65 research outputs found
An Effective Model of Magnetoelectricity in Multiferroics
An effective model is developed to explain the phase diagram and the
mechanism of magnetoelectric coupling in multiferroics, . We show
that the nature of magnetoelectric coupling in is a coupling between
two Ising-type orders, namely, the ferroelectric order in the b axis, and the
coupled magnetic order between two frustrated antiferromagnetic chains. The
frustrated magnetic structure drives the system to a
commensurate-incommensurate phase transition, which can be understood as a
competition between a collinear or col-plane order stemming from the `order by
disorder' mechanism and a chiral symmetry order. The low energy excitation is
calculated and the effect of the external magnetic field is analyzed. Distinct
features in the electromagnon spectrums in the incommensurate phase are
predicted
Magnetic properties of (FeCo)B alloys and the effect of doping by 5 elements
We have explored, computationally and experimentally, the magnetic properties
of \fecob{} alloys. Calculations provide a good agreement with experiment in
terms of the saturation magnetization and the magnetocrystalline anisotropy
energy with some difficulty in describing CoB, for which it is found that
both full potential effects and electron correlations treated within dynamical
mean field theory are of importance for a correct description. The material
exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations
between and . A simple model for the temperature dependence of
magnetic anisotropy suggests that the complicated non-monotonous temperature
behaviour is mainly due to variations in the band structure as the exchange
splitting is reduced by temperature. Using density functional theory based
calculations we have explored the effect of substitutional doping the
transition metal sublattice by the whole range of 5 transition metals and
found that doping by Re or W elements should significantly enhance the
magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed
in enhancing the magnetic anisotropy due to formation of other phases. On the
other hand, doping by Ir and Re was successful and resulted in magnetic
anisotropies that are in agreement with theoretical predictions. In particular,
doping by 2.5~at.\% of Re on the Fe/Co site shows a magnetocrystalline
anisotropy energy which is increased by 50\% compared to its parent
(FeCo)B compound, making this system interesting, for
example, in the context of permanent magnet replacement materials or in other
areas where a large magnetic anisotropy is of importance.Comment: 15 pages 17 figure
A multi-stage, first-order phase transition in LaFe11.8Si1.2: interplay between the structural, magnetic and electronic degrees of freedom
Alloys with a first-order magnetic transition are central to solid-state
refrigeration technology, sensors and actuators, or spintronic devices. The
discontinuous nature of the transition in these materials is a consequence of
the coupling between the magnetic, electronic and structural subsystems, but in
a real experiment, it is difficult to observe and analyze the simultaneous
evolution of all the subsystems. As a result, it is very hard to determine the
main mechanisms of the transition and purposefully develop these advanced
magnetic materials. To resolve this issue, we changed the existing paradigm and
conducted simultaneous measurements of the macroscopic properties -
magnetization, temperature change of the sample, longitudinal and transversal
magnetostrictions - to reveal the rich details of the magneto-structural,
first-order transition occurring in the prototypical alloy LaFe11.8Si1.2. We
complement these findings with experiments on the atomistic scale, i.e., x-ray
absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD) and
M\"ossbauer spectroscopy, and then combine them with first-principles
calculations to reveal the full complexity and two-stage nature of the
transition. This new approach can be successfully extended to a large class of
advanced magnetic materials that exhibit analogous transformations.Comment: 24 page
Low-temperature ferroelectric phase and magnetoelectric coupling in the underdoped La_2CuO_(4+x)
We report the discovery of a ferroelectric ground state below 4.5 K in highly
underdoped La_2CuO_(4+x) accompanied by slow charge dynamics which develop
below T~40 K. An anisotropic magnetoelectric response has also been observed,
indicating considerable spin-charge coupling in this lightly doped "parent"
high temperature copper-oxide superconductor. The ferroelectric state is
proposed to develop from polar nanoregions, in which spatial inversion symmetry
is locally broken due to non-stoichiometric carrier doping.Comment: 7 Pages, 6 Figures, supplementary materia
Lewis Acids and Heteropoly Acids in the Synthesis of Organic Peroxides
Organic peroxides are an important class of compounds for organic synthesis, pharmacological chemistry, materials science, and the polymer industry. Here, for the first time, we summarize the main achievements in the synthesis of organic peroxides by the action of Lewis acids and heteropoly acids. This review consists of three parts: (1) metal-based Lewis acids in the synthesis of organic peroxides; (2) the synthesis of organic peroxides promoted by non-metal-based Lewis acids; and (3) the application of heteropoly acids in the synthesis of organic peroxides. The information covered in this review will be useful for specialists in the field of organic synthesis, reactions and processes of oxygen-containing compounds, catalysis, pharmaceuticals, and materials engineering
Evidence for filamentary superconductivity nucleated at antiphase domain walls in antiferromagnetic CaFeAs
Resistivity, magnetization and microscopic As nuclear magnetic
resonance (NMR) measurements in the antiferromagnetically ordered state of the
iron-based superconductor parent material CaFeAs exhibit anomalous
features that are consistent with the collective freezing of domain walls.
Below K, the resistivity exhibits a peak and downturn, the bulk
magnetization exhibits a sharp increase, and As NMR measurements reveal
the presence of slow fluctuations of the hyperfine field. These features in
both the charge and spin response are strongly field dependent, are fully
suppressed by T, and suggest the presence of filamentary
superconductivity nucleated at the antiphase domain walls in this material.Comment: 6pages, 6 figure
In-beam fast-timing measurements in 103,105,107Cd
Fast-timing measurements were performed recently in the region of the
medium-mass 103,105,107Cd isotopes, produced in fusion evaporation reactions.
Emitted gamma-rays were detected by eight HPGe and five LaBr3:Ce detectors
working in coincidence. Results on new and re-evaluated half-lives are
discussed within a systematic of transition rates. The states in
103,105,107Cd are interpreted as arising from a single-particle excitation. The
half-life analysis of the states in 103,105,107Cd shows no change in
the single-particle transition strength as a function of the neutron number
Towards engineering the perfect defect in high-performing permanent magnets
Permanent magnets draw their properties from a complex interplay, across
multiple length scales, of the composition and distribution of their
constituting phases, that act as building blocks, each with their associated
intrinsic properties. Gaining a fundamental understanding of these interactions
is hence key to decipher the origins of their magnetic performance and
facilitate the engineering of better-performing magnets, through unlocking the
design of the "perfect defects" for ultimate pinning of magnetic domains. Here,
we deployed advanced multiscale microscopy and microanalysis on a bulk
Sm2(CoFeCuZr)17 pinning-type high-performance magnet with outstanding thermal
and chemical stability. Making use of regions with different chemical
compositions, we showcase how both a change in the composition and distribution
of copper, along with the atomic arrangements enforce the pinning of magnetic
domains, as imaged by nanoscale magnetic induction mapping. Micromagnetic
simulations bridge the scales to provide an understanding of how these
peculiarities of micro- and nanostructure change the hard magnetic behaviour of
Sm2(CoFeCuZr)17 magnets. Unveiling the origins of the reduced coercivity allows
us to propose an atomic-scale defect and chemistry manipulation strategy to
define ways toward future hard magnets
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