72 research outputs found
Magneto-optic Kerr effect in a spin-polarized zero-moment ferrimagnet
The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to
the magnetisation of a magnetically ordered metallic sample; in metallic
ferrimagnets with chemically distinct sublattices, such as rare-earth
transition-metal alloys, it depends on the difference between the sublattice
contributions. Here we show that in a highly spin polarized, fully compensated
ferrimagnet, where the sublattices are chemically similar, MOKE is observed
even when the net moment is strictly zero. We analyse the spectral ellipsometry
and MOKE of Mn 2 Ru x Ga, and show that this behaviour is due to a highly
spin-polarized conduction band dominated by one of the two manganese
sublattices which creates helicity-dependent reflectivity determined by a broad
Drude tail. Our findings open new prospects for studying spin dynamics in the
infra-red.Comment: 7 pages, 7 figure
Metabolite profiling of somatic embryos of Cyclamen persicum in comparison to zygotic embryos, endosperm, and testa
Winkelmann T, Ratjens S, Bartsch M, Rode C, Niehaus K, Bednarz H. Metabolite profiling of somatic embryos of Cyclamen persicum in comparison to zygotic embryos, endosperm, and testa. Frontiers in Plant Science. 2015;6: 597.Somatic embryogenesis has been shown to be an efficient in vitro plant regeneration system for many crops such as the important ornamental plant Cyclamen persicum, for which this regeneration pathway of somatic embryogenesis is of interest for the vegetative propagation of parental lines as well as elite plants. However, somatic embryogenesis is not commercially used in many crops due to several unsolved problems, such as malformations, asynchronous development, deficiencies in maturation and germination of somatic embryos. In contrast, zygotic embryos in seeds develop and germinate without abnormalities in most cases. Instead of time-consuming and labor-intensive experiments involving tests of different in vitro culture conditions and plant growth regulator supplements, we follow a more directed approach. Zygotic embryos served as a reference and were compared to somatic embryos in metabolomic analyses allowing the future optimization of the in vitro system. The aims of this study were to detect differences in the metabolite profiles of torpedo stage somatic and zygotic embryos of C. persicum. Moreover, major metabolites in endosperm and testa were identified and quantified. Two sets of extracts of two to four biological replicates each were analyzed. In total 52 metabolites were identified and quantified in the different tissues. One of the most significant differences between somatic and zygotic embryos was that the proline concentration in the zygotic embryos was about 40 times higher than that found in somatic embryos. Epicatechin, a scavenger for reactive oxygen species, was found in highest abundance in the testa. Sucrose, the most abundant metabolite was detected in significantly higher concentrations in zygotic embryos. Also, a yet unknown trisaccharide, was significantly enriched in zygotic embryos
Quasi-static magnetization dynamics in a compensated ferrimagnetic half-metal -- MnRuGa
Exploring anisotropy and diverse magnetization dynamics in specimens with
vanishing magnetic moments presents a significant challenge using traditional
magnetometry, as the low resolution of existing techniques hinders the ability
to obtain accurate results. In this study, we delve deeper into the examination
of magnetic anisotropy and quasi-static magnetization dynamics in \mrg\,(MRG)
thin films, as an example of a compensated ferrimagnetic half-metal, by
employing anomalous Hall effect measurements within a tetragonal crystal
lattice system. Our research proposes an innovative approach to accurately
determine the complete set of anisotropy constants of these MRG thin films. To
achieve this, we perform anomalous Hall voltage curve fitting, using torque
models under the macrospin approximation, which allow us to obtain out-of-plane
anisotropy constants J m (\,T) and
J m (\,T), along with a weaker
in-plane anisotropy constant J m
(\,T). By additionally employing first-order reversal curves
(FORC) and classical Preisach hysteresis (hysterons) models, we are able to
validate the efficacy of the macrospin model in capturing the magnetic behavior
of MRG thin films. Furthermore, our investigation substantiates that the
complex quasi-static magnetization dynamics of MRG thin films can be
effectively modelled using a combination of hysteronic and torque models. This
approach facilitates the exploration of both linear and non-linear quasi-static
magnetization dynamics, in the presence of external magnetic field and/or
current-induced effective fields, generated by the spin-orbit torque and spin
transfer torque mechanisms.Comment: 14 pages, 10 figure
Effects of disorder on the magnetic properties of the Heusler alloy VFeAl
Magnetic properties of multicomponent alloys depend sensitively on the degree
of atomic order on the different crystallographic sites. In this work we
demonstrate the magnetic contrast between bulk and thin-film samples of the
Heusler alloy VFeAl. Arc-melted bulk ingots show practically no site
preference of the elements (A2 structure), whereas magnetron-sputtered
thin-film samples display a higher degree of atomic ordering with a tendency
towards XA-type order. Electronic structure calculations favour ferrimagnetic
XA-type ordering, and the effect of different pairwise atomic disorder on the
element specific and net magnetic moments are evaluated to reproduce
experimental observations. XA-type thin-films with iron moment of 1.24
determined by X-ray magnetic circular dichroism are in
agreement with calculation, but the measured net moment of 1.0
per formula unit and average vanadium moment are smaller
than expected from calculations. The measured Curie temperature is
approximately 500 K. Films with a higher degree of disorder have a
T close to 300 K with a net moment of 0.1 at
low temperature. The large calculated vanadium moments are destroyed by partial
disorder on vanadium sites. By contrast, the arc-melted and annealed bulk
alloy with a fully-disordered A2 structure shows no spontaneous magnetization;
it is a Pauli paramagnet with dimensionless susceptibility
.Comment: 10 pages, 7 figure
Designing a fully compensated half-metallic ferrimagnet
Recent experimental work on Mn2RuxGa demonstrates its potential as a compensated ferrimagnetic
half-metal (CFHM). Here we present a set of high-throughput ab initio density functional
theory calculations and detailed experimental characterisation, that enable us to correctly describe
the nominal Mn2RuxGa thin films, in particular with regard to site-disorder and defects. We then
construct models that accurately capture all the key features of the Mn-Ru-Ga system, including
magnetic compensation and the spin gap at the Fermi level. We find that electronic doping is neccessary,
which is achieved with a Mn/Ga ratio smaller than two. Our study shows how composition
and substrate-induced biaxial strain can be combined to design a ferrimagnetic half-metal with a
compensation point close to room temperature
Magnetism of noncolinear amorphous DyCo3 and TbCo3 thin films
The magnetization of amorphous DyCo3 and TbCo3 is studied by magnetometry,
anomalous Hall effect and magneto-optic Kerr effect to understand the
temperature-dependent magnetic structure. A square magnetic hysteresis loop
with perpendicular magnetic anisotropy and coercivity that reaches 3.5 T in the
vicinity of the compensation temperature is seen in thin films. An anhysteretic
soft component, seen in the magnetization of some films but not in their Hall
or Kerr loops is an artefact due to sputter-deposition on the sides of the
substrate. The temperature-dependence of the net rare earth moment from 4-300K
is deduced, using the cobalt moment in amorphous YxCo1-x. The single-ion
anisotropy of the quadrupole moments of the 4f atoms in the randomly-oriented
local electrostatic field gradient overcomes their exchange coupling to the
cobalt subnetwork, resulting in a sperimagnetic ground state where spins of the
noncollinear rare-earth subnetwork are modelled by a distribution of rare earth
moments within a cone whose axis is antiparallel to the ferromagnetic axis z of
the cobalt subnetwork. The reduced magnetization (Jz)/J at T=0 is calculated
from an atomic Hamiltonian as a function of the ratio of anisotropy to exchange
energy per rare-earth atom for a range of angles between the local anisotropy
axis and -z and then averaged over all directions in a hemisphere. The
experimental and calculated values of (J-z)/J are close to 0.7 at low
temperature for both Dy and Tb. On increasing temperature, the magnitude of the
rare earth moment and the local random anisotropy that creates the cone are
reduced; the cone closes and the structure approaches collinear ferrimagnetism
well above ambient temperature. An asymmetric spin flop of the exchange-coupled
subnetworks appears in the vicinity of the magnetization compensation
temperatures of 175K for amorphous Dy0.25Co0.75 and 200 K for amorphous TbCo3.Comment: 23 pages, 12 figure
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