Приводятся результаты экспериментальных и теоретических исследований магнитокалорических свойств ленточного образца сплава Ni45Co5Mn31Al19 в интервале T=80-350 K в магнитных полях до 8 Т. Данный сплав демонстрирует магнитоструктурный фазовый переход (МСФП) 1-го рода в области температур 270 K, а также переход II рода --- при температуре Кюри 294 K. Магнитокалорический эффект (МКЭ) исследовался как прямым методом модуляции магнитного поля в циклических полях до 8 T, так и классическим экстракционным методом. Полевые зависимости МКЭ имеют различный характер для фазовых переходов I и II родa. Вблизи МСФП обратный МКЭ необратим, т. е. конечная температура образца ниже начальной на 0.75 K. Теоретические исследования магнитных свойств и МКЭ исследуемого образца выполнено с помощью ab initio расчетов и моделирования методом Монте-Карло. Теоретические температурные зависимости МКЭ характеризуются схожим интервалом проявления эффекта в области мартенситного превращения и более узким интервалом в области температуры Кюри аустенита по сравнению с экспериментом, что обусловлено наличием неоднородного смешанного состояния аустенита в экспериментальном образце. В целом, теоретические данные качественно и количественно воспроизводят экспериментальные зависимости. Ключевые слова: магнитокалорический эффект, циклические поля, сплав Гейслера, метод Монте-Карло.

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.

The properties of Heusler alloys of the Mn2ScZ family (Z = Al, Si, P, Ga, Ge, As, In, Sn, Sb) are investigated within the framework of the density functional theory. The PBE GGA and meta-GGA SCAN functionals were used for approximation of the exchange-correlation interactions. Calculations show that PBE does not predict ideal half-metallic behavior, unlike SCAN. It is shown that at Z = P, Si, a transition from the half-metallic state to the metallic one is observed. This effect can be used to develop tunable spintronic devices.

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.

С помощью расчетов из первых принципов исследованы вопросы стабильности серии сплавов Гейслера Ni2Mn1+xSb1-x (x=0, 0.25, 0.5, 0.75, 1) с шахматным и послойным упорядочением атомов Mn. Показано, что композиции с избытком Mn являются устойчивыми по отношению к распаду на составляющие элементы и неустойчивыми по отношению к распаду на двухфазную систему, состоящую из ферромагнитной кубической L21-фазы Ni2MnSb и антиферромагнитной тетрагональной L10-фазы NiMn. Таким образом, все нестехиометрические составы в аустенитной и мартенситной фазе с учетом различного магнитного и атомного упорядочения проявляют тенденцию к сегрегации. Стабильность сплавов возможна только в стехиометрических составах (x = 0 и 1). Ключевые слова: Сплавы Гейслера, первопринципные вычисления, сегрегация, фазовая стабильность.

This work presents an ab initio study of the effect of a small addition of the third element of III and IV groups on the elastic and magnetoelastic properties of Fe75Ga25 alloy. The dependencies of the tetragonal elastic modulus C', magnetoelastic constant –b1, and the tetragonal magnetostrictive constant λ001 on the concentration of the Z-element in the cubic crystal structures A2 and D03 were obtained with the help of the density functional theory and the magnetic torque method in Fe75Ga25-xZx (Z = Al, Ge, Si) alloys (0≤x≤6 at.%). It is shown that the addition of Al and Si atoms leads to an increase in the tetragonal elastic modulus compared to the Fe75Ga25 binary alloy. A correlation was established in the dependence equilibrium lattice constants a0(x) and λ001(x) in the studied ternary alloys for the A2 structure.

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.

In present work, on the basis of structural and magnetic phase transition temperatures estimated theoretically from the first principles, the concentration phase diagram for Fe100-xSix (9.375 ≤ x ≤ 25.0 at.%) was plotted. Structural phase transition temperatures for the experimentally observed crystal structures were obtained from the structural optimization. The Curie temperatures were estimated from mean field approximation using ab initio calculated exchange coupling constants. Both structural and magnetic phase transitions temperatures are found in qualitative agreement with the experiment. For the whole considered concentration range, structural transitions from the ordered cubic to fully disordered through partially disordered one are appeared with temperature increasing. As for magnetic transformations, ferromagnetic-paramagnetic transition is observed for all compositions, however, it happens in different crystal phases.

The work presents the investigations of the functional properties of the Heusler alloy Mn2NiGa, made by argon arc melting. The direct experimental methods were used to study: electrical resistance, magnetization, magnetocaloric effect, and shape memory effect in the wide temperature range of 100–400 K. The inverse magnetocaloric effect was found in both the martensitic and austenitic phases, which did not experience saturation in pulsed magnetic field up to 50 T. The values of reversible deformation in the alloy up to 0.35% were obtained with bending mechanical stress up to 247 MPa.

Abstract The first-principles studies have been performed for the electronic and magnetic properties of DyFe_4Ge_2 alloys near the P 4_2/ mmm – Cmmm phase transition. The calculations are carried out in a local spin density approximation taking into account the Coulomb interaction within the limit of strong localization in a mean field approximation. The electronic and magnetic properties of the tetragonal structure are shown to be weakly changed in the dependence on the Coulomb and exchange interactions and also on the choice of the approximations. In the case of the orthorhombic structure, a change in the parameters of the Coulomb and exchange interactions leads to a change in the magnetic ordering: from the ferromagnetic to ferrimagnetic in the strong localization limit and from the ferromagnetic to paramagnetic in the mean field approximation.

This work is devoted to studying the structure and magnetic properties of FeRhl-xIrx (x = 0.5, 0.625, 0.75, 0.875, 1) alloys using first-principle methods using the VASP software package. Two types of structure (CuAu and CsCl) were considered in the work. The equilibrium lattice parameters, total energies and magnetic moments, and also total and partial densities of states for Fe-Rh-Ir were obtained. It was shown that an antiferromagnetic structure with a spin configuration of the AFM-III type is energetically favorable for all the studied alloys.

AbstractThe results of first-principles studies of the structure and magnetic properties of Fe–Rh–Z alloys conducted using the VASP package are reported. The magnetic moments and lattice parameters are determined, and the possibility of existence of stable tetragonal distortions in Fe–Rh–Z alloys with various magnetic configurations set by the level of doping with Mn, Pt, and Co is examined. It is demonstrated that the equilibrium lattice parameter and the type of magnetic ordering change with the concentration of the third element.

AbstractThe results of examination of the structural, magnetic, electronic, and thermodynamic properties of Pd_2MnZ (Z = Ga, Ge, As) Heusler alloys obtained in ab initio and Monte Carlo modeling are presented. It is demonstrated that a stable martensitic state is possible for Pd_2MnGa and Pd_2MnAs alloys. The equilibrium lattice parameter increases in the considered series of alloys with the number of valence electrons per atom ( e / a ). The Curie temperature of Pd_2MnZ (Z = Ga, Ge, As) alloys is determined using the calculated parameters of exchange interaction and total magnetic moments.

The magnetization, the electrical resistivity, the specific heat, the thermal conductivity, and the thermal diffusion of a polycrystalline Heusler alloy Ni45.37Mn40.91In13.72 sample are studied. Anomalies, which are related to the coexistence of martensite and austenite phases and the change in their ratio induced by a magnetic field and temperature, are revealed and interpreted. The behavior of the properties of the alloy near Curie temperature T (C) also demonstrates signs of a structural transition, which suggests that the detected transition is a first-order magnetostructural phase transition. The nontrivial behavior of specific heat detected near the martensite transformation temperatures is partly related to a change in the electron density of states near the Fermi level. The peculiar peak of phonon thermal conductivity near the martensitic transformation is interpreted as a consequence of the appearance of additional soft phonon modes, which contribute to the specific heat and the thermal conductivity.