Curie Temperature Of Austenitic Phase Research Articles

Interplay of austenite and martensite phase inside martensite transition regime and its role on magnetocaloric effect and magnetoresistance in Ni-Mn-Sn based Heusler alloy

The first order martensite transition has been observed clearly through magnetic, electronic- and magneto-transport studies in Ni45Mn44Sn11 disordered ferromagnetic shape memory Heusler alloy. The alloy also shows second order ferromagnetic → paramagnetic phase transition, near the Curie temperature of austenite phase. From calorimetric measurement, the lattice entropy due to martensite transformation is found to be 38 Jkg−1K−1, both for heating and cooling cycle. Technologically important noticeably large positive magnetic entropy change has also been clearly noticed in this alloy near martensite phase transition. Maximum isothermal magnetic entropy changes at 275 K for the application of 5 T magnetic field are found to be 15 Jkg−1K−1 and 24 Jkg−1K−1, for field increasing and field decreasing protocol, respectively. A maximum temperature change −5.1 K is observed at 276 K under 5 T magnetic field in adiabatic condition. The noticeable maximum magnetoresistance of −38% for the field change of 5 T is observed at 266 K. The austenite and martensite phase fractions in the sample depend on the magnetic and thermal path history, which are the most possible reasons to observe different values of magnetic entropy change and magnetoresistance in Ni45Mn44Sn11 Heusler alloy at 275 K.

Magnetocaloric effect and critical behavior study in NiMnSnIn Heusler alloys

In the present work, polycrystalline Ni50Mn36Sn14-xInx (x = 0, 1, 2 and 3) Heusler alloys were prepared. The influence of In substitution for Sn on the crystal structure, martensitic transition (MT) and magnetic properties have been investigated for these alloys. Structural analysis indicate that the samples possess L21 structure at room temperature. With increasing In content, the martensitic transition shifts to a higher temperature, while the Curie temperature of austenite phase moves to a lower temperature. The maximum entropy change (ΔSM)max near the martensitic transition is found to be 8.4 J/kgK, 9.8 J/kgK, 10.1 J/kgK and 19.2 J/kgK for x = 0, 1, 2 and 3 respectively, for a field change of 50 kOe. The entropy change around the Curie temperature of the austenite phase is found to be – 2.9 J/kgK, −1.8 J/kgK, −4.1 J/kgK and −3.9 J/kgK respectively for x = 0, 1, 2 and 3 alloys, for a field change of 50 kOe. The effective RCP values are 80 J/kg, 15.6 J/kg, 65.8 J/kg and 69.4 J/kg for x = 0, 1, 2 and 3 alloys respectively, near the first order transition region. The RCP values are higher around the second order transition region due to the broad nature of ΔSM curves. The corresponding values are 114 J/kg, 92 J/kg, 121 J/kg and 132 J/kg for x = 0, 1, 2 and 3 alloys, respectively. Moreover, the critical behavior of Ni50Mn36Sn14-xInx (x = 1, 2 and 3) Heusler alloys around the ferromagnetic-paramagnetic transition region of austenite phase is studied from the field dependence of magnetic entropy change. The critical exponent values are close to the theoretical prediction of mean field model with a slight deviation.

Electron spin resonance probed competing states in NiMnInSi Heusler alloy

Shape memory Heusler alloy Ni50Mn35In12Si3 is investigated with electron spin resonance (ESR) technique in a temperature range of 200–300K. ESR is a dynamic probe allowing us to separate the responses from various magnetic phases, thus to study the complex phase transitions. The sample shows three transition temperatures: TcA (271K), TM (247K) and TcM (212K), where TcA is the Curie temperature of austenitic phase, TM and TcM are the temperatures of magnetostructural martensitic transition and the Curie temperature of martensitic phase, respectively. Furthermore, ESR data reveals the coexistence of two magnetic modes in whole temperature range of 200–300K. Particularly in martensitic phase, two magnetic modes are attributed to two different kinds of lattice deformation, the slip and twinning deformations.

Investigation of multifunctional properties of Mn50Ni40−xCoxSn10 (x = 0–6) Heusler alloys

A series of Co doped Mn50Ni40−xCoxSn10 (x=0, 1, 2, 2.5, 3, 4 and 6) Heusler alloys has been investigated for their structural, magnetic, magnetocaloric and exchange bias properties. The martensitic transition temperatures are found to decrease with the increase in Co concentration due to the decrease in valence electron concentration (e/a ratio). The Curie temperature of austenite phase increases significantly with increasing Co concentration. A large positive magnetic entropy change (ΔSM) of 8.6 and 10.5J/kgK, for a magnetic field change of 50kOe is observed for x=0 and 1 alloys, and ΔSM values decreases for higher Co concentrations. The relative cooling power shows a monotonic increase with the increase in Co concentration. Large exchange bias fields of 920 Oe and 833 Oe have been observed in the alloys with compositions x=0 and 1, after field cooling in presence of 10kOe. The unidirectional anisotropy arising at the interface between the frustrated and ferromagnetic phases is responsible for the large exchange bias observed in these alloys. With increase in Co, the magnetically frustrated phase diminishes in strength, giving rise to a decrease in the exchange bias effect for larger Co concentration. The exchange bias fields observed for compositions x=0 and 1, in the present case are larger than that reported for Co doped Ni–Mn–Z (Z=Sn, Sb, and Ga) alloys. Temperature and cooling field dependence of the exchange bias field and coercivity have been related to the change in the unidirectional anisotropy. This study shows that an optimum Co doping results in the tuning and enhancement of the multifunctional properties in Mn rich Ni–Mn–Sn alloys.

Effect of excess Ni on martensitic transition, exchange bias and inverse magnetocaloric effect in Ni2+xMn1.4−xSn0.6 alloy

The martensitic transition, exchange bias (EB) and inverse magnetocaloric effect (IMCE) of bulk Ni2+xMn1.4−xSn0.6 (x=0, 0.06, 0.12, 0.18) Heusler alloy is investigated in this paper. Substitution of Mn by Ni causes an increase in the martensite transition temperature (TM), decrease in Curie temperature of austenite phase (TCA) and also a decrease in the saturation magnetic moment (Msat). While the decrease in TCA and Msat is explained by the dilution of the magnetic subsystems and on the other hand the increase in TM is due to the increase of valence electron concentration per atom (e/a). All the alloys shows EB effect below a certain temperature (T∗) and EB field (HEB) value is almost thrice in magnitude for x=0.18 sample compared to x=0 sample at 5K. In these alloys, Ni/Mn atoms at regular site couples antiferromagnetically (AFM) with the excess Ni atoms at Mn or Sn sites and this AFM coupling plays the key role in the observation of EB. For the IMCE, the change in magnetic entropy (ΔSM) initially increased with excess Ni concentration upto x=0.12 but then a drastic fall in ΔSM value is observed for the sample x=0.18 but the relative cooling power (RCP) value is increased continuously with the excess Ni concentration.

Microstructure, magnetic and electrical transport properties of melt-spun Ni-Mn-Sb ribbons

The Ni50Mn37Sb13 ribbons were prepared by melt-spinning technique to overcome the brittleness of bulk. A single phase austenite with L21 structure was confirmed in the as-spun and annealed ribbons at room temperature similar to bulk. We observe increments in the martensite transformation temperature (TM) from 238 K to 252 K and in Curie temperature of austenite phase (TCA) from 336 K to 342 K after annealing the as-spun ribbon. The exchange bias, magnetic entropy change (ΔSM), and magnetoresistance are found to increase from 380 Oe to 415 Oe, 1 J/kg-K to 3 J/kg-K (ΔH=50 kOe), and −4% to −13% (ΔH=50 kOe), respectively, after annealing the as-spun ribbon. The possible reasons for observed behaviour are discussed.

The influence of Ge substitution on the magnetostucture transition and magnetocaloric effect of Mn–Ni–Sn–Ge alloys

This paper investigates the effects of Ge addition on the martensitic transition (MT) and magnetic properties in the Mn-rich ferromagnetic shape memory alloys of Mn2Ni1.6Sn0.4−xGex. The increase of Ge content leads to a rapid increase of the MT temperature but a gradual decrease of the Curie temperature of austenitic phase. Field-induced metamagnetic phase transition was confirmed and the MT temperatures decreased by about 7 K under the magnetic field of 30 kOe for x = 0.02. Large magnetic entropy changes were observed around MT temperatures, which would be ascribed to the temperature- and field-induced MT in Mn2Ni1.6Sn0.4−xGex alloys. The hysteresis losses and effective refrigerant capacities for these alloys were discussed in this paper.

The Phase Transformation and Magnetic Properties in Ni-Mn-In Alloys

The phase transformation and magnetic properties of Heusler-type Ni50Mn50-xInx (x=10~16) ferromagnetic shape-memory alloys have been systemically investigated by differential scanning calorimetry and vibrating sample magnetometry. It is found that the phase transition temperatures show a linear relationship with the In concentration and the curie temperatures of austenite phases are not sensitive to the alloy composition. The existence of large magnetization change during the magneto-structural transition is the origin of magnetic-field-induced transformation effect.

Effect of post-annealing on the phase transitions and magnetocaloric properties in bulk Ni44Mn45Sn11 alloy

The effect of low temperature post-annealing on the phase transitions and magnetocaloric properties in bulk Ni44Mn45Sn11 alloy has been investigated. The results indicate that with the increase of annealing temperature, the martensite start temperature decreases rapidly, while the Curie temperature of austenitic phase shows a small decrease. The variations of phase transition temperatures can be attributed to the decrease of atomic order degree characterized by the increases of cell volume. Although the peak value of magnetic entropy change decreases rapidly, the refrigerant capacity shows a slight increase at the annealing temperature ≤300 °C due to the wider peaks.

The influence of Al substitution on the phase transitions and magnetocaloric effect in Ni 43Mn 46Sn 11− xAl x alloys

The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni 43Mn 46Sn 11− x Al x ( x=0–2) ferromagnetic shape memory alloys were investigated by X-ray diffraction and magnetization measurements. With the increase of Al content, the cell volume decreases due to the smaller radius of Al, and the martensitic transformation temperature increases rapidly, while the Curie temperature of austenitic phase shows a small increase. A large positive and a negative magnetic entropy change were observed near the first-order martensitic transition and the second-order magnetic transition, respectively. The magnetic entropy changes, hysteresis behavior, and refrigerant capacity near the two transitions are compared.

Martensitic transition and magnetocaloric properties in Ni 45Mn 44− xFe xSn 11 alloys

The structural, martensitic transition and magnetocaloric effect in Heusler alloys Ni 45Mn 44− x Fe x Sn 11 ( x = 0, 2, 5 and 8) have been studied by means of XRD, SEM/EDS and magnetization measurements. The alloys with x = 2 and 5 exhibit the co-existence of orthorhombic (S.G. Pmma) martensite phase and cubic L2 1 structure (S.G. Fm-3m) austenite phase at room temperature. The martensitic transition temperature, M s , shows a non-monotonic dependence on the Fe content. M s increases from 265 K in the undoped alloy to 310 K with the addition of 2 at.% Fe, whereas further increase in Fe content to 5 and 8 at.% leads to the decrease of M s to 290 and 207 K. Substitution of Fe for Mn slightly increases the Curie temperature of austenite phase and the thermal hysteresis. Associated with the martensitic transition, the maximum magnetic entropy changes under a magnetic field change of 20 kOe are 10.6, 6.4 and 5.6 J/kg K for x = 0, 5 and 8, respectively.

Magnetic and electrical properties of Ni50Mn35In15−xSix Heusler alloys

We have studied the magnetic and electrical properties of the polycrystalline ferromagnetic Ni50Mn35In15−xSix (1≤x≤5) Heusler alloys through magnetization, thermal expansion, and resistivity measurements. It was observed that an increase in Si concentration strongly affects the ground state of the martensitic phase and the magnetic properties of compounds. A magnetic phase diagram has been constructed for these alloys. It was found that both martensitic transition temperature (TM) and Curie temperature of austenitic phase (TC) decrease, while ferromagnetic ordering temperature of the martensitic phase increases with increasing Si concentration. The magnetoresistance (Δρ/ρ) associated with martensitic transformation was found to vary from –47% for x=2 at T=261 K to −26% for x=5 at T=230 K for a magnetic field change of 5 T.