Multilayers of Zinc-Blende Half-Metals with Semiconductors

We report on first-principles calculations of multilayers of zinc-blende half-metallic ferromagnets CrAs and CrSb with III-V and II-VI semiconductors, in the [001] orientation. We examine the ideal and tetragonalised structures, as well as the case of an intermixed interface. We find that, as a rule, half-metallicity can be conserved throughout the heterostructures, provided that the character of the local coordination and bonding is not disturbed. At the interfaces with semiconductors, we describe a mechanism that can give also a non-integer spin moment per interface transition atom, and derive a simple rule to evaluate it

Ferromagnetism in Nitrogen-doped MgO

The magnetic state of Nitrogen-doped MgO, with N substituting O at concentrations between 1% and the concentrated limit, is calculated with density-functional methods. The N atoms are found to be magnetic with a moment of 1 Bohr magneton per Nitrogen atom and to interact ferromagnetically via the double exchange mechanism. The long-range magnetic order is established above a finite concentration of about 1.5% when the percolation threshold is reached. The Curie temperature increases linearly with the concentration, and is found to be about 30 K for 10% concentration. Besides the substitution of single Nitrogen atoms, also interstitial Nitrogen atoms, clusters of Nitrogen atoms and their structural relaxation on the magnetism are discussed. Possible scenarios of engineering a higher Curie temperature are analyzed, with the conclusion that an increase of the Curie temperature is difficult to achieve, requiring a particular attention to the choice of chemistry

Thermal collapse of spin-polarization in half-metallic ferromagnets

The temperature dependence of the magnetization and spin-polarization at the Fermi level is investigated for half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab-initio calculations. The thermal properties are studied by mapping ab-initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method

Non-collinear Korringa-Kohn-Rostoker Green function method: Application to 3d nanostructures on Ni(001)

Magnetic nanostructures on non-magnetic or magnetic substrates have attracted strong attention due to the development of new experimental methods with atomic resolution. Motivated by this progress we have extended the full-potential Korringa-Kohn-Rostoker (KKR) Green function method to treat non-collinear magnetic nanostructures on surfaces. We focus on magnetic 3d impurity nanoclusters, sitting as adatoms on or in the first surface layer on Ni(001), and investigate the size and orientation of the local moments and moreover the stabilization of non-collinear magnetic solutions. While clusters of Fe, Co, Ni atoms are magnetically collinear, non-collinear magnetic coupling is expected for Cr and Mn clusters on surfaces of elemental ferromagnets. The origin of frustration is the competition of the antiferromagnetic exchange coupling among the Cr or Mn atoms with the antiferromagnetic (for Cr) or ferromagnetic (for Mn) exchange coupling between the impurities and the substrate. We find that Cr and Mn first-neighbouring dimers and a Mn trimer on Ni(001) show non-collinear behavior nearly degenerate with the most stable collinear configuration. Increasing the distance between the dimer atoms leads to a collinear behavior, similar to the one of the single impurities. Finally, we compare some of the non-collinear {\it ab-initio} results to those obtained within a classical Heisenberg model, where the exchange constants are fitted to total energies of the collinear states; the agreement is surprisingly good.Comment: 11 page

First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors

Employing first principles electronic structure calculations in conjunction with the frozen-magnon method we calculate exchange interactions, spin-wave dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin gapless semiconductor (SGS) compounds Mn$_2$CoAl, Ti$_2$MnAl, Cr$_2$ZnSi, Ti$_2$CoSi and Ti$_2$VAs. We find that their magnetic behavior is similar to the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice exchange interactions play an essential role in the formation of the magnetic ground state and in determining the Curie temperature, $T_\mathrm{c}$. All compounds, except Ti$_2$CoSi possess a ferrimagnetic ground state. Due to the finite energy gap in one spin channel, the exchange interactions decay sharply with the distance, and hence magnetism of these SGSs can be described considering only nearest and next-nearest neighbor exchange interactions. The calculated spin-wave dispersion curves are typical for ferrimagnets and ferromagnets. The spin-wave stiffness constants turn out to be larger than those of the elementary 3$d$-ferromagnets. Calculated exchange parameters are used as input to determine the temperature dependence of the magnetization and $T_\mathrm{c}$ of the SGSs. We find that the $T_\mathrm{c}$ of all compounds is much above the room temperature. The calculated magnetization curve for Mn$_2$CoAl as well as the Curie temperature are in very good agreement with available experimental data. The present study is expected to pave the way for a deeper understanding of the magnetic properties of the inverse-Heusler-based SGSs and enhance the interest in these materials for application in spintronic and magnetoelectronic devices.Comment: Accepted for publ;ication in Physical Review

Ballistic Spin Injection from Fe into ZnSe and GaAs with a (001), (111), and (110) orientation

We present first-principles calculations of ballistic spin injection in Fe/GaAs and Fe/ZnSe junctions with orientation (001), (111), and (110). We find that the symmetry mismatch of the Fe minority-spin states with the semiconductor conduction states can lead to extremely high spin polarization of the current through the (001) interface for hot and thermal injection processes. Such a symmetry mismatch does not exist for the (111) and (110) interfaces, where smaller spin injection efficiencies are found. The presence of interface states is found to lower the current spin polarization, both with and without a Schottky barrier. Finally, a higher bias can also affect the spin injection efficiency.Comment: 12 pages, 18 figure

Introduction to half-metallic Heusler alloys: Electronic Structure and Magnetic Properties

Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to the high Curie temperatures and the structural similarity to the binary semiconductors. In this review we present an overview of the basic electronic and magnetic properties of both Heusler families: the so-called half-Heusler alloys like NiMnSb and the the full-Heusler alloys like Co$_2$MnGe. \textit{Ab-initio} results suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The total spin magnetic moment $M_t$ scales linearly with the number of the valence electrons $Z_t$, such that $M_t=Z_t-24$ for the full-Heusler and $M_t=Z_t-18$ for the half-Heusler alloys, thus opening the way to engineer new half-metallic alloys with the desired magnetic properties.Comment: 28 pages, submitted for a special issue of 'Journal of Physics D: Applied Physics' on Heusler alloy

Half-metallic ferromagnets for magnetic tunnel junctions

Using theoretical arguments, we show that, in order to exploit half-metallic ferromagnets in tunneling magnetoresistance (TMR) junctions, it is crucial to eliminate interface states at the Fermi level within the half-metallic gap; contrary to this, no such problem arises in giant magnetoresistance elements. Moreover, based on an a priori understanding of the electronic structure, we propose an antiferromagnetically coupled TMR element, in which interface states are eliminated, as a paradigm of materials design from first principles. Our conclusions are supported by ab-initio calculations

Simulation of the enhanced Curie temperature in Mn_5Ge_3C_x compounds

Mn_5Ge_3C_x films with x>0.5 were experimentally shown to exhibit a strongly enhanced Curie temperature T_C compared to Mn_5Ge_3. In this letter we present the results of our first principles calculations within Green's function approach, focusing on the effect of carbon doping on the electronic and magnetic properties of the Mn_5Ge_3. The calculated exchange coupling constants revealed an enhancement of the ferromagnetic Mn-Mn interactions mediated by carbon. The essentially increased T_C in Mn_5Ge_3C is well reproduced in our Monte Carlo simulations and together with the decrease of the total magnetisation is found to be predominantly of an electronic nature