Ab initio study of the interface properties of Fe/GaAs(110)

We have investigated the initial growth of Fe on GaAs(110) by means of density functional theory. In contrast to the conventionally used (001)-surface the (110)-surface does not reconstruct. Therefore, a flat interface and small diffusion can be expected, which makes Fe/GaAs(110) a possible candidate for spintronic applications. Since experimentally, the actual quality of the interface seems to depend on the growth conditions, e.g., on the flux rate, we simulate the effect of different flux rates by different Fe coverages of the semiconductor surface. Systems with low coverages are highly diffusive. With increasing amount of Fe, i.e., higher flux rates, a flat interface becomes more stable. The magnetic structure strongly depends on the Fe coverage but no quenching of the magnetic moments is observed in our calculations.Comment: 9 pages, 8 figure

Computational design of rare-earth reduced permanent magnets

Multiscale simulation is a key research tool in the quest for new permanent magnets. Starting with first principles methods, a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition. Iron (Fe)-rich magnetic phases suitable for permanent magnets can be found by means of adaptive genetic algorithms. The intrinsic properties computed by ab intro simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with a realistic structure. Using machine learning techniques, the magnet's structure can be optimized so that the upper limits for coercivity and energy density product for a given phase can be estimated. Structure property relations of synthetic permanent magnets were computed for several candidate hard magnetic phases. The following pairs (coercive field (T), energy density product (kJ.m(-3))) were obtained for iron-tin-antimony (Fe3Sn0.75Sb0.25): (0.49, 290), L1(0) -ordered iron-nickel (L1(0) FeNi): (1, 400), cobalt-iron-tantalum (CoFe6Ta): (0.87, 425), and manganese-aluminum (MnAl): (0.53, 80).Web of Science6215314

Tuning magnetocrystalline anisotropy of Fe3_{3}Sn by alloying

The electronic structure, magnetic properties and phase formation of hexagonal ferromagnetic Fe$_{3}$Sn-based alloys have been studied from first principles and by experiment. The pristine Fe$_{3}$Sn compound is known to fulfill all the requirements for a good permanent magnet, except for the magnetocrystalline anisotropy energy (MAE). The latter is large, but planar, i.e. the easy magnetization axis is not along the hexagonal c direction, whereas a good permanent magnet requires the MAE to be uniaxial. Here we consider Fe$_{3}$Sn$_{0.75}$M$_{0.25}$, where M= Si, P, Ga, Ge, As, Se, In, Sb, Te and Bi, and show how different dopants on the Sn sublattice affect the MAE and can alter it from planar to uniaxial. The stability of the doped Fe$_{3}$Sn phases is elucidated theoretically via the calculations of their formation enthalpies. A micromagnetic model is developed in order to estimate the energy density product (BH)max and coercive field $\mu_{0}$H$_{c}$ of a potential magnet made of Fe$_{3}$Sn$_{0.75}$Sb$_{0.25}$, the most promising candidate from theoretical studies. The phase stability and magnetic properties of the Fe$_{3}$Sn compound doped with Sb and Mn has been checked experimentally on the samples synthesised using the reactive crucible melting technique as well as by solid state reaction. The Fe$_{3}$Sn-Sb compound is found to be stable when alloyed with Mn. It is shown that even small structural changes, such as a change of the c/a ratio or volume, that can be induced by, e.g., alloying with Mn, can influence anisotropy and reverse it from planar to uniaxial and back

First-principles prediction of spin-density-reflection symmetry driven magnetic transition of CsCl-type FeSe

Based on results of density functional theory (DFT) calculations with the local spin density approximation (LSDA) and the generalized gradient approximation (GGA), we propose a new magnetic material, CsCl-type FeSe. The calculations reveal the existence of ferromagnetic (FM) and antiferromagnetic (AFM) states over a wide range of lattice constants. At 3.12\,{\AA} in the GGA, the equilibrium state is found to be AFM with a local Fe magnetic moment of $\pm 2.69\,\mu_\mathrm{B}$. A metastable FM state with Fe and Se local magnetic moments of $2.00\,\mu_\mathrm{B}$ and $-0.032\,\mu_\mathrm{B}$, respectively, lies 171.7\,{meV} above the AFM state. Its equilibrium lattice constant is $\sim 2$\,{\%} smaller than that of the AFM state, implying that when the system undergoes a phase transition from the AFM state to the FM one, the transition is accompanied by volume contraction. Such an AFM-FM transition is attributed to spin-density $z$-reflection symmetry; the symmetry driven AFM-FM transition is not altered by spin-orbit coupling. The relative stability of different magnetic phases is discussed in terms of the local density of states. We find that CsCl-type FeSe is mechanically stable, but the magnetic states are expected to be brittle.Comment: LaTeX,16 pages, 6 figure

Iron porphyrin molecules on Cu(001): Influence of adlayers and ligands on the magnetic properties

The structural and magnetic properties of Fe octaethylporphyrin (OEP) molecules on Cu(001) have been investigated by means of density functional theory (DFT) methods and X-ray absorption spectroscopy. The molecules have been adsorbed on the bare metal surface and on an oxygen-covered surface, which shows a $\sqrt{2}\times2\sqrt{2}R45^{\circ}$ reconstruction. In order to allow for a direct comparison between magnetic moments obtained from sum-rule analysis and DFT we calculate the dipolar term $7$, which is also important in view of the magnetic anisotropy of the molecule. The measured X-ray magnetic circular dichroism shows a strong dependence on the photon incidence angle, which we could relate to a huge value of $7$, e.g. on Cu(001) $7$ amounts to -2.07\,\mbo{} for normal incidence leading to a reduction of the effective spin moment $m_s + 7$. Calculations have also been performed to study the influence of possible ligands such as Cl and O atoms on the magnetic properties of the molecule and the interaction between molecule and surface, because the experimental spectra display a clear dependence on the ligand, which is used to stabilize the molecule in the gas phase. Both types of ligands weaken the hybridization between surface and porphyrin molecule and change the magnetic spin state of the molecule, but the changes in the X-ray absorption are clearly related to residual Cl ligands.Comment: 17 figures, full articl

Heusler compounds -- how to tune the magnetocrystalline anisotropy

Tailoring and controlling magnetic properties is an important factor for materials design. Here, we present a case study for Ni-based Heusler compounds of the type Ni$_2$YZ with Y = Mn, Fe, Co and Z = B, Al, Ga, In, Si, Ge, Sn based on first principles electronic structure calculations. These compounds are interesting since the materials properties can be quite easily tuned by composition and many of them possess a non-cubic ground state being a prerequisite for a finite magnetocrystalline anisotropy (MAE). We discuss systematically the influence of doping at the Y and Z sublattice as well of lattice deformation on the MAE. We show that in case of Ni$_2$CoZ the phase stability and the MAE can be improved using quaternary systems with elements from group 13 and 14 on the Z sublattice whereas changing the Y sublattice occupation by adding Fe does not lead to an increase of the MAE. Furthermore, we studied the influence of the lattice ratio on the MAE. Showing that small deviations can lead to a doubling of the MAE as in case of Ni$_2$FeGe. Even though we demonstrate this for a limited set of systems the findings may carry over to other related systems.Comment: 27pages (preprint style), 10 figures, regular pape