Showing papers on "Half-metal published in 2013"

Spin injection from Heusler alloys into semiconductors: A materials perspective

Abstract: The notion of using electron spins as bits for highly efficient computation coupled with non-volatile data storage has driven an intense international research effort over the past decade. Such an approach, known as spin-based electronics or spintronics, is considered to be a promising alternative to charge-based electronics in future integrated circuit technologies. Many proposed spin-based devices, such as the well-known spin-transistor, require injection of spin polarized currents from ferromagnetic layers into semiconductor channels, where the degree of injected spin polarization is crucial to the overall device performance. Several ferromagnetic Heusler alloys are predicted to be half-metallic, meaning 100% spin-polarized at the Fermi level, and hence considered to be excellent candidates for electrical spin injection. Furthermore, they exhibit high Curie temperatures and close lattice matching to III-V semiconductors. Despite their promise, Heusler alloy/semiconductor heterostructures investigated in the past decade have failed to fulfill the expectation of near perfect spin injection and in certain cases have even demonstrated inferior behavior compared to their elemental ferromagnetic counterparts. To address this problem, a slew of theoretical and experimental work has emerged studying Heusler alloy/semiconductor interface properties. Here, we review the dominant prohibitive materials challenges that have been identified, namely atomic disorder in the Heusler alloy and in-diffusion of magnetic impurities into the semiconductor, and their ensuing detrimental effects on spin injection. To mitigate these effects, we propose the incorporation of half-metallic Heusler alloys grown at high temperatures (>200 °C) along with insertion of a MgO tunnel barrier at the ferromagnet/semiconductor interface to minimize magnetic impurity in-diffusion and potentially act as a spin-filter. By considering evidence from a variety of structural, optical, and electrical studies, we hope to paint a realistic picture of the materials environment encountered by spins upon injection from Heusler alloys into semiconductors. Finally, we review several emerging device paradigms that utilize Heusler alloys as sources of spin polarized electrons.

Spintronics : From Materials to Devices

Abstract: Heusler compounds at a glance.- New Heusler compounds and their properties.- Crystal structure of Heusler compounds.- Substitution effects in double Perovskites: How the crystal structure influences the electronic properties.- Half-metallic ferromagnets.- Correlation and chemical disorder in Heusler compounds: a spectroscopical study.- Theory of the half-metallic Heusler compounds.- Electronic structure of complex oxides.- Local structure of highly spin polarized Heusler compounds revealed by nuclear magnetic resonance spectroscopy.- New materials with high spin polarization investigated by X-ray magnetic circular dichroism.- Hard X-ray photoelectron spectroscopy of new materials for spintronics.- Characterization of the surface electronic properties of Co2Cr1-xFexAl.- Magneto-optical investigations and ion beam-induced modification of Heusler compounds.- Co2Fe(Al1-xSix) Heusler alloys and their applications to spintronics.- Transport properties of Co2(Mn,Fe)Si thin films.- Preparation and investigation of interfaces of Co2Cr1-xFexAl thin films.- Tunnel magnetoresistance effect in tunnel junctions with Co2MnSi Heusler alloy electrode and MgO barrier.

Novel half-metal and spin gapless semiconductor properties in N-doped silicene nanoribbons

Abstract: We carry out a spin polarized first-principles study on the energetic and electronic properties of zigzag silicene nanoribbons (ZSiNRs) doped with N atoms, as well as N and Si vacancy (VSi) complexes The formation energy analysis shows that the doped N atom and N-VSi complex prefer the edge sites in ZSiNRs Due to breaking the degeneracy of the spin-polarization in ZSiNR, the substitution of N for Si atom exhibits a spin gapless semiconductor (SGS) property When the N-VSi complex is introduced forming so called pyridine- and pyrrole-like structure in ZSiNR, they also exhibit half-metal or SGS behaviors with 100% spin-polarized currents in the Fermi level These interesting properties may further stimulate potential applications of silicene-based nanostructures in nanoelectronics

Ab initio studies of disorder in the full Heusler alloy Co2FexMn1−xSi

Abstract: Co-based full Heusler alloys have been predicted to be half-metals, with 100% spin polarisation at the Fermi level, yet this has yet to be realised in practice. Heusler thin-films often exhibit a degree of atomic disorder, and this is believed to be one cause of the low magnetoresistance in Heusler-based spin valves' devices. We present an ab initio density functional theory + U investigation into the effect of disorder on the electronic properties of Co2Fe0.5Mn0.5Si. It is found that the half-metallicity depends strongly on the kind of disorder present, with low or even reversed spin-polarisation when the Co sublattice mixes with either the Fe/Mn or Si sublattice, but keeps the high spin polarisation when the Mn/Fe and Si sublattices are mixed. Calculations of the formation energy show that this latter kind of disorder is by far the most likely to occur, an encouraging result which means that it may not be necessary to produce perfectly ordered Heusler alloys in order to achieve 100% spin polarisation.

Magnetic Heusler Compounds

Abstract: Heusler compounds are a remarkable class of intermetallic materials with 1:1:1 (often called Half-Heusler) or 2:1:1 composition comprising more than 1500 members. New properties and potential fields of applications emerge constantly; the prediction of topological insulators is the most recent example. Surprisingly, the properties of many Heusler compounds can easily be predicted by the valence electron count or within a rigid band approach. The wide range of the multifunctional properties of Heusler compounds is reflected in extraordinary magnetooptical, magnetoelectronic, and magnetocaloric properties. Co 2 -Heusler compounds are predicted and proven half-metallic ferromagnets showing Slater–Pauling type behavior. The recently discovered Mn 2 -Heusler compounds are another class of half metallic Heusler compounds which even can be designed to be compensated ferrimagnets. Tetragonal Heusler compounds Mn 2 YZ as potential materials for STT applications can be easily designed by positioning the Fermi energy at the van Hove singularity in one of the spin channels.

Control of spin in a La(Mn,Zn)AsO alloy by carrier doping

Abstract: The control of spin without magnetic field is one of the challenges in developing spintronic devices. In an attempt to solve this problem, we proposed a novel hypothetical La(Mn0.5Zn0.5)AsO alloy from two experimentally synthesized rare earth element transition metal arsenide oxides, i.e. LaMnAsO and LaZnAsO. On the basis of the first-principles calculations with strong-correlated correction, we found that the La(Mn0.5Zn0.5)AsO alloy is an antiferromagnetic semiconductor at the ground state, and a bipolar magnetic semiconductor at the ferromagnetic state. Both electron and hole doping in the La(Mn0.5Zn0.5)AsO alloy induce the transition from antiferromagnetic to ferromagnetic, as well as semiconductor to half metal. In particular, the spin-polarization direction is switchable depending on the doped carrier type. As carrier doping can be realized easily in experiment by applying a gate voltage, the La(Mn0.5Zn0.5)AsO alloy stands as a promising spintronic material to generate and control the spin-polarized carriers with an electric field.

Minority-spin t(2g) states and the degree of spin polarization in ferromagnetic metallic La(2-2x)Sr(1+2x)Mn(2)O(7) (x = 0.38).

Abstract: A half-metal is a material with conductive electrons of one spin orientation. This type of substance has been extensively searched for due to the fascinating physics as well as the potential applications for spintronics. Ferromagnetic manganites are considered to be good candidates, though there is no conclusive evidence for this notion. Here we show that the ferromagnet La2−2xSr1+2xMn2O7 (x = 0.38) possesses minority-spin states, challenging whether any of the manganites may be true half-metals. However, when electron transport properties are taken into account on the basis of the electronic band structure, we found that the La2−2xSr1+2xMn2O7 (x = 0.38) can essentially behave like a complete half metal.

A first principles study of novel one-dimensional organic half-metal vanadium-cyclooctatetraene wire

Abstract: The structural, electronic, and magnetic properties of one-dimensional vanadium-cyclooctatetraene[(V-COT)]∞ wire and sandwich clusters are investigated by means of density functional theory. It is found that the (V-COT)∞ SMW is half-metallic. Through the spin transportation calculations, the system for V-COT clusters coupled to gold electrodes performs nearly perfect spin filters. In addition, the I-V curve shows obviously negative differential resistance effects. These results suggest the potential applications of (V-COT)∞ in spintronics.

Exchange bias in thin Heusler alloy films in contact with antiferromagnet

Abstract: Exchange bias is studied in thin film systems, in which three types of Heusler alloys (Ni2MnSn, Co2MnSn, and Co2FeSi) are in contact with an antiferromagnet. Magnetic exchange interactions between the constituting atoms (i.e., Ni-Mn, Mn-Mn, Co-Mn, and Co-Fe, or Co-Co) differ substantially in these Heusler alloys. We explain the influence of the exchange stiffness A within the Heusler alloys and of the exchange coupling between Heusler alloy and an antiferromagnet in a finite interface volume. Insertion of an ultrathin Co layer at interfaces brings about an enhancement of the exchange bias in Heusler alloy/antiferromagnet layer system.

Excess manganese as the origin of the low-temperature anomaly in NiMnSb

Abstract: The archetype of half-metallic magnetism, NiMnSb, has been reported to show an anomaly at low temperature. The high degree of spin polarization of the conduction electrons, characteristic of a half metal, is lost above this temperature. Recently reported experiments show that this anomaly is not an intrinsic property of NiMnSb: it requires an excess of (interstitial) manganese. Electronic structure calculations reported here show that the excess manganese orders antiferromagnetically with respect to the host magnetization, reduces the half-metallic band gap, and pushes the top of the valence band up to 36 meV below the Fermi level. Thermal excitations from minority to majority spin channel induce an avalanche effect, leading to the disordering of the magnetic moments of the excess manganese. This mechanism is supported by measurements of the magnetization as a function of temperature on NiMn${}_{1.05}$Sb: It shows a maximum in the magnetization measured in a field of 400 Oe.

New Heusler Compounds Cr 2 YSb (Y=Fe, Co and Ni): Magnetic and Electronic Properties

Abstract: The electronic and magnetic properties of the Cr2YSb (Y=Co, Fe, and Ni) Heusler alloys with both CuHg2Ti-type and AlCu2Mn-type structures have been investigated using first-principles calculations based on density functional theory (DFT). Two nearly half-metallic ferrimagnets (HMFs), Cr2CoSb, and Cr2FeSb in CuHg2Ti-type structure, are predicted. The energy gap lies in the minority spin band for both alloys. The calculated total spin magnetic moments are 2μ B and 1μ B per unit cell for Cr2CoSb and Cr2FeSb alloys, respectively, which are in good agreement with the Slater–Pauling relation. For these alloys, the magnetic moments of Y and Cr(B) are antiparallel to that of Cr(A) and all of these moments increase with increasing lattice constant. It was also found that the half-metallic properties of Cr2CoSb and Cr2FeSb are unaffected to the lattice distortion and the half-metallicity can be obtained within the wide range of 5.52–6.07 A and 5.96–6.16 A for Cr2CoSb and Cr2FeSb alloys, respectively.

Pressure induced semiconductor to half metal transition in Sr2NiReO6

Abstract: Electronic and magnetic properties of Sr2NiReO6 have been studied by using the density functional method. Experimentally, Sr2NiReO6 is a semiconductor at ambient conditions. By applying external pressure, a transition from semiconductor to half metal is realized. Compared with ambient conditions, both the spin magnetic moments and the orbital moments of Ni and Re remain almost unchanged after pressure, indicating that no spin state transition is found. The estimated transition pressure is no more than 22.9 GPa.

A First-Principles Study of Heusler Alloy Cr 2 ZrGe

Abstract: The band structures and magnetic properties of Heusler alloy Cr2ZrGe with Cu2MnAl -type have been investigated by first-principles calculations. The alloy is predicted to be close to half-metal ferromagnet. The alloy has a total magnetic moment of 3.98μB per unit cell on first-principles calculations which is in agreement with the SlaterPauling (SP) rule. The magnetic moments of Cr(A) atom and Cr(C) atom which are both much larger than that of atom Zr(C) are same. This similarity comes from similar atom coordination surroundings of Cr(A) and Cr(C) atoms in crystal structure.

Coulomb‐enhanced spin–orbit coupling and semiconductor to half‐metal transition under pressure in Sr2CrReO6

Abstract: Ordered Sr2CrReO6 has been synthesized recently. It is measured to be ferrimagnetic semiconductor, in contrary to the previous reports of metallic properties. To solve the discrepancy, we have investigated the compound by using the density functional theory. The semiconducting behavior is reproduced by including the electron correlation and spin-orbit coupling simultaneously. The calculated band gap is 0.22 eV, close to the experimental value of 0.21 eV. A large orbital moment of 0.69 mu(B) is found for Re, which is caused by the Coulomb-enhanced spin-orbit coupling. By applying pressure, a semiconductor to half-metal transition is observed through 5% volume compression. [GRAPHICS] (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Control of Spin in La(Mn,Zn)AsO Alloy by Carrier Doping

Abstract: The control of spin without magnetic field is one of challenges in developing spintronic devices. In an attempt to solve this problem, we proposed a novel hypothetic LaMn0.5Zn0.5AsO alloy from two experimentally synthesized rare earth element transition metal arsenide oxides, i.e. LaMnAsO and LaZnAsO. On the basis of the first-principles calculations with strong-correlated correction, we found that the LaMn0.5Zn0.5AsO alloy is an antiferromagnetic semiconductor at ground state, while bipolar magnetic semiconductor at ferromagnetic state. Both electron and hole doping in the LaMn0.5Zn0.5AsO alloy induces the transition from antiferromagnetic to ferromagnetic, as well as semiconductor to half metal. In particular, the spin-polarization direction is switchable depending on the doped carrier's type. As carrier doping can be realized easily in experiment by applying a gate voltage, the LaMn0.5Zn0.5AsO alloy stands for a promising spintronic material to generate and control the spin-polarized carriers with electric field.

Structural and magnetic properties of Fe 2 CrSi Heusler Alloy and Tunneling magnetoresistance of its magnetic tunneling junctions

Abstract: We report the magnetic properties, microstructure and surface morphology of epitaxially grown Fe2CrSi films. Highly ordered B2 films were obtained by deposition at room temperature followed by annealing at 400°C. Magnetic tunnel junctions using Fe2CrSi show a tunnelling magnetoresistance (TMR) of 2.5%. The low TMR is ascribed to the oxidation of Fe2CrSi at the interface with MgO. An enhancement of TMR to 8.1% was achieved by inserting a 0.3nm Mg between Fe2CrSi and MgO to prevent the oxidation of Fe2CrSi.

ELECTRONIC STRUCTURE AND HALF-METALLICITY OF HEUSLER ALLOY Mn2RhAl

Abstract: The site preference, electronic structure and magnetic properties of Mn2RhAl have been studied by first-principles calculations. Both the Cu2MnAl-structure and the Hg2CuTi-type have been tested. For the compound Mn2RhAl, the Hg2CuTi structure is the more stable one with a lattice parameter of 5.80 A. The Mn2RhAl alloy is predicted to be a half-metal with 100% spin polarization of the conduction electrons at the Fermi level (EF). The calculated total magnetic moment is 2.00 μB per unit cell, which is in line with the Slater–Pauling curve of Mt = Zt-24. The Mn(A) and Mn(B) atom-projected spin moments are -1.54 μB and 3.16 μB, respectively. The resulting moment is mainly determined by the antiparallel aligned Mn(A) and Mn(B) spin moment. Whereas, the small spin magnetic moment of Rh is small and only 0.38 μB and the Al atom is almost nonmagnetic. Such an alloy may be a promising material for future spintronics devices.

Epitaxial growth and physical properties of Heusler/perovskite heterostructures

Abstract: Multiferroic heterostructures of the ferromagnetic, half-metallic Heusler Fe2CrSi (FCS) and the ferroelectric perovskite Ba0.7Sr0.3TiO3 (BSTO) have been formed by magnetron sputtering, and their magnetic and ferroelectric properties have been investigated. FCS/BSTO bilayer structures were epitaxially grown on LaAlO3 substrates with epitaxial relationships of FCS (001)[110]//BSTO (001)[100]. Multiferroic properties with a remanent polarization of 10.6 µC/cm2 and a saturation magnetization of 417 emu/cc were observed for the FCS/BSTO heterostructures at room temperature. These results suggest that the Heusler/perovskite epitaxial heterostructure is a promising candidate for fabricating multiferroic devices.

Magnetic Surface States in High Polarization Materials

Abstract: Surfaces are often different materials, and typically have a different electronic structure from the bulk and since the dawn of surface science, surface-localized electronic states, surface states, have been extensively studied and investigated with growing accuracy. Of particular importance to spintronics are magnetic surface states. Interfaces will play a very important role in many spintronics devices, yet the interface properties are often ignored, poorly understood or badly characterized. For many nominally half metal materials, materials that in some ground state calculations exhibit 100% spin polarization, the magnetic surface states may significantly reduce the effective spin polarization. We review the magnetic surface states of several well known and often highly touted high spin polarized materials such as NiMnSb, Fe 3 O 4 , CoS 2 and CrO 2 . Finally, we summarize surface state measurements of magnetoelectric antiferromagnets Cr 2 O 3 , which has electrically controllable net surface spins, a major complication to the study of CrO 2 by photoemission.