V. I. Kamenev

Magnetostrictive and magnetocaloric effects in Mn0.89Cr0.11NiGe

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.

Magnetic and magnetocaloric properties of the alloys Mn2−xFexP0.5As0.5 (0⩽x⩽0.5)

The results of investigations of the magnetic and magnetocaloric properties of alloys from the system Mn2−xFexP0.5As0.5 (0⩽x⩽0.5) are presented. The magnetization measurements are performed in the temperature interval 4.2–700K in magnetic fields up to 8T. The entropy changes ΔS with the magnetic field changing from 0 to 2, 4, 5, and 8T are determined from the magnetization isotherms obtained near temperatures of the spontaneous appearance of the ferromagnetic state (TC,TAF–FM1), and the curves ΔS(T0) are constructed. It is found that TC and TAF–FM1 decrease monotonically with increasing manganese concentration and that the ferromagnetic phase is completely suppressed in Mn1.5Fe0.5P0.5As0.5. It is found that the concentration dependences of the maximum entropy jump (and the corresponding cold-storage capacity) and the magnitudes of the ferromagnetic moment of the unit cell with maxima for x=0.9 and 0.8 show extremal behavior. The data obtained are compared with the ferromagnetic moments calculated from fir...

Influence of pressure on the stability of the magnetically ordered states in alloys of the system Mn2−xFexAs0.5P0.5

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 cha...

Visualization of the antiferromagnetic insulator–ferromagnetic metal phase transition in manganite Nd0.5Sr0.5MnO3

It is found that the first-order phase transition from a nonmetallic antiferromagnetic (AFM) state to a metallic ferromagnetic (FM) state in manganite Nd0.5Sr0.5MnO3 is accompanied by a significant change in the reflectivity of the visible light. This effect is used for visualizing the AFM–FM phase transition in Nd0.5Sr0.5MnO3. The coexistence of AFM and FM phases was observed visually during spontaneous and field-induced AFM–FM transitions. In both cases, the transition occurs through nucleation and expansion of domains of the phase that is favorable from the energy point of view. However, the periodic domain structure of the intermediate magnetic state was not formed during the phase transition. A striped domain structure was formed in the AFM state while the FM phase had a uniform structure.

Stability characteristics of the low-temperature ferrimagnetism in the Mn2−xZnxSb system

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...

Features of the magnetic behavior of Mn2−xCrxSb alloys in the low-temperature state

The spontaneous and magnetic-field-induced first-order magnetic phase transitions in single-crystal samples of the alloy system Mn2−xCrxSb (symmetry space group P4/nmm) are investigated in samples with x=0.06 and x=0.12. It is found that the spontaneous first-order transitions from the high-temperature to the low-temperature phase are not accompanied by complete vanishing of the magnetization. At the induced transitions the magnetization before and after the transition is an increasing linear function of field. It is concluded on the basis of the results that the ground state in these alloys cannot be one of collinear antiferromagnetism, as had been thought previously. Other possibilities for interpreting the ground state and the mechanisms of order–disorder transitions inherent to magnets with itinerant carriers of magnetism are examined. It is conjectured that the local magnetic moment has a ferromagnetic and a periodic component which are formed by itinerant d electrons and coexist in low- and high-tem...

Structural features of the ferromagnetic order formation in the Mn1–x Cr x NiGe system

Within the phenomenological model of the interacting parameters of magnetic and structural orders, magnetic and structural transitions in magnetocaloric alloys of the Mn1–xCrxNiGe system are analyzed. Based on the calculated isobaric temperature dependences of the parameters of magnetic and structural orders, a magnetic susceptibility jump in the first-order structural transition region is predicted and confirmed experimentally; the change in the magnetic ordering type during the approach of magnetic and structural transitions is justified. The change in the phase transition type during the reverse change in the temperature and magnetic field, which is observed in a number of samples of the system under study, is explained. The efficiency of the use of the transitions induced by the magnetic field in magnetocaloric applications is analyzed.

Features of the low-temperature magnetic-field-induced order–order transitions in alloys of the system Fea−xMnxAs with a⩽1.6

Spontaneous and magnetic-field-induced first-order transitions of the order–order type in Fe0.5Mn1.1As and Fe0.55Mn1.04As single crystals with the C38 lattice (symmetry space group P4/nmm) are investigated at hydrostatic pressures up to 7 kbar in the temperature range from 4.2 to 300 K. The temperature and field dependences of the magnetization at different values of the pressure are measured in static and pulsed magnetic fields. It is established on the basis of the results that the first-order transitions from a low-magnetic to a high-magnetic phase induced by magnetic field pulses at low temperatures are irreversible, but with a finite existence time of the high-magnetic phase. A qualitative explanation of the observed effect is proposed on the basis of the theory of magnetostrictive blocking of the formation of new-phase nuclei at first-order phase transitions.

Phase transitions in the Mn-Zn fluorosilicate hexahydrates

The effect of chemical substitution on the stability of structural phases in Mn1-xZnxSiF6?6H2O fluorosilicate hexahydrates has been investigated by means of x-ray diffraction. Under ambient conditions the substitution of heavier Zn atoms for Mn atoms leads to a phase transition between two trigonal phases with the loss of the mirror plane symmetry. It has been found that external pressure and Zn doping have very similar effects on the phase stability of the fluorosilicate hexahydrates, and the established x-T phase diagram mirrors the generalized pressure-temperature phase diagram for the fluorosilicate hexahydrates series. The variety of the phases found and the character of the phase transitions between them are discussed in terms of a qualitative model, which takes into consideration the order-disorder processes in the material.

Change of the magnetic properties of CoSiF6⋅6(H2O) at structural transformations under pressure. Determination of the g factor

The interrelationships between structural phase transformations and the magnetic characteristics of cobalt fluorosilicate hexahydrate are determined in the temperature interval 400–15 K under hydrostatic pressure up to 220 MPa. It is shown that the values of the magnetization and magnetic susceptibility in the different structural phases realized in P–T space are practically independent of the amount of compression but undergo jumps when the symmetry of the crystal lattice changes. The results obtained are used to determine the values of the g factors along two crystallographic directions for the phases studied.