and it is characterized by the change of temperature (∆T) and change of the total entropy (∆Stot) upon influence an external magnetic field. Recent research has shown that ferromagnetic (FM) Heusler alloys such as Ni-Mn-X (X = Ga, In, Sn, Sb) are also prospective materials by way of refrigerants in cooling devices. In these compounds two types of large MCE (positive (∆S < 0, ∆T > 0) and negative (∆S > 0, ∆T < 0)) in the vicinity of the room temperature are observed experimentally [1-4]. A giant MCE is observed in these as well as in Gd-Ge-Si, Mn-As, La-Fe-Si compounds. The reason for a negative MCE is the following: recent experimental and ab initio studies of non-stoichiometric Ni2Mn1+xX1-x alloys have shown antiferromagnetic (AFM) interactions between Mn atoms on their own sites and on X sites and FM interactions between Mn atoms on their own sites [2–7]. These competing interactions can induce new phases in the magnetic subsystem with a primary, AFM, order and a phase transition from a FM phase to an AFM-like one. In this work we study theoretically the conditions that bring about positive and negative MCE in magnetic systems with competing AFM-FM interactions by means of a Monte Carlo method. Examples of such systems may be non- stoichiometric the Heusler Ni-Mn-X (X = In, Sn, Sb) alloys in which FM and AFM interactions coexist. THEORETICAL MODEL In the simple model proposed, we consider a q-state Potts model on a three-dimensional hypercubic lattice with nearest-neighbor (nn) antiferromagnetic and next-nearest-neighbor (nnn) ferromagnetic interactions. Here, q is a number of spin states, and we consider q is equal to 5. The choice of 5 spin components is related to the fact that, for example, in Ni-Mn-X (X = Ga, In,
Sn, Sb) alloys the magnetic moment of the Mn atoms is much larger than the Ni magnetic moment, and X atoms are non-magnetic atoms. So, we consider 5 spin states due to
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