749 research outputs found
Magnetization dynamics in dysprosium orthoferrites via inverse Faraday effect
The ultrafast non-thermal control of magnetization has recently become
feasible in canted antiferromagnets through photomagnetic instantaneous pulses
[A.V. Kimel {\it et al.}, Nature {\bf 435}, 655 (2005)]. In this experiment
circularly polarized femtosecond laser pulses set up a strong magnetic field
along the wave vector of the radiation through the inverse Faraday effect,
thereby exciting non-thermally the spin dynamics of dysprosium orthoferrites. A
theoretical study is performed by using a model for orthoferrites based on a
general form of free energy whose parameters are extracted from experimental
measurements. The magnetization dynamics is described by solving coupled
sublattice Landau-Lifshitz-Gilbert equations whose damping term is associated
with the scattering rate due to magnon-magnon interaction. Due to the inverse
Faraday effect and the non-thermal excitation, the effect of the laser is
simulated by magnetic field Gaussian pulses with temporal width of the order of
hundred femtoseconds. When the field is along the z-axis, a single resonance
mode of the magnetization is excited. The amplitude of the magnetization and
out-of-phase behavior of the oscillations for fields in z and -z directions are
in good agreement with the cited experiment. The analysis of the effect of the
temperature shows that magnon-magnon scattering mechanism affects the decay of
the oscillations on the picosecond scale. Finally, when the field pulse is
along the x-axis, another mode is excited, as observed in experiments. In this
case the comparison between theoretical and experimental results shows some
discrepancies whose origin is related to the role played by anisotropies in
orthoferrites.Comment: 10 pages, 6 figure
Large angle magnetization dynamics measured by time-resolved ferromagnetic resonance
A time-resolved ferromagnetic resonance technique was used to investigate the
magnetization dynamics of a 10 nm thin Permalloy film. The experiment consisted
of a sequence of magnetic field pulses at a repetition rate equal to the
magnetic systems resonance frequency. We compared data obtained by this
technique with conventional pulsed inductive microwave magnetometry. The
results for damping and frequency response obtained by these two different
methods coincide in the limit of a small angle excitation. However, when
applying large amplitude field pulses, the magnetization had a non-linear
response. We speculate that one possible cause of the nonlinearity is related
to self-amplification of incoherence, known as the Suhl instabilities.Comment: 23 pages, 8 figures, submitted to PR
Quantized spin excitations in a ferromagnetic microstrip from microwave photovoltage measurements
Quantized spin excitations in a single ferromagnetic microstrip have been
measured using the microwave photovoltage technique. Several kinds of spin wave
modes due to different contributions of the dipole-dipole and the exchange
interactions are observed. Among them are a series of distinct dipole-exchange
spin wave modes, which allow us to determine precisely the subtle spin boundary
condition. A comprehensive picture for quantized spin excitations in a
ferromagnet with finite size is thereby established. The dispersions of the
quantized spin wave modes have two different branches separated by the
saturation magnetization.Comment: 4 pages, 3 figure
Controlled switching of intrinsic localized modes in a 1-D antiferromagnet
Nearly steady-state locked intrinsic localized modes (ILMs) in the quasi-1d
antiferromagnet (C2H5NH3)2CuCl4 are detected via four-wave mixing emission or
the uniform mode absorption. Exploiting the long-time stability of these locked
ILMs, repeatable nonlinear switching is observed by varying the sample
temperature, and localized modes with various amplitudes are created by
modulation of the microwave driver power. This steady-state ILM locking
technique could be used to produce energy localization in other atomic
lattices.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. v.2 :
clarifications of text and figures in response to comment
Giant Magnetoelastic Effects in BaTiO3-based Extrinsic Multiferroic Hybrids
Extrinsic multiferroic hybrid structures consisting of ferromagnetic and
ferroelectric layers elastically coupled to each other are promising due to
their robust magnetoelectric effects even at room temperature. For a
quantitative analysis of these magnetoelectric effects, a detailed knowledge of
the piezoelectric and magnetoelastic behavior of both constituents as well as
their mutual elastic coupling is mandatory. We here report on a theoretical and
experimental study of the magnetic behavior of BaTiO3-based extrinsic
multiferroic structures. An excellent agreement between molecular dynamics
simulations and the experiments was found for Fe50Co50/BaTiO3 and Ni/BaTiO3
hybrid structures. This demonstrates that the magnetic behavior of extrinsic
multiferroic hybrid structures can be determined by means of ab-initio
calculations, allowing for the design of novel multiferroic hybrids
Field-induced magnetic anisotropy in La0.7Sr0.3CoO3
Magnetic anisotropy has been measured for the ferromagnetic La0.7Sr0.3CoO3
perovskite from an analysis of the high-field part of the magnetization vs.
field curves, i.e., the magnetic saturation regime. These measurements give a
magnetic anistropy one order of magnitude higher than that of reference
manganites. Surprisingly, the values of the magnetic anisotropy calculated in
this way do not coincide with those estimated from measurements of coercive
fields which are one order of magnitude smaller. It is proposed that the reason
of this anomalous behaviour is a transition of the trivalent Co ions under the
external magnetic field from a low-spin to an intermediate-spin state. Such a
transition converts the Co3+ ions into Jahn-Teller ions having an only
partially quenched orbital angular momentum, which enhances the intra-atomic
spin-orbit coupling and magnetic anisotropy.Comment: Accepted of publication in Europhysics Letters, 11 pages, 5 figure
Portfolio Entrepreneurs: Structure, Strategy and Management of Business Groups
This paper draws on the experiences of portfolio entrepreneurs and develops new insights
into this important mode of business development. Portfolio entrepreneurs own and
manage multiple businesses simultaneously, providing an alternative growth process and
the prospect of enduring entrepreneurship. Previous research has focused on either the
genealogy of businesses in a portfolio or the human capital attributes that determine who
becomes a portfolio entrepreneur and how they perform relative to other types of
entrepreneurs. Key issues involving the structure, strategy and management of portfolios
need further exploration and development. This multiple-case study conducted in New
Zealand features eleven entrepreneurs with portfolios of different ages and sizes, each
reflecting a series of opportunistic responses to different situations. Our results show that
some entrepreneurs use structure as an internal seedbed to spawn new ventures, others
seek opportunities to acquire more businesses, while some use both means to build and
maintain their business groups. Structure is the dominant construct, providing a flexible
canvas upon which entrepreneurs enact growth ambitions by creating and re-creating
their portfolios over time. We found no evidence of portfolio-level decision making or
performance measurement with the lead entrepreneur’s attention largely focused at the
business unit level
Anisotropy effects on the magnetic excitations of a ferromagnetic monolayer below and above the Curie temperature
The field-driven reorientation transition of an anisotropic ferromagnetic
monolayer is studied within the context of a finite-temperature Green's
function theory. The equilibrium state and the field dependence of the magnon
energy gap are calculated for static magnetic field applied in plane
along an easy or a hard axis. In the latter case, the in-plane reorientation of
the magnetization is shown to be continuous at T=0, in agreement with free spin
wave theory, and discontinuous at finite temperature , in contrast with
the prediction of mean field theory. The discontinuity in the orientation angle
creates a jump in the magnon energy gap, and it is the reason why, for ,
the energy does not go to zero at the reorientation field. Above the Curie
temperature , the magnon energy gap vanishes for H=0 both in the
easy and in the hard case. As is increased, the gap is found to increase
almost linearly with , but with different slopes depending on the field
orientation. In particular, the slope is smaller when is along the hard
axis. Such a magnetic anisotropy of the spin-wave energies is shown to persist
well above ().Comment: Final version accepted for publication in Physical Review B (with
three figures
Single cell imaging of nuclear architecture changes
This is the final version. Available from Frontiers Media via the DOI in this record.Data and materials availability: Data obtained in this work are available upon request.The dynamic architecture of chromatin, the macromolecular complex comprised primarily of
DNA and histones, is vital for eukaryotic cell growth. Chemical and conformational changes to
chromatin are important markers of functional and developmental processes in cells. However,
chromatin architecture regulation has not yet been fully elucidated. Therefore, novel approaches
to assessing chromatin changes at the single-cell level are required. Here we report the use of
FTIR imaging and microfluidic cell-stretcher chips to assess changes to chromatin architecture
and its effect on the mechanical properties of the nucleus in immune cells. FTIR imaging enables
label-free chemical imaging with subcellular resolution. By optimizing the FTIR methodology
and couple it with cell segmentation analysis approach, we have identified key spectral changes
corresponding to changes in DNA levels and chromatin conformation at the single cell level. By
further manipulating live single cells using pressure-driven microfluidics, we found that
chromatin decondensation – either during general transcriptional activation or during specific
immune cell maturation – can ultimately lead to nuclear auxeticity which is a new biological
phenomenon recently identified. Taken together our findings demonstrate the tight and,
potentially bilateral, link between extra-cellular mechanotransduction and intra-cellular nuclear
architecture.Engineering and Physical Sciences Research Council (EPSRC)Biotechnology and Biological Sciences Research Council (BBSRC)Academy of Medical SciencesRoyal Societ
First principles study of the multiferroics BiFeO, BiFeCrO, and BiCrO: Structure, polarization, and magnetic ordering temperature
We present results of an {\it ab initio} density functional theory study of
three bismuth-based multiferroics, BiFeO, BiFeCrO, and
BiCrO. We disuss differences in the crystal and electronic structure of
the three systems, and we show that the application of the LDA+ method is
essential to obtain realistic structural parameters for BiFeCrO. We
calculate the magnetic nearest neighbor coupling constants for all three
systems and show how Anderson's theory of superexchange can be applied to
explain the signs and relative magnitudes of these coupling constants. From the
coupling constants we then obtain a mean-field approximation for the magnetic
ordering temperatures. Guided by our comparison of these three systems, we
discuss the possibilities for designing a multiferroic material with large
magnetization above room temperature.Comment: 8 Pages, 4 Figure
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