Arjun K. Pathak

Large magnetic entropy change in Ni50Mn50- xInx Heusler alloys

The magnetocaloric properties of polycrystalline Ni50Mn50−xInx (15⩽x⩽16) associated with the second order magnetic transition at the Curie temperature and the first order martensitic transition were studied using magnetization measurements. The refrigeration capacity and magnetic entropy change were found to depend on the In concentration and reach a maximum value of refrigeration capacity of 280J∕kg with a magnetic entropy change of −6.8J∕kgK at 318K for a magnetic field change of 5T. These values of the magnetocaloric parameters are comparable to that of the largest values reported near the second order transition of metallic magnets near room temperature.

Cerium: An Unlikely Replacement of Dysprosium in High Performance Nd–Fe–B Permanent Magnets

Replacement of Dy and substitution of Nd in NdFeB-based permanent magnets by Ce, the most abundant and lowest cost rare earth element, is important because Dy and Nd are costly and critical rare earth elements. The Ce, Co co-doped alloys have excellent high-temperature magnetic properties with an intrinsic coercivity being the highest known for T ≥ 453 K.

The effect of partial substitution of In by Si on the phase transitions and respective magnetic entropy changes of Ni50Mn35In15 Heusler alloy

The effect of the partial substitution of In by Si on the crystal structure, phase transition temperatures and respective magnetic entropy changes in Ni50Mn35In15−xSix with 1 ≤ x ≤ 5 have been determined using room temperature x-ray diffraction and magnetization measurements in the temperature interval 5–400 K and in magnetic fields up to 5 T. The results show that small amount of Si in the In position results a significant influence in the inverse magnetocaloric effect accompanied with the martensitic transitions in these compounds. The peak values of positive magnetic entropy change for magnetic field changes H = 5 T is found to depend on composition and vary from 82 J Kg−1 K−1 for x = 1 (at T = 275 K) to 124 J Kg−1 K−1 for x = 3 (at T = 239 K). It is found that the substitution of 20% In atoms by Si in Ni50Mn35In15 results in an increase in positive magnetic entropy changes of more than 300%. This system also shows a negative entropy change at the Curie temperature, making this a candidate material for application in a refrigeration cycle that exploits both positive and negative magnetic entropy changes.

Tunable Electronics in Large-Area Atomic Layers of Boron-Nitrogen―Carbon

We report on the low-temperature electrical transport properties of large area boron and nitrogen codoped graphene layers (BNC). The temperature dependence of resistivity (5 K < T < 400 K) of BNC layers show semiconducting nature and display a band gap which increases with B and N content, in sharp contrast to large area graphene layers, which shows metallic behavior. Our investigations show that the amount of B dominates the semiconducting nature of the BNC layers. This experimental observations agree with the density functional theory (DFT) calculations performed on BNC structures similar in composition to the experimentally measured samples. In addition, the temperature dependence of the electrical conductivity of these samples displays two regimes: at higher temperatures, the doped samples display an Arrhenius-like temperature dependence thus indicating a well-defined band gap. At the lowest temperatures, the temperature dependence of the conductivity deviates from activated behavior and displays a conduction mechanism consistent with Motts two-dimensional (2D) variable range hopping (2D-VRH). The ability to tune the electronic properties of thin layers of BNC by simply varying the concentration of B and N will provide a tremendous boost for obtaining materials with tunable electronic properties relevant to applications in solid state electronics.

Phase transitions and magnetoresistance in Ni50Mn50−xInx Heusler alloys

The phase transitions and magnetoresistance in polycrystalline ferromagnetic Ni50Mn50−xInx (15⩽x⩽16.2) Heusler alloys were studied through ac susceptibility, magnetization, thermal expansion, and resistivity measurements in the temperature interval of 5–400K. The temperatures of the martensitic transformations were found to be strongly dependent on In concentration and on the strength of the applied external magnetic field. We observed large magnetoresistance (MR) Δρ∕ρo≈−80% for x=16 at T≈125K and Δρ∕ρo≈−56% for x=15 at T≈309K for ΔH=5T. In addition to large MR, the Ni50Mn50−xInx system exhibits ferromagnetic shape-memory effect and a large magnetic entropy change. Hence this system has potential to be a multifunctional applied material.

Large inverse magnetic entropy changes and magnetoresistance in the vicinity of a field-induced martensitic transformation in Ni50−xCoxMn32−yFeyGa18

Significantly large inverse magnetic entropy changes (ΔSM) and magnetoresistance (MR) were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys: Ni50−xCoxMn32−yFeyGa18. The crystal structures of alloys were tetragonal at 300 K and the phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion. The maximum peak values of the ΔSM and MR at H=5 T were determined as ≈(+)31 J Kg−1 K−1 and ≈−21%, respectively, for x=8 and y=2. The relatively small hysteretic loss and large refrigeration capacity observed in this system make these compounds promising materials for applications.

Magnetoresistance and magnetocaloric effect at a structural phase transition from a paramagnetic martensitic state to a paramagnetic austenitic state in Ni50Mn36.5In13.5 Heusler alloys

It is established, using magnetization measurements, that Ni50Mn36.5In13.5 is in a paramagnetic state (PS) above and below the martensitic transition temperature (TM). Magnetoresistance (MR) and magnetic entropy changes (ΔSM) in the vicinity of TM were studied. MR and ΔSM at TM were found to be ≈−8% and ≈+24 J Kg−1 K−1, respectively, at ΔH=5 T. Although MR and ΔSM values were lower than compared to those found in other Heusler systems, the significantly smaller hysteresis observed in Ni50Mn36.5In13.5 makes this compound, and other such compounds that undergo a martensitic transition in a PS, promising for the study and applications of magnetocaloric magnetic materials.

Direct measurements of field-induced adiabatic temperature changes near compound phase transitions in Ni–Mn–In based Heusler alloys

The adiabatic temperature changes (ΔTad) in the vicinity of the Curie and martensitic transition temperatures of Ni50Mn35In15 and Ni50Mn35In14Z (Z=Al and Ge) Heusler alloys have been studied using an adiabatic magnetocalorimeter of 250–350 K temperature interval for applied magnetic field changes up to ΔH=1.8 T. The largest measured changes were ΔTad=−2 and 2 K near the martensitic (first-order) and ferromagnetic (second-order) transitions for ΔH=1.8 T, respectively. It was observed that |ΔTad|≈1 K for relatively small field changes (ΔH=1 T) for both types of transitions. The results indicate that these materials should be further explored as potential working materials in magnetic refrigeration applications.

The effect of partial substitution of In by X = Si, Ge and Al on the crystal structure, magnetic properties and resistivity of Ni50Mn35In15 Heusler alloys

The effect of partial substitution of In by X = Si, Ge and Al on the structural, electrical and magnetocaloric properties of Ni50Mn35In15 Heusler alloys was studied using x-ray diffraction, resistivity, magnetization and thermal expansion measurements. It was observed that the presence of X atoms (~7% X) at In sites strongly affects the crystal structure and electric and magnetic behaviours of Ni50Mn35In15. Martensitic (Pmmm space group) and austenitic ( ) crystal structures were observed at room temperature for Ni50Mn35In14Ge, Ni50Mn35In14Al and Ni50Mn35In14Si. All samples were found to show unidirectional anisotropy at low temperatures (T < 150 K). Giant positive and negative magnetic entropy changes, as large as ΔSM, + 82 J Kg−1 K−1 (at T = 274 K) and −6.6 J Kg−1 K−1 (at T = 307 K) were found for a magnetic field change of 5 T with X = Si. The net refrigeration capacity was also observed to change significantly. The maximum negative magnetoresistance was found to be 50% for Ni50Mn35In14Si at H = 5 T and T = 284 K. Phase transition temperatures attributed to martensitic transformation and to ferromagnetic ordering in martensitic and austenitic phases were determined. The influence of the interatomic distance on the properties of Ni50Mn35In15 was discussed.

Magnetism and magnetocaloric effects in Ni50Mn35−xCoxIn15 Heusler alloys

The effects of the partial substitution of Mn by Co on the magnetic, magnetoelastic, and magnetocaloric properties of Ni50Mn35In15 Heusler alloys were studied using x-ray diffraction, temperature and field dependences of the magnetization [M(T,H)], and strain gauge techniques. It was observed that the presence of ≈3% Co atoms in Mn sites strongly affected the magnetic and magnetoelastic behaviors of Ni50Mn35In15. At H=5 T, a maximum magnetoelastic strain of ≈0.6% was observed for Ni50Mn35In15. It was found that Co substituted into the Mn position does not affect the ΔSM in the vicinity of the second order transitions (SOTs), however ΔSM near the first order transitions (FOTs) it slightly decreases. The net refrigeration capacities in the vicinity of both the FOT and SOT were found to significantly increase by introducing Co into Mn sites. The net refrigeration capacity in the vicinity of the FOT and SOT was found to be 167 J/kg (T=277–293 K) and 229 J/kg (T=305–354 K), respectively, at a magnetizing field...