Mn-based Heusler Alloys Research Articles

Structural and Magnetic Properties of Mn<sub>2</sub>NiZ (Z = Ga, In, Sn, Sb) Heusler Alloys from <i>Ab Initio</i> Calculations

The structural and magnetic properties of Mn-based stoichiometric Heusler alloys have investigated by means of ab initio calculations in framework of the density functional theory. First principles electronic structure calculations have shown that Mn2NiZ (Z = Ga, In, Sn, Sb) alloys are ferrimagnets with antiparallel alignment between the Mn atoms. The martensitic transition can be realized in Mn2NiGa and Mn2NiSn alloys with tetragonal ratio of 1.27 and 1.16, respectively. Calculated properties are in a good agreement with available experimental data.

Effect of substitution on elastic stability, electronic structure and magnetic property of Ni–Mn based Heusler alloys: An ab initio comparison

First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson’s ratio and Young’s modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt’s and Reuss’s method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively.

Spin Stiffness Constant of Half-metallic Ferrimagnet in Mn-based Heusler Alloys

In order to investigate the spin stiffness constant, D, of Mn-based Heusler alloys with half-metallic density of states, magnetic measurements were properly carried out. The Curie temperature, TC, of Mn2VAl, Mn50V20Al30 and Mn2CoGa were 768, 738 and 740K, respectively. The value of D was evaluated from the temperature, T, dependence of the spontaneous magnetization in each specimen below 150K, where linear dependence of T3/2 was observed. The D values for these materials were obtained to be 5.34, 4.38 and 5.67 meV·(nm)2 and significantly larger than that for Co-based Heusler alloys reported previously. The strong exchange interaction would be the reason that the Mn-based Heusler alloys indicate high TC even though they have not so large total magnetic moment comparing to that of Co-based Heusler alloys.

Extraordinary magnetic and structural properties of the off-stoichiometric and the Co-doped Ni2MnGa Heusler alloys under high pressure

To show and to describe a contrast between the magnetic and structural properties of the stoichiometric Ni2MnGa alloy and the off-stoichiometric and the Co-doped Ni–Mn–Ga alloys, we report the effect of high hydrostatic pressure (up to 1.2GPa) on magnetization and the critical temperatures of structural and magnetic transitions in the off-stoichiometric Ni50−xMn25+x+yGa25−y and the Co-doped Ni50−xCoxMn25+yGa25−y Heusler alloys. A huge decrease in their saturation magnetization MM(5K) under pressure was observed with an extreme value of pressure parameter dlnMM(5K)/dP=−56×10−3GPa−1 in the case of the Ni39Co11Mn34Ga16 alloy. The Curie temperature TCA increases under pressure in all the studied alloys. Almost identical values of dTCA/dP=+4.7÷+5.9KGPa−1 point to a normalized steepness of a dependence of exchange interactions on interatomic distances. The values of paramagnetic effective moments, meff, are identical in both structural phases of the alloys and they lie in a narrow interval from 4.6μB to 5.7μB. Furthermore, the Co-doped alloys show a very strong pressure dependence on the structural critical temperature TM–A, with values of dTM–A/dP up to +34KGPa−1, the highest reported values for the Ni–Mn-based Heusler alloys. The results are discussed in light of the fundamental magnetic interactions in the Heusler alloys and from the point of view of a further exploration of their multifunctional applications.

Magnetocaloric effect and multifunctional properties of Ni–Mn-based Heusler alloys

The studies of magnetocaloric properties, phase transitions, and phenomena related to magnetic heterogeneity in the vicinity of the martensitic transition (MT) in Ni–Mn–In and Ni–Mn–Ga off-stoichiometric Heusler alloys are summarized. The crystal structure, magnetocaloric effect (MCE), and magnetotransport properties were studied for the following alloys: Ni50Mn50−xInx, Ni50−xCoxMn35In15, Ni50Mn35−xCoxIn15, Ni50Mn35In14Z (Z=Al, Ge), Ni50Mn35In15−xSix, Ni50−xCoxMn25+yGa25−y, and Ni50–xCoxMn32−yFeyGa18. It was found that the magnetic entropy change, ΔS, associated with the inverse MCE in the vicinity of the temperature of the magneto-structural transition, TM, persists in a range of (125-5)J/(kgK) for a magnetic field change ΔH=5T. The corresponding temperature varies with composition from 143 to 400K. The MT in Ni50Mn50−xInx (x=13.5) results in a transition between two paramagnetic states. Associated with the paramagnetic austenite-paramagnetic martensite transition ΔS=24J/(kgK) was detected for ΔH=5T at T=350K. The variation in composition of Ni2MnGa can drastically change the magnetic state of the martensitic phase below and in the vicinity of TM. The presence of the martensitic phase with magnetic moment much smaller than that in the austenitic phase above TM leads to the large inverse MCE in the Ni42Co8Mn32−yFeyGa18 system. The adiabatic change of temperature (ΔTad) in the vicinity of TC and TM of Ni50Mn35In15 and Ni50Mn35In14Z (Z=Al, Ge) was found to be ΔTad=−2K and 2K for ΔH=1.8T, respectively. It was observed that |ΔTad|≈1K for ΔH=1T for both types of transitions. The results on resistivity, magnetoresistance, Hall resistivity in some In-based alloys are discussed.

Effect of electron density on the martensitic transition in Ni–Mn–Sn alloys

The structure and martensitic transition (MT) temperatures of Ni 44Mn 45Sn 10R (R = Al, Ga, In and Sn) alloys were investigated. Valence electron concentration and cell volume are two main factors affecting the MT temperatures in Ni–Mn based Heusler alloys. Our results indicated that the MT temperatures do not vary monotonously with either of the two factors, but increase with the increasing of electron density. Combining the two factors, electron density might be a more appropriate parameter to describe the variations of MT temperatures.

Structural-induced antiferromagnetism in Mn-based full Heusler alloys: The case of Ni2MnAl

We employ ab initio electronic structure calculations and a model Heisenberg Hamiltonian, and show that the Heusler alloy Ni2MnAl exhibits a ferrromagnetic–antiferromagnetic phase transition upon Mn–Al disorder. The transition is triggered by the smaller Mn–Mn nearest-neighbors distance in accordance to the Bethe–Slater curve. Our results explain available experimental data and show that the prevention of disorder is essential to achieve maximum performance in Heusler-based devices.

First-principles investigation of the electronic structure and magnetism of Heusler alloys CoMnSb and Co 2MnSb

The spin-polarized electronic band structures, density of states (DOS), and magnetic properties of Co–Mn-based Heusler alloys CoMnSb and Co 2MnSb have been studied by first-principles method. The calculations were performed by using the full-potential linearized augmented plane wave (FP-LAPW) within the spin-polarized density functional theory and generalized gradient approximation (GGA). Calculated electronic band structures and the density of states are discussed in terms of the contribution of Co 3d 74s 2, Mn 3d 54s 2, and Sb 5s 25p 3 partial density of states and the spin magnetic moments were also calculated. The results reveal that both CoMnSb and Co 2MnSb have stable ferromagnetic ground state. They are ideal half-metallic (HM) ferromagnet at their equilibrium lattice constants. The calculated total spin magnetic moments are 3μ B for CoMnSb and 6μ B for Co 2MnSb per unit cell, which agree with the Slater–Pauling rule quite well.

A first principles study on the full-Heusler compound Mn2CuSb

Electronic structure calculations from first-principles are employed to design half-metallic ferrimagnets susceptible of finding applications in spintronics. The electronic structure and magnetism properties of a new Mn-based Heusler alloy Mn2CuSb are studied and both possible L21 structures (CuHg2Ti and AlCu2Mn types) are taken into account. It is found that the CuHg2Ti-type structure is energetically more favorable than the AlCu2Mn-type structure and presents half-metallic character. Calculations show that their total spin moment is −2.00μB for a wide range of equilibrium lattice constants and the total spin magnetic moment is attributed mainly to the two Mn atoms. Simultaneously, the small spin magnetic moments of Cu and Sb atoms are parallel and the compound is ferrimagnets. The total spin moment mainly origins from the antiparallel configurations of the Mn partial moments. The CuHg2Ti-type Mn2CuSb alloy keeps a 100% of spin polarization of states at the Fermi level. Our findings suggest that Mn2CuSb may be a promising material for future spintronic applications.

Half-metallic ferrimagnetism in Full-Heusler alloy Mn 2CuMg

In the paper Ab initio electronic structure calculations are applied to study the electronic structure and magnetism properties of a new Mn-based Heusler alloy Mn 2CuMg. We take into account both possible L 2 1 structures (CuHg 2Ti and AlCu 2Mn types). The CuHg 2Ti-type structure is found to be energetically more favorable than the AlCu 2Mn-type structure and presents half-metallic ferrimagnetism. However, the case of exchanging X with Y atoms in generic formula loses its half-metallicity due to the symmetric surroundings. Calculations show that their total spin moment is − 1 μ B for a wide range of equilibrium lattice constants and the total spin magnetic moment is attributed mainly to the two Mn atoms, while the Cu atom is almost non-magnetic. A small total spin moment origins from the antiparallel configurations of the Mn partial moments. The CuHg 2Ti-type Mn 2CuMg alloy keeps a 100% of spin polarization of conduction electrons at the Fermi level, thus opening the way to engineer new half-metallic alloys with the desired magnetic properties.

Half-metallic ferrimagnetism in Mn 2CuGe

We study magnetism properties and the electronic structure of a new Mn-based Heusler alloys Mn 2CuGe using ab initio electronic structure calculations. We take into account both possible L 2 1 structures (CuHg 2Ti and AlCu 2Mn types). The CuHg 2Ti-type structure is found to be energetically more favorable than the AlCu 2Mn-type structure and exhibits half-metallic ferrimagnetism. Calculations show that their total spin moment is − 1 μ B for a wide range of equilibrium lattice constants and magnetic moment mainly comes from the two Mn atoms, while the Cu atom is almost nonmagnetic. The small total moment comes from the antiparallel configurations of the Mn partial moments. And the CuHg 2Ti-type Mn 2CuGe alloy keeps a 100% of spin polarization at the Fermi level. Thus, the Mn 2CuGe is the compound of choice for further experimental investigations.

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.

Energies and Lifetimes of Magnons in Complex Ferromagnets: A First-Principle Study of Heusler Alloys

The energies and lifetimes of magnons in several Mn-based Heusler alloys are studied using linear response density functional theory. The number of the spin wave branches in Co(2)MnSi corresponds to the number of its magnetic sublattices in contrast with the NiMnSb case in which the induced Ni sublattice cannot support optical magnons. The half-metallicity of these systems results in long-living acoustic spin waves. The example of non-half-metallic Cu(2)MnAl shows that the hybridization with Stoner continuum leads not only to the damping of magnons but also to a renormalization of their energies.

Ferromagnetism in the Mn-based Heusler alloy Mn2NiSb

A Mn-based Heusler alloy Mn2NiSb has been synthesized. First-principles calculations and experimental measurements were employed to investigate its electronic structure and magnetism. Theoretical calculation indicates a ferromagnetic coupling in Mn2NiSb and a total spin moment of 4.21μB/f.u., which agrees quite well with the saturation moment at 5 K. This ferromagnetic coupling arises from the larger lattice constant and more valence electrons compared with other Mn-based Heusler alloys. The partial spin moments for Mn (A), Mn (B), and Ni are 0.6, 3.30, and 0.3μB, respectively. The Curie temperature of Mn2NiSb is 647 K, indicating a strong exchange interaction between Mn atoms.

Electronic structure and magnetism of Mn2CuAl: A first-principles study

Abstract The electronic structure and magnetism of a new Mn-based Heusler alloys Mn2CuAl has been studied by first-principles calculations. The calculations suggest that Mn2CuAl crystallizes in the Hg2CuTi type of structure, in which the Cu atoms have Al as nearest neighbors. As a consequence, the Mn atoms occupy two nearest neighbor sublattices A and B. Like the well known Heusler alloy Cu2MnAl, the magnetic moment of Mn2CuAl also comes from the two Mn atoms in the lattice, while the Cu atom is almost nonmagnetic. At equilibrium lattice constant, Mn2CuAl is a ferrimagnet with moment of 0.22μB. The partial spin moments of Mn (A) and Mn (B) are −3.52μB and 3.74μB, respectively. The small total moment comes from the antiparallel configurations of the Mn partial moments. With a small contraction of the lattice, the total moment becomes near zero and a half-metallic antiferromagnetic state is observed.

Magnetocaloric effects in Ni–Mn–X based Heusler alloys with X=Ga, Sb, In

The Mn-based Heusler alloys encompass a rich collection of useful materials from highly spin-polarized systems to shape memory alloys to magnetocaloric materials. In this work we have summarized our studies of magnetostructural transitions from paramagnetic austenite to ferromagnetic martesite phases at TMC in Ni 2MnGa-based alloys (Ni 2Mn 0.75Cu 0.25-xCo xGa, Ni 2Mn 0.70Cu 0.30Ga 0.95Ge 0.05, Ni 2Mn 1-xCu xGa, Ni 2+xMn 1-xGa, and Ni 2Mn 0.75-xCu xGa), and martensitic transitions from the ferromagnetic austenite to the martesite state in off-stoichiometric Ni–Mn–(In/Sb) Heusler alloys. The phase transition temperatures and respective magnetic entropy changes (Δ S) depend on composition in these systems and have been determined from magnetization measurements in the temperature interval 5–400 K, and in magnetic fields up to 5 T. It is shown that, depending on the composition and doping scheme the “giant” Δ S=40–60 J/(kgK) (for a field change of 5 T) can be observed in the temperature range (300–360 K) for the Ga-based alloys. The interplay between or coupling of the various transitions in Ni 2Mn(Mn,X) systems with X=Sb and In leads to exchange bias effects, giant magnetoresistance, and both inverse and “normal” magnetocaloric effects.

Role of conduction electrons in mediating exchange interactions in Mn-based Heusler alloys

Because of the large spatial separation of the Mn atoms in Heusler alloys $({d}_{\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Mn}}>4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$, the $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}3d$ states belonging to different atoms do not overlap considerably. Therefore, an indirect exchange interaction between Mn atoms should play a crucial role in the ferromagnetism of the systems. To study the nature of the ferromagnetism of various Mn-based semi- and full-Heusler alloys, we perform a systematic first-principles calculation of the exchange interactions in these materials. The calculation of the exchange parameters is based on the frozen-magnon approach. The Curie temperature is estimated within the mean-field approximation. The calculations show that the magnetism of the Mn-based Heusler alloys depends strongly on the number of conduction $sp$ electrons, their spin polarization, and the position of the unoccupied $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}3d$ states with respect to the Fermi level. Various magnetic phases are obtained depending on the combination of these characteristics. The magnetic phase diagram is determined at zero temperature. The results of the calculations are in good agreement with available experimental data. Anderson's $s\text{\ensuremath{-}}d$ model is used to perform a qualitative analysis of the obtained results. The conditions leading to a diverse magnetic behavior are identified. If the spin polarization of the conduction electrons at the Fermi energy is large and the unoccupied $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}3d$ states lie well above the Fermi level, a Ruderman-Kittel-Kasuya-Yoshida-type ferromagnetic interaction is dominating. On the other hand, the contribution of the antiferromagnetic superexchange becomes important if unoccupied $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}3d$ states lie close to the Fermi energy. The resulting magnetic behavior depends on the competition of these two exchange mechanisms. The calculation results are in good correlation with the conclusions made on the basis of the Anderson $s\text{\ensuremath{-}}d$ model which provides useful framework for the analysis of the results of first-principles calculations and helps to formulate the conditions for high Curie temperature.

Defects-driven appearance of half-metallic ferrimagnetism in Co–Mn-based Heusler alloys

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co 2MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co 1− x Mn x ] 2MnSi is of particular interest since Mn 3Si is known to crystallize in the Heusler L 2 1 lattice structure of Co 2MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.

Electronic aspects of the martensitic transition in Ni–Mn based Heusler alloys

The martensitic transformation temperature M s depends linearly on the valence electron concentration for Ni–Mn–X Heusler systems where X is a group III–group V element. However, the slopes of the linearity are different for alloys with different X species and increases either as X changes from Al to In (isoelectronic) or from In to Sb (increase in number of p electrons). We discuss the features in the M s vs e/a diagram and the relative stability of the various crystallographic phases of Ni–Mn based Heusler alloys.

Electronic structure and magnetic properties of Co- and Mn-based Heusler alloys and thin films

Electronic structure and magnetic properties of Co- and Mn-based Heusler alloys and thin films