Shane Stadler

Magnetocaloric properties of Ni2Mn1−xCuxGa

We present the magnetocaloric properties of the substituted Heusler alloy Ni2Mn1−xCuxGa, which shows a maximum magnetic entropy change of ΔSM≈−64J∕KgK=−532mJ∕cm3K at 308K for a magnetic field change ΔH=5T. The dependence of ΔSM on ΔH is approximately linear and does not reach saturation at 5T. It is demonstrated that the temperature at which ΔSM occurs can be tuned through subtle variations in composition.

Exchange bias behavior in Ni–Mn–Sb Heusler alloys

The authors report the observation of exchange bias in bulk polycrystalline Ni50Mn25+xSb25−x Heusler alloys. Shifts in hysteresis loops of up to 248Oe were observed in the 5T field cooled samples. The observed exchange bias behavior in Ni50Mn25+xSb25−x is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions in the system. Such behavior is an addition to the multifunctional properties of the Ni50Mn25+xSb25−x Heusler alloy system.

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.

Inverse Magnetocaloric Effect in Ferromagnetic Ni50Mn37+xSb13−x Heusler Alloys

A study of the magnetocaloric effect (MCE) in the ferromagnetic Heusler alloys Ni50Mn37+xSb13−x (x=0,0.5,1) has been carried out through magnetization measurements. An inverse magnetocaloric effect was observed in the vicinity of the first order martensitic transition. A maximum positive magnetic entropy change of ΔSm≈19J∕kgK at approximately 297K for a magnetic field change of 5T was observed. It is demonstrated that the martensitic transformation temperature, and the corresponding ΔSm, can be tuned through a slight variation in composition.

Exchange bias in bulk Mn rich Ni–Mn–Sn Heusler alloys

An experimental study on the exchange bias properties of bulk polycrystalline Ni50Mn50−xSnx Heusler alloys has been performed. Martensitic transformations have been observed in the alloys for some critical Sn concentrations. The alloys, while in their respective martensitic phases, are found to exhibit exchange bias effect. Shifts in hysteresis loops of up to 225Oe were observed in the 50kOe field cooled samples. The observed exchange bias behavior in Ni50Mn50−xSnx is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions in the system.

Magnetostructural phase transitions in Ni50Mn25+xSb25-x Heusler alloys

The structural and magnetic properties of Ni(50)Mn(25+x)Sb(25-x) (0≤x≤18) Heusler alloys have been investigated by x-ray diffraction, magnetization, thermal expansion, and electrical resistivity measurements. Austenitic phases with the L2(1) cubic crystal cell and martensitic phases with Pmm2 orthorhombic structures have been observed at room temperature in alloys with 0≤x≤12.5 and 13≤x≤14, respectively. The Curie temperatures of the austenitic phases decrease linearly with increasing x and change from 370 (x = 0) to 340xa0K (x = 12.5). In the concentration 7<x≤10, the system is found to undergo martensitic phase transitions above 150xa0K. The martensitic transition temperatures increase rapidly with increasing x and for x≥13.5 the temperature exceeds the Curie temperature of the austenitic phase (340xa0K). The alloys in their martensitic phases exhibit complex magnetic behavior. The phase diagram with respect to temperature and composition of the Ni(50)Mn(25+x)Sb(25-x) system has been determined, where the following magnetic phases have been observed in the martensitic state: antiferromagnetic phase (14<x); paramagnetic and antiferromagnetic (13≤x≤14, above 120xa0K) mixed phase; ferromagnetic and antiferromagnetic (10≤x≤14, above 20xa0K) mixed phase. The mixed phases result in observation of the exchange bias effect at low temperatures (below 120xa0K).

Magnetic and structural phase transitions in Heusler type alloys Ni2MnGa1−xInx

X-ray diffraction, magnetization, electrical resistivity and thermal expansion measurements were done on a series of Heusler alloys of the form Ni2MnGa1−xInxxa00< x<0.25. X-ray diffraction patterns indicate that the samples possess a highly ordered Heusler alloy L 21 type structure. Due to the larger radii of the In ion, the lattice parameters were found to expand with increasing In concentration. The negative volume anomaly () is found to accompany the martensitic transition. With increasing In content, a linear decrease of the martensitic transition and the Curie temperatures were observed. The compositional–transition temperature phase diagram is evaluated. Possible reasons for the structural transformations are discussed.

The Structural and Magnetic Properties of Ni2Mn1−xMxGa (M = Co, Cu)

In Ni2MnGa (cubic structure of L21 type) a first order martensitic structural transition, from the parent cubic (austenitic) phase to a low temperature complex tetragonal structure, takes place at TM=202K, and ferromagnetic order in the austenitic phase sets at TC=376K. In this work, the Mn sites in Ni2MnGa have been partially substituted with magnetic Co and nonmagnetic Cu, and the influence of these substitutions on the structural and magnetic properties of Ni2Mn1−xMxGa (M=Co and Cu) have been studied by XRD and magnetization measurements. X-ray diffraction patterns indicate that the Co doped system possess a highly ordered Heusler alloy L21 type structure for 0.05<x<0.12, and the Cu doped compounds possess L21 structure for 0.05<x<0.10. The ferromagnetic ordering temperature increases with increasing Co concentration for this system, and rapidly decreases with increasing Cu concentration. Both systems show the increase in TM with increasing Co and Cu concentration. (T-x) phase diagrams have been plotte...

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.

The magnetism of a buried La0.7Sr0.3MnO3 interface

Using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) techniques, we have investigated the electronic structure and magnetic properties of La0.7Sr0.3MnO3 (LSMO) as a function of YBa2Cu3O7−δ (YBCO) cap layer thickness. The Mn L2,3 XAS and XMCD data clearly show that the magnetic properties and electronic structure of the LSMO are adversely affected by the YBCO overlayer caused by cation displacement/exchange that effectively reduces the concentration of the La atoms in the LSMO near the YBCO/LSMO interface.