Reverse first-order magnetostructural phase transitions have been theoretically analyzed within the model of interacting parameters of magnetic and structural orders. A characteristic feature of ...these transitions is stepwise occurrence of the magnetic order upon cooling (as in the case of the first-order phase transition) and its smooth disappearance upon heating (as in the conventional second-order phase transition). These transitions are observed in some alloys of the Mn
1 –
x
Cr
x
NiGe magnetocaloric systems under pressure (
x
= 0.11) and without it (
x
= 0.18) and are accompanied by specific magnetic and magnetocaloric features. These specific features are phenomenologically described within the concept of soft mode for a structural subsystem undergoing the first-order structural phase transition (
P
6
3
/
mmc
–
Pnma
) and the Heisenberg model for a spin subsystem. It is shown for systems with magnetostructural instability within the molecular-field approximation for a spin subsystem and the approximation of biased harmonic oscillator for a lattice subsystem that reverse phase transitions occur when the magnetic disordering temperature is in the range of temperature hysteresis of the
P
6
3
/
mmc
–
Pnma
first-order structural phase transition. It is also shown that the two-peak form of isothermal entropy (characteristic of reverse transitions) is due to separation of contributions from the structural and magnetic entropies.
The magnetostructural phase transition in Mn(As,P) compounds was studied by a combination of X-ray diffraction on single crystals and by the magnetisation measurements. The mentioned transition is ...accompanied by anisotropic deformation of the crystal lattice, magnetisation hopping and heat evolving due to a large magnetocaloric effect (MCE). It is shown that this transition is a transformation from the hexagonal ferromagnetic to the orthorhombic paramagnetic phase at heating above the Curie temperature (TC). The values of the magnetic and lattice entropy changes at phase transition are obtained and the adiabatic temperature change is estimated in the frame of Clapeyron-Clausius relations. It is found that the MCE in the magnetic field 140 kOe reaches the value of 13.5 K. It is shown that a superparamagnetic structure (as a conglomerate of microscale areas) is formed above Curie temperature in zero magnetic field. On the other hand at the temperatures above the TC the ferromagnetic state is induced by magnetic fields exceeding the certain critical value Hcr while the above-mentioned superparamagnetic state is destroyed by the same magnetic fields. The magnetic properties of the bulk Mn(As,P) and the polymeric composite based on Mn(As,P) compound are compared.
•The increase of P content to 0.05 suppresses the jump of the lattice parameter.•A high magnetic field induces the ferromagnetic state at temperatures above TC.•The maximum of ΔT in MnAs0.97P0.03 in a field of 14 T reaches the value of 13.5 K.
In this work, BiFeO
3
powders were synthesized by a sol–gel method. The influence of annealing temperature on the structure and magnetic properties of the samples has been discussed. X-ray ...diffraction studies showed that the purest phase was formed in the temperature range of 400 °C to 550 °C and the samples annealed at a temperature below 550 °C were of nanocrystalline character. Mössbauer spectroscopy and magnetization measurements were used as complementary methods to investigate the magnetic state of the samples. In particular, the appearance of weak ferromagnetic properties, significant growth of magnetization, and spin-glass-like behavior were observed along with the drop of average grain size. Mössbauer spectra were fitted by the model assuming cycloidal modulation of spins arrangement and properties of the spin cycloid were determined and analyzed. Most importantly, it was proved that the spin cycloid does not disappear even in the case of the samples with a particle size well below the cycloid modulation period
λ
= 62 nm. Furthermore, the cycloid becomes more anharmonic as the grain size decreases. The possible origination of weak ferromagnetism of the nanocrystalline samples has also been discussed.
Ni
Mn
In
(close to 2-1-1 system) Heusler alloy was studied by magnetization measurement dependence on the temperature in magnetic fields of up to 13.5 T. The magnetocaloric effect measured by the ...direct method in quasi-adiabatic conditions showed a maximum value of ∆T
= -4.2 K at a temperature T = 212 K in a magnetic field of 10 T in the region of martensitic transformation. The structure of the alloy was studied by transmission electron microscopy (TEM) as a function of the temperature and the thickness of the sample foil. In the temperature range from 353 to 215 K, at least two processes were established. The results of the study indicate that the concentration stratification occurs according to the mechanism of spinodal decomposition (conditionally spinodal decomposition) into nanoscale regions. At a temperature of 215 K and lower, martensitic phase with 14 M modulation is observed in the alloy at thicknesses greater than 50 nm. Some austenite is also observed. In foils with thickness of less than 50 nm in a temperature range from 353 to 100 Km only the initial austenite, which has not transformed, was found.
•Magnetic field-induced phase transitions found in the Mn1-xCoxNiGe system (0.15 ≤ x < 0.8) at T ∼ 5 K.•The mechanism of low-temperature magnetostructural transitions of solid solutions of the ...Mn1-xCoxNiGe system (0.15 ≤ x ≤ 0.80) is considered.•In the Mn1-xCoxNiGe (0.15 ≤ x < 0.30) system, an inverse magnetocaloric effect (IMCE) was found at T ∼ 5 K.•The observed features can be explained by the realization of a metastable orthorhombic crystal state at low temperatures.
Magnetic field-induced phase transitions found in the Mn1-xCoxNiGe system (0.15 ≤ x < 0.8) at T = 5 K may differ in nature and magnitude of the saturation magnetization of the initial weakly magnetic phase. These differences also affect the phenomena accompanying the transitions, specifically, the inverse magnetocaloric effect (MCE). In the current work, the mechanisms of these phenomena are analyzed within the framework of the magnetostructural model, which takes into account the difference in the limiting values of the magnetic order parameters in competing orthorhombic and hexagonal crystal structures. As follows from the analysis performed, the features observed in Mn1-xCoxNiGe can be explained by the realization at low temperatures of a metastable orthorhombic crystal state in magnetically ordered phases with a stable high-temperature hexagonal crystal structure
•The equiatomic MnZnSb compound were prepared.•Its magnetic properties and magnetocaloric effect (MCE) were systematically studied.•A considerable reversible MCE was observed for MnZnSb around its ...own Curie temperature.•The origin of MCE and its potential application in MnZnSb were discussed.•The field dependence of the MCE was described in frames of the thermo-dynamic theory.
The equiatomic intermetallic alloy MnZnSb with a tetragonal Cu2Sb-type crystal structure (space group P4/nmm) was melted in the resistance furnace in the evacuated quartz ampoule. The adiabatic temperature change (ΔTad) and the isothermal variation of the magnetic entropy (ΔSM) of the MnZnSb compound near to the magnetic phase transition were studied. The ΔTad in a magnetic field up to 1.25 T was studied by the direct method. It was found that temperature dependencies of both ΔTad and ΔSM show a sharp peak near room temperature with a maximum at Curie temperature TC = 317 K. It was shown that there is no temperature hysteresis of the ΔTad in MnZnSb, and the maximum of ΔTad when the compound is heated or cooled is detected at the same temperature. The estimated value of ΔTad in a field of 14 T is 4.5 K. It was shown that near the Curie temperature, the field dependence of the maximum of magnetic entropy change is adequately described by the thermodynamic Landau theory for magnetic second-order phase transitions.
A theoretical analysis of the features of first-order structural and magnetostructural transitions in magnetocaloric helimagnetic alloys of the Mn
1 –
x
Cr
x
NiGe system is performed. The observed ...structural transitions of the displacement type hex(
P
6
3
/
mmc
) ↔ orth(
Pnma
) are described using the local soft mode model in the approximation of a displaced harmonic oscillator. In the absence of a magnetic field, the emergence of a helimagnetic order as a structurally induced second-order transition is described within the Heisenberg model with allowance for the dependence of the exchange integrals on the structural order parameters and elastic strains. In the presence of a magnetic field, it was found that the mutual approach of the characteristic temperatures for the helimagnetic state (HM(
Pnma
)) and the lability temperatures of the hexagonal paramagnetic state (PM(
P
6
3
/
mmc
)), due to the action of the magnetic field, leads to the appearance of previously unexplored peripheral first-order magnetostructural transitions with insignificant magnetization jumps, increasing with increasing magnetic induction field. In this case, as the pressure increases to 4 kbar with a constant magnetic induction field, the peripheral transitions are transformed into reversible first-order magnetostructural transitions and, at even higher pressures (10–14 kbar), into full-scale first-order magnetostructural transitions with magnetization jumps comparable with the maximum magnetization. Experimental baric studies of the temperature dependences of the magnetization in static magnetic fields with an induction up to 1 T and pressure up to 14 kbar confirm the theoretical results.
The isobaric temperature dependences of the magnetization and the magnetocaloric characteristics of alloys of the Mn
1 –
x
Cr
x
NiGe system are studied in a constant magnetic field up to 10 kOe in ...the range of hydrostatic pressures up to 12 kbar. It has been established that, with increasing pressure, the implementation of helimagnetic ordering undergoes qualitative changes from the smooth hysteresis-free second-order phase transition to the first-order phase transitions accompanied by the appearance of a temperature hysteresis and an increase in the magnetocaloric effect. Based on the exchange-structural model, an explanation is given for the mechanism of baric transformation of magnetic and magnetocaloric properties.
Magnetic and Mössbauer measurements were performed for Mn1-xFexNiGe solid solutions. The Mössbauer data obtained suggest that the iron atoms at small concentrations x < 0.20 prefer to fill trigonal ...bipyramidal positions substituting the nickel atoms and thus do not participate in magnetic interactions. This is consistent with a decrease of magnetization in the x < 0.20 samples, evident from the magnetic measurements data. As the concentration increases above x > 0.20 in Mn1-xFexNiGe, the iron atoms replace both the nickel atoms in trigonal bipyramidal and the manganese atoms in octahedral positions.
•Magnetic and Mössbauer measurements were performed for Mn1-xFexNiGe solid solutions.•The iron atoms at small concentrations x < 0.20 prefer to fill trigonal bipyramidal positions substituting the nickel atoms.•At x > 0.20, the Fe atoms replaces both the nickel atoms in trigonal bipyramidal and the Mn atoms in octahedral positions.
The magnetic and magnetocaloric characteristics of the Mn
1.9
Cu
0.1
Sb alloy were studied. The presence of a relatively sharp decrease in the magnetization in the region of 100 K is established, ...which, according to ab initio calculations, can be interpreted as antiferromagnetism–ferrimagnetism transitions. The presence of a magnetic phase transition from a ferrimagnetic to an antiferromagnetic state (F ↔ AF) leads to the appearance of an inverse magnetocaloric effect, which is preserved in magnetic fields up to 10 T.
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