35 research outputs found
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 XYZ-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 FeNiZn, FeNiTi and
NiCoFe are potential candidates for permanent magnets with large
out-of-plane magnetic anisotropy (1.23, 0.97 and 0.82 MJ/m 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
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
The role of pressure-induced stacking faults on the magnetic properties of gadolinium
Experimental data show that under pressure, Gd goes through a series of
structural transitions hcp to Sm-type (close-packed rhombohedral) to dhcp that
is accompanied by a gradual decrease of the Curie temperature and magnetization
till the collapse of a finite magnetization close to the dhcp structure. We
explore theoretically the pressure-induced changes of the magnetic properties,
by describing these structural transitions as the formation of fcc stackings
faults. Using this approach, we are able to describe correctly the variation of
the Curie temperature with pressure, in contrast to a static structural model
using the hcp structure.Comment: Preprint (no peer-reviewed
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 FeSn by alloying
The electronic structure, magnetic properties and phase formation of
hexagonal ferromagnetic FeSn-based alloys have been studied from first
principles and by experiment. The pristine FeSn 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 FeSnM, 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 FeSn
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 H of a potential
magnet made of FeSnSb, the most promising candidate
from theoretical studies. The phase stability and magnetic properties of the
FeSn 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 FeSn-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
Elucidating the 3d Electronic Configuration in Manganese Phthalocyanine
To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controversial, we performed photoelectron measurements both in the gas phase and as thin film. With the purpose of explaining the experimental results, three different electronic configurations close in energy to one another were studied by means of density functional theory. The comparison between the calculated valence band density of states and the measured spectra revealed that in the gas phase the molecules exhibit a mixed electronic configuration, while in the thin film, manganese phthalocyanine finds itself in the theoretically computed ground state, namely, the b12ge3ga1gb01g electronic configuration