Heusler Alloy System Research Articles

Phase transformations, inverse magnetocaloric effect and critical behavior of Ni50Mn36Sn14−xSix Heusler alloys

The magnetic, magnetocaloric and critical behavior properties are systematically investigated for the Heusler alloy system Ni50Mn36Sn14−xSix (x=1, 2 and 3). For all the samples both the normal and inverse magnetocaloric effects (MCE) are observed. The magnetic entropy change and relative cooling power are evaluated near the structural phase transformation and magnetic transition regions under an applied magnetic field of 50 kOe. The maximum magnetic entropy change associated with the martensitic transition are 3.4 J kg−1 K−1, 2.5 J kg−1 K−1 and 1.3 J kg−1 K−1 for x=1, 2 and 3 respectively. The entropy change for x=1, 2 and 3 alloys near the Curie temperature of the austenite phase is found to be −2.8 J kg−1 K−1, −2.7 J kg−1 K−1 and −2.3 J kg−1 K−1 respectively. Relative cooling power is calculated in the vicinity of the Curie temperature of the austenite phase, as well as in the martensitic transition temperature regime. The critical parameters (β, γ and δ) of the samples were determined from the static magnetic data at the second-order ferromagnetic-paramagnetic transition region using the field dependence of magnetic entropy change.

Martensitic transition, spin glass behavior and enhanced exchange bias in Si substituted Ni50Mn36Sn14 Heusler alloys

The martensitic transition and exchange bias properties of the Heusler alloy system Ni50Mn36Sn14−xSix (x = 0, 1, 2 and 3) have been investigated.

V 2 NiSe: a First-Principles Study

The Heusler alloy system is a rich source of functional materials. We studied the ternary alloy V2NiSe by first-principles calculations to explore for a new functional alloy. We performed geometry optimization for the alloy with Hg2CuTi-type structure and obtained the equilibrium lattice parameter a0. The magnetic moment of the compound is 0.707 µB in 1-unit cell. The total density of states and the partial density of states were calculated. The band structure was also studied. The magnetic moment of the two vanadium atoms in 1-unit cell in different space positions is different.

Large exchange bias in polycrystalline ribbons of Ni56Mn21Al22Si1

Large exchange bias (EB) effect is demonstrated in a Ni56Mn21Al22Si1 polycrystalline ribbon that shows spin-glass behaviour below 68K and superparamagnetic (SPM) behaviour above this temperature. The average magnetic moment μ of the SPM clusters is estimated to be considerably smaller than that reported for other Heusler alloy systems that demonstrate spin glass behaviour and EB effect. The M–H loops measured in field-cooled (FC) conditions show significant asymmetry that is observed to be strongly field and temperature dependant. The maximum EB field (HEB ~2.6kOe at 20kOe and 2K) is much larger than that reported for any metallic alloy system with conventional EB effect. The dependence of HEB on temperature shows an exponential decrease. An approach towards enhancement of the EB effect in this conventional EB system can be realized via manipulation of SPM or superferromagnetic (SFM) cluster size with variation of cooling magnetic field.

Critical behavior and magnetocaloric effect in Co50−xNixCr25Al25 (x = 0 and 5) full Heusler alloy system

This work reports the investigation of critical behavior of Co50−xNixCr25Al25 (x=0 and 5) and magneto caloric effect (MCE) of bulk Co2CrAl full Heusler alloy system. The alloy series of Co50−xNixCr25Al25 (x=0, 1, 2, 3, 4 and 5) have been prepared using arc melting technique. The magnetic properties of all the samples have been studied in the temperature range of 5–300K. The value of Curie temperature (TC) is found to decrease with increasing doping concentration of the Ni (substitution of Ni at Co site). The critical exponents behavior and scaling relation have been investigated using magnetic isotherms in Co50−xNixCr25Al25 (x=0 and 5) alloys. The critical exponents are estimated by various techniques such as, Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. The value of critical exponents vicinity to the second order magnetic phase transition of Co50Cr25Al25 were found to be β=0.488 (7), γ=1.144 (16) and δ=3.336 (5) with TC=328.64 (5)K whereas for Co50Ni5Cr25Al25 the values are β=0.522 (13), γ=1.014 (6) and δ=3.043 (7) with TC=285.71 (11). The critical exponent values for both the samples are almost similar to the value as predicted by mean field theory. This has been best explained by long range mean field like ferromagnetic interaction in the entire Co50−xNixCr25Al25 (x=0, 5) highly spin polarized full Heusler alloy system. Under an external magnetic field maximum up to 5T, normal magneto caloric effect (MCE) have been observed with a maximum magnetic entropy change of 2.55J/kgK at 328.5K (near the ferromagnetic→paramagnetic second order magnetic phase transition in the austenite phase) in Co2CrAl full Heusler alloy system.

Magnetocaloric properties, exchange bias, and critical behavior of Ge substituted Ni50Mn36Sn14 Heusler alloys

The effect of Ge substitution on the magnetic, magnetocaloric, and exchange bias properties of Heusler alloy system Ni50Mn36Sn14-xGex (x = 1, 2) has been investigated. With the increase of Ge content, the cell volume decreases due to the smaller Ge radius and the martensitic transition temperature increases, while the Curie temperature of the austenite phase shows a small decrease. Metamagnetic behavior is observed in the low temperature magnetization isotherms for x = 1, whereas it is less pronounced in x = 2. The maximum magnetic entropy changes associated with the martensitic transition are 7.8 J/kg K and 2.3 J/kg K for x = 1 and 2, respectively, for a field change of 5 T. Relative cooling power is found to be more in the vicinity of the Curie temperature of the austenite phase, compared to that at the martensitic transition temperature in both x = 1 and 2. At low temperatures, both the samples exhibit exchange bias effect, with x = 2 showing higher value of exchange bias field. This is ascribed to the coexistence of ferromagnetic and antiferromagnetic interactions in these alloys. Further, the critical behavior of the austenite phase of both the alloys is studied and the values of the critical exponents are found to be different from the mean field values, which is attributed to the magnetic inhomogeneity originated due to Ge substitution.

A New Titanium-Based Heusler Alloy

The Heusler alloy system is a rich source of functional materials. We studied the ternary alloy Ti2NiSb by first-principles calculations to explore for new functional alloy. We performed geometry optimization for the alloy with Hg2CuTi-type structure and the obtained equilibrium lattice parameter is a0=6.21 Å. The magnetic moments of atom Ni is 0.22μB in one cell. The alloy is a ferromagnet. The little magnetic moment of atom Ni comes from the characteristic spatial occupation of the atoms in the space structure of the alloy.

Influence of Si substitution on the structure, magnetism, exchange bias and negative magnetoresistance in Ni48Mn39Sn13 Heusler alloys

The effect of partial Si substitution for Sn on the structure, magnetism, magnetic entropy change, exchange bias and negative magnetoresistance in Ni48Mn39Sn13−xSix(1 ⩽ x ⩽ 4) Heusler alloy systems is investigated. A small amount of Si in the Sn position influences the characteristic transition temperatures and the magnetocaloric effect. A maximum positive entropy change of 5.13 J kg−1 K−1 is observed for a field change of 9 T in the x = 1 alloy. An increase in Si content increases the exchange bias field. The exchange bias field and the coercive field strongly depend on the Si content. For the x = 4 alloy, an exchange bias field of 354 Oe is observed. In addition, a negative magnetoresistance is observed in this alloy system, which decreases with Si content. The increase in resistivity at the martensitic transition could be attributed to the formation of superzone boundary gaps that changes the density of states near the Fermi surface.

Intermartensitic Transformation in Ni54.8Mn23.2Ga21.7 Ferromagnetic Shape Memory Alloy Nano Powder

The Ni-Mn-Ga Ferromagnetic shape memory alloys are becoming an important element of sensor and actuator materials due to their large magnetic-field-induced strain and shape memory effect in recent years. The martensitic transformation temperature of the Heusler alloy system has been intensively studied. The Ni54.8Mn23.2Ga21.7 irregular agglomerated nanoparticles were prepared by ball milling method, and characterized by x-ray diffraction, differential scanning calorimetry and transmission electron microscopy techniques. These particles reveal a disordered non modulated face centered tetragonal structure. For the first time, nanopowder has been found to undergo a sequence of temperature-induced intermartensitic transformation in addition to the martensitic transformation on cooling. It is believed that relaxation of internal stresses is the main issue in this case. The resistivity of the material has been measured on cooling from 450 K to 300 K. It is observed that the resistivity is maximum in the nano crystalline state and it decreases substantially with decrease of the temperature. Both amorphous and nanocrystalline material coexisted in the ball-milled sample, such a nanomagnetic structure is considered to be ideal for soft nanomagnetic material.

Interdiffusion in Heusler film epitaxy on GaAs(001)

We report the role of interdiffusion in molecular beam epitaxy of the binary Heusler alloy system Fe${}_{1\ensuremath{-}x}$Si${}_{x}$/GaAs(001), employing a variety of complementary techniques that, in their combination, provide quantitative insight into the dynamics of the involved processes. The main properties of the investigated Fe${}_{0.84}$Si${}_{0.16}$ and Fe${}_{0.76}$Si${}_{0.24}$ films---growth, epitaxy, crystallographic order, interface quality, saturation magnetization, coercive field, and magnetic anisotropy---are in perfect agreement with the literature. Additionally, our results reveal a strong interdiffusion of Fe and Si into the GaAs substrate as well as of As and Ga into the Fe${}_{1\ensuremath{-}x}$Si${}_{x}$ films, creating intermixed layers of 2--3-nm thickness in both film and substrate. Interdiffusion is dominant already at moderate growth temperatures required for crystallographic ordering, thus demanding new concepts including appropriate diffusion barriers for the development of ferromagnet/semiconductor hybrid systems.

Influence of Ni/Mn concentration on the structural, magnetic and magnetocaloric properties in Ni50−xMn37+xSn13 Heusler alloys

We report the structure, magnetism and magnetic entropy change in a Mn-rich Ni50−xMn37+xSn13 Heusler alloy system in the composition range 0 ⩽ x ⩽ 4. An excess Mn content stabilizes the cubic austenite phase at room temperature. Martensitic transition decreases from 305 to 100 K with increasing Mn concentration (x: 0 → 4) and also it was found to shift to a lower temperature with the application of a higher magnetic field. The exchange bias blocking temperature was found to decrease drastically from 149 to 9 K with increasing Mn concentration. A large magnetic entropy change (ΔSM) of 32 J kg−1 K−1 has been achieved for a field change of 5 T in the x = 3 alloy.

Magneto-thermal and magneto-transport behavior around the martensitic transition in Ni 50− xCo xMn 40Sb 10 ( x = 9, 9.5) Heusler alloys

The structural, magnetic, magnetocaloric and magnetoresistance behavior in Ni 50− x Co x Mn 40Sb 10 ( x = 9 and 9.5) has been studied around the magnetostructural transition. The X-ray diffraction results at room temperature show that the compound with x = 9 is predominantly martensitic whereas the one with x = 9.5 is austenite in nature. The maximum magnetic entropy change of 9.2 J kg −1 K −1 was achieved in x = 9.5 at 246 K in a field of 50 kOe and a value of 8.2 J kg −1 K −1 was found near room temperature for x = 9. The effective refrigeration capacity of 106 J/kg has been calculated for x = 9, which is larger than the same observed in other Ni–Mn based Heusler alloy systems. A large magnetoresistance of 25% was obtained around room temperature for x = 9. The large difference in the magnetization between the martensitic and austenite phases is responsible for the large magnetocaloric effect and magnetoresistance in these compounds.

Structure Stability and Magnetic Properties of Ni<sub>2</sub>XGa (X = Mn, Fe, Co) Ferromagnetic Shape Memory Alloys by DFT Approach

In this paper, we report some ab initio calculation results of three perfect stoichiometric alloy systems Ni2XGa (X = Mn, Fe, Co). The calculations have been performed on optimizing the crystal structures in both austenitic and martensitic phases, the electronic density of states (DOS), the magnetic properties and the difference of charge distributions on considering that Mn, Fe, Co are adjacent to each other within one period in the periodic table of elements, and they are all ferro-magnetic. We have shown the impact of these magnetic elements on several properties of each alloy. Study on these three Heusler alloy systems is important in view of property prediction.

Observation of enhanced exchange bias behaviour in NiCoMnSb Heusler alloys

We report the observation of large exchange bias (EB) in Ni50−xCoxMn38Sb12 Heusler alloys with x = 0, 2, 3, 4, 5, which is attributed to the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases in the martensitic phase. The phase coexistence is possibly due to the supercooling of the high temperature FM phase and the predominant AFM component in the martensitic phase. The presence of EB is well supported by the observation of the training effect. The EB field increases with Co concentration. The maximum value of 480 Oe at T = 3 K is observed in x = 5 after field cooling in 50 kOe, which is almost double the highest value reported so far in any Heusler alloy system. Increase in the AFM coupling after Co substitution is found to be responsible for the increase in the EB.

Large anisotropic magnetoresistance of ruthenium-based Heusler alloys

Anomalous magnetic behavior was found in the polycrystalline Heusler alloy system Ru2Mn1−xFexGe. Samples of x=0 and 1.0 show no magnetoresistance (MR); however, the anisotropic MR effect is observed for x=0.5 (MR ratios of −4% and +2% are observed under parallel and perpendicular configurations of applied field and applied current, respectively). Moreover, magnetic measurement revealed that the sample of x=0 is antiferromagnetic, whereas the samples of x=0.5 and 1.0 are ferromagnetic with Curie temperatures of 270 and 490 K, respectively. The saturation magnetization and coercivity at 5 K are 3.1μB/f.u. and negligibly small for x=1.0 and 1.8μB/f.u. and 0.1 T for x=0.5, respectively. The MR effect is explained by a model in which antiferromagnetic and ferromagnetic domains coexist.

Hybrid Spintronic Structures With Magnetic Oxides and Heusler Alloys

Hybrid spintronic structures, integrating half-metallic magnetic oxides and Heusler alloys with their predicted high spin polarization, are important for the development of second-generation spintronics with high-efficient spin injection. We have synthesized epitaxial magnetic oxide Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> on GaAs(100) and the unit cell of the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> was found to be rotated by 45 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> to match the gallium arsenide GaAs. The films were found to have a bulk-like moment down to 3-4 nm and a low coercivity indicating a high-quality magnetic interface. The magnetization hysteresis loops of the ultrathin films are controlled by uniaxial magnetic anisotropy. The dynamic response of the sample shows a heavily damped precessional response to the applied field pulses. In the Heusler alloy system of Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnGa on GaAs, we found that the magnetic moment was reduced for thicknesses down to 10 nm, which may account for the lower than expected spin-injection efficiency from the spin-light-emitting diode structures. Using the element-specific technique of X-ray magnetic circular dichroism (XMCD), the reduced spin moments were found to originate from the Mn rather than the Co atoms, and the improvement of the interface is thus needed to increase the spin injection efficiency in this system. Further studies of the I-V characteristics of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /GaAs(100) and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO/GaAs(100) show that the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -based spintronic structures have a well-defined Schottky or tunneling barrier of moderate barrier height, which is encouraging for high-efficient spin injection.

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.

Phase equilibria and stability of B2 and L2 1 ordered phases in the Co–Fe–Ga Heusler alloy system

The phase equilibria, A2/B2 and B2/L2 1 order–disorder transitions and ferromagnetic/paramagnetic transition on the Co–Fe side of the Co–Fe–Ga system were examined by electron probe microanalysis (EPMA), differential scanning calorimetric (DSC) measurement and vibrating sample magnetometer (VSM). The equilibrium compositions of interrelations mainly among γ (A1: disordered fcc-Co), α (A2: disordered bcc-Fe), β (B2: ordered bcc-CoGa or FeGa) and β′ (L2 1 or D0 3: ordered bcc-Co 2FeGa or Fe 3Ga) phases were determined using diffusion couples and two-phase bulk specimens. It was confirmed that a bcc single-phase region composed of α, β and β′ at 700–1000° C exists in a wide composition range and that the ferromagnetic region appearing on the Co–Fe side extends to the center of the Gibbs triangle with decreasing temperature. On the other hand, the β′ phase appears along the Fe 3Ga–Co 3Ga section and the maximal critical temperature of the B2/L2 1 order–disorder transformation was determined to be 825 °C at a stoichiometric composition expressed as Co 2FeGa. The obtained phase diagram was examined in comparison with that of the Co–Fe–Al system.

Phase equilibria and stability of B2 and L2 1 ordered phases in the vicinity of half-metallic composition of Co–Cr–Ga Heusler alloy system

Recently, half-metallic properties have been extensively investigated from theoretical and practical viewpoints. In the present study, the phase equilibria, A2/B2 and B2/L2 1 order–disorder transition, and ferromagnetic/paramagnetic transition in the Co-rich portion, that is, in the vicinity of half-metallic composition, for Co–Cr–Ga system were examined by electron probe microanalysis (EPMA), differential scanning calorimetric (DSC) measurement, and transmission electron microscope (TEM). The compositions of the γ (A1; fcc-Co), α (A2; bcc-Cr), β (B2; CoGa), ɛ (A3; hcp-Co), and σ (D8 b) phases in equilibrium, the critical compositions corresponding to the A2/B2 continuous order–disorder transition, and the critical temperatures of the B2/L2 1 order–disorder transition and the paramagnetic/ferromagnetic transition in the L2 1 phase region have been determined. It has been confirmed that the β single-phase region at 1000 °C exists in a wide composition range, and the B2/L2 1 order–disorder transition temperature is higher than that of the Co–Cr–Al system. Furthermore, it has been disclosed that the phase stability of the CoGa B2 phase, estimated from the Bragg–Williams–Gorsky (BWG) calculation as a linear extrapolation of the continuous order–disorder transition temperatures, is lower than that of CoAl.

Approach of the state from antiferromagnetic to heavy fermion in CeIn(Ag 1− xCu x) 2 under pressure

The electrical resistivity has been studied in the Heusler alloy system CeIn(Cu 1− x Ag x ) 2 under pressure up to 20 kbar. Under pressure, the Néel temperature, T N , decreases for x = 0.75 and 0.85 but increases for N = 0.50, T max (resistivity maximum temperature) decreases for x = 0.50 and 0.75 while for x = 0.85 it increases above 10 kbar. These results are analyzed and discussed according to Doniach's Kondo-necklace model. The result shows that the system approaches to the pure heavy-fermion state (for x = 0.75 and 0.85) while it approaches to the manifested antiferromagnetic state (for x = 0.50) due to the pressure-induced hybridization.