Ajaya K. Nayak

Giant inverse magnetocaloric effect near room temperature in Co substituted NiMnSb Heusler alloys

The effect of Co on the structural, magnetic and magnetocaloric effect (MCE) of Ni50-xCoxMn38Sb12 (x=0,2,3,4,5) Heusler alloys was studied. Using x-ray diffraction, we show the evolution of the martensitic phase from the austenite phase. The martensitic transition temperature is found to decrease monotonically with Co concentration. Remarkable enhancement of MCE is observed near room temperature upon Co substitution. The maximum magnetic entropy change of 34 Jkg-1K-1 was achieved in x=5 at 262 K in a field of 50 kOe and a value of 29 Jkg-1K-1 found near room temperature. The significant increase in the magnetization associated with the reverse martensitic transition is responsible for the giant MCE in these compounds.

Pressure induced magnetic and magnetocaloric properties in NiCoMnSb Heusler alloy

The effect of pressure on the magnetic and the magnetocaloric (MC) properties around the martensitic transformation temperature in NiCoMnSb Heusler alloy has been studied. The martensitic transition temperature has significantly shifted to higher temperatures with pressure, whereas the trend is opposite with the application of applied magnetic field. The maximum magnetic entropy change around the martensitic transition temperature for Ni45Co5Mn38Sb12 is 41.4 J/kg K at the ambient pressure, whereas it is 33 J/kg K at 8.5 kbar. We find that by adjusting the Co concentration and applying suitable pressure, NiCoMnSb system can be tuned to achieve giant MC effect spread over a large temperature span around the room temperature, thereby making it a potential magnetic refrigerant material for applications.

Observation of enhanced exchange bias behaviour in NiCoMnSb Heusler alloys

We report the observation of large exchange bias (EB) in Ni50−xCoxMn38Sb12 Heusler alloys with x = 0, 2, 3, 4, 5, which is attributed to the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases in the martensitic phase. The phase coexistence is possibly due to the supercooling of the high temperature FM phase and the predominant AFM component in the martensitic phase. The presence of EB is well supported by the observation of the training effect. The EB field increases with Co concentration. The maximum value of 480 Oe at T = 3 K is observed in x = 5 after field cooling in 50 kOe, which is almost double the highest value reported so far in any Heusler alloy system. Increase in the AFM coupling after Co substitution is found to be responsible for the increase in the EB.

Large entropy change associated with the elastocaloric effect in polycrystalline Ni-Mn-Sb-Co magnetic shape memory alloys

We report on compressive strain measurements in polycrystalline magnetic shape memory alloys aimed at determining the entropy change associated with their elastocaloric effect. It is shown that for a maximum applied stress of 100 MPa, the stress-induced entropy change amounts to ΔS=21 J/kg K. This value compares well to the values reported for nonmagnetic shape memory alloys, and it is of the same order as those reported for the best giant magnetocaloric materials at moderate magnetic fields.

Effect of Fe substitution on the magnetic, transport, thermal and magnetocaloric properties in Ni50Mn38−xFexSb12 Heusler alloys

The structural, magnetic, transport, thermal, and magnetothermal properties of quaternary Heusler alloys Ni50Mn38−xFexSb12 have been studied. Powder x-ray diffraction and temperature dependence of magnetization studies reveal that with the addition of Fe in the Mn site, the martensitic transition shifts to low temperatures. It is also found that the martensitic transition becomes broader with the increase of Fe concentration. The metamagnetic transition in M(H) isotherms becomes very prominent in x = 2 and vanishes for x = 3 and 4. A maximum positive magnetic entropy change of 14.2 J/kg K is observed for x = 2 at 288 K for 50 kOe. Electrical resistivity data show an abrupt decrease across the martensitic transition in all the alloys, except x = 6, which does not have a martensitic transition. A maximum negative magnetoresistance of 21% has been obtained for x = 2 at 50 kOe. The same alloy also shows an exchange bias field of 288 Oe.

Magnetic, electrical, and magnetothermal properties in Ni-Co-Mn-Sb Heusler alloys

We have studied the magnetic, magnetoresistance, and thermal properties of Ni50−xCoxMn38Sb12 for x=0–7. The martensitic transition temperature decreases with increase in x and there is no martensitic transition observed for x=8. The martensitic transition is accompanied by a large change in magnetization as well as in electrical resistance. Due to the large change in magnetization, a large magnetic entropy change in 68 and 43 J kg−1 K−1 is observed for x=5 and 4, respectively, around the room temperature. A large magnetoresistance of 34% is observed for x=7. Both the magnetocaloric effect and the magnetoresistance are associated with the martensitic transition, which can be tuned significantly by varying the Ni/Co composition. The results obtained in this system suggest that it may act as a potential magnetic refrigerant as well as a magneto resistive material.

Irreversibility of field-induced magnetostructural transition in NiCoMnSb shape memory alloy revealed by magnetization, transport and heat capacity studies

The effects of magnetic field on the martensitic transition have been studied in Ni45Co5Mn38Sb12. We find a large field-induced irreversibility in this system, as revealed by the field dependence of resistivity, magnetization, and heat capacity data. At the critical temperature, the field-induced conversion of the martensitic to austenite phase is not reversible under any field variation. At this temperature any energy fluctuation induces nucleation and growth of the equilibrium austenite phase at the expense of the metastable martensitic phase and gets arrested. All these three measurements completely rule out the coexistence of austenite and martensitic phases in the irreversibility regime.

Structural, magnetic, magnetocaloric and magnetotransport properties in Ge doped Ni–Mn–Sb Heusler alloys

Abstract The effect of Ge substitution on the magnetic, magnetocaloric and transport properties of Ni 45 Co 5 Mn 38 Sb 12− x Ge x ( x =0–3) has been investigated. The decrease in the exchange interaction brought by Ge substitution can be seen from the reduction in the magnetization of austenite phase and the increase in the martensitic transition temperature. The magnetocaloric effect and the magnetoresistance values are found to be quite sensitive to small changes in Sb/Ge ratio. Taking into account various properties, the present series seems to be a promising multifunctional system.

Anisotropy induced large exchange bias behavior in ball milled Ni–Co–Mn–Sb alloys

We report the effect of decrease in the grain size on the structural, magnetic and exchange bias (EB) behavior in ball milled Ni50−xCoxMn38Sb12 (x=0 and 5) Heusler alloys. The existence of a wide range of grain sizes in the ball milled samples results in dramatic changes in the structural and magnetic properties. For x=0, a large EB field of 3.2 kOe is observed in the ball milled sample, compared to a value of 245 Oe of the bulk sample. This increase is attributed to the enhanced exchange coupling between the soft and hard magnetic particles.We report the effect of decrease in the grain size on the structural, magnetic and exchange bias (EB) behavior in ball milled Ni50−xCoxMn38Sb12 (x=0 and 5) Heusler alloys. The existence of a wide range of grain sizes in the ball milled samples results in dramatic changes in the structural and magnetic properties. For x=0, a large EB field of 3.2 kOe is observed in the ball milled sample, compared to a value of 245 Oe of the bulk sample. This increase is attributed to the enhanced exchange coupling between the soft and hard magnetic particles.

Large reversible magnetocaloric effect in Er3Co compound

Magnetic and magnetocaloric properties of the intermetallic compound Er3Co have been studied. Temperature dependence of magnetization data shows that it exhibits ferromagneticlike bulk magnetic ordering at 14 K. For field changes (ΔH) of 20 and 50 kOe, the maximum values of isothermal magnetic entropy change are found to be 9 and 17 J/kg K, respectively. For ΔH=20 and 50 kOe, the relative cooling powers are found to be 140 and 450 J/kg, respectively. At temperatures above Tord, the spin fluctuations are found to affect the magnetocaloric properties of Er3Co.