The study of the magnetic and magnetocaloric properties of Ni2+xMn1-xGa (x = 0.16, 0.18 and 0.3) Heusler alloys are presented. The research was performed using a model based on the Malygin theory of smeared phase transitions, Bean–Rodbell theory of first-order phase transitions, and mean-field theory. The temperature dependences of deformation, magnetization, and isothermal entropy change are studied. It is shown that the largest change in magnetic entropy is observed in the Ni2.18Mn0.82Ga alloy, in which the martensitic transition is accompanied by a change in magnetic ordering. The smallest change in entropy is demonstrated by the Ni2.3Mn0.7Ga alloy, in which magnetocaloric the effect is observed in the martensitic phase upon a change in magnetic ordering. However, the refrigeration capacity of this alloy is twice as much as for the other considered compositions.
This work is devoted to the Monte Carlo simulation of the kinetics of phase transitions of the order-disorder type in the Fe80.5Ga19.5 alloy using the Blume-Emery-Griffiths Hamiltonian. The study of phase transformations was carried out in two stages: (1) different cooling rates (different number of Monte Carlo steps at a constant value of the binding energy); (2) isothermal annealing (the binding energy depends on the number of Monte Carlo steps at a fixed temperature). In the first case, a high cooling rate leads to one phase transition A2-> D03 + A2 at 700oС, while slow cooling leads to a transition A2-> D03 + A2 at 700oС through a mixture of phases A2, D03, B2 and L12 in the temperature range from 850 to 700oС. In the second case, an increase in the volume fraction of the L12 phase is shown with an increase in the duration of annealing at 750oС. Based on the data obtained, a thermokinetic diagram of phase transformations was constructed.
С помощью первопринципного подхода исследованы структурные и магнитны свойства сплавов Гейслера Co2Ni1+xSn1-x (x=0, 0.25, 0.5, 0.75, 1). Получены зависимости полной энергии кристаллической структуры от степени тетрагональных искажений и выявлено наиболее выгодное кристаллическое упорядочение. Для всех рассматриваемых композиций вычислена энергия магнитокристаллической анизотропии. Показано, что все композиции с обратной гейслеровской структурой обладают анизотропией типа "легкая плоскость", тогда как в случае регулярной гейслеровской структуры наблюдается смена типа анизотропии с типа "легкая плоскость" на "легкую ось" с увеличением содержания Ni. Наибольшая величина анизотропии обнаружена для нестехиометрических композиций Co2Ni1.5Sn0.5 и Co2Ni1.75Sn0.25. Ключевые слова: сплавы Гейслера, магнитокристаллическая анизотропия, структурные свойства, устойчивость сплавов.
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