Showing papers on "Half-metal published in 2014"

Atomic-scale structure and properties of highly stable antiphase boundary defects in Fe 3 O 4

Abstract: The complex and intriguing properties of the ferrimagnetic half metal magnetite (Fe3O4) are of continuing fundamental interest as well as being important for practical applications in spintronics, magnetism, catalysis and medicine. There is considerable speculation concerning the role of the ubiquitous antiphase boundary (APB) defects in magnetite, however, direct information on their structure and properties has remained challenging to obtain. Here we combine predictive first principles modelling with high-resolution transmission electron microscopy to unambiguously determine the three-dimensional structure of APBs in magnetite. We demonstrate that APB defects on the {110} planes are unusually stable and induce antiferromagnetic coupling between adjacent domains providing an explanation for the magnetoresistance and reduced spin polarization often observed. We also demonstrate how the high stability of the {110} APB defects is connected to the existence of a metastable bulk phase of Fe3O4, which could be stabilized by strain in films or nanostructures.

Tunable electronic and magnetic properties of graphene-like ZnO monolayer upon doping and CO adsorption: a first-principles study

Abstract: Graphene-like zinc oxide monolayer (g-ZnO) is a new class of two-dimensional nanomaterials with unique new properties that is still largely unknown. This work studies the tunability of the electronic and magnetic properties of g-ZnO upon chemical doping (with B, N and C) and CO adsorption by using first-principles calculations. Both electronic and magnetic properties of g-ZnO exhibit strong dependency on its structural change and molecular adsorption. The g-ZnO with oxygen atom substituted by a C or N atom (one atom per supercell) are ferromagnetic (FM) half metal (HM), while that substituted by a B atom is an FM semiconductor. The doped g-ZnO shows strong chemisorption to CO molecule by forming A–CO bond (A = B, N or C), in contrast to the weak physisorption of the intrinsic g-ZnO. Furthermore, CO adsorption converts the N- and C-doped g-ZnO to n-type semiconductor with nonmagnetic (NM) ground states, while B-doped g-ZnO becomes a ferromagnetic half metal (FM-HM). The mechanism for property change has been investigated by analyzing their partial density of states (PDOS) upon different conditions. This study provides insights in the physical properties and chemical reactivity of g-ZnO, which could help in realizing their diverse potentials in electronic and magnetic devices.

Electronic and phonon properties of the full-Heusler alloys X2YAl (X = Co, Fe and Y = Cr, Sc): a density functional theory study

Abstract: First-principle calculations have been carried out on the structural, electronic, elastic, and phonon properties of the full-Heusler alloys X2YAl (X = Co, Fe and Y = Cr, Sc). The calculations predict that the Fe2CrAl and Co2CrAl are half-metallic ferromagnets at the equilibrium lattice constant with a minority-spin energy gap of 0.2912 and 0.668 eV, respectively. Fe2ScAl exhibit a gap in the majority density of states, with a few states at the Fermi level and about 0.217 states eV−1, unlike the other Heusler compounds; due to this, it is considered a false half metal, and Co2ScAl is considered a non-magnetic compound. The elastic constants were derived from the slopes of the acoustic branches in the phonon-dispersion curve. The calculated lattice constants, bulk modulus, and first-order pressure derivative of the bulk modulus are reported for the L21 structure and compared with previous values. Phonon-dispersion curves were obtained using the first-principle linear-response approach of the density-functional perturbation theory. The specific heat capacity at a constant volume C V of X2YAl (X = Co, Fe and Y = Cr, Sc) alloys is calculated and discussed.

Half metal in two-dimensional hexagonal organometallic framework

Abstract: Two-dimensional (2D) hexagonal organometallic framework (HOMF) made of triphenyl-metal molecules bridged by metal atoms has been recently shown to exhibit exotic electronic properties, such as half-metallic and topological insulating states. Here, using first-principles calculations, we investigate systematically the structural, electronic, and magnetic properties of such HOMFs containing 3d transition metal (TM) series (Sc to Cu). Two types of structures are found for these HOMFs: a buckled structure for those made of TMs with less half-filled 3d band and a twisted structure otherwise. The HOMFs show both ferromagnetic and antiferromagnetic properties, as well as nonmagnetic properties, due to the electronic configuration of the TM atoms. The V, Mn, and Fe lattices are ferromagnetic half metals with a large band gap of more than 1.5 eV in the insulating spin channel, making them potential 2D materials for spintronics application.

Efficient spin-light emitting diodes based on InGaN/GaN quantum disks at room temperature: a new self-polarized paradigm.

Abstract: A well-behaved spin-light emitting diode (LED) composed of InGaN/GaN multiple quantum disks (MQDs), ferromagnetic contact, and Fe3O4 nanoparticles has been designed, fabricated, and characterized. The degree of circular polarization of electroluminescence (EL) can reach up to a high value of 10.9% at room temperature in a low magnetic field of 0.35 T, which overcomes a very low degree of spin polarization in nitride semiconductors due to the weak spin-orbit interaction. Several underlying mechanisms play significant roles simultaneously in this newly designed device for the achievement of such a high performance. Most of all, the vacancy between nanodisks can be filled by half-metal nanoparticles with suitable energy band alignment, which enables selective transfer of spin polarized electrons and holes and leads to the enhanced output spin polarization of LED. Unlike previously reported mechanisms, this new process leads to a weak dependence of spin relaxation on temperature. Additionally, the internal strain in planar InGaN/GaN multiple quantum wells can be relaxed in the nanodisk formation process, which leads to the disappearance of Rashba Hamiltonian and enhances the spin relaxation time. Our approach therefore opens up a new route for the further research and development of semiconductor spintronics.

Magnetic properties of Co2Fe(Ga1−xSix) alloys

Abstract: Magnetic and crystallographic properties of bulk Co2Fe(Ga1−xSix) alloys with 0≤x≤1 are reported in this work. The alloys with x=0.75 and 1.00 exhibit L21 structure whereas the alloys with x=0, 0.25 and 0.50 crystallized in the disordered A2 phase. Unit cell volume of this series of alloys decreased from 189.1 to 178.5 A3 as x was increased from 0 to 1.00. All alloy compositions exhibit ferromagnetic behavior with a high Curie temperature (TC) which showed a systematic variation with x (1089 K, 1075 K, 1059 K, 1019 K and 1015 K for x=0, 0.25, 0.5, 0.75 and 1.00, respectively). The saturation magnetization moment Ms for the alloys with x=0, 0.25 and 0.50 are 5.05μB, 5.23μB, 5.49μB, respectively, in accordance with the Slater–Pauling rule, but alloys with x=0.75 and 1.00 deviated from this rule. The effective moment per magnetic atom (pc) of the alloys was estimated from the inverse DC magnetic susceptibility data above TC. A comparison of Ms with pc reveals the half-metallic character of the alloys.

Half Antiperovskites VI: On the Substitution Effects in Shandites InxSn2–xCo3S2 †‡

Abstract: In the shandite type solid solution InxSn2–xCo3S2 the transition from half metal ferromagnetic Sn2Co3S2 to the new thermoelectric InSnCo3S2 is related to A = In, Sn on different crystallographic sites. Effects and origin of crystal and electronic structure changes induced by A = In are now investigated within the solid solution 0 ≤ x ≤ 2 including In2Co3S2. Effects are studied from X-ray data, 119Sn Mosbauer spectroscopy, and ab initio calculations. Their origin is explored by DFT modeling on site preference of In and Sn in a supercell, electric field gradients (EFG), spin polarization, band structures, and direct space analyses (ELF, AIM). Indium is found to cause the crystal structure distortion on one A site, the electronic structure distortion to the other, as a consequence of inverted anisotropic bonding.

Origin and effect of In–Sn ordering in InSnCo3S2: a neutron diffraction and DFT study

Abstract: The solid solution In2−xSnxCo3S2 is attractive due to a variety of interesting properties depending on the In/Sn content, i.e. half metal ferromagnetic Sn2Co3S2, low dimensional metal In2Co3S2, and semiconducting thermoelectric InSnCo3S2. For the latter, crystal structure effects and a metal to insulator transition are not only related to electron counting but also to ordering of In and Sn within and between Co Kagome nets. These observations have not been adequately understood to date. The degree of ordering is now evaluated from neutron diffraction data to distinguish In and Sn. The origin and effects on crystal and electronic structures are studied by DFT calculations on a superstructure model. Relations of local bonding (electron localization function ELF and Bader's AIM theory), In/Sn site preference, crystal structure distortions, and the opening of the gap are explored. Results are generalised from predictions on isoelectronic compounds.

Magnetic Properties of SrO Doped with 3d Transition Metals

Abstract: Based on the density functional theory and using the Korringa-Kohn-Rostoker coherent potential approxima- tion (KKR-CPA) method, we study the (Sr, TM)O doped systems where TM = V, Cr, Mn, Fe, Co, and Ni atoms. In particular, we start first by relaxing the parameters of the corresponding structures. Then we discuss its electronic structures, magnetic stabilities, and half-metal properties us- ing 3d transition metals. Among others, it has been shown that doping with Cr, Mn, Fe, and Co, the ferromagnetic phase can be stabilized using a double exchange mechanism. Moreover, we find that the half-metallic properties of these compounds are formed due to a large exchange splitting and the delocalized properties of the majority spin eg state and the minority spin teg states.

Fate of half-metallicity near interfaces: the case of NiMnSb/MgO and NiMnSi/MgO.

Abstract: The electronic and magnetic properties of the interfaces between the half-metallic Heusler alloys NiMnSb, NiMnSi, and MgO have been investigated using first-principles density-functional calculations with projector augmented wave potentials generated in the generalized gradient approximation. In the case of the NiMnSb/MgO (100) interface, the half-metallicity is lost, whereas the MnSb/MgO contact in the NiMnSb/MgO (100) interface maintains a substantial degree of spin polarization at the Fermi level (∼60%). Remarkably, the NiMnSi/MgO (111) interface shows 100% spin polarization at the Fermi level, despite considerable distortions at the interface, as well as rather short Si/O bonds after full structural optimization. This behavior markedly distinguishes NiMnSi/MgO (111) from the corresponding NiMnSb/CdS and NiMnSb/InP interfaces.

Electronic structure and half-metallicity of the Heusler alloy Co 2 ZrGe

Abstract: The site preference, the electronic structure and the magnetic properties of Co2ZrGe have been studied by using first-principles calculations, and the stabilities of the Cu2MnAl-type and the Hg2CuTi-type structures have been tested in this respect. The Cu2MnAltype structure is more favorable than the Hg2CuTitype structure for the Co2ZrGe compound, and the equilibrium lattice parameter of the Cu2MnAltype Co2ZrGe alloy is 6.06 A. The Co2ZrGe alloy is found to have an energy gap in the minority spin direction at the Fermi level (E F ) and the majority spin band shows strongly metallic characteristic. As a result, the Co2ZrGe alloy is predicted to be a half-metal with 100% spin polarization of the conduction electrons at the E F . The calculated total magnetic moment is 2.00µ B per unit cell, which is in line with the Slater-Pauling curve of M t = Z t − 24. The Co atom-projected spin moment is 1.02µ B , which mainly determines the total moment. Simultaneously, the Zr and the Ge atom moments are −0.08µ B and 0.04µ B , respectively. The Co2ZrGe alloy may be a promising material for application in future spintronics devices.

Transition from half metal to semiconductor in Li doped g-C4N3

Abstract: We have investigated the structural and magnetic properties of Li doped graphitic carbon nitride (g-C4N3) using the van der Waals density functional theory. A free standing g-C4N3 was known to show a half metallic state with buckling geometry, but this feature completely disappears in the presence of Li doping. Besides this structural modification, very interestingly, we have obtained that the Li doped g-C4N3 shows dramatic change in its electronic structure. Both ferromagnetic and nonmagnetic states are almost degenerated in one Li atom doped system. However, the transition from half metallic state to semiconductor is observed with further increase of Li concentration and the calculated energy gap is 1.97 eV. We found that Li impurity plays as a donor element and charge transfer from the Li atom to neighboring N atoms induces a band gap. Overall, we have observed that the electronic and magnetic properties of g-C4N3 are substantially modified by Li doping.

Ferri-to-ferro-magnetic and ferro-to-para-magnetic transitions in Ni48Co2Mn35In13Ga2 Heusler alloy

Abstract: Heusler alloys feature both conventional and inverse magnetocaloric effects near room temperature as they undergo two different transitions. In this paper, new data are presented and analyzed and a new mechanism to explain the complex hysteretic behavior of a Ni48Co2Mn35In13Ga2 Heusler alloy is developed. This mechanism explains isothermal loops near room temperature. The various descriptions and classifications of these transitions, however, is not critical to this analysis.

Insulator–half metal transition driven by hole doping: a density functional study of Sr-doped La2VMnO6

Abstract: La2VMnO6 is an insulating ferrimagnet experimentally. By substituting La with Sr, La2-xSrxVMnO6 (x = 0.5, 1.0, 1.5, 2.0) was investigated using the density functional theory. Our results indicated that half metallic properties are obtained for x = 0.5, 1.5, 2.0. For x = 1.0, it is insulating. With the increase of hole doping, the holes initially go to V 3d orbitals for x = 0.5, 1.0, and after that, the holes go to the Mn 3d orbitals for x = 1.5, 2.0. Ferrimagnetic coupling between V and Mn is found to be the ground state for x = 0.5, 1.5, while for x = 1.0 and 2.0, ferromagnetic and antiferromagnetic couplings between Mn and Mn are competitive for the ground state.

Pressure dependence of half metallic behavior of Co2VZ (Z=Si, Ge)-An ab initio Study

Abstract: The structural, electronic and magnetic properties of Co-based Heusler compounds Co2VZ (Z=Si, Ge) under pressure are studied using first-principles density functional theory. The total magnetic moment decreases on compression. Under application of external pressure, the valence band and conduction band are shifted downward which leads to modification of electronic structure. True half metallic to nearly half metallic transition is observed at 30 GPa, 50 GPa for Co2VSi and Co2VGe respectively. Around 100 GPa, Co2VGe shows complete metallic behavior. The half metal to metal transition is accompanied by quenching of magnetic moment.

Nmr spectroscopy on heusler thin films — a review

Abstract: Heusler compounds exhibit different electronic ground states and functionalities, making them attractive materials for studies of their fundamental properties and for their technological exploitation. The high spin polarization, predicted in particular for Co2-based Heusler compounds, renders them prime candidates for electrode materials in spintronic devices such as giant magnetoresistance (GMR) elements or magnetic tunnel junctions and requires their implementation in thin film stacks. The growth of high quality Heusler films, however, demands their careful characterization. Typical issues in Heusler thin films are, besides the type and degree of structural order, the control of the film composition and the conservation of smooth interfaces between different layers in the film, e.g., between the Heusler layer and the tunneling barrier, while at the same time enabling high structural order. This review illustrates how nuclear magnetic resonance spectroscopy contributes to those issues by discussing recent examples of nuclear magnetic resonance studies of Heusler thin films.

Investigation of the temperature-dependence of ferromagnetic resonance and spin waves in Co2FeAl0.5Si0.5

Abstract: Co2FeAl0.5Si0.5 (CFAS) is a Heusler compound that is of interest for spintronics applications, due to its high spin polarization and relatively low Gilbert damping constant. In this study, the behavior of ferromagnetic resonance as a function of temperature was investigated in CFAS, yielding a decreasing trend of damping constant as the temperature was increased from 13 to 300 K. Furthermore, we studied spin waves in CFAS using both frequency domain and time domain techniques, obtaining group velocities and attenuation lengths as high as 26 km/s and 23.3 um, respectively, at room temperature.

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

Abstract: 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.