S B Roy

Large inverse magnetocaloric effect in Ni50Mn34In16

We report a study of the magnetocaloric effect in the ternary alloy system Ni50Mn34In16. This system undergoes an austenite–martensite phase transition, and the change in magnetic entropy is found to be quite large across this martensitic transition. This entropy change is due to an increase in entropy induced by the application of an external magnetic field and can lead to a large inverse magnetocaloric effect. Isothermal magnetic field variation of magnetization exhibits field hysteresis in Ni50Mn34In16 across the martensitic transition. But in spite of the hysteresis losses, a large effective refrigerant capacity can be obtained in this material over a wide temperature interval.

Reducing the operational magnetic field in the prototype magnetocaloric system Gd5Ge4 by approaching the single cluster size limit

We studied polycrystalline samples of the prototype magnetocaloric system Gd5Ge4 using scanning Hall imaging and global magnetometry. The magnetic field required to complete the phase transition that is critical for magnetocaloric application is reduced by up to 20% when small fragments, each consisting of several randomly oriented crystallites, are removed from the bulk. The removal of competing strain fields from neighboring grains when the fragment is embedded in the bulk underlies the observation. We support the results by showing local Hall imaging of phase separation in the bulk. When the bulk is ground into a fine powder, the effect vanishes.

Temperature and magnetic field induced strain in Ni50Mn34In16 alloy

We report a study of temperature and magnetic field induced strain across the martensite (MST)–austenite (AST) transition in Ni50Mn34In16 alloy. Both the isothermal magnetic field variation and the variation of temperature at constant field can induce strain of greater than 0.1% in the MST–AST phase transition region. Furthermore, our results reveal the influence of the path traversed in field-temperature phase space on the strain generated in this Ni50Mn34In16 alloy. Depending on the thermomagnetic history of the sample the induced strain can be reversible or irreversible in nature.

Thermomagnetic history dependence of magnetocaloric effect in Ni50Mn34In16

A large inverse magnetocaloric effect has been reported to be associated with the austenite to martensite phase transition in Ni50Mn34In16. It is shown here that the magnitude of the observed magnetocaloric effect as well as effective refrigerant capacity depend significantly on the thermo-magnetic history of the sample.

Elevating the temperature regime of the large magnetocaloric effect in a Ni–Mn–In alloy towards room temperature

We have prepared a Ni50(Mn,2%Cr)34In16 alloy by substituting Mn by Cr in a Ni50Mn34In16 alloy. A large isothermal magnetic entropy change is observed across the austenite?martensite phase transition in this new alloy for a moderate field change of 50?kOe. In comparison with the parent Ni50Mn34In16 alloy where the peak in isothermal entropy change amounts to nearly 19?J?kg?1?K?1, in this Ni50(Mn,2%Cr)34In16 alloy the peak value of the isothermal entropy change is much larger, 24.4?J?kg?1?K?1. Moreover, this large magnetocaloric effect in the Ni50(Mn,2%Cr)34In16 alloy occurs at ambient temperature near 294?K as compared with the temperature regime of the magnetocaloric effect being near 240?K in the parent Ni50Mn34In16 alloy. Thus the partial substitution of Mn by Cr in the Ni50Mn34In16 alloy is shown to be an effective method for enhancing the potential of the alloy system for near-room temperature applications.

The effect of external pressure on the magnetocaloric effect of Ni?Mn?In alloy

The martensitic transition in Ni(50)Mn(34)In(16) alloy has been studied by measuring the magnetization of the alloy as a function of temperature, magnetic field and pressure. Magnetic field and pressure have opposite effects on the martensitic transition in this alloy; the martensitic transition temperature decreases with increasing magnetic field but it increases with increasing pressure. The effect of pressure on the magnetocaloric properties of this large magnetocaloric effect alloy has been investigated in detail. The magnitude of the peak in the isothermal magnetic entropy change in Ni(50)Mn(34)In(16) increases with pressure. The temperature at which the magnetocaloric effect reaches the peak value in this alloy increases from near 240 K under ambient pressure to near 280 K under an external pressure of 9.5 kbar. The temperature corresponding to the peak in the isothermal magnetic entropy change increases with increasing pressure at a rate which matches the rate of increase of the martensite start temperature with increasing pressure. The temperature dependence of the isothermal magnetic entropy change under different pressures is found to follow a universal curve for a particular magnetic field change. These results show that pressure as a control parameter can be used to tune the temperature regime of the magnetocaloric effect in the alloy. The effect of pressure on the martensitic transition also gives a clue as regards the possibility of tuning this temperature regime with elemental substitution.

Metamagnetic transition and the anomalous virgin magnetization curve in Ce(Fe0.96Ru0.04)2

We present results of dc magnetization measurements investigating the magnetic field-induced metamagnetic transition in Ce(Fe0.96Ru0.04)2. In the temperature region where this material undergoes a ferromagnetic to antiferromagnetic first-order phase transition, the isothermal virgin magnetization curve lies distinctly outside the envelope magnetization curve obtained in the subsequent field cycles. We show that this anomalous behaviour of the virgin magnetization curve arises due to the dependence of the initial zero field magnetic state on the temperature–field history of the sample in the concerned temperature regime. This origin of the anomalous virgin magnetization curve is distinctly different from the very similar behaviour observed earlier in Al- and Os-doped CeFe2 alloys, where this feature was associated with the kinetic arrest of the first-order ferromagnetic to antiferromagnetic transition.

Large magnetocaloric effect in Ni50Mn33.66Cr0.34In16 alloy

A large isothermal magnetic entropy change with a peak value of 17.7?J?kg?1?K?1 has been observed across the austenite?martensite phase transition in Ni50Mn33.66Cr0.34In16 alloy near 270?K. In comparison with the parent Ni50Mn34In16 alloy this temperature regime of the peak in isothermal entropy change is closer to the ambient temperature by nearly 30?K. The effective refrigerant capacity is calculated to be 254?J?kg?1, which is 15% higher than that of the parent alloy.

Contrasting magnetic behavior of Ni50Mn35In15 and Ni50Mn34.5In15.5 alloys

We have studied the electrical resistivity, magnetization, and heat capacity of the off-stoichiometric Heusler alloys Ni50Mn35In15 and Ni50Mn34.5In15.5 as functions of temperature and magnetic field. The results show that the alloy system is more sensitive to the composition than what is apparent from the established phase diagram. We have found that the ground states as well as the nature of phase transitions strongly depend on concentration differences as low as 0.5 at. %. While in the case of Ni50Mn34.5In15.5 we do observe a magnetic field induced martensite to austenite phase transition, there is no detectable signature of any field induced transition in the Ni50Mn35In15 alloy even up to fields as high as 80 kOe. Accordingly, the functional properties of these two alloys are also drastically different.

Local probing of arrested kinetics in Gd5Ge4

We present spatially localized magnetic relaxation measurements of the first-order magnetostructural transition in Gd 5 Ge 4 using a scanning Hall probe imaging technique. Relaxation measurements were performed at 6, 10 and 35 K to probe the field-increasing antiferromagnetic (AFM) to ferromagnetic (FM) transition and, when it can occur, the field-decreasing FM to AFM transition. We demonstrate that localized regions relax with time towards the ground state for a given field and temperature and observe different magnetic behaviours at the three measurement temperatures. In particular, the observed magnetic relaxation at 6 K is consistent with the idea of an arrested state at this temperature. Our scanning Hall probe imaging data give an insight into the phase nucleation and growth process of the martensitic-like magnetostructural first-order transition in Gd 5 Ge 4 .