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 = 202 K, and ferromagnetic order in the austenitic phase sets at TC = 376 K. 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\u3cx\u3c0.12, and the Cu doped compounds possess L21 structure for 0.05\u3cx\u3c0.10. 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 plotted. The results are discussed in terms of 3d-electron concentration variation

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 248 Oe were observed in the 5 T 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

Phase Transitions and Corresponding Magnetic Entropy Changes in Ni2Mn0.75Cu0.25−xCoxGa Heusler Alloys

Detailed studies of room temperature crystal structures, phase transitions, and related magnetic entropy changes (ΔSm) in shape memory alloys Ni2Mn0.75Cu0.25−xCoxGa (x = 0.0, 0.01, 0.02,0.025 0.03, 0.05) have been carried out by x-ray diffraction, magnetization, and thermal expansion measurements in magnetic fields of up to 5 T and in a temperature interval of 5–400 K. The high temperature austenitic cubic phase passes through a magnetic transition to ferromagnetic state and a structural transition to martensitic phase at the same temperature for all samples of the Ni2Mn0.75Cu0.25−xCoxGa system. The first order magnetostructural transition temperature increases from 308 to 345 K with increasing Co concentration. All of the alloys in the Ni2Mn0.75Cu0.25−xCoxGa system were found to possess large magnetic entropy changes. The maxima in the magnetic entropy changes ranged from ΔSmmax = −48 J/kg K to −64 J/kg K in a temperature range of 308–345 K

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 225 Oe were observed in the 50 kOe 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

Magnetic and Electrical Properties of Ni50Mn35In15−xSix Heusler Alloys

We have studied the magnetic and electrical properties of the polycrystalline ferromagnetic Ni50Mn35In15−xSix (1 ≤ x ≤ 5) Heusler alloys through magnetization, thermal expansion, and resistivity measurements. It was observed that an increase in Si concentration strongly affects the ground state of the martensitic phase and the magnetic properties of compounds. A magnetic phase diagram has been constructed for these alloys. It was found that both martensitic transition temperature (TM) and Curie temperature of austenitic phase (TC) decrease, while ferromagnetic ordering temperature of the martensitic phase increases with increasing Si concentration. The magnetoresistance (Δρ/ρ) associated with martensitic transformation was found to vary from –47% for x = 2 at T = 261 K to −26% for x = 5 at T = 230 K for a magnetic field change of 5 T

Magnetic and magnetocaloric properties of Gd6X 2Si3 (X = Ni, Co) and Ln6Co2Si 3 (Ln = Pr, La)

Phase compositions and crystal structures of Gd6X 2Si3 (X = Ni, Co,) and Ln6Co2Si 3 (Ln = Pr, La) have been studied. The magnetic properties of Gd 6X2Si3 (X = Ni, Co) and Ln6Co 2Si3 (Ln = Pr, La) have been evaluated from magnetization measurements performed by a superconducting quantum interference device magnetometer in a temperature interval of 5-400 K, and at magnetic fields up to 5T. The crystal structures of Gd6X2Si3 (X = Co, Ni) and Ln6Co2Si3 (Ln = Pr, La) were found to be hexagonal at 300 K. The Curie temperature (TC), effective and magnetic moment of the compounds (at5 K) have been determined for Gd 6X2Si3 (X = Ni, Co), and Ln6Co 2Si3 (Ln = Pr, La). TC was found to depend on composition, and reaches maximum value of about 300 K for the Gd 6X2Si3 system. Themagnetocaloric effect (magnetic entropy changes and relative cooling power (RCP) was found to depend on X and Ln, and the maximum RCP values were found to be larger than 500 J/kg near room temperature for the Gd6Ni2Si3 and Gd6Co2Si3 compounds. This value of RCP is comparable to the prototype magnetic refrigeration material, Gd. © 2011 American Institute of Physics

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\u3eαbulk of 3/2. Thus the spin wave does not follow the Bloch law in the case of nano particles of La0.8Sr0.2MnO3−δ

Magnetic and Magnetocaloric Properties of the New Rare-Earth-Transition-Metal Intermetallic Compound Gd3Co29Ge4B10

The compounds Gd3-xYxCo29Ge4B10 (x = 0, 0.5, 1.0, 1.5, and 3.0), Gd3Co29Al4B10, and Gd3Co29Al4B10 were synthesized by arc melting, and their magnetic properties investigated as a function of temperature and applied magnetic field. X-ray measurements showed primarily single-phase samples with the tetragonal crystal structure P4/nmm. It was found that Gd3Co29Ge4B10 orders ferromagnetically at TC  = 212 K and shows a compensation point at 128 K, indicating a ferrimagnetic ordering of the Co and Gd moments. An entropy change of −ΔS = 0.5 J/kgK was observed in a 5-T field at TC for this sample, while a change in sign for this quantity was observed both at the maximum value of magnetization (around 200 K) and then again at the compensation point. Substitution of Y for Gd in Gd3Co29Ge4B10 does not affect the Curie temperature, but shifts the compensation point to lower temperatures. This indicates that a decrease in Gd concentration does not affect the d-d exchange interaction, but has a pronounced effect on the f-d exchange interaction