We propose a ferromagnetic Heusler alloy that can switch between a metal and a half-metal. Thiseffect can provide tunable spintronics properties. Using the density functional theory (DFT) withreliable implementations of the electron correlation effects, we find Mn2ScSi total energy curvesconsisting of distinct branches with a very small energy difference. The phase at low lattice crystalvolume is a low magnetic half-metallic state while the phase at high lattice crystal volume is a highmagnetic metallic state. We suggest that the transition between half-metallic and metallic statescan be triggered by a triaxial contraction/expansion of the crystal lattice or by an external magneticfield if we assume that the lattice is cubic and remains cubic under expansion/contraction. However,the phase at high volume can also undergo an austenite-martensite phase transition because of thepresence of Jahn-Teller active3delectrons on the Mn atoms.
The magnetocaloric effect in the metamagnetic Ni43Mn37.9In12.1Co7 Heusler alloy is directly studied experimentally under the adiabatic and quasi-isothermal conditions in a magnetic field with induction of up to 14 T.
In this letter, we investigate the magnetic properties, electronic structures, Slater-Pauling behaviours of some quanternary Heusler alloys with 4d and 3d transition metal elements. The energy levels of the minority-spin electronic band structures for all our calculated alloys are discussed. We have summaried 21 quanternary Heusler alloys. Our calculations indicate that the half metals ZrCoCrSi, ZrCoCrIn, ZrCoFeSi, ZrFeVAl, ZrFeVGa and NbFeCrAl have large spin-flip gaps.
X-ray emission Kα1,2 spectra of Mn in Heusler alloys Co2MnMe (Me = Al, Ga, Sb), Ni2MnIn, Cu2MnAl were studied. Shifts of Mn Kα1,2 lines relatively pure Mn in high-energy region and low-energy shifts of binding energy Mn 2p XPS is detected. X-ray emission and XPS shifts are in qualitative agreement.
Half-Heusler alloys (MgAgSb structure) are promising thermoelectric materials. RNiSn half-Heusler phases (R=Hf, Zr, Ti) are the most studied in view of their thermal stability. The highest dimensionless figure of merit (ZT) obtained is ~1 in the temperature range ~450-900oC, primarily achieved in nanostructured alloys. Through proper annealing, ZT~1.2 has been obtained in a previous ZT~1 n-type (Hf,Zr)NiSn phase without the nanostructure. There is an appreciable increase in the power factor, decrease in charge carrier density, and increase in carrier mobility. The findings are attributed to th…
By using a multiscale approach based on first-principles density functional theory combined with atomistic spin dynamics, we investigate the electronic structure and magnetization dynamics of an inverse Heusler and a Heusler compound and their alloys, i. e. Mn$_{2-x}Z_x$CoAl and Mn$_{2-x}Z_x$VAl, where $Z$ = Mo, W, Os and Ru, respectively. A signature of the ferrimagnetic ordering of Mn$_{2}$CoAl and Mn$_{2}$VAl Heusler alloys is reflected in the calculated Heisenberg exchange constants. They decay very rapidly with the interatomic distance and have short range, which is a consequence of the e…
The ferromagnetic Heusler alloy $Ni_2MnGa$ had been of major interest in the past few years because of its magnetic properties which can be easily tuned. The $Ni_2MnGa$ Heusler alloys are intermetallic alloy with $L2_1$ structure. Here we report a detailed investigation of the effect of doping of Co and Mn in Ni2MnGa. Magnetic properties and electronic structure of $Ni_{2-x}Co_xMnGa_{1-y}Mn_y$ Heusler alloys have been studied by using Green's function-based KKR-CPA method based DFT calculations. We will show the magnetization can be tuned depending on the Co and Mn occumencies. We will also di…
Half-metallic ferromagnetic (HMF) materials show high spin polarization and are therefore interesting to researchers due to their possible applications in spintronic devices. In these materials, while one spin sub band has a finite density of states at the Fermi level, the other sub band has a gap. Because of their high Curie temperature and tunable electronic structure, HMF Heusler alloys have a special importance among the HMF materials. Full Heusler alloys with the stoichiometric composition X2YZ (where X and Y are the transition metals and Z is a sp element) have the cubic structure with f…
The authors demonstrated that L21-ordered full-Heusler Co2FeSi (CFS) alloy film with thickness of 100 nm were formed by facing targets sputtering (FTS) method at a substrate temperature TS = 300 deg C. Degrees of L21- and B2- order for the film were 0.37, and 0.96, respectively. Furthermore, full-Heusler CFS alloy thin films with perpendicular magnetic anisotropy (PMA) induced by MgO-interface magnetic anisotropy were successfully formed by the FTS method. The CFS/MgO stacking layers showed PMA when dCFS was 0.6 nm <= dCFS <= 1.0 nm. The PMA in these structures resulted from the CFS/MgO interf…
In this study we report a method for the preparation of freestanding magnetocaloric thin films. Non-stoichiometric Heusler alloys Ni-Mn-Sn, Ni-Co-Mn-Sn and Ni-Co-Mn-Al are prepared via sputter deposition. A sacrificial vanadium layer is added between the substrate and the Heusler film. By means of selective wet-chemical etching the vanadium layer can be removed. Conditions for the crystallization of Vanadium layers and epitaxial growth of the Heusler films are indicated. Magnetic and structural properties of freestanding and as-prepared films are compared in detail. The main focus of this stud…
MgO-barrier magnetic tunnel junctions with half-metallic Heusler alloy electrodes attracted much attentions for spintronics applications. However, a couples of issues related to materials still remain to be resolved for practical uses. Recently, quarterly equiatomic Heusler alloys attracted attentions as advanced Heusler alloys. CoIrMnZ (Z = Al, Si, Ga, and Ge) half-metallic Heusler alloys were designed and predicted to have moderate Curie temperatures and to be a lattice-matched with the MgO barrier, being advantageous to traditional Co2 Heusler alloys [T. Roy et al., J. Magn. Magn. Mater. 49…
We report a systematic investigation on the magnetization relaxation properties of iron-based half-metallic Heusler alloy Fe$_{2}$Cr$_{1-x}$Co_${x}$Si (FCCS) thin films using broadband angular-resolved ferromagnetic resonance. Band structure engineering through Co doping (x) demonstrated by first-principles calculations is shown to tune the intrinsic magnetic damping over an order of magnitude, namely 0.01-0.0008. Notably, the intrinsic damping constants for samples with high Co concentration are among the lowest reported for Heusler alloys and even comparable to magnetic insulator yttrium iro…
Heusler alloy Co2FeAl was employed as ferromagnetic layers in Co2FeAl/Ru/Co2FeAl synthetic antiferromagnet structures. The experimental results show that the structure with a Ru thickness of 0.45 nm takes on strongly antiferromagnetic coupling, which maintains up to 150 oC annealing for 1 hour. The structure has a very low saturation magnetization Ms of 425 emu/cc, a low switching field Hsw of 4.3 Oe and a high saturation field Hs of 5257 Oe at room temperature, which are favorable for application in ultrahigh density magnetic read heads or other magnetic memory devices. XRD study testifies th…
The electronic band structure-controlled ultrafast demagnetization mechanism in Co2FexMn1-xSi Heusler alloy is underpinned by systematic variation of composition. We find the spin-flip scattering rate controlled by spin density of states at Fermi level is responsible for non-monotonic variation of ultrafast demagnetization time (τM) with x with a maximum at x = 0.4. Furthermore, Gilbert damping constant exhibits an inverse relationship with τM due to the dominance of inter-band scattering mechanism. This establishes a unified mechanism of ultrafast spin dynamics based on Fermi level position.
The direct control of carriers spin by electric field under room temperature is one of the most important challenges in the field of spintronics. For this purpose, we here propose a quaternary Heusler alloy FeVTiSi. Based on first principles calculations, FeVTiSi alloy is found to be an intrinsic bipolar magnetic semiconductor in which the valence band and conduction band approach the Fermi level through opposite spin channels. Thus FeVTiSi alloy can conduct completely spin-polarized currents with tunable spin-polarization direction simply by applying a gate voltage. Furthermore, by Monte Carl…
The authors developed a new analysis technique for atomic disorder structures in full-Heusler alloys using x-ray diffraction (XRD) with Co-Ka and Cu-Ka sources. The developed technique can quantitatively evaluate all the atomic disorders for the exchanges between X, Y, and Z atoms in full-Heusler X2YZ alloys. In particular, the technique can treat the DO3 disorder that cannot be analyzed by ordinary Cu-Ka XRD. By applying this technique to full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing (RTA), RTA-temperature (TA) dependence of the atomic disorders was revealed. The sit…
In this work, we study the equiatomic quaternary Heusler Alloy CoFeCrP using two methods: DFT and Monte Carlo simulations. The DFT method allowed us to illustrate the structural, electronic and magnetic properties of this alloy. The ground state phase diagrams have been presented to show the stable configurations in different physical parameter planes. On the other hand, the Monte Carlo simulations, performed under the Metropolis algorithm, permitted to deduce the critical the behavior of the equiatomic quaternary Heusler alloy CoFeCrP. The structural properties results show that the phase of …
We present a study of the dynamic magnetic properties of TiN-buffered epitaxial thin films of the Heusler alloy Fe$_{1.5}$CoGe. Thickness series annealed at different temperatures are prepared and the magnetic damping is measured, a lowest value of $α=2.18\times 10^{-3}$ is obtained. The perpendicular magnetic anisotropy properties in Fe$_{1.5}$CoGe/MgO are also characterized. The evolution of the interfacial perpendicular anisotropy constant $K^{\perp}_{\rm S}$ with the annealing temperature is shown and compared with the widely used CoFeB/MgO interface. A large volume contribution to the per…
Spintronics is the fast growing field that will play a key role in optimizing power consumption, memory, and processing capabilities of nanoelectronic devices. Heusler alloys are potential candidates for application in spintronics due to their room temperature (RT) half-metallicity, high Curie temperature, low lattice mismatch with most substrates, and strong control on electronic density of states at Fermi level. In this work, we investigate the effect of {substitutional doping and disorder} on the half-metallicity, phase stability, and magnetism of Heusler alloys using density functional the…
In the present work, two successive magneto-structural transformations (MSTs) consisting of martensitic and intermartensitic transitions have been reported in polycrystalline Ni55.8Mn18.1Ga26.1 Heusler alloy. Benefiting from the additional latent heat contributed from intermediate phase, this alloy exhibits a large transition entropy change ΔStr with the value of ~28 J/kg K. Moreover, the magnetocaloric effect (MCE) has been also evaluated in terms of Maxwell relation. For the magnetic field change of 3 T, it is found that the calculated value of refrigeration capacity for Ni55.8Mn18.1Ga26.1 a…
We study the exchange interactions in half-metallic Heusler alloys using first-principles calculations in conjunction with the frozen-magnon approximation. The Curie temperature is estimated within both mean-field (MF) and random-phase-approximation (RPA) approaches. For the half-Heusler alloys NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In this case the MF and RPA estimations differ strongly. The RPA approach provides better agreement with experiment. The exchange interactions are more complex in the case of full-Heusler alloys Co$_2$MnSi and Co$_2$CrAl where t…
The performance of Heusler based magnetoresistive multilayer devices depends crucially on the spin polarization and thus on the structural details of the involved surfaces. Using low energy electron diffraction (LEED), one can non-destructively distinguish between important surface terminations of Co2XY full-Heusler alloys. We present an analysis of the LEED patterns of the Y-Z ,the vacancy-Z, the Co and the disordered B2 and A2 terminations. As an example, we show that the surface geometries of bulk L21 ordered Co2MnSi and bulk B2 disordered Co2Cr0.6Fe0.4Al can be determined by comparing the …
The strongly constrained and appropriately normed (SCAN) semi-local functional for exchange-correlation is deployed to study the ground-state properties of ternary Heusler alloys transforming martensitically. The calculations are performed for ferromagnetic, ferrimagnetic, and antiferromagnetic phases. Comparisons between SCAN and generalized gradient approximation (GGA) are discussed. We find that SCAN yields smaller lattice parameters and higher magnetic moments compared to the GGA corresponding values for both austenite and martensite phases. Furthermore, in the case of ferromagnetic and no…
The structural, electronic, and magnetic properties of half-Heusler alloys CKMg and SiKMg are studied by using first-principle density functional theory. The calculations reveal the SiKMg alloy is a half-metallic ferromagnet with the magnetic moment of 1 $μ_B$ per formula unit at equilibrium lattice constant. The magnetic moment mainly originates from the strong spin-polarization of $p$ electrons of Si atom and partial involvement of $d$ electrons of K atom. The half-metallic gap is 0.105 eV. The robustness of half-metallic against the lattice constants for SiKMg is also calculated. CKMg alloy…
High lattice thermal conductivity in half-Heusler alloys has been the major bottleneck in thermoelectric applications. Disordered half-Heusler alloys could be a plausible alternative to this predicament. In this paper, utilizing first-principles simulations, we have demonstrated the low lattice thermal conductivity in two such phases, NbFe$_{0.5}$Ni$_{0.5}$Sn and TaFe$_{0.5}$Ni$_{0.5}$Sn, in comparison to well-known half-Heusler alloy TiCoSb. We trace the low thermal conductivity to their short phonon lifetime, originating from the interaction among acoustic and low-lying optical phonons. We r…
Based on first-principles electronic structure calculations, we analyze the chemical and magnetic mechanisms stabilizing the cubic phase in Fe$_2$-based Heusler materials, which were previously predicted to be tetragonal when being chemically ordered. In agreement with recent experimental data, we found that these compounds crystallize within the so-called "inverted" cubic Heusler structure perturbed by a certain portion of the intrinsic chemical disorder. Understanding these mechanisms is a necessary step to guide towards the successful future synthesis of the stable Fe$_2$-based tetragonal p…
Recent first-principles calculations have predicted Mn$_{2}$Ni{\it X} ({\it X}=Al, Ga, In, Sn) alloys to be magnetic shape memory alloys. Moreover, experiments on Mn$_{2}$NiGa and Mn$_{2}$NiSn suggest that the alloys deviate from the perfect inverse Heusler arrangement and that there is chemical disorder at the sublattices with tetrahedral symmetry. In this work, we investigate the effects of such chemical disorder on phase stabilities and magnetic properties using first-principles electronic structure methods. We find that except Mn$_{2}$NiAl, all other alloys show signatures of martensitic t…
For electrical spin injection and detection of spin-polarized electrons in silicon, we explore highly epitaxial growth of ferromagnetic full-Heusler-alloy Co2FeSi thin films on silicon substrates using low-temperature molecular beam epitaxy (LTMBE). Although in-situ reflection high energy electron diffraction images clearly show two-dimensional epitaxial growth for growth temperatures T_G of 60, 130, and 200 C, cross-sectional transmission electron microscopy experiments reveal that there are single-crystal phases other than Heusler alloys near the interface between Co_2FeSi and Si for T_G = 1…
The large magnetocaloric effect (MCE) observed in Ni-Mn based shape-memory Heusler alloys put them forward to use in magnetic refrigeration technology. It is associated with a first-order magnetostructural (martensitic) phase transition. We conducted a comprehensive study of the MCE for the off-stoichiometric Heusler alloy Ni$_{2.2}$Mn$_{0.8}$Ga in the vicinity of its first-order magnetostructural phase transition. We found a reversible MCE under repeated magnetic field cycles. The reversible behavior can be attributed to the small thermal hysteresis of the martensitic phase transition. Based …
The effect of ball milling on the structural, magnetic and exchange bias properties of Ni50Mn36Fe2Sb12 Heusler alloys was studied. The ball milled samples exhibited coexisting austenite and martensite phases at room temperature, while annealing supresses the austenite phase completely. Ball milling was found to reduce the grain size, which resulted in the weakening of the ferromagnetic properties. An exchange bias field of 111 Oe and coercivity of 826 Oe were observed at 5 K in the as-milled sample, in contrast to the bulk alloy values of 288 Oe and 292 Oe, respectively. Annealing causes an in…
Using density functional theory based calculations, we have carried out in-depth studies of effect of Co substitution on the magnetic properties of Ni and Pt-based shape memory alloys. We show the systematic variation of the total magnetic moment, as a function of Co doping. A detailed analysis of evolution of Heisenberg exchange coupling parameters as a function of Co doping has been presented here. The strength of RKKY type of exchange interaction is found to decay with the increase of Co doping.
Parameters of the energy gap and, consequently, electronic, magnetic and galvanomagnetic properties in different X$_2$YZ Heusler alloys can vary quite strongly. In particular, half-metallic ferromagnets (HMFs) and spin gapless semiconductors (SGSs) with almost 100% spin polarization of charge carriers are promising materials for spintronics. The changes in the electrical, magnetic and galvanomagnetic properties of the Co$_2$YSi (Y = Ti, V, Cr, Mn, Fe) and Co$_2$MnZ Heusler alloys (Z = Al, Si, Ga, Ge) in possible HMF and/or SGS states were followed and their interconnection was established. Sig…
Perpendicular magnetic anisotropy (PMA) in ultrathin magnetic structures is a key ingredient for the development of electrically controlled spintronic devices. Due to their relatively large spin-polarization, high Curie temperature and low Gilbert damping the Co-based full Heusler alloys are of special importance from a scientific and applications point of view. Here, we study the mechanisms responsible for the PMA in Pt/Co-based full Heusler alloy/MgO thin films structures. We show that the ultrathin Heusler films exhibit strong PMA even in the absence of magnetic annealing. By means of ferro…
