The results of measurements of magnetization, magnetostriction and magnetocaloric effect and their correlation analysis near the critical temperature TC of the ferromagnet-paramagnet phase transition of the LaFe11.1Mn0.1Co0.7Si1.1 alloy are presented. In finite magnetic field the character of magnetization M dependence of magnetostriction lambda exhibits strong temperature dependence in the vicinity of critical temperature TC: in the vicinity of critical temperature T = TC standard quadratic dependence of lambda(M) is insufficient to explain experimental data, instead we find lambda = AM2 + BM4, in a qualitative agreement with the self-consistent spin renormalization theory by Moriya and Takahashi, where A and B are temperature dependent coefficients and A vanishes near critical temperature but not exactly at T = TC. Correlation analysis of temperature dependences of magnetocaloric effect Delta SM and longitudinal magnetostriction lambda||showed that in magnetic fields of 18, 40 and 80 kOe near & Tcy;& Scy; a linear relationship is observed between Delta SM and lambda||, Delta SM =- kappa lambda||, with proportionality coefficient kappa=9.52 & sdot;103 J/kg K. Within the framework of the Heisenberg model taking into account magnetostriction the value of proportionality coefficient between Delta SM and lambda|| is estimated.

The work is devoted to the study of the dependence of the structure, magnetic properties, and functional properties such as shape memory effect (SME) and magneto caloric effect (MCE) of Heusler alloys of the Ni51 – xMn33.4In15.6Vx family on the degree of vanadium doping x. Studies using scanning electron microscope (SEM), differential scanning calorimetry (DSC) and magnetometry revealed in all studied samples of Heusler Ni51 – xMn33.4In15.6Vx alloys the presence of a Curie point type magnetic phase transition (PT) and a metamagnetostructural phase transition (MMSPT), of the first order which is accompanied by a sharp decrease in magnetization in the low-temperature martensitic phase. The temperature of the magnetic PT is weakly dependent on x, the temperature of the MMSPT decreases with increasing x. The temperature of the MMSPT is very sensitive to the magnetic field. For the Ni50Mn33.4In15.6V1 sample, the sensitivity is—5 K/T. Direct measurements of the MCE in alternating magnetic fields of 0.62 and 1.2 T showed the presence of a direct MCE at the point of magnetic PT and an inverse MCE at the point of MMSPT. There is a strong decrease in the MCE with an increase in the frequency of the alternating magnetic field from 1 to 30 Hz both near the magnetic PT and near the MMSPT. Measurements of the dependence of bending deformation on load and temperature have shown that all studied alloys exhibit SME near MMSPT. It can be concluded that due to the combination of high sensitivity of MMSPT to the field and SME, this family of alloys is promising for the creation of magnetically controlled actuators.

Magnetocaloric effect (MCE) and its potential most simple and evident application in magnetic cooling technology represent urgent issues that have been the subject of intensive global investigation over recent decades. The practical use of a material exhibiting a magneto-caloric effect in cooling technology necessitates a cyclic exposure of the material to a magnetic field. It is therefore imperative to undertake a comprehensive study of the magnetocaloric properties of materials subjected to alternating (or cyclic) magnetic fields. This paper presents an overview of the fundamental concepts pertaining to the effect, methodologies for the measurement of MCE in alternating magnetic fields, and the outcomes of MCE studies depending on the frequency of the magnetic field and the duration of exposure of materials to the alternating magnetic field.

The effect of microstructure and the frequency of change in an alternating magnetic field on the adiabatic temperature change ΔTad in manganite Pr0.7Sr0.2Ca0.1MnO3 has been studied. It has been shown that the transformation of specimens from a denser to looser microstructure leads to both a decrease in ΔTad and a stronger frequency dependence of the adiabatic temperature change. For the specimen with an annealing temperature of 1300°C, ΔTad is 0.6 K at a frequency of 1 Hz in a field of 1.2 T to decrease to 0.3 K at 20 Hz (by more than 50

The results of direct measurements for the adiabatic temperature change ΔTad in the Ni47Mn40Sn13 alloy in cyclic magnetic fields by the magnetic field modulation method are presented. In the temperature dependence of the magnetocaloric effect (MCE), direct (ΔTad > 0) and inverse (ΔTad < 0) MCE are detected. The inverse effect value in a cyclic magnetic field depends on the temperature scanning rate. An increase in the frequency of a cyclic magnetic field with an induction of 1.2 T from 1 to 30 Hz decreases the direct effect value by more than 2 times. In a cyclic magnetic field with an induction of 1.2 T at frequencies f ≥ 1 Hz, complete disappearance (“collapse”) is observed for the inverse magnetocaloric effect, while ΔТad during the one-time actuation of magnetic field is –0.49 K. The dependence of the inverse effect value on the temperature scanning rate, along with its strong frequency dependence, results from both the manifestation of irreversibility in the magnetostructural phase transition due to hysteresis and the presence of phase inhomogeneities influencing the phase transition kinetics.

We investigated the effect of high pressure on the field dependences of magnetoresistance (MR) in La_{0.8}Ag_{0.1}MnO_{3} near the metal-insulator transition temperature. Our results showed that an increase in pressure results in a decrease in the magnitude of negative MR. At pressures $P\geqslant5.6$ GPa and magnetic fields up to 4 kOe, we observed a positive MR. However, with a further increase in magnetic field (>4 kOe), the MR again became negative. Therefore, we discovered a "negative-positive" MR crossover induced by high pressure near the transition temperature. We supported our experimental findings with a qualitative theoretical interpretation using the electron-hole model of MR. This theory explains observed the MR sign change.

The results of the study of the effect of partial substitution of Fe by Mn in the LaFe11.2−xMnxCo0.7Si1.1 system on magnetization, specific heat, magnetostriction and magnetocaloric effect are presented. Direct measurements of the adiabatic temperature change (∆Tad) were carried out in alternating magnetic fields (AMF) using the magnetic field modulation method. Partial substitution of Fe atoms by Mn atoms leads to a shift in the Curie temperature (TC) towards lower temperatures without a noticeable deterioration in magnetic properties. A correlation was found between the structural component of the magnetocaloric effect and the stability of the frequency of the ∆Tad in the AMFs—an increase in the manganese concentration leads to a decrease in magnetostriction and to a lower dependence of ∆Tad on the frequency of the magnetic field. Estimates of the specific cooling power QC as a function of the frequency of the AMF showed that the highest value of QC at f = 20 Hz in a magnetic field of 12 kOe is 26.3 W g−1 and is observed for the composition with x = 0.1. This value is higher than that of Gd, for which, under the same conditions, QC = 21.6 W g−1. All the samples studied show stability of the value of ΔTad without any sign of deterioration of the effect up to 60,000 cycles of switching on/off of the magnetic field of 12 kOe. The discovered frequency and cyclic stability of ΔTad of the studied samples increase their prospects for application in magnetic cooling technology.

The results of direct measurements for the adiabatic temperature change ∆Tad in a rapidly quenched ribbons Heusler Ni44Co6Mn32Al18 alloy specimen within a temperature range of 100–350 K in a cyclic magnetic field with a strength of 18 kOe and a frequency of 0.2 Hz are presented. It is shown that, in the case of one-time magnetic field actuation, the inverse effect value is –0.25 K at a magnetostructural transition temperature TS. The second and following on/off cycles lead to a decrease in the effect by an order of magnitude to –0.03 K. Such a behavior is associated with the effect of kinetic relaxation in the martensite phase and phase transition irreversibility in the used magnetic fields. Near TC, the stable and direct effect ∆Tad = 0.23 K without degradation signs is observed.

We present the results of direct measurements of the adiabatic temperature change (ΔTad) for the Fe50Rh50 alloy in a cyclic magnetic field (CMF) of 1.2 T. It is shown that increasing the frequency of the CMF from 1 to 30 Hz is accompanied by a shift of the position of temperature dependence ΔTad(T) maximum, Tmax, toward low temperatures. With an increase in the CMF frequency from 1 to 5 Hz, the ΔTmax value decreases by ∼12%. A further increase in frequency leads to stabilization of the effect. In the vicinity of the antiferromagnetic-ferromagnetic phase transition point TC = 370 K, ΔTad exhibits unconventional frequency behavior: while at T well above TC, the value of ΔTad monotonously decreases as frequency increases, at T = 370.4 K; an interval of frequency-independent ΔTad up to 10 Hz is observed, and at 368 K < T < TC, the maximum of ΔTad(f) dependence is found in the interval 1 < f < 10 Hz. Such behavior in the future can be applied in magnetic cooling technology due to large values of ΔTad and the frequency stability of the effect in alternating fields. The specific cooling power reaches giant values of ∼22 W/g at 20 Hz, which is comparable to the values under the same conditions for Gd −21.6 W/g. The unconventional behavior of ΔTad in the CMF is discussed in the context of the role of secondary phase localization, which leads to an enhanced internal local magnetic field and dynamic effects of ΔTad.

Some results of studying the temperature dependence of the magnetocaloric effect ΔТad, thermal expansion, and magnetostriction in the Mn1 – хFexAs system (х = 0.003, 0.006) in magnetic fields up to 8 T are presented. It has been shown that an increase in the iron concentration in the Mn1 – хFexAs system leads to the shift of the phase transition temperature towards lower values by 15 K. In a field of 8 T, ΔТad = 8.3 K for Mn0.997Fe0.003As at an initial temperature T0 = 318 K, and ΔТad = 7.7 K for Mn0.994Fe0.006As at T0 = 307 K. The thermal expansion and magnetostriction data show that the magnetostriction decreases with increasing iron concentration, which also leads to a decrease in the magnetocaloric effect.

It is shown that the phase shift between an applied weak alternating magnetic field and the magnetocaloric response signal of the magnetic material is drastically sensitive to the order of phase transition. Namely, at the second-order phase transition, the phase shift does not depend on the magnetic field magnitude, while in the first-order phase transition this one depends significantly on the field strength. We have shown that this effect follows from the general critical dynamics theory.

The magnetization and magnetostriction of polycrystalline LaFe11.2 – хMnxCo0.7Si1.1 alloys (x = 0.1, 0.2, 0.3) were measured in pulsed magnetic fields up to 180 kOe in the temperature region of 80–270 K. The substitution of Fe atoms by Mn atoms shifts ТС towards lower temperatures and has no effect on the saturation magnetization. The observed magnetization–field dependence M(H) near ТС is typical of the second-order phase transitions, whereas the magnetization–temperature dependence M(Т) above ТС in strong magnetic fields indicates the occurrence of a first-order phase transition. The magnetovolume effect ∆V/V attains 0.81

First-principles studies on phase stability and resistance with respect to the segregation of austenitic and martensitic phases of Ni2 – xCoxMn1 + yZ1 – y Heusler alloys (x = 0, 0.25, 0.5 and y = 0, 0.25, 0.5, 0.75; Z = Ga, In, Sb, Sn) with different types of magnetic ordering. Among all the considered compounds, the stability has been demonstrated only by the Ni1.5Co0.5MnGa and Ni2MnGa alloys in the cubic and tetragonal structures having a ferromagnetic ordering, respectively, as well as by Ni2Mn2 in the tetragonal structure with a staggered and layer-by-layer AFM ordering. For the case of these compositions, the presence of zero energy of the convex hull, as well as the absence of reactions with positive decomposition energy has been shown. The remaining compounds appear to be metastable, both owing to the presence of stable reactions with negative decomposition energy, and decomposition reactions with positive decomposition energy. The number of decomposition reactions exhibits an increase with increasing chemical disorder, i.e., with deviations from stoichiometry.

The results of a study of the resistivity, thermal conductivity, thermoelectric power, and thermal expansion of a polycrystalline Ni47Mn40Sn13 Heusler alloy as a function of temperature (80-350 K) and magnetic field (0-8 T) are presented. In addition, ab initio calculations of the structural, electronic magnetic and thermal properties of Ni-Mn-Sn alloy are performed. It is shown that the sensitivity of the magnetostructural phase transition (MSPT) to a magnetic field is Delta T/Delta H approximate to 2 K/T. The temperature dependence of the thermoelectric power near the temperature of the magnetostructural phase transition has a minimum, which is due to a competition between the contributions of electrons and holes to the thermoelectric power. In the region of the martensitic transition, a jump-like increase in thermal conductivity Delta kappa approximate to 4.2 W/mxK was detected, which is associated with both an increase in the electronic component due to an increase in the mobility of charge carriers, and an increase in the free path of phonons as a result of narrowing of the phonon relaxation channel upon transition to a less defective austenitic phase. The ab initio calculations reproduce well a jump-like behavior in the electronic and phonons components of thermal conductivity, providing Delta kappa of 4.63 W/mxK for electron-doped compound. It is shown that the martensitic transformation between the austenitic and martensitic phases is accompanied by a slight change in the carrier concentration, which is about 3 %. This result confirms the experimental suggestion about the weak change in the charge carriers at the martensitic transformation. The martensite-austenite phase transition is accompanied by a sharp increase in the linear dimensions of the sample Delta l/l(0)approximate to 1.5x10(-3). A direct relationship was found between Delta rho/Delta rho(0) and magnetostriction near the MSPT temperature in a magnetic field of 1.8 T.

The main aim of this work is to study heat transfer in mechanical thermal switch under conditions close to a real magnetic refrigeration. This study examines the thermal behavior of a mechanical thermal switch which comprise a detachable pair of copper - copper contact bulks, incorporating an indium foil thermal interface with a 100 mu m thickness. We investigated the time it took to reach thermal equilibrium from initial temperature span of 3 K, 5 K, and 10 K and explored the influence of the indium foil thermal interface within a temperature range of 15 to 300 K. The experimental data provided the heat dissipation values required to maintain the specified temperature of the object being cooled. As the results showed, the use an indium thermal interface significantly reduces the time until thermal equilibrium occurs.

The paper presents the results of a study of magnetization, specific heat (CP), thermal diffusivity (1), thermal conductivity (x) and direct measurements magnetocaloric effect of polycrystalline samples Pr0.7Sr0.3_xBaxMnO3 (x = 0-0.3). An increase in the Ba concentration leads to a decrease in the Curie temperature (TC), an increase in the disorder parameter (02) and a noticeable increase in thermal conductivity of the ferromagnetic phase (FM). The & kappa;(T) data near Tc revealed small minima and a sharp jump-like change in thermal conductivity upon transition to the FM phase. These results suggest that the anomalies in the temperature dependences of & eta;(T) and & kappa;(T) are associated with the scattering of thermal phonons both from distortions of the crystal lattice and from spin fluctuations. The giant change of & eta; (& UDelta;& eta;/& eta; -67%) and & kappa; (& UDelta;& kappa;/& kappa; -33%) was found in a magnetic field of 1.8 T for the x = 0 (Pr0.7Sr0.3MnO3) sample. The maximum values of & UDelta;T, & UDelta;S and relative cooling power (RCP), observed for this sample in a field of 1.8 T, are equal to 1.81 K, 3.38 J/kg K and 74.02 J/kg, respectively. In the studied Pr0.7Sr0.3_ xBaxMnO3 samples, the specific cooling power value at f = 0.2 Hz is 0.35, 0.29, 0.21, and 0.11 W/g for x = 0, 0.1, 0.2, and 0.3 sample, respectively.

—The results of studying the temperature dependence of the magnetocaloric effect (ΔТad), thermal expansion and magnetostriction in the Mn1 – хFexAs (х = 0.003, 0.006) system in magnetic fields up to 8 T are presented. It is shown that an increase in the iron concentration in the Mn1 – xFexAs system leads to a shift in the phase transition temperature towards low temperatures by 15 K. In a field of 8 T, the value ΔTad = 8.3 K for the Mn0.997Fe0.003As sample at the initial temperature T0 = 318 K, and ΔTad = 7.7 K for Mn0.994Fe0.006As at T0 = 307 K. The data on thermal expansion and magnetostriction show that the magnetostriction decreases with increasing iron concentration, which also leads to a decrease in the magnetocaloric effect.

This study presents the results of direct measurements of the adiabatic temperature change (& UDelta;Tad) in Ni50Mn28Ga22_x(Cu, Zn)x (x = 0; 1,5) alloys in cyclic magnetic fields of 0.62 and 1.2 T at frequencies f= 1, 5, 10, and 20 Hz. The substitution of zinc and copper for the initial composition resulted in a decrease in & UDelta;Tad and a stronger frequency dependence. The strong frequency dependence of & UDelta;Tad near TC in the Ni50Mn28Ga22_x(Cu, Zn)x system is due to the inhomogeneous microstructure and the coexistence of martensite-austenite phases, which give rise to two simultaneously acting mechanisms. Firstly, the addition of Zn and Cu increases the coercive force HC, resulting in an increase in the viscosity coefficient. Secondly, an increase in the frequency of the cyclic magnetic field (i.e., an increase in the field sweep rate) leads to the same effect, increasing the viscosity coefficient. Which, in our opinion, is the reason for the frequency dependences of & UDelta;Tad near TC. The study found that the specific cooling power (SCP) for the Ni50Mn28Ga22 sample in a cyclic magnetic field of 1.2 T increased with frequency, reaching 4.75 W/g at 20 Hz, which is 14.1 times greater than at 1 Hz. This suggests that, under conditions of sufficient heat exchange, the cooling efficiency of a magnetic refrigerator can be greatly improved by increasing the operating frequency.

Direct studies of the adiabatic temperature change (ΔTad) in the Ni47Mn40Sn12.5Cu0.5 Heusler alloy in steady magnetic fields up to 8 T by the extraction method and in pulsed magnetic fields up to 50 T were carried out in this paper. The alloy Ni47Mn40Sn12.5Cu0.5 demonstrates a magnetostructural phase transition (MSPT) of the first order in the 254–283 K temperature range as well as a second order phase transition near the Curie temperature TC = 313 K. An inverse magnetocaloric effect (MCE) was found in the region of the MSPT, and it reaches the maximum value ΔTad = −12 K in 20 T at the initial temperature T0 = 275 K. The irreversible part of the MCE reached ΔTir = −10 K when the field is completely removed. We consider the dynamics of the MCE in the vicinity of the MSPT and discuss the mechanisms that cause the giant irreversibility of the MCE as well as the possibilities of its application in hybrid cooling systems.