42 research outputs found
First-principles studies on graphene-supported transition metal clusters
Theoretical studies on the structure, stability, and magnetic properties of icosahedral TM13 (TM = Fe, Co, Ni) clusters, deposited on pristine (defect free) and defective graphene sheet as well as graphene flakes, have been carried out within a gradient corrected density functional framework. The defects considered in our study include a carbon vacancy for the graphene sheet and a five-membered and a seven-membered ring structures for graphene flakes (finite graphene chunks). It is observed that the presence of defect in the substrate has a profound influence on the electronic structure and magnetic properties of graphene-transition metal complexes, thereby increasing the binding strength of the TM cluster on to the graphene substrate. Among TM13 clusters, Co-13 is absorbed relatively more strongly on pristine and defective graphene as compared to Fe-13 and Ni-13 clusters. The adsorbed clusters show reduced magnetic moment compared to the free clusters
Epitaxial strain adaption in chemically disordered FeRh thin films
Strain and strain adaption mechanisms in modern functional materials are of
crucial importance for their performance. Understanding these mechanisms will
advance innovative approaches for material properties engineering. Here we
study the strain adaption mechanism in a thin film model system as function of
epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V
buffer layers, which allow for large variation of the preset lattice constants,
e.g. epitaxial boundary condition. It is shown by means of high resolution
X-ray reciprocal space maps and transmission electron microscopy that the
system reacts with a tilting mechanism of the structural units in order to
adapt to the lattice constants of the buffer layer. This response explained by
density functional theory calculations, which evidence an energetic minimum for
structures with a distortion of c/a =0.87. The experimentally observed tilting
mechanism is induced by this energy gain and allows the system to remain in the
most favorable structure. In general, it is shown that the use of epitaxial
model heterostructures consisting of alloy buffer layers of fully miscible
elements and the functional material of interest allows to study strain
adaption behaviors in great detail. This approach makes even small secondary
effects observable, such as the directional tilting of the structural domains
identified in the present case study
Magnetic nanostructures by adaptive twinning in strained epitaxial films
We exploit the intrinsic structural instability of the Fe70Pd30 magnetic
shape memory alloy to obtain functional epitaxial films exhibiting a
self-organized nanostructure. We demonstrate that coherent epitaxial straining
by 54% is possible. The combination of thin film experiments and large-scale
first-principles calculations enables us to establish a lattice relaxation
mechanism, which is not expected for stable materials. We identify a low twin
boundary energy compared to a high elastic energy as key prerequisite for the
adaptive nanotwinning. Our approach is versatile as it allows to control both,
nanostructure and intrinsic properties for ferromagnetic, ferroelastic and
ferroelectric materials.Comment: Final version. Supplementary information available on request or at
the publisher's websit
Anisotropic ferromagnetism in carbon doped zinc oxide from first-principles studies
A density functional theory study of substitutional carbon impurities in ZnO
has been performed, using both the generalized gradient approximation (GGA) and
a hybrid functional (HSE06) as exchange-correlation functional. It is found
that the non-spinpolarized C impurity is under almost all
conditions thermodynamically more stable than the C impurity which
has a magnetic moment of , with the exception of very O-poor
and C-rich conditions. This explains the experimental difficulties in sample
preparation in order to realize -ferromagnetism in C-doped ZnO. From GGA
calculations with large 96-atom supercells, we conclude that two
C-C impurities in ZnO interact ferromagnetically, but
the interaction is found to be short-ranged and anisotropic, much stronger
within the hexagonal -plane of wurtzite ZnO than along the c-axis. This
layered ferromagnetism is attributed to the anisotropy of the dispersion of
carbon impurity bands near the Fermi level for C impurities in
ZnO. From the calculated results, we derive that a C
concentration between 2% and 6% should be optimal to achieve
-ferromagnetism in C-doped ZnO.Comment: 9 pages, 7 figure
Spatio-Temporal Electron Propagation Dynamics in Au/Fe/MgO(001) in nonequilibrium: Revealing Single Scattering Events and the Ballistic Limit
Understanding the microscopic spatio-temporal dynamics of nonequilibrium
charge carriers in heterosystems promises optimization of process and device
design towards desired energy transfer. Hot electron transport is governed by
scattering with other electrons, defects, and bosonic excitations. Analysis of
the energy dependence of scattering pathways and identification of diffusive,
super-diffusive, and ballistic transport regimes are current challenges. We
determine in femtosecond time-resolved two-photon photoelectron emission
spectroscopy the energy-dependent change of the electron propagation time
through epitaxial Au/Fe(001) heteostructures as a function of Au layer
thickness for energies of 0.5 to \unit[2.0]{eV} above the Fermi energy. We
describe the laser-induced nonequilibrium electron excitation and injection
across the Fe/Au interface using real-time time-dependent density functional
theory and analyze the electron propagation through the Au layer by microscopic
electron transport simulations. We identify ballistic transport of minority
electrons at energies with a nascent, optically excited electron population
which is determined by the combination of photon energy and the specific
electronic structure of the material. At lower energy, super-diffusive
transport with 1 to 4 scattering events dominates. The effective electron
velocity accelerates from 0.3 to \unit[1]{nm/fs} with an increase in the Au
layer thickness from 10 to 100~nm. This phenomenon is explained by electron
transport that becomes preferentially aligned with the interface normal for
thicker Au layers, which facilitates electron momentum / energy selection by
choice of the propagation layer thickness
Magnetic properties of small Pt-capped Fe, Co and Ni clusters: A density functional theory study
Theoretical studies on M (M = Fe, Co, Ni) and MPt (for
= 3, 4, 5, 20) clusters including the spin-orbit coupling are done using
density functional theory. The magnetic anisotropy energy (MAE) along with the
spin and orbital moments are calculated for M icosahedral clusters. The
angle-dependent energy differences are modelled using an extended classical
Heisenberg model with local anisotropies. From our studies, the MAE for
Jahn-Teller distorted Fe, Mackay distorted Fe and nearly
undistorted Co clusters are found to be 322, 60 and 5 eV/atom,
respectively, and are large relative to the corresponding bulk values, (which
are 1.4 and 1.3 eV/atom for bcc Fe and fcc Co, respectively.) However, for
Ni (which practically does not show relaxation tendencies), the
calculated value of MAE is found to be 0.64 eV/atom, which is
approximately four times smaller compared to the bulk fcc Ni (2.7
eV/atom). In addition, MAE of the capped cluster (FePt) is
enhanced compared to the uncapped Jahn-Teller distorted Fe cluster
Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films
We present phonon dispersions, element-resolved vibrational density of states
(VDOS) and corresponding thermodynamic properties obtained by a combination of
density functional theory (DFT) and nuclear resonant inelastic X-ray scattering
(NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and
thin epitaxial films. We see distinct differences in the VDOS of the
antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic
proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular
sensitivity to the slight tetragonal distortions present in epitaxial films,
which is not encountered in the AF phase. This results in a notable change in
lattice entropy, which is important for the comparison between thin film and
bulk results. Our calculations confirm the recently reported lattice
instability in the AF phase. The imaginary frequencies at the -point depend
critically on the Fe magnetic moment and atomic volume. Analyzing these non
vibrational modes leads to the discovery of a stable monoclinic ground state
structure which is robustly predicted from DFT but not verified in our thin
film experiments. Specific heat, entropy and free energy calculated within the
quasiharmonic approximation suggest that the new phase is possibly suppressed
because of its relatively smaller lattice entropy. In the bulk phase, lattice
degrees of freedom contribute with the same sign and in similar magnitude to
the isostructural AF-FM phase transition as the electronic and magnetic
subsystems and therefore needs to be included in thermodynamic modeling.Comment: 15 pages, 12 figure