Half-metallic Ferromagnetic Heusler Research Articles

Large magnetoresistance and high spin-transfer torque efficiency of Co2MnxFe1−xGe (0 ≤ x ≤ 1) Heusler alloy thin films obtained by high-throughput compositional optimization using combinatorially sputtered composition-gradient film

Half-metallic ferromagnetic Heusler alloys having high spin polarization are promising candidates to realize large magnetoresistance (MR) ratio and high spin-transfer torque (STT) efficiency in next-generation spintronic devices. Since the Heusler alloy properties are sensitive to composition, optimizing the composition is crucial for enhancing device performance. Here, we report the fabrication of high-performance current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices using Co2MnxFe1−xGe (0 ≤ x ≤ 1) Heusler alloy, employing a high-throughput and detailed composition optimization method. The method combined composition-gradient films and local measurements to enable the composition variation from Co2FeGe to Co2MnGe to be efficiently studied on a single library sample with a small composition interval. The CPP-GMR devices fabricated from stacks annealed at 250 °C showed a clear composition dependence of MR with the maximum of MR ratio ∼38% in the Mn-rich region of x = 0.85. By increasing the annealing temperature to 350 °C, the MR ratio increased to ∼45% along with high STT efficiency ∼0.6 in the broad composition range of 0.2 ≤ x ≤ 0.7. The optimal composition for the highest MR changed with annealing temperature because of the stability of the GMR stack being higher in the lower x range. The record high MR for the all-metal CPP-GMR devices, at low annealing temperature of 250 °C was achieved by the detailed composition optimization. These results present the high potential of Co2MnxFe1−xGe and provide a comprehensive guidance on the composition optimization for achieving large MR ratio and high STT efficiency in the CPP-GMR devices.

Selective electronic excitations in nearly half-metallic Heusler alloy NiFeMnSn—A Raman spectroscopic study

Half-metallic ferromagnetic full Heusler alloy NiFeMnSn is a promising material in spintronic device fabrication as it carries high spin polarization and high Curie temperature (Tc = 405 K). Understanding electronic excitations in a spin-polarized band structure is essential for the further use of this material in spin-based devices and to optimize the spin-dependent electronic structure in related compounds. In this Letter, we report electronic Raman scattering of NiFeMnSn with spectral signatures at a higher wavenumber than expected from the calculated phonon modes of the system. Temperature-dependent Raman spectroscopy demonstrates a non-monotonic behavior of the Raman shift with temperature across Tc. The orbital resolved electronic density of states and electronic band structure for both spin channels of the system, as obtained from the first principles density functional theory calculations, suggest that the observed Raman signatures originate from the Fe d orbital and its splitting due to the crystal field near the Fermi level. Furthermore, a strong magnetic field dependence of the spectral profile is observed. The study not only exhibits electronic Raman scattering in a Heusler alloy compound, which was unexplored to date, but also establishes Raman scattering as a promising probe to study the orbital-resolved partial density of states in the band structure near the Fermi level of a Heusler alloy.

First-principles investigations of the half-metallic ferromagnetic LaCoTiIn equiatomic quaternary Heusler alloy for spintronics

We have evaluated the structural and mechanical stability, electronic structure, total spin magnetic moment, and Curie temperature of LaCoTiIn Equiatomic Quaternary Heusler Alloy (EQHA) using first-principles studies. The Generalized Gradient Approximation (GGA) and GGA+U schemes have been used as exchange-correlation functional for the above calculations. From the ground state calculation, LaCoTiIn EQHA with a Type-III structure in the ferromagnetic (FM) state is found to be stable. The electronic structure of LaCoTiIn EQHA depicts half-metallic behavior which has metallic overlap in the spin up ([Formula: see text] channel and a semiconductor band gap in the other channel. The spin–orbit coupling of LaCoTiIn has a great influence on the band gap of the material. The computed band gap values for the spin down ([Formula: see text] channel are 0.480 eV and 0.606 eV by using the GGA and GGA+U schemes. The total spin magnetic moment is 1 [Formula: see text], according to the Slater–Pauling rule, [Formula: see text] = ([Formula: see text] - 18) [Formula: see text]. These results obtained can be used as a valuable reference for future research, or they will be used to further motivate the experimental synthesis of the corresponding alloy.

Investigation of the New Half Metallic Ferromagnetic Quaternary Heusler Compounds LiX′NO (X′=Ba, Ca and Sr)

In this paper, we present an ab initio study, based on the density functional theory (DFT) approach and GGA-PBE approximation, of the structural, electronic and magnetic properties of new quaternary Heusler compounds LiX[Formula: see text]NO ([Formula: see text], Ca and Sr). We searched the structural stability of these quaternary Heusler among three different atomic arrangements (type-1, type-2 and type-3) by calculating of the cohesive energy, elastic constant and phonon dispersion. We found that the type-2 structure is the most stable structure in the ferromagnetic phase. The results of the magnetic and electronic properties show that the LiX[Formula: see text]NO quaternary compound exhibits a half-metallic character with 100% spin polarization at the Fermi level and obey the Slater Pauling rule. It is also divulged that half-metallicity comes mainly from N and O atoms. These theoretical results make these new quaternary alloys as very promising materials for spintronic device.

Band structure tuning of Heusler compounds: Spin- and momentum-resolved electronic structure analysis of compounds with different band filling

Half-metallic ferromagnetic Heusler compounds represent an important class of materials for spintronic applications. We experimentally observe the dispersion of electronic bands with spin resolution in three representative Heusler compounds (${\mathrm{Co}}_{2}\mathrm{MnGa}, {\mathrm{Co}}_{2}\mathrm{MnSi}$, and ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$). Bulk-sensitive measurements at very low photon energy are complemented by spin-integrated hard x-ray angular resolved photoemission spectroscopy. The dispersion of majority and minority electrons allows us to link exchange splitting and band filling to the number of valence electrons ${N}_{v}$. Photoexcitation at $h\ensuremath{\nu}=6.05\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ gives access to the spin-polarization texture $\mathbit{P}({E}_{B},{k}_{x},{k}_{y})$ of the bulk bands covering a $({k}_{x},{k}_{y})$ range of 60% of the Brillouin zone. We find that ${\mathrm{Co}}_{2}\mathrm{MnSi}$ and ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ exhibit minority band gaps of 0.5 and 0.35 eV, i.e., decreasing band gap with increasing ${N}_{\mathrm{V}}$ in contrast to the rigid-band expectation. For ${\mathrm{Co}}_{2}\mathrm{MnGa}$, the minority valence state maximum lies at approx. 0.2 eV above ${E}_{\mathrm{F}}$.

Effect on Annealing Temperature (Ta) of Ternary Full Fe2CrSi Heusler Alloy Nanoparticles for Spin-Based Device Applications

The ternary full Heusler alloy nanoparticle Fe2CrSi compounds were prepared using mechanical alloy method. The as-prepared sample was observed as amorphous in nature and further increases various annealing temperatures (like 700 °C, 800 °C, and 900 °C) in nitrogen gas atmosphere for 5 h in tubular furnace using quartz tube. X-ray diffraction studies confirmed the formation of Cu2MnAl-type structure instead of the conventional L21 disordered structure. Through Williamson-Hall plot (W-H plot), the uniform deformation model was used to get low value of strain-induced broadening. In the surface morphology, elemental composition studies were carried out using HRSEM with EDS. The presence of Fe2CrSi with the particle size of ~ 35 nm was also observed using high-resolution scanning electron microscopy and elemental composition study confirmed the presence of Fe, Cr, and Si. The magnetic properties of Fe2CrSi nanoparticles were studied showing ferromagnetic behavior and low coercivity of soft magnetic nature. Half metallic ferromagnetic Heusler alloys have wide range of applications from medical imaging to data storage for its profound performance on spintronics.

Anomalous Hall effect in the half-metallic Heusler compound Co2TiX ( X=Si , Ge)

Though Weyl fermions have recently been observed in several materials with broken inversion symmetry, there are very few examples of such systems with broken time reversal symmetry. Various Co$_{2}$-based half-metallic ferromagnetic Heusler compounds are lately predicted to host Weyl type excitations in their band structure. These magnetic Heusler compounds with broken time reversal symmetry are expected to show a large momentum space Berry curvature, which introduces several exotic magneto-transport properties. In this report, we present systematic analysis of experimental results on anomalous Hall effect (AHE) in Co$_2$Ti$X$ ($X$=Si and Ge). This study is an attempt to understand the role of Berry curvature on AHE in Co$_2$Ti$X$ family of materials. The anomalous Hall resistivity is observed to scale quadratically with the longitudinal resistivity for both the compounds. The detailed analysis indicates that in anomalous Hall conductivity, the intrinsic Karplus-Luttinger Berry phase mechanism dominates over the extrinsic skew scattering and side-jump mechanism.

Investigation of phase transition in electronic structure and magnetic properties of Fe2-x Co x TiSn Heusler alloys

ABSTRACT The structural, electronic, mechanical and magnetic properties of Fe2-x Co x TiSn (x = 0, 0.5, 1, 1.5, 2) Heusler alloys in L21 (Cu2MnAl prototype) and XA (Hg2CuTi) phases are analysed. The structural optimization confirms the stability of these alloys in L21 phase. As the pressure is increased, the structural phase transition from L21 to XA phase is predicted at the pressures of 123.34, 74.27, 47.22, 43.18 and 31.25 GPa in Fe2TiSn, Fe1.5Co0.5TiSn, Fe1.0Co1.0TiSn, Fe0.5Co1.5TiSn and Co2TiSn, respectively. Fe2TiSn is identified as a non-magnetic semiconductor using electronic structure calculation. The effect of substitution of Co for Fe on electronic structure and magnetic property of Fe2TiSn is investigated. It is found that Co substituted Fe1.5Co0.5TiSn, Fe1.0Co1.0TiSn and Fe0.5Co1.5TiSn are found to be half-metallic ferromagnetic Heusler alloys. The elastic constants are calculated to ensure mechanical stability. This study opens a way out for the potential application of these alloys as spintronic materials.

Soft Ferromagnetic Properties of Half-Metallic Mn2CoAl Heusler Alloy Nanoparticles for Spintronics Applications

The ternary full Heusler alloy Mn2CoAl nanoparticles were synthesized by using co-precipitation technique. The as-prepared sample was observed for amorphous nature and further increased at various annealing temperatures (like 700 °C, 800 °C and 900 °C) in argon gas atmosphere for 5 h. X-ray diffraction studies were performed, and the pattern confirms the formation of Hg2CuTi-type structure of the XA or Xα disorder structure. Williamson-Hall plot (W-H plot) was used to find the uniform deformation model so as to get low value of strain-induced broadening. The presence of Mn2CoAl with the particle size ≈ 50 nm was obtained from high-resolution scanning electron microscopy (HR-SEM), and the elemental analysis study confirms the presence of Mn, Co, and Al. UV-Visible absorption spectrum shows the wide absorption peak at 470 nm and the estimated band gap energy value at Eg = 0.8 eV (from Tauc plot) for 900 °C annealed sample. Synthesized Mn2CoAl nanoparticles induced ferromagnetic behavior due to composition deviation and as a result displayed low coercivity of soft magnetic behavior. Half-metallic ferromagnetic Heusler alloys have a wide range of applications from medical imaging to data storage, and also it has profound spintronics applications.

Peculiarities of Electronic Transport and Magnetic State in Half-Metallic Ferromagnetic and Spin Gapless Semiconducting Heusler Alloys

A brief survey of experimental and theoretical studies of half-metallic ferromagnets (HMFs) and spin gapless semiconductors is given, the possible candidates being the X$_2$YZ (X = Mn, Fe, Co; Y = Ti, V, Cr, Mn, Fe, Co, Ni; Z = Al, Si, Ga, Ge, In, Sn, Sb) Heusler alloys. The data on the electrical resistivity, normal and anomalous Hall Effect, and magnetic properties are presented. It is shown that the Co$_2$FeZ alloys demonstrate properties of conventional ferromagnets, the HMF properties being also manifested at the variation of the Z-component. The Fe$_2$YAl and Mn$_2$YAl alloys show at the variation of the Y-component both metallic and semiconducting electronic characteristics, the magnetic properties, changing from the ferromagnetic to compensated ferrimagnetic state. The HMF and spin gapless semiconductor states are supposed to exist in these Heusler alloys systems.

Experimental observation of anomalies in the electrical, magnetic, and galvanomagnetic properties of cobalt-based Heusler alloys with varying transition elements

The residual resistivity, Hall effect, and magnetization of Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys were considered at T = 4.2 K and in fields up to 100 kOe. It is shown that as the number of valence electrons z ranges from 26 to 32, significant changes in the residual resistivity ρ0, magnetization Ms, sign and magnitude of the normal R0 and anomalous RS Hall effect coefficients are observed during the transition from Co2TiSi to Co2NiSi. It is established that there is a clear correlation between the values ρ0, R0, RS and Ms, depending on the number z, which can be associated with the appearance of a half-metal ferromagnetic state and/or spin gapless semiconductor. As z changes, the anomalous Hall effect coefficient has a power-law dependence on the residual electrical resistivity with an exponent of k = 3.1, which diverges with existing theories but agrees well with the experimental data obtained earlier for similar half-metallic ferromagnetic Heusler alloys.

Spin-dependent thermoelectric effect in Co2Fe0.4Mn0.6Si thin film with perpendicular magnetic anisotropy

We report the anomalous Nernst effect in trilayers containing a thin film of the half-metallic ferromagnetic Heusler alloy Co2Fe0.4Mn0.6Si with perpendicular magnetic anisotropy. The structure is MgO/CFMS/Pd and we have studied the variation of anomalous Hall and Nernst effects as a function of CFMS and Pd thickness. The anomalous Nernst coefficient reaches 0.5 μV/K at room temperature and we have observed a strong dependence of the anomalous Nernst coefficient on the thickness of both layers. Our results indicate that inducing perpendicular magnetic anisotropy in a strongly spin-polarising Heusler alloy such as CFMS is very promising for new thermoelectric devices based on exploiting the anomalous Nernst effect.

Stability of Weyl points in magnetic half-metallic Heusler compounds

We employ {\it ab-initio} fully-relativistic electronic structure calculations to study the stability of the Weyl points in the momentum space within the class of the half-metallic ferromagnetic full Heusler materials, by focusing on Co$_2$TiAl as a well-established prototype compound. Here we show that both the number of the Weyl points together with their $k$-space coordinates can be controlled by the orientation of the magnetization. This alternative degree of freedom, which is absent in other topological materials (e.g. in Weyl semimetals), introduces novel functionalities, specific for the class of half-metallic ferromagnets. Of special interest are Weyl points which are preserved irrespectively of any arbitrary rotation of the magnetization axis.

Perpendicular magnetic anisotropy in Co2Fe0.4Mn0.6Si

We report perpendicular magnetic anisotropy (PMA) in the half-metallic ferromagnetic Heusler alloy Co2Fe0.4Mn0.6Si (CFMS) in a MgO/CFMS/Pd trilayer stack. PMA is found for CFMS thicknesses between 1 and 2 nm, with a magnetic anisotropy energy density of KU=1.5×106 erg/cm3 for tCFMS=1.5 nm. Both the MgO and Pd layer are necessary to induce the PMA. We measure a tunable anomalous Hall effect, where its sign and magnitude vary with both the CFMS and Pd thickness.

Specific features of the electrical resistivity of half-metallic ferromagnets Co2 MeAl (Me = Ti, V, Cr, Mn, Fe)

The transport properties of half-metallic ferromagnetic Heusler alloys Co2 MeAl (Me = Ti, V, Cr, Mn, Fe are transition 3d metals) have been measured in the temperature range of 4–900 K. The specific features of the behavior of the resistivity have been considered in the framework of the two-current model of conductivity that takes into account the existence of the energy gap in the electronic spectra of the alloys near the Fermi level of one of electron subbands that differs in the spin direction.

Electronic, Elastic, and Magnetic Properties of the Full-Heusler with the 4d Transition Metal Element, Co2YSi, Co2ZrSi, and Co2Y0.5Zr0.5Si: a First-Principle Study

The full-potential all-electron linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K, has been used to systematically investigate the structural, electronic, elastic, and magnetic properties of the half-metallic ferromagnetic Heusler compounds with 4d transition elements Co2YSi, Co2ZrSi, and alloy Co2Y0.5Zr0.5Si presented. The theoretical formalism used is the generalized gradient approximation to density functional theory (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional. The calculated lattice parameters and magnetic moments agree well with the available theoretical results. The spin-polarized density of states (DOS) of Co2YSi and Co2ZrSi indicate a half-metallic behavior with vanishing electronic density of states for minority spin at the Fermi level, which yields a perfect spin polarization while for Co2Y0.5Zr0.5Si shows a nearly half-metallic behavior with small spin-down electronic density of states at the Fermi level.

Peculiarities of the electronic transport in Co2CrAl and Co2CrGa half-metallic ferromagnets

We present results on the transport properties of the half-metallic ferromagnetic Heusler alloys Co2CrAl and Co2CrGa in the temperature range from 4 to 900 K. The peculiarities of the resistivity and the absolute differential thermoelectric power are considered within a two-current model of conductivity, taking into account the energy gap at the Fermi level in the electronic spectrum of alloys for electrons with spin opposite to the direction of the magnetization vector.

Specific features of the properties of half-metallic ferromagnetic Heusler alloys Fe2MnAl, Fe2MnSi, and Co2MnAl

The structural, magnetic, and electrical properties of half-metallic Heusler alloys Fe2MnAl, Fe2MnSi, and Co2MnAl have been investigated in the temperature range of 4–900 K. According to the X-ray diffraction analysis, these alloys have the B2 and L21 structures with different degrees of atomic order. The magnetic state of the alloys is considered as a two-sublattice ferrimagnet. The electrical resistivity and thermoelectric power have been discussed in the framework of the two-current conduction model taking into account the existence of an energy gap in the electronic spectrum of the alloys near the Fermi level for the subband with spin-down (minority) electrons.

Electronic transport in Co-based half-metallic ferromagnetic Heusler alloys

The electroresistivity of the Co-based half-metallic ferromagnetic Co2CrGa, Co2CrAl, Co2VAl, Co2MnAl Heusler alloys and the Ni2MnGa ferromagnets were measured in the temperature range from 4.2 to 900 K. Specific features in the electrical resistivity of the half-metallic ferromagnetic Heusler alloys were observed, which can be explained in terms of the two-current conduction model, taking into account the existence of an energy gap in the electron spectrum near the Fermi level.

Specific features of the electrical resistivity of half-metallic ferromagnets Fe2MeAl (Me = Ti, V, Cr, Mn, Fe, Ni)

The transport properties of half-metallic ferromagnetic Heusler alloys Fe2MeAl (where Me = Ti, V, Cr, Mn, Fe, and Ni are 3d transition elements) have been measured in the temperature range of 4–900 K. The specific features in the behavior of the electrical resistivity have been considered in terms of the two-current conduction model, which takes into account the presence of an energy gap in the electron spectrum of the alloys near the Fermi level.