Full Heusler Research Articles

Electronic, magnetic, and optical properties in bulk and various (111), (001) surfaces of Sc2CoSn full Heusler compound

The electronic, magnetic, and optical characteristics of the bulk and (111), (001) surfaces of the Sc2CoSn Heusler compound are calculated using first principle calculations. Investigating these properties is performed with FPLAPW, which takes into account the approximate function correlation within the Generalized Gradient Approximation (GGA). The results show that the bulk Sc2CoSn compound achieves a ferromagnetic half-metallic characteristic with an energy gap of 0.54 eV. The density of states for Sc(1)-, Sc(2)-, Co-terminations at the (111) surface, and the Sc(1)Co- and Sc(2)Sn-terminations at the (001) surface show metallic properties at the spin up and spin down channels, whereas the Sn-termination at (111) surface has a half-metallic nature with full spin polarization. A matter that paves the way for many applications related to constructing a massive magnetic resistance. At all ends, the values of magnetic moments of (111) and (001) surfaces demonstrate that Co atoms contribute the most. The optical investigation of the compound shows high reflectivity in the infrared region of the photon energy. The optically non-metallic behavior of the Sc2CoSn compound was revealed by the imaginary component of the dielectric function, with an optical band gap of roughly 1 eV. Based on the obtained results, the investigated material could be utilized in many photo-electronics applications including photo-senses, photo-catalyst, and photo-electrochemical areas.

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.

Co[formula omitted]Fe[formula omitted]Ge[formula omitted]: A single-phase full Heusler alloy with highest magnetic moment and Curie temperature

Elemental substitution by a different element is a well utilized technique of stabilizing a single-phase compound, in case the parent alloy is multi-phase, and this has been demonstrated specifically in a number of Heusler systems. This can, however, give an increased propensity for chemical disorder as well as adding complexity to synthesis and characterization.In this paper, we present the successful synthesis of a single-phase compound, Co2Fe1.25Ge0.75, by tuning the parent Co2FeGe stoichiometry (which exhibits multi-phase structure) rather than introducing a fourth element. The compound is found to crystallize in L21 structure (space group # 225). Magnetization measurements reveal Co2Fe1.25Ge0.75 has a saturation magnetization as high as 6.7±0.1μB/f.u. at 5 K and a Curie temperature of 1135 ± 5 K – both being the highest reported to date for cubic full Heusler alloys to our knowledge. Thin films of Co2Fe1.25Ge0.75 deposited on Al2O3(110) and MgAl2O4(100) substrates show excellent expitaxial quality, among the reported for Heusler films to date, and exhibit magnetic properties comparable to bulk samples. First principle calculations suggest the system exhibits total energy minimum at the experimentally-observed lattice parameter. Furthermore, the calculations corroborate the observed enhancement in magnetization and point to the importance of on-site Coulomb interactions. While our novel approach of substitution led to the discovery of stable Co2Fe1.25Ge0.75 alloy with very high moment and Curie temperature that can be readily grown as a high-quality epitaxial thin film, making it a candidate for device applications, this approach can be taken as a new paradigm for the discovery of novel single-phase Heusler compounds with enhanced magnetic properties.

Longitudinal fluctuations of Co spin moments and their impact on the Curie temperature of the Heusler alloy Co2FeSi

The magnetism of the full Heusler alloy Co2FeSi with its high magnetic ordering temperature is studied on a first-principles basis employing the disordered local moment approximation, the magnetic force theorem and single-site spin fluctuation theory as formulated recently in the framework of the Local Spin Density Approximation. We find that the magnetic moments of Fe and Co in Co2FeSi exhibit a quite distinctive behavior at high temperatures. The Fe moments are well localized and keep their magnitude unchanged with temperature, whereas the Co moments are itinerant and show a temperature dependence. We find that the effects of magnetic disorder strongly renormalize the Fe-Co inter-atomic exchange interactions. Our results suggest a deficiency of the classical Heisenberg model with rigid localized atomic spin moments for the description of magnetism in Co2FeSi. An accurate estimation of the Curie temperature is obtained by taking into the account the thermal longitudinal fluctuations the Co moments.

Critical Behavior and Magnetocaloric Effect in Co2CrAl Heusler Alloy

The analysis of structural, electrical transport, critical magnetic behavior, and magnetocaloric properties of Co2CrAl Heusler alloys has been conducted here. Co2CrAl full Heusler alloy has been prepared by the conventional arc melting method in Ar atmosphere and post‐annealing in a vacuum‐sealed quartz tube. X‐ray diffraction (XRD) pattern reveals the crystallization of the alloy to the Fmm space group with L21 crystal symmetry. The correspondence between the magnetic and electrical transport properties of the Co2CrAl alloy has been observed. This alloy exhibits paramagnetic to ferromagnetic transition (Curie temperature) just above room temperature at around 340 K. Metallic to semiconducting transition occurs near the magnetic Curie temperature. Critical magnetic behavior has been analyzed by employing the Arrott plot, Kouvel–Fisher methods, and critical isotherm analysis. These conclude that the exchange interaction in Co2CrAl alloy follows between mean‐field theory having spin–spin long‐range interaction and the 3D Heisenberg model. The magnetocaloric effect has been explored from the isothermal magnetization data by employing Maxwell's equations. The highest magnetic entropy change is found to be ≈0.99 J (kg K)−1 for a magnetic field change of 30 kOe. A large working temperature window of 57 K is the advantage of the present alloy.

Theoretical prediction of mechanics, transport, and thermoelectric properties of full Heusler compounds Na2KSb and X2CsSb (X=K,Rb)

Full Heusler compounds have become a research hotspot in the thermoelectric (TE) field, because they have the remarkable electronic properties and high-TE power factor. Nevertheless, the inherent high thermal conductivity ${\ensuremath{\kappa}}_{L}$ hinders their further application as TE device. Hence, we investigate the mechanical, transport, and thermoelectric properties in ${\mathrm{Na}}_{2}\mathrm{KSb}$ and ${\mathrm{X}}_{2}\mathrm{CsSb}$ (X = K, Rb) with eight valence electrons per formula unit (f.u.) by using the first-principles calculations combined with self-consistent phonon (SCP) theory, compressive sensing (CS) techniques, and Boltzmann transport equation (BTE). Due to the strong three phonon scattering combined with small phonon group velocity, we obtain the ultralow lattice thermal conductivities. An evaluation of their mechanical properties reveals that they are all brittle compounds, and ${\mathrm{Rb}}_{2}\mathrm{CsSb}$ has the strongest elastic anisotropy. To capture rational electron transport properties, we include the effects of the acoustic deformation potential (ADP) scattering, polar optical phonon (POP) scattering, and ionized impurity (IMP) scattering on the electron relaxation time. Finally, a high p-type $\mathit{ZT}$ values of 2.74 (2.48) at 600 (900) K are captured in the cubic ${\mathrm{K}}_{2}\mathrm{CsSb}$ $({\mathrm{Na}}_{2}\mathrm{KSb})$. These findings not only help us to comprehensively understand the physical properties of full Heusler compounds with eight valence electrons per f.u., but also support them as potential candidates for thermal management and thermoelectric applications.

Effects of Electron Correlations on the Magnetic Stability of Rh2TMSn Full Heusler Alloys (TM=Cr, Mn, and Fe)

Effects of Electron Correlations on the Magnetic Stability of Rh2TMSn Full Heusler Alloys (TM=Cr, Mn, and Fe)

Effect of Structural Disorder on the Physical Properties of the Pd Based Heusler Alloys: PdCoMnAl, Pd2CoAl and Pd2MnAl

In this manuscript, we investigate the effect of structural disorder on the physical properties of Pd-based Heusler alloys: PdCoMnAl, Pd2CoAl and Pd2MnAl using first-principles calculations. In fact, we are studying the structural properties of the equatorial quaternary Heusler alloy PdCoMnAl for three phases: type 1, type 2 and type 3, we have carried out this property to determine the most stable phases. The structural properties results show that the phase of type I, of this alloy is the most stable configuration. In addition, the band structures, and density of states calculations results show that this compound exhibits a half-metallic character with a 100% of spin polarization (SP) at the Fermi-level. The total magnetic moment of the Heusler compound is found to be 5.00 µB. Moreover, it is found that the Slater-Pauling is well described for this alloy. On the other hand, concerning total energy of the full Heusler Pd2YAl (Y = Co or Mn) alloys. It is found that such material in L21-type structure is always smaller than in the XA-type structure during the whole lattice variation variety and the L21-type structure is more energetically preferable for the both full alloys. In addition, the density of states at the Fermi level within the spin up and down states show the metallicity of these compounds. Our results show that the quaternary Heusler is a potential candidate for the spintronic applications.

Spin Hall conductivity and anomalous Hall conductivity in full Heusler compounds

The spin Hall conductivity (SHC) and anomalous Hall conductivity (AHC) in about 120 full Heusler compounds are calculated using the density functional theory in a high-throughput way. The electronic structures are mapped to the Wannier basis and the linear response theory is used to get the conductivity. Our results show that the mechanism under the SHC or AHC cannot be simply related to the valence electron numbers or atomic weights. It is related to the very details of the electronic structures, which can only be obtained by calculations. A high-throughput calculation is efficient to screen out the desired materials. According to our present results, Rh2MnAl and Cu2CoSn, as well as Co2MnAl and Co2MnGa are candidates in spintronic materials regarding their high SHC and AHC values, which can benefit the spin-torque-driven nanodevices.

In Situ Evolution of Secondary Metallic Phases in Off-Stoichiometric ZrNiSn for Enhanced Thermoelectric Performance.

The full-Heusler (FH) inclusions in the half-Heusler (HH) matrix is a well-studied approach to reduce the lattice thermal conductivity of ZrNiSn HH alloy. However, excess Ni in ZrNiSn may lead to the in situ formation of FH and/or HH alloys with interstitial Ni defects. The excess Ni develops intermediate electronic states in the band gap of ZrNiSn and also generates defects to scatter phonons, thus providing additional control to tailor electronic and phonon transport properties synergistically. In this work, we present the implication of isoelectronic Ge-doping and excess Ni on the thermoelectric transport of ZrNiSn. The synthesized ZrNi1.04Sn1-xGex (x = 0-0.04) samples were prepared by arc-melting and spark plasma sintering, and were extensively probed for microstructural analysis. The in situ evolution of minor secondary phases, i.e., FH, Ni-Sn, and Sn-Zr, primarily observed post sintering resulted in simultaneous optimization of the electrical power factor and lattice thermal conductivity. A ZT of ∼1.06 at ∼873 K was attained, which is among the highest for Hf-free ZrNiSn-based HH alloys. Additionally, ab initio calculations based on density functional theory (DFT) were performed to provide comparative insights into experimentally measured properties and understand underlying physics. Further, mechanical properties were experimentally extracted to determine the usability of synthesized alloys for device fabrication.

Superconductivity in Heusler compound ScAu2Al

We report superconducting state properties and electronic structure of a full Heusler material ScAu2Al. The resistivity measurement indicates a zero-field (at nominal Earth’s field) superconducting transition temperature, T c = 5.12 K (in contrary to the previously reported value of 4.4 K), which falls in the highest T c -regime among the Heusler superconductors. The magnetization data shows that ScAu2Al is a moderate type-II superconductor, where the critical field values can be estimated from the Ginzburg–Landau–Abrikosov–Gorkov theory. The field-dependent magnetization response further shows signatures of flux jump in ScAu2Al. A sharp jump in the temperature dependent specific heat (C p ) data confirms bulk superconductivity. We report that the electron–phonon coupling constant, λ e–ph = 0.77, suggesting a moderate electron–phonon coupling in ScAu2Al. Further, we show that the observed λ e–ph value in ScAu2Al is the highest amongst the reported Heusler superconductors, indicating strong correlation between T c and λ e–ph values and significant role of electron–phonon coupling in mediating superconductivity in Heusler superconductors. Finally, we discuss the electronic properties and reveal the existence of van Hove singularity near the Fermi level in ScAu2Al.

High-pressure studies of a biphasic NiTiSn/Ni2TiSn Heusler alloy by in situ X-ray diffraction and first principle calculations.

The present work studies the effect of hydrostatic pressure on a biphasic Heusler alloy using in situ X-ray diffraction. A nanostructured biphasic NiTiSn/Ni2TiSn Heusler alloy was synthesized by four hours of mechanical alloying using a vibrational high-energy mill and characterized by x-ray diffraction measurements and Rietveld method. The sample was submitted to high-pressure conditions (up to 11.9 GPa) using a diamond anvil cell (DAC) loaded with He, and the structural evolutions were followed by in situ synchrotron x-ray diffraction. The results validate the stability of the half and full-Heusler alloys, as no pressure-induced structural phase transition was observed over the entire pressure range. The behavior of the crystallographic parameters, equation of state and the compressibility factors for both the Heusler phases were obtained from the experimental results as well from the density functional theory (DFT) calculations. The second-order Birch–Murnaghan equation of state determined from our experimental results yields bulk moduli of 143(9) GPa and 156(6) GPa for the Half and Full Heusler phases, respectively. According to the DFT calculations, the corresponding values are 131.0(9) GPa and 161.8(8) GPa.

Influence of ferromagnetic layer thickness on the Gilbert damping and magnetocrystalline anisotropy in PLD grown epitaxial Co2FeSi Heusler alloy thin films

In the present work, full Heusler alloy (FHA) Co2FeSi (CFS) epitaxial thin films with thickness (tCFS) ranging from 5–25 nm were grown by Pulse laser deposition (PLD) on MgO (001) substrates at an optimized deposition temperature of 600°C. Dynamic magnetization response of PLD-grown CFS thin films was systematically investigated as a function of tCFS. Thickness-dependent structural and magnetic correlation exhibit a non-monotonic variation in Gilbert damping parameter, α. The ferromagnetic resonance (FMR) investigation revealed that the presence of lattice strain and a lower degree of atomic ordering in the thinner FHA CFS films, results in additional enhancement of α. The lowest α value of 4×10−3 is obtained from the theoretical fit of experimental data in a 20 nm thick film. This low value of α is suitable in magnonic devices, such as spin-transfer torque magnetic random-access memories (STT-MRAMs) where an efficient spin injecting source from a FHA ferromagnet to a non-magnetic interface is required. Further, in-plane angle-dependent (φ-variation) FMR measurements of the sample yield an intrinsic four-fold magnetocrystalline anisotropy (MCA) along with the presence of uniaxial anisotropy that has non-monotonicity with tCFS. A significant value of cubic anisotropy constant of 5.16×105erg/cm3 has been obtained in the film with a thickness of around 15nm. Further investigation suggests that uniaxial anisotropy in the CFS/MgO film is a combination of strain-induced magneto-elastic and interface-related contribution that is intrinsic in nature. The experimental observations have been verified by using micromagnetic simulations run on the epitaxial crystal structure. FMR mode position and linewidth of varied thickness CFS films are generated by micromagnetic simulations utilizing the Landau–Lifshitz–Gilbert formalism.

A Comparative Study of Spin Magnetic Moment, Spin Polarization and Curie Temperature of Heusler type Compounds (A<sub>2-X-Y</sub>Z<sub>y</sub>)Mnz (A = Ni, Pd, Pt And Z = Sb, Sn)

The Heusler type compounds (A2-x-yZy)MnZ (A = Ni, Pd, Pt; Z = Sb, Sn; x = 1, 0; and y = 0, antisite defects of Z atoms) are investigated to understand the induced magnetic properties. The structural phases C1b type half Heusler (x = 1, y = 0) compounds AMnZ and the corresponding L21 type full Heusler (x = y = 0) compounds A2MnZ and some disordered compounds (A1-yZy)MnZ and (A2-yZy)MnZ (y = 0.01, 0.05) were calculated using the Korringa-Kohn-Rostoker Green’s function method. The antisite doping of 1% (5%) Sb retained (suppressed) the half metallicity at (Ni1-ySby)MnSb, whereas the other compounds were found to be gapless in both spin directions. The density of states at the Fermi level exhibits the explicit spin polarization in the compounds. The net magnetic moments of the compounds are closer in their values, wherein the manganese atom is responsible for the major contribution to the magnetic moments, which is much larger than the partial moment of the other constituents. The Curie temperature (TC) of the ordered ferromagnetic compounds was estimated using the mean-field approximation. The TC higher than the room temperature was found for the ordered compounds, except for the cases of Pt2MnSb and Pt2MnSn. No literature value of TC has been reported yet for these two compounds. Calculated spin moments and TC agree well with the experimental results, where available. J. Bangladesh Acad. Sci. 45(2); 217-229: December 2021

First-principles calculations to investigate stability, electronic properties and anisotropy of half-metallic full Heusler alloy Co2NbGa

The phase structures, magnetic properties, electronic structures, and anisotropy of the full Heusler alloy Co2NbGa were studied using the first-principles method, and the results were compared with experiments. The L21-type cubic structure was the most stable, which was in good agreement with the experimental measurement. The Curie temperature obtained by the mean field theory was also very close to the experimental value. The electronic structures and magnetic properties showed that the Co2NbGa alloy was a half-metallic ferromagnet with 100% spin polarization at equilibrium, and the robustness of the half-metallic feature was preserved in lattice ranges of 5.79 to 6.42 Å. The Co2NbGa alloy was an anisotropic material with stable mechanical properties at L21-type cubic and XA-type tetragonal structures, and the influence of Hubbard U and vacancy defects on the physical nature was also studied and discussed. The 100% spin polarization, high Curie temperature, and stable equilibrium structure make Co2NbGa alloy a promising candidate in spintronics and magnetoelectronic.

Antibonding induced anharmonicity leading to ultralow lattice thermal conductivity and extraordinary thermoelectric performance in CsK2X (X = Sb, Bi)

A first principles-based machine learning potential is developed to model the phonon transport properties in full Heusler CsK2Sb and CsK2Bi taking account of all higher-order phonon interactions.

NiMnSn half Heusler alloy: Critical phenomena at the ferromagnetic to paramagnetic phase transition

The critical magnetic analysis and magnetocaloric effect of NiMnSn half Heusler alloy have been explored in this article. This alloy undergoes ferromagnetic to paramagnetic second order phase transition above room temperature at around 388 K. To understand the exchange interaction, magnetic ordering and magnetic phase transition, the critical magnetic analysis has been carried out around the transition temperature. Critical exponents are turned out to be β = 0.44, γ = 1.1 and δ = 3.1. These values imply that the exchange interaction is following towards mean field theory i.e., long-range magnetic ordering is present in the system. The absence of one Ni atom from full Heusler Ni2MnSn alloy does not affect much the magnetic ordering of half Heusler NiMnSn. For environment-friendly solid state cooling applications, the magnetocaloric effect of NiMnSn has been studied. The magnetic entropy change is calculated to be high as 0.81 J/kg-K near the transition temperature for a moderate field change of 30 kOe.

Enhanced thermoelectric performance of n-type Zr0.66Hf0.34Ni1+xSn Heusler nanocomposites

By exploiting the benefit of the half-Heusler (HH) structure, the nanocomposite strategy has been an effective mechanism for most HH-based thermoelectric materials. We have successfully developed HH-based nanocomposites by economically feasible solid-state reaction (SSR) route followed by Spark Plasma Sintering (SPS). X-ray diffraction (XRD) analysis confirms the single HH phase for Zr0.66Hf0.34NiSn composition, and full-Heusler (FH) as an inclusion phase in HH matrix for Zr0.66Hf0.34Ni1+xSn (x = 0.01, 0.03, 0.05, 0.10) alloys. Energy Dispersive Spectroscopy (EDS) also reveals the nominal stoichiometric compositions to be in close matching with experimental compositions. Thermoelectric measurements have been performed for all the compositions Zr0.66Hf0.34Ni1+xSn (x = 0, 0.01, 0.03, 0.05, 0.10) from 323 to 773 K. Interestingly, increasing FH concentration in the HH matrix (up to Zr0.66Hf0.34Ni1.03Sn) leads to the simultaneous increase in Seebeck coefficient due to optimization of carrier concentration and increase in electrical conductivity due to increased mobility of carriers. Therefore, an optimized power factor of 35.31 μW K−2 cm−1 at 773 K was obtained for Zr0.66Hf0.34Ni1.03Sn alloy. In addition to this, a reduced lattice thermal conductivity of 1.26 Wm−1 K−1 was optimized for Zr0.66Hf0.34Ni1.1Sn composition at 773 K. Thus, enhanced power factor and reduced thermal conductivity have finally resulted in an increased figure of merit of 0.93 at 773 K for the optimized composition Zr0.66Hf0.34Ni1.03Sn, which is nearly 260% more as compared to the pristine HH composition Zr0.66Hf0.34NiSn.

Magnetocaloric properties at the austenitic Curie transition in Cu and Fe substituted Ni-Mn-In Heusler compounds

We present a study on the Curie transition of austenitic Ni-Mn-In full Heusler compounds when Mn atoms are replaced by Fe or Cu. The substituted compounds are designed to present a relevant magnetocaloric effect at the second order Curie transition of the austenitic phase near room temperature. We show that Fe and Cu modify the magnetic moments and interactions responsible of the localized magnetism in the L21 ordered cubic structure, resulting in a change of the Curie temperature and saturation magnetization of the compound. Neutron diffraction experiments and electron microscopy analysis were used to study the sites occupancy of doping atoms and the presence of secondary phases, thus possibly optimizing the annealing protocols to obtain homogeneous samples even at high Cu/Fe concentrations. On this basis, a series of quinary compounds with a tunable Curie temperature and high values of saturation magnetization (100–110 Am2/kg at 80 K) was successfully synthesized. The obtained results show the feasible fine tuning of the Curie temperature at which the peak of the magnetocaloric effect is realized, highlighting a new promising strategy to design graded regenerators for room temperature magnetocaloric applications.

Effects of composition variation and site disorder on the magnetic interactions in Ru2Fe(Si1-xGex) alloys

X-ray diffraction studies on arc melted Ru2Fe(Si1-xGex) (0 ≤ x ≤ 1) alloys did not show the characteristic (111) and (200) super-lattice peaks of the stable L21 structure of full Heusler alloys. The alloy with x = 0 exhibited antiferromagnetic behaviour. However, with increase in x, the alloys became ferromagnetic. In order to understand the origin of the novel magnetic behavior of these alloys and its correlation with their crystal structure, ab initio study was carried out using spin-polarized relativistic Korringa-Kohn-Rostoker Green’s function method. The study shows that the antiferomagnetism in the alloy with x = 0 is due to the presence of B2 disorder. Due to this, the 2nd nearest neighbour (NN) exchange interaction between the Fe atoms at proper and improper sites, respectively, becomes strongly antiferromagnetic which dominates over the 1st NN ferromagnetic exchange interaction. With increase in x, the alloys become ferromagnetic with an improvement in L21 ordering which in turn makes the 1st NN interaction strongly ferromagnetic. The electronic structure calculations show no half metallic gap in either spin direction for all the alloys. Comparison of experimental and theoretically estimated properties of Ru2Fe(Si1-xGex) alloys provides insight to the variation in the magnetic interaction with the composition and the disorder present in the alloys.