Metamagnetic Alloys Research Articles

Experimental study on coupled caloric effect driven by dual fields in metamagnetic alloy ErCo2

This study presents an experimental investigation of the coupled caloric effect driven by dual-fields in metamagnetic alloy ErCo2 with strong magneto-structural coupling. Magnetic measurements were conducted under different pressures, revealing that the application of hydrostatic pressure stabilizes a small volume of paramagnetism (PM) phase, resulting in a shift of the phase transition temperature towards the low-temperature region. This shift is opposite to the temperature associated with the magnetic field-driven phase transition. As pressure increases, the metamagnetic transition in ErCo2 is suppressed, and the hysteresis disappears. However, the produced cross-coupling caloric effect compensates the decrease in entropy change caused by the disappearance of the metamagnetic transition. As a result, a reversible giant magnetocaloric effect of 46.2 J/(kg·K) without hysteresis is achieved at a pressure of 0.910 GPa. Moreover, we propose that the temperature span of ErCo2 can be significantly widened by optimizing the thermodynamic pathway of the magnetic and pressure fields, overcoming the defect of a narrow temperature range.

Determination of magnetic, electronic and lattice contributions to low-temperature specific heat: Procedure and its application to metamagnetic alloys

Procedure of determination of magnetic part of the specific heat of ferromagnetic (FM) solid from the results of the low-temperature measurements is proposed and applied to the number of non-stoichiometric Ni50Mn50-xInx alloys. It is shown that the disregard of magnetic contribution to the specific heat of FM solid results in the noticeable underestimation of Debye temperature and overestimation of the specific heat of electron subsystem of FM solid. The magnetic contribution to the low-temperature specific heat of antiferromagnetic (AFM) solid is negligibly small if Néel temperature exceeds the Debye temperature. In the opposite case the magnetic part of the specific heat of AFM solid may be comparable in value with non-magnetic part, but separation of these parts is an open problem.

Testing the Applicability of 119Sn Mössbauer Spectroscopy for the Internal Stress Study in Ternary and Co-Doped Ni-Mn-Sn Metamagnetic Alloys

The influence of both the Co addition and the internal stress on the atomic level magnetism is comparatively studied in Ni50Mn37Sn13 and Ni45Mn38Sn13Co4 alloys by magnetic measurements and 119Sn Mössbauer spectroscopy. The results show that the saturation magnetization and the hyperfine field follow the same temperature trend. The internal stress state is investigated by subjecting the samples to milling and annealing treatments, and tracking the singlet component revealed by 119Sn Mössbauer spectroscopy. Contrary to what was expected, in the Co-doped Ni-Mn-Sn sample the singlet component can be resolved between the milled and annealed states in both martensite and austenite phases. Therefore, the results demonstrate the feasibility of tracking the singlet component upon the structural recovery in Co-doped Ni-Mn-Sn alloys in a much wider range than in ternary alloys. In addition, it is concluded that the transferred dipolar field at Sn from the neighbor magnetic atoms depends very strongly on the stress field and on the microstructural order surrounding Sn atoms. The observed sensitivity of Sn Mössbauer probe atoms to slight microstructural distortions make 119Sn a powerful technique for the characterization of the stress present in Sn containing metamagnetic shape memory alloys.

Large magnetic entropy change and magnetostrain in a directionally solidified Ni45.7Co4.2Mn37.3Sb12.8 alloy

Heusler-type Ni-(Co)-Mn-X (X = In, Sn, Sb) meta-magnetic alloys can exhibit various magneto-functional behaviors due to the strong coupling between the magnetic and structural orders. Here, we demonstrate large magnetocaloric effect and magnetic shape memory effect in a directionally solidified Ni45.7Co4.2Mn37.3Sb12.8 alloy with 〈5 5 3〉A preferred orientation. It is shown that large magnetic entropy change up to 27.4 J kg−1 K−1 can be achieved under the field change of 5 T. Moreover, a large magnetic field-induced strain of 0.42% was also obtained with no-prestrain through inverse martensitic transformation induced by applying a magnetic field of 7 T. The present magnetostain is much higher than those in polycrystalline Ni-(Co)-Mn-Sb based alloys reported previously. Such enhanced magneto-functional behavior could be attributed to the highly textured microstructure and enlarged lattice distortion through Co addition.

Properties of metamagnetic alloy Fe48Rh52 in high magnetic fields

A technique for differential scanning calorimetry (DSC) of materials in high magnetic fields is developed. Based on Peltier elements, a differential calorimeter is designed to work in Bitter coil magnet fields (up to 140 kOe). Calorimetric studies are conducted for Fe48Rh52 alloy with the reverse magnetocaloric effect (MCE) in the vicinity of the metamagnetic structural phase transformation (PT). It is shown that the transition latent heat falls during both forward and reverse transformation as the magnetic field grows.

Stress-induced resistivity changes in a Ni-Mn-In alloy

The elastoresistive effect is reported in polycrystalline Ni45.4Mn40In14.6 metamagnetic alloy at uniaxial compression: abrupt growth of electric resistance by more than 30% occurs when the applied mechanical stress achieves 16 MPa. In the process of deformation at temperature 23 K above martensitic transition, the alloy demonstrated superelastic behavior. Surface morphology of specimen at different strain levels was monitored in situ by optical microscopy. The observations showed that after compressive deformation up to 5% followed by unloading specimen fully reverted back to parent phase.

A phase-field model of phase transitions and domain structures of NiCoMnIn metamagnetic alloys

We developed a computational model to investigate the magnetic and structural phase transitions in metamagnetic shape memory alloys. The model combined the phase-field method with micromagnetic simulations. The model was used to calculate the transition temperature from ferromagnetic austenite to antiferromagnetic martensite, the Curie temperature, and their response to an external magnetic field, for the typical metamagnetic alloy NiCoMnIn. The calculated magnetization curves at different temperatures are consistent with reported experimental measurements. The simulations show that the walls of martensite twins are superimposed with the 90° magnetic domain walls of the low-temperature martensite phase, because of magnetostructural order parameter coupling.

Contributions to the Transformation Entropy Change and Influencing Factors in Metamagnetic Ni-Co-Mn-Ga Shape Memory Alloys

Ni-Co-Mn-Ga ferromagnetic shape memory alloys show metamagnetic transition from ferromagnetic austenite to paramagnetic (or weak-magnetic) martensite for a limited range of Co contents. The temperatures of the structural and magnetic transitions depend strongly on composition and atomic order degree, in such a way that combined composition and thermal treatment allows obtaining martensitic transformation (MT) between any magnetic state of austenite and martensite. The entropy change ΔS measured in the magnetostructural transition comprises a magnetic contribution which depends on the type and degree of magnetic order of the related phases. Consequently, both the magnetization jump across the MT (ΔM) and ΔS are composition and atomic order dependent. Both ΔS and ΔM determine the effect of applied magnetic fields on the MT, hence knowledge and understanding of their behavior can help to approach the best conditions for magnetic field induced MT and related effects. In previous papers, we have reported findings regarding the behavior of the transformation entropy in relation to composition and atomic order in Ni50−xCoxMn25+yGa25−y (x = 3–8, y = 5–7) alloys. In the present paper we will review our recent results, summarizing the key findings and drawing general conclusions regarding the magnetic contribution to ΔS and the effect of different factors on the magnetic and structural properties of these metamagnetic alloys.

Barocaloric effect in metamagnetic shape memory alloys

AbstractWe report on calorimetric measurements under hydrostatic pressure in a series of composition related metamagnetic shape memory alloys. We show that metamagnetic shape memory alloys exhibit a barocaloric effect whose magnitude compares well to the magnetocaloric effect exibited by this kind of alloys. While in metamagnetic alloys the magnetocaloric effect is inverse, the barocaloric effect has been found to be conventional. The values obtained for the pressure‐induced entropy changes at moderate pressures are in the range of those reported for giant caloric materials.

Magnetic field and atomic order effect on the martensitic transformation of a metamagnetic alloy

The martensitic transformation (MT) of metamagnetic shape memory alloys is very sensitive to the applied magnetic field and atomic order. We analyze the alloy Ni50Mn34.5In15.5 in magnetic fields up to 13 T. The alloy has been prepared both in an ordered state by slow cooling, and in a disordered state by rapid quenching. In both cases the dependence of the martensitic transition temperature on the field is highly nonlinear. Such departure from linearity is due to a decrease of the entropy change at the transition, ΔS, with the applied field. This can be explained by the ordering effect of the magnetic field on the frustrated magnetic structure of the alloy in the martensitic phase. Compliance with a recent model, relying on the strong magnetoelastic interactions in these compounds, is very satisfactory.

Entropy Changes in Ferromagnetic Shape Memory Alloys

Several features of the entropy change S related to the martensitic transformation (MT) in metamagnetic alloys are discussed. In these alloys a change in magnetic order is concomitant with the MT, as it occurs between ferromagnetic austenite and non-magnetic (weakly magnetic) martensite. In this case it has been shown that S strongly decreases as the MT temperature range go far below the Curie temperature of austenite. The behavior of S can be understood considering the different signs of the lattice and magnetic contributions to the total entropy change. It has been shown that the so called kinetic arrest of the MT is directly related to the decrease of S, which in the limit S → 0 leads to the suppression of the driving force for the transformation.

Effect of post-annealing on martensitic transformation and magnetocaloric effect in Ni45Co5Mn36.7In13.3 alloys

The metamagnetic alloy Ni45Co5Mn36.7In13.3 was fabricated by conventional arc-melting technique. Subsequent annealing may relax the stress and modify the atom ordering, thus influencing the magnetic properties and martensitic transformation behaviors. Our studies demonstrate that post-annealing at temperatures ≤ 300 °C can lead to a significant change in the magnetic properties and martensitic temperature (TM). Annealing the sample at 300 °C for 3 h can cause a decrease of as much as 30 K in TM (from 319 to 289 K) while retaining strong metamagnetic behaviors. The field-induced metamagnetic transition is accompanied with a large magnetocaloric effect. With an increase in the annealing temperature, the magnitude of the effective magnetic entropy change decreases somewhat, while the refrigeration capacity shows a slight increase.

Isothermal martensitic transformation in metamagnetic shape memory alloys

We show that in metamagnetic shape memory alloys exhibiting a magnetostructural first order phase transition the direct transition from ferromagnetic austenite to nonmagnetic martensite is isothermal. In contrast to the direct transformation, the reverse one (nonmagnetic martensite–ferromagnetic austenite) is athermal, just as are athermal both direct and reverse martensitic transformations in conventional ferromagnetic shape memory alloys. The observed asymmetry of properties of the direct and reverse phase transitions in metamagnetic alloys, together with the data on entropy change during the magnetostructural transition, evidences that the magnetostructural transition is driven by the first order lattice modification. The change in magnetic ordering is an effect accompanying the lattice modification, opposing the direct transformation and promoting the reverse one. It has been shown that relaxation effects in metamagnetic shape memory alloys are intrinsic in the direct transformation itself and do not require the “arrest” of the transformation.

Large magnetic entropy change with small thermal hysteresis near room temperature in metamagnetic alloys Ni51Mn49−xInx

Magnetic properties and magnetic entropy change ΔS have been investigated in Heusler alloys Ni51Mn49−xInx (x=15.6, 16.0, and 16.2). By tuning Ni, Mn, and In contents around composition Ni50Mn34In16, large ΔS with small thermal hysteresis near room temperature can be achieved. Martensitic temperature Tm shifts from 308 K to 253 K with x varying from 15.6 to 16.2. The thermal hysteresis around Tm is small, <2 K, for all samples. Because of the fundamental difference in magnetization around Tm, an external magnetic field induces a metamagnetic transition from the martensitic to the austenitic state. As a result, a large magnetic entropy change with positive sign appears in a wide temperature range. The size of ΔS reaches 19, 20, and 33 J/kg K under 5 T magnetic field at 253, 262, and 308 K for samples x=15.6, 16.0, and 16.2, respectively.

Anomalous transport in itinerant metamagnets with structural disorder

We report on low-temperature transport in magnetic conductors with structural disorder. The primary motivation for this work was large positive magnetoresistance (MR) found in magnetically ordered ground state of some itinerant metamagnetic alloys. The positive MR suggests that external magnetic field enhances static magnetic disorder δ M → ( r → ) = M → ( r → , T , H → ) - 〈 M → ( r → , T , H → ) 〉 , whereas standard approach assumes suppression of magnetic fluctuations by external magnetic field as a source of negative MR. We review the relevant experimental data, mostly the properties of RCo 2 -based alloys and discuss a phenomenological model developed for the interpretation of the experimental results. This model includes new mechanism of magnetoresistivity in structurally disordered itinerant metamagnetic alloys.