S. Pramanick

Barocaloric and magnetocaloric effects in Fe49Rh51

We report on calorimetry under applied hydrostatic pressure and magnetic field at the antiferromagnetic-ferromagnetic (AFM/FM) transition of Fe49Rh51. Results demonstrate the existence of a giant barocaloric effect in this alloy, a functional property that adds to the magnetocaloric and elastocaloric effects previously reported for this alloy. All caloric effects originate from the AFM/FM transition which encompasses changes in volume, magnetization, and entropy. The strong sensitivity of the transition temperatures to both hydrostatic pressure and magnetic field confers to this alloy outstanding values for the barocaloric and magnetocaloric strengths (|?S|/?p ~ 12 J kg-1K-1kbar-1 and |?S|/µ0?H~ 12 J kg-1K-1T-1). Both barocaloric and magnetocaloric effects have been found to be reproducible upon pressure and magnetic field cycling. Such a good reproducibility and the large caloric strengths make Fe-Rh alloys particularly appealing for solid-state cooling technologies at weak external stimuli.

Magnetocaloric effect in the low hysteresis Ni-Mn-In metamagnetic shape-memory Heusler alloy

We have studied magnetocaloric properties of a Ni-Mn-In metamagnetic shape-memory alloy especially designed in order to display low thermal hysteresis. Magnetization and calorimetric measurements under a magnetic field have been used in order to determine isothermal magnetic field-induced entropy changes. Results obtained indirectly from magnetization data, quasi-directly from isofield calorimetric measurements, and directly from isothermal calorimetric runs are systematic and agree well with each other. We have analyzed the reproducibility of magnetocaloric properties with cycling from direct isothermal calorimetric measurements. Due to low thermal hysteresis, we have found that about 80% of the transition entropy change, ΔSt ≃ 25 J/kg K, can be reversibly induced under successive application and removal of a field of 6 T.

Reversible adiabatic temperature changes at the magnetocaloric and barocaloric effects in Fe49Rh51

We report on the adiabatic temperature changes (ΔT) associated with the magnetocaloric and barocaloric effects in a Fe49Rh51 alloy. For the magnetocaloric effect, data derived from entropy curves are compared to direct thermometry measurements. The agreement between the two sets of data provides support to the estimation of ΔT for the barocaloric effect, which are indirectly determined from entropy curves. Large ΔT values are obtained at relatively low values of magnetic field (2 T) and hydrostatic pressure (2.5 kbar). It is also shown that both magnetocaloric and barocaloric effects exhibit good reproducibility upon magnetic field and hydrostatic pressure cycling, over a considerable temperature range.

Hydrostatic pressure effect on the magnetocaloric behavior of Ga-doped MnNiGe magnetic equiatomic alloy

The magnetocaloric properties of a new class of ferromagnetic shape memory alloys of nominal composition MnNiGe0.928Ga0.072 have been investigated in ambient conditions as well as in the presence of external hydrostatic pressure. Both inverse (6.35 Jkg−1K−1 for 0 − 50 kOe around 160 K) and conventional (−4.54 Jkg−1 K−1 for 0–50 kOe around 210 K) magnetocaloric effects (MCEs) have been observed around the structural and magnetic transitions respectively. The sample can be thought of as being derived from the parent MnNiGe alloy, where Ga was doped at the expense of the Ge atom. Ga doping at Ge sites brings down the martensitic transition temperature to below room temperature and induces ferromagnetism by affecting the lattice volume of the alloy. However, below the first-order martensitic transition the alloy loses its ferromagnetism. Application of external hydrostatic pressure results in a revival of ferromagnetic interactions in the martensitic phase of the alloy and a considerable increase in the refrigeration capacity around the conventional MCE region.

Excess Ni-doping induced enhanced room temperature magneto-functionality in Ni-Mn-Sn based shape memory alloy

Present work reports on the observation of large magnetoresistance (∼−30% at 80 kOe) and magnetocaloric effect (∼12 J·kg−1·K−1 for 0–50 kOe) near room temperature (∼290 K) on the Ni-excess ferromagnetic shape memory alloy Ni2.04Mn1.4Sn0.56. The sample can be thought of being derived from the parent Ni2Mn1.4Sn0.6 alloy, where excess Ni was doped at the expense of Sn. Such Ni doping enhances the martensitic transition temperature and for the Ni2.04Mn1.4Sn0.56 it is found to be optimum (288 K). The doped alloy shows enhanced magneto-functional properties as well as reduced saturation magnetization as compared to the undoped counterpart at low temperature. A probable increment of antiferromagnetic correlation between Mn-atoms on Ni substitution can be accounted for the enhanced magneto-functional properties as well as reduction in saturation moment.

Cooperative spin freezing and the pinning assisted thermoremanent magnetization in Ni2.04Mn1.36Sn0.6 alloy

Detailed investigation on the ground-state magnetic properties of Ni2.04Mn1.36Sn0.6 alloy is reported. The sample undergoes martensitic type structural transformation from a cubic austenite phase to an orthorhombic martensite phase on cooling around TM = 220 K. It orders ferromagnetically just above room temperature with TC = 310 K. The phases above and below TM are predominantly ferromagnetic, although incipient antiferromagnetic correlations have been mooted between the Mn atoms sitting at two different crystallographic sites. The zero-field-cooled magnetic state shows a step like anomaly around TB = 100 K, and the sample is found to show clear signature of spin glass like behaviour below this point. It is also associated with considerable exchange bias effect below TB, where horizontal shift of isothermal magnetization loop is observed in the field-cooled state. Apart from exchange bias, there exists large thermoremanent magnetization. Interestingly, the thermoremanent magnetization obtained by cooling...

Spin-glass–like ground state and observation of exchange bias in Mn0.8Fe0.2NiGe alloy

The ground-state magnetic properties of hexagonal equiatomic alloy of nominal composition Mn_{0.8}Fe_{0.2}NiGe were investigated through dc magnetization and heat capacity measurements. The alloy undergoes first order martensitic transition below 140 K with simultaneous development of long range ferromagnetic ordering from the high temperature paramagnetic phase. The undoped compound MnNiGe has an antiferromagnetic ground state and it shows martensitic like structural instability well above room temperature. Fe doping at the Mn site not only brings down the martensitic transition temperature, it also induces ferromagnetism in the sample. Our study brings out two important aspects regarding the sample, namley (i) the observation of exchange bias at low temperature, and (ii) spin glass like ground state which prevails below the martensitic and magnetic transition points. In addition to the observed usual relaxation behavior the spin glass state is confirmed by zero field cooled memory experiment, thereby indicating cooperative freezing of spin and/or spin clusters rather than uncorrelated dynamics of superparamagnetic like spin clusters. We believe that doping disorder can give rise to some islands of antiferromagnetic clusters in the otherwise ferromagnetic background which can produce interfacial frustration and exchange pinning responsible for spin glass and exchange bias effect. A comparison is made with doped rare-earth manganites where similar phase separation can lead to glassy ground state.

Hydrostatic pressure tuned magneto-structural transition and occurrence of pressure induced exchange bias effect in Mn0.85Fe0.15NiGe alloy

The magnetic and magneto-functional behavior of a Fe-doped MnNiGe alloy with nominal composition Mn0.85Fe0.15NiGe have been investigated in ambient as well as in high pressure conditions. The alloy undergoes a first order martensitic phase transition (MPT) around 200 K and also shows a large conventional magnetocaloric effect (MCE) ( J kg−1 K−1 for magnetic field (H) changing from 0–50 kOe) around the transition in ambient conditions. The application of external hydrostatic pressure (P) results in a shift in MPT towards the lower temperature and a clear decrease in the saturation moment of the alloy at 5 K. The peak value of MCE is also found to decrease with increasing external P (~18 J kg−1 K−1 decrease in has been observed for P = 12.5 kbar). The most interesting observation is the occurrence of the exchange bias effect (EBE) on application of external P. The competing ferromagnetic and antiferromagnetic interaction in the presence of external P plays the pivotal role towards the observation of P induced EBE.

Anomalous giant positive magnetoresistance and heavy fermion like behaviour in Mn11Ge8

Mn11Ge8 undergoes long range ferromagnetic ordering at 274 K followed by non-collinear antiferromagnetic structure below 150 K. Our investigations indicate the existence of large positive magnetoresistance which changes sign depending upon the applied field and temperature. In the temperature variation of resistivity and heat capacity, the sample shows very large coefficients of quadratic and linear terms, respectively. This indicates the existence of giant spin fluctuation despite the fact that the sample is in a magnetically ordered state. The Mn-moments possibly have both localized and itinerant character and the anomaly appears to be associated with the local moment spin fluctuations mediated via itinerant electrons. The large positive magnetoresistance is believed to be originated from the possible development of short range antiferromagnetic clusters on application of magnetic field.

Anomalous pressure effect on the magnetic properties of Ni-Mn based shape memory alloys

The magnetic behavior of three Ni-Mn-(Sn,In) based shape memory alloys are investigated under hydrostatic pressure. Among them, Ni 51.2Mn 32.8In 16 (NM-In 16) and Ni 51Mn 35Sn 14 (NM-Sn 14) have their martensitic transition close to room temperature and undergo several magnetic transitions at lower temperatures. They order ferromagnetically at T C A, which is just above room temperature. However, the ferromagnetism is destroyed by the martensitic transition at T M S ( < T C A). The ferromagnetic order is revoked further at a lower temperature T C M ( < T M S) in the martensite phase. The third alloy, namely, Ni 50Mn 34Sn 16 (NM-Sn 16), has a relatively stable ferromagnetic state (Curie point T C A = 347 K), and it survives below the martensitic transition temperature, T M S. Our magnetic study under hydrostatic pressure indicates that T M S and T C M (only for the first two alloys) increase systematically with pressure. However, the saturation magnetization at base temperature is found to be an increasing...