The results of magnetic measurements and x-ray analysis of the Mn0.89Cr0.11NiGe alloy are presented. It is shown that the temperatures of the first-order paramagnetic structural transition from hexagonal to orthorhombic phase and paramagnetic–ferromagnetic magnetic phase transition in the orthorhombic phase can be aligned by quenching the sample. This alignment results in the change of the magnetic phase transformation order: the initially isostructural phase transition of the second order becomes a magnetostructural transition of the first order (hexagonal paramagnet–orthorhombic ferromagnet). Moreover, the character of the low-temperature ferromagnetic–antiferromagnetic transition observed in magnetic fields below 3.5 T does not change substantially. The mechanisms of the giant anisotropic magnetostrictive (up to 10%) and magnetocaloric (up to 28 J/(kg·K) upon changing the magnetic field from 0 to 5 T) effects are discussed.

В рамках феноменологической модели взаимодействующих параметров магнитного и структурного порядков проведен анализ магнитных и структурных переходов в магнитокалорических сплавах системы Mn1-xCrxNiGe. На основе рассчитанных изобарических температурных зависимостей параметров магнитного и структурного порядков предсказано и подтверждено экспериментально cкачок магнитной восприимчивости в области структурного перехода первого рода, обосновано изменение рода магнитного упорядочения при сближении магнитного и структурного переходов. Дано объяснение изменению рода фазового перехода при реверсивном изменении температуры и магнитного поля, которое наблюдается в ряде образцов исследуемой системы. Дан анализ эффективности использования индуцированных магнитным полем переходов для магнитокалорических приложений. DOI: 10.21883/FTT.2017.02.44046.202

The magnetic properties of the system Mn2−xZnxSb are investigated experimentally in static and pulsed magnetic fields. It is found that the spontaneous first-order transitions from the ferro=imagnetic phase Fi to the weakly ferrimagnetic phase If which are observed on decreasing temperature in samples with x=0.1 and 0.2 are accompanied by an almost twofold decrease of the magnetization and magnetostriction with no change of the easy-plane character of the anisotropy. Studies of the effect of a pulsed magnetic field in the temperature region of stability of the weakly ferrimagnetic phase reveal reversible magnetic-field-induced first-order phase transitions If↔Fi. Quenching of the samples from 570K into liquid nitrogen enhances the stability of the weakly ferrimagnetic phase with respect to the influences of temperature and external magnetic field. This is manifested in an increase of the temperature of the spontaneous transition by 45 degrees and an increase in the critical field for its induction from 80 to 200kOe. As the quenched sample is heated to room temperature, the initial characteristics of the weakly ferrimagnetic phase are gradually restored. The results are analyzed in a model of itinerant magnetism with the use of first-principles calculations of the electron density of states and total energy of the system. It is shown that the weakly ferrimagnetic phase with the doubled crystal-chemical cell can be described by the Hubbard model. The properties of the quenched samples can be explained by assuming the presence of two stable states of the system, separated by an energy barrier and associated with the possibility of a redistribution of the zinc ions over two crystallographically inequivalent lattice positions.

The phase diagram of the system Mn2−xFexAs0.5P0.5 under pressure is investigated experimentally and theoretically. It is found that the spontaneous and magnetic-field-induced low-temperature phase in the region 0.5⩽x<0.8 does not suffer significant changes under hydrostatic pressure up to 2kbar. Based on ab initio calculations of the electronic structure of the alloys Mn1.5Fe0.5As0.5P0.5, it is established that the degree of filling of the 3d electron band changes upon ferromagnetic polarization and compression of the crystal lattice. A model is proposed by which one can take into account the main features of the antiferromagnetic and canted ferromagnetic structures. The parameters of the model are the degree of filling of the d band, the nonmagnetic electronic density of states, and the intra-atomic exchange integral. Their values are estimated directly from the data of the first-principles electronic structure calculations. It is shown in the framework of the model that the stability of the magnetic characteristics of the canted ferromagnetic structure with respect to pressure is due to an increase of the number of electrons in the magnetically active band upon a decrease of the unit cell volume.

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