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

Exploration of all-3d Heusler alloys for permanent magnets: an ab initio based high-throughput study

Heusler alloys have attracted interest in various fields of functional materials since their properties can quite easily be tuned by composition. Here, we have investigated the relatively new class of all-3d Heusler alloys in view of its potential as permanent magnets. To identify suitable candidates, we performed a high-throughput study using an electronic structure database to search for X$_2$YZ-type Heusler systems with tetragonal symmetry and high magnetization. For the alloys which passed our selection filters, we have used a combination of density functional theory calculations and spin dynamics modelling to investigate their magnetic properties including the magnetocrystalline anisotropy energy and exchange interactions. The candidates which fulfilled all the search criteria served as input for the investigation of the temperature dependence of the magnetization and determination of Curie temperature. Based on our results, we suggest that Fe$_2$NiZn, Fe$_2$NiTi and Ni$_2$CoFe are potential candidates for permanent magnets with large out-of-plane magnetic anisotropy (1.23, 0.97 and 0.82 MJ/m$^3$ respectively) and high Curie temperatures lying more than 200 K above the room temperature. We further show that the magnitude and direction of anisotropy is very sensitive to the strain by calculating the values of anisotropy energy for several tetragonal phases. Thus, application of strain can be used to tune the anisotropy in these compounds

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

Deposited Transition Metal‐Centered Porphyrin and Phthalocyanine Molecules: Influence of the Substrates on the Magnetic Properties

The field of molecular spintronics has gained much attention since molecules withmagnetic centers form natural magnetic units, which do not suffer from the size limitations of conventional electronics, opening a new path towards miniaturization. To fabricate devices, the molecules have to be deposited on a substrate. The key questions are the interaction of the molecules with the substrate and the control of the magnetic properties. Considering molecule‐substrate hybrid interfaces as building blocks for spintronic devices, a deep understanding of the electronic structure and the coupling mechanisms is central to future applications. The orientation and reconstruction of the substrates can strongly affect the electronic and magnetic characteristics of the adsorbed molecule and drastically change the properties of the free molecules. In this chapter, we will discuss the interaction of transition metal‐centered porphyrins and phthalocyanines with different types of substrates, for example, ferromagnetic transition metals or graphene sheets, in the framework of state‐of‐the‐art density functional theory methods plus insights gained from X‐ray absorption/X‐ray magnetic circular dichroism experiments. The goal is to give an insight into the relevant processes on the atomic scale and to present possible routes to tailor magnetic properties in molecule‐substrate hybrid structures

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