Igor Dubenko

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.

Size Induced Variations in Structural and Magnetic Properties of Double Exchange La0.8Sr0.2MnO3−δ Nano-Ferromagnet

A detailed study on the influence of particle size varied from 8 nm to 53 nm on the structural and magnetic properties of La0.8Sr0.2MnO3−δ has been done. The unit cell volume increases and the microstrain in the compound shows peak formation as the particle size decreases. Nano particles of La0.8Sr0.2MnO3−δ exhibit superparamagnetism whose blocking temperature has a nonlinear and logarithmic decreasing tendency as function of particle size and applied magnetic field, respectively. Evidence of formation of a magnetically dead layer at the surface has been found and the ratio of the thickness of the dead layer to the particle size increases exponentially with particle size. The coercivity of the nanoparicles increases manifold as particle size varies from 53 nm to 21 nm. In the single domain region the coercivity exhibits a d−1.125 behavior. The temperature dependence of the saturation magnetization shows strong collective excitation due to the spin wave that varies as Tα with α>αbulk of 3/2. Thus the spin ...

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.

Giant magnetocaloric effects near room temperature in Mn1 − xCuxCoGe

Magnetic and structural transitions were found to coincide by substituting Cu for Mn in Mn1−xCuxCoGe, leading to a giant magnetocaloric effects associated with the first-order magnetostructural phase transition from the ferromagnetic to paramagnetic state. Maximum magnetic entropy changes of −ΔSM = 52.5 and 53.3 J/kg K for a magnetic field change of ΔH = 5 T have been observed at 302 and 316 K, for x = 0.080 and 0.085, respectively. The giant magnetocaloric effects with tunable phase transition temperatures through subtle variations in composition make this system promising for room-temperature magnetic cooling applications.

Magnetostructural phase transitions and magnetocaloric effects in MnNiGe1−xAlx

The thermomagnetic and magnetocaloric properties of the MnNiGe1−xAlx system have been investigated by magnetization and differential scanning calorimetry (DSC) measurements. The presence of first-order magnetostructural transitions (MSTs) from hexagonal ferromagnetic to orthorhombic antiferromagnetic phases has been detected for x = 0.085 and 0.09 at 193 K and 186 K, respectively. The values of latent heat (L = 6.6 J/g) and corresponding total entropy changes (ΔST = 35 J/kg K) have been evaluated for the MST (x = 0.09) from DSC measurements. The magnetic entropy change for x = 0.09 (ΔSM = 17.6 J/kg K for 5 T) was found to be comparable with well-known giant magnetocaloric materials, such as Gd5Si2Ge2, MnFeP0.45As0.55, and Ni50Mn37Sn13.

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...

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.

Magnetic and crystallographic properties of SmMn/sub 6-x/Fe/sub x/Ge/sub 6/ intermetallics

The magnetic and crystallographic properties of induction-melted SmMn/sub 6-x/Fe/sub x/Ge/sub 6/ compounds, where x is 0, 0.5, 1.0, and 1.5, have been studied using X-ray diffraction and bulk magnetic measurements between 30 and 490 K. All of the samples crystallized in the YCo/sub 6/Ge/sub 6/-type structure with the space group P6/mmm. A small amount, less than 9 mol%, of Sm(MnFe)/sub 2/Ge/sub 2/ was present as an impurity in all samples. The unit cell volume decreases with increasing iron content at an average rate of 2.4% per substituted atom at room temperature. The a and c lattice parameters decrease at an average rate of 1.1 and 0.1% per substituted atom at room temperature, respectively. The magnetic properties deteriorate rapidly as the iron concentration increases beyond x=0.5. Magnetic behavior of SmMn/sub 5/Fe/sub 1/Ge/sub 6/ and SmMn/sub 4.5/Fe/sub 1.5/Ge/sub 6/ is indicative of antiferromagnetic or ferrimagnetic coupling. According to X-ray diffraction patterns of magnetically aligned powders, the easy direction of magnetization at room temperature for all of the samples lies close to the basal plane of the unit cell.