Half-metallic full-Heusler Alloy Research Articles

Enhancement of the thermoelectric performance of half-metallic full-Heusler Mn2VAl alloys via antisite defect engineering and Si partial substitution

A half-metallic full-Heusler Mn2VAl alloy is a potential p-type thermoelectric material that can directly generate electricity from waste heat via the Seebeck effect. For practical use, the Seebeck coefficient S of Mn2VAl should be increased while maintaining a high electrical conductivity σ from its half-metallic character. Herein, we achieved this objective through antisite defect engineering. Theoretically, it was predicted that the S was maximized by regulating partial density of states of majority-spin sp-electrons through the control of the fraction of antisite defect, fAD, between V and Al atoms in Mn2VAl. Experimentally, a significant increase in S and a slight decrease in σ were observed for an Mn2VAl sample with an optimal fAD = 33%, enhancing the thermoelectric power factor PF by 2.7 times from an Mn2VAl sample with fAD = 14%. Furthermore, we combined the antisite defect engineering with a partial substitution method. An Mn2V(Al0.96Si0.04) sample with fAD = 33% exhibited the highest PF = 4.5 × 10−4 W·m−1·K−2 at 767 K among the samples. The maximum dimensionless figure-of-merit zT of the Mn2V(Al0.96Si0.04) sample with fAD = 33% was measured to be 3.4 × 10−2 at 767 K, which is the highest among the p-type half-metallic full-Heusler alloys.

Temperature dependent partially compensated to nearly fully compensated magnetic state in half-metallic full Heusler alloy, Mn1.2Fe1.18V0.62Al

We report experimental and theoretical studies of magnetization compensation phenomenon in quaternary Heusler alloy Mn1.2Fe1.18V0.62Al by employing dc magnetization, neutron depolarization, neutron diffraction, specific heat, electrical resistivity, and electronic structure calculation. In this ferrimagnetic alloy system with ordering temperature TC = 360 K, the asymmetric thermal variations of the site magnetic moments provide an evidence of the partially compensated magnetic state of the system down to ∼ 48 K. However, below 48 K, the system exhibits its magnetic ground state with the negligibly small ordered moment of 0.05(5) μB/f.u. at 10 K, as obtained from the neutron diffraction study. Nearly full recovery of neutron beam polarization at 4 K in the neutron depolarization study also infers a nearly fully compensated magnetization state in this Heusler alloy. Temperature dependent specific heat and resistivity measurements also corroborate with the dc magnetization study representing 48 K as a turning point at which the system reaches its ground state. The electronic structure calculations, using the SPR-KKR Green’s function approach, verify the half-metallic nature of the present system. The net magnetic moment (0.06 μB/f.u.) obtained from the theoretical calculations as well as neutron diffraction experiments (0.05 μB/f.u.) is in good agreement with the Slater-Pauling rule (0.06 μB/f.u.). This small magnetic moment of the system (Mn0.82Fe1.18)(Mn0.38V0.62)Al proclaims to be a nearly compensated ferrimagnetic system with 23.94 valence electrons. Such magnetically compensated systems with finite spin polarization could be favorable for spintronics applications.

Computational discovery of the novel half-metallic full-Heusler alloys Mn2LiAs and Mn2LiSb

The structural, elastic, electronic and magnetic properties of the novel full-Heusler alloys Mn2LiAs and Mn2LiSb are determined to explore their structural stability and half-metallicity. The method used is the first-principles method based on density functional theory. Calculations show that the Hg2CuTi-type structure is more energetically favourable than the conventional Cu2MnAl-type structure and is thermodynamically and mechanically stable for both alloys. The Hg2CuTi-type Mn2LiAs and Mn2LiAs alloys are half-metallic ferrimagnets and have a total magnetic moment of 2.000 μB per formula unit. They follow the Slater-Pauling rule and have an indirect energy gap in the spin-down channels with values of 1.176 and 1.269 eV at equilibrium lattice constants of 5.793 and 6.132 Å, respectively. The half-metallicity is kept within the limits of a wide lattice constant range. The spin polarisation ratio is 100% and the total magnetic moment is 2.000 μB. Mn2LiAs and Mn2LiSb alloys, which can be synthesised via experiments, are promising candidates for spintronic applications.

Investigation of new d0 half-metallic full-heusler alloys N2BaX (X=Rb, Cs, Ca and Sr) using first-principle calculations

The first-principle density functional theory (DFT) calculations were employed to investigate the structural, elastic, electronic, magnetic properties and half-metallicity of N2BaX (X = Rb, Cs, Ca and Sr) new d0 half-metallic full-Heusler compounds with Hg2CuTi-type and Cu2MnAl-type structures within generalized gradient approximation Perdew, Burke, and Ernzerhof GGA-PBE and the modified Becke-Johnson potential mBJ-GGA-PBE for the exchange correlation potential. Total energy calculations show that formation of these alloys in Cu2MnAl-type structure are energetically favorable and are more stable in the ferromagnetic phase than Hg2CuTi-type structure. We have found that N2BaX (X = Rb, Cs, Ca and Sr) alloys in Cu2MnAl-type structure are half-metallic ferromagnets with an integer total magnetic moment at the equilibrium lattice constant, which were in agreement with Slater–Pauling rule (Mtot = 16-Ztot). The half-metallicity was discussed for N2BaX using the energy band structure, our calculations show that the minority spin channel has metallic nature and the majority spin channel has a wide semiconducting gap of 3.78, 4.60, 5.44 and 3.74eV and they has a huge half-metallic gap of 1.56, 2.33, 2.39 and 1.59 eV for N2BaCa, N2BaCs, N2BaRb and N2BaSr respectively with the mBJ-GGA-PBE.

First-principles calculation of the effects of partial alloy disorder on the static and dynamic magnetic properties ofCo2MnSi

On the basis of fully relativistic Korringa-Kohn-Rostoker calculations and in conjunction with the coherent potential approximation and the linear response formalism, we present a complete ab initio study of the influence of alloy disorder on the static and dynamic (Gilbert damping) magnetic properties and on the electronic structure of the half-metallic full-Heusler alloy ${\mathrm{Co}}_{2}\mathrm{MnSi}$. We discuss in particular partial atomic disorders intermediate between the main crystal phases $\text{L}{2}_{1}$, B2, A2, and $\text{D}{0}_{3}$ of this alloy. We compare our results with homemade experiments and measurements from the literature, and conclude that the presence of a partial $\text{D}{0}_{3}$-like disorder could explain the relatively high value of the Gilbert damping parameter and the lack of half-metallicity measured in real samples, in which alloy disorder cannot be totally avoided.

High-pressure and high-temperature physical properties of half-metallic full-Heusler alloy Mn2RuSi by first-principles and quasi-harmonic Debye model

First-principles calculations based on density functional theory and quasi-harmonic Debye model are used to investigate the high-pressure and high-temperature physical properties, including the lattice constant, magnetic moment, density of states, pressure-volume-temperature relationship, bulk modulus, thermal expansivity, heat capacity, and Grüneisen parameter for the new Mn-based full-Heusler alloy Mn2RuSi in CuHg2Ti-type structure. The optimized equilibrium lattice constant is consistent with experimental and other theoretical results. The calculated total spin magnetic moment remains an integral value of 2.0μB in the lattice constant range of 5.454–5.758Å, and then decreases very slowly with the decrease of lattice constant to 5.333Å. By the spin resolved density of states calculations, we have shown that Mn2RuSi compound presents half-metallic ferrimagnetic properties under the equilibrium lattice constant. The effects of temperature and pressure on bulk modulus, thermal expansivity, heat capacity, and Grüneisen parameter are opposite, which are consistent with a compression rate of volume. Furthermore, the results show that the effect of temperature is larger than pressure for heat capacity and the effect of high temperature and pressure on thermal expansion coefficient is small. All the properties of Mn2RuSi alloy are summarized in the pressure range of 0–100GPa and the temperature up to 1200K.

Structural and Thermoelectric Properties of Ternary Full-Heusler Alloys

The thermoelectric properties of ternary full-Heusler alloys, Co2YZ, which are in a ferromagnetic state up to high temperature above 300 K, were measured and are discussed in terms of the crystal structure and electronic states. Among the full-Heusler alloys studied, the Co2MnSi sample exhibited the highest absolute value of Seebeck coefficient and also the highest electrical conductivity in the temperature range from 300 K to 1023 K. The highest power factor of 2.9 × 10−3 W/m-K2 was obtained for the Co2MnSi sample at 550 K, demonstrating the potential of half-metallic full-Heusler alloys as thermoelectric materials.

Magnetic properties of a Pt/Co2FeAl/MgO structure with perpendicular magnetic anisotropy

Microstructures and magnetic properties of Ta/Pt/Co2FeAl (CFA)/MgO multilayers are studied to understand perpendicular magnetic anisotropy (PMA) of half-metallic full-Heusler alloy films. PMA is realized in a 2.5-nm CFA film with B2-ordered structure observed by a high resolution transmission electron microscope. It is demonstrated that a high quality interface between the ferromagnetic layer and oxide layer is not essential for PMA. The conversions between in-plane anisotropy and PMA are investigated to study the dependence of magnetic moment on temperature. At the intersection points, the decreasing slope of the saturation magnetization (Ms) changes because of the conversions. The dependence of Ms on the annealing temperature and MgO thickness is also studied.

Effect of interfacial strain on spin injection and spin polarization of Co2CrAl/NaNbO3/Co2CrAl magnetic tunneling junction

First-principles calculations were carried out to investigate interfacial strain effects on spin injection and spin polarization of a magnetic tunnel junction consisting of half-metallic full-Heusler alloy Co2CrAl and ferroelectric perovskite NaNbO3. Spin-dependent coherent tunneling was calculated within the framework of non-equilibrium Green's function technique. Both spin polarization and tunnel magnetoresistance (TMR) are affected by the interfacial strain but their responses to compressive and tensile strains are different. Spin polarization across the interface is fully preserved under a compressive strain due to stronger coupling between interfacial atoms, whereas a tensile strain significantly enhances interface states and leads to substantial drops in spin polarization and TMR.

First-principles study of half-metallic properties of the Heusler alloy Ti2CoGe

First-principles calculations have been performed on the electronic structures and magnetic properties of a new Ti2Co-based full-Heusler alloy Ti2CoGe. The calculations predict the Ti2CoGe is a half-metallic ferromagnet at the equilibrium lattice constant with the minority-spin energy gap of 0.60eV. It is found that the total magnetic moment (Mt) and the number of valence electrons (Zt) in Ti2CoGe obey a new Slater–Pauling (SP) rule of Mt=Zt−18 and the rule also can be applied to other Ti2Co-based half-metallic full-Heusler alloys. The Ti2CoGe alloy keeps a 100% polarization at Fermi level and maintains the half-metallic character for lattice constants ranging between 6.05 and 6.67Å.

Chemical ordering dependence of interlayer exchange coupling in Co-Mn-Si/Cr/Co-Mn-Si trilayer structures

The effect of chemical ordering of a half-metallic full-Heusler alloy ${\text{Co}}_{2}\text{MnSi}$ (CMS) on interlayer exchange coupling (IEC) behavior was investigated in Co-Mn-Si(20 nm)/Cr(1.2 nm)/Co-Mn-Si(7 nm) trilayers. Two types of experimental investigations were employed, namely, the variations in IEC with changing the annealing temperature of the bottom CMS from room temperature to $500\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and with changing the Co concentration in top and bottom Co-Mn-Si layers. The $90\ifmmode^\circ\else\textdegree\fi{}$ coupling parameter ${J}_{2}$ and the bilinear coupling parameter ${J}_{1}$ were estimated from the numerical simulations of magnetization curves. Only the strong $90\ifmmode^\circ\else\textdegree\fi{}$ coupling with almost the same strength was observed in both the completely $B2$ ordered and in a mixture of ordered $L{2}_{1}$ and $B2$ structures. The degree of $B2$ ordering as well as the strength of $90\ifmmode^\circ\else\textdegree\fi{}$ coupling was observed to decrease simultaneously with the Co concentration in Co-Mn-Si and $180\ifmmode^\circ\else\textdegree\fi{}$ coupling appeared in highly Co antisite defective $(A2)$ structures, i.e., for the Co concentration over $73\text{ }\text{at}\text{.}\text{ }\mathrm{%}$. This observation showed a clear relationship between the degree of $B2$ ordering and the IEC behavior in full-Heusler Co-Mn-Si-based structures.

Formation of Si- and Ge-based Full-Heusler Alloy Thin Films using SOI and GOI Substrates for the Half-metallic Source and Drain of Spin Transistors

The paper presents a novel preparation technique for Si- and Ge-based half-metallic full-Heusler alloy thin films, utilizing silicon-on-insulator (SOI) and germanium-on-insulator (GOI) substrates, respectively. Full-Heusler Co2FeSi (Co2FeGe) alloy thin films were successfully formed by thermally activated silicidation (germanidation) reaction between an ultra-thin SOI (GOI) layer and Co/Fe layers deposited on it. This technique can easily produce fully ordered L21 structure that is necessary for the half-metallicity of full-Heusler alloys. The proposed technique is compatible with metal source/drain formation process in advanced CMOS technology and would be applicable to the fabrication of the half-metallic source/drain of MOSFET type of spin transistors.

Polarization reduction in half-metallic Heusler alloys: the effect of point defects andinterfaces with semiconductors

Half-metallic full-Heusler alloys represent a promising class of materials for spintronicapplications. However, (i) intrinsic point defects in Heusler compounds can be detrimentalwith respect to their predicted 100% spin polarization at the Fermi level and (ii) whenjoined to mainstream semiconductors the presence of interface states—whichdestroys half-metallicity—can degrade their performance. Here, we present anoverview of recent first-principles calculations performed to explore both theseissues. In particular, we focus on ab initio FLAPW calculations performed forCo2MnGe and Co2MnSi in the presence of intrinsic defects (such as stoichiometric atomic swaps as well asnon-stoichiometric antisites) and when interfaced with GaAs and Ge. Our findings showthat Mn antisites, due to their low formation energies, can easily occur, in excellentconsistency with experimental observations, and that they do not destroy half-metallicity.On the other hand, Co antisites, which also show a modest formation energy, give rise todefect states at the Fermi level. As for the [001]-ordered interfaces, we show that the stronghybridization in proximity to the junction gives rise to rather broad interfacestates that locally destroy half-metallicity. However, the bulk gaps (both in theminority spin channel for the Heusler alloy and for both spin channels in thesemiconducting side) are fully recovered within a few layers away from the junction.

Effect of substitution of Mn with Fe or Cr in Heusler alloy ofCo2MnSn

Crystallographic and magnetic properties ofCo2Mn1−xFexSn and Co2Mn1−yCrySn Heusler alloys were studied using x-ray diffraction, scanning electron microscopy withenergy dispersive x-ray analysis, magnetization measurements and Mössbauer spectroscopy.We found that the intermetallic compounds crystallize into a single phase with theL 21-type Heusler structure at the concentration of0≤x≤0.5 for Co2Mn1−xFexSn and 0≤y≤0.3 for Co2Mn1−yCrySn. The spontaneous magnetization increases when Mn is substituted with Fe and decreaseswhen Mn is substituted with Cr. The values of hyperfine field at the Sn nuclear sites inCo2Mn1−xFexSn and Co2Mn1−yCrySn also increase or decrease in combination with the size of magnetization. Thesemeasurements suggest that the Slater–Pauling behaviour in half-metallic full-Heusleralloys provides a good description of the substitution effect of Mn with Fe or Cr.