160 research outputs found
Quantum States of Neutrons in Magnetic Thin Films
We have studied experimentally and theoretically the interaction of polarized
neutrons with magnetic thin films and magnetic multilayers. In particular, we
have analyzed the behavior of the critical edges for total external reflection
in both cases. For a single film we have observed experimentally and
theoretically a simple behavior: the critical edges remain fixed and the
intensity varies according to the angle between the polarization axis and the
magnetization vector inside the film. For the multilayer case we find that the
critical edges for spin up and spin down polarized neutrons move towards each
other as a function of the angle between the magnetization vectors in adjacent
ferromagnetic films. Although the results for multilayers and single thick
layers appear to be different, in fact the same spinor method explains both
results. An interpretation of the critical edges behavior for the multilyers as
a superposition of ferromagnetic and antifferomagnetic states is given.Comment: 6 pages, 5 figure
AND/R: Advanced neutron diffractometer/reflectometer for investigation of thin films and multilayers for the life sciences
An elastic neutron scattering instrument, the advanced neutron diffractometer/reflectometer (AND/R), has recently been commissioned at the National Institute of Standards and Technology Center for Neutron Research. The AND/R is the centerpiece of the Cold Neutrons for Biology and Technology partnership, which is dedicated to the structural characterization of thin films and multilayers of biological interest. The instrument is capable of measuring both specular and nonspecular reflectivity, as well as crystalline or semicrystalline diffraction at wave-vector transfers up to approximately 2.20 Ã…(-1). A detailed description of this flexible instrument and its performance characteristics in various operating modes are given.D. J. M. is supported
through a NSF NIRT grant Contract No. 0304062
Probing vertically graded anisotropy in FePtCu films
Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (TA). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300º C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low TA. A fundamentally different mechanism is found after annealing at 400ºC, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500º C,a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated TA
Polarized neutron reflectivity studies of magnetic semiconductor superlattices
Abstract Polarized neutron reflectivity studies of EuS/PbS, EuS/YbSe and GaMnAs/GaAs superlattices performed at the NIST Center for Neutron Research are presented. Pronounced antiferromagnetic (AFM) interlayer coupling has been found in EuS/PbS superlattices for a very broad range of PbS spacer thicknesses. Similar, but weaker, AFM coupling is also present in EuS/YbSe, although only for relatively thin YbSe layers. Neutron polarization analysis shows distinct in-plane asymmetry of the magnetization directions of EuS layers in both systems under investigation. For GaMnAs/GaAs superlattices, ferromagnetic (FM) interlayer correlations have been observed. Polarized neutron reflectometry investigations of several GaMnAs/GaAs superlattices have revealed that the manganese magnetic moments in individual GaMnAs layers, in spite of low Mn concentration, form a truly long range, that is in certain cases a single domain, ferromagnetic state.
The High-Flux Backscattering Spectrometer at the NIST Center for Neutron Research
We describe the design and current performance of the high-flux
backscattering spectrometer located at the NIST Center for Neutron Research.
The design incorporates several state-of-the-art neutron optical devices to
achieve the highest flux on sample possible while maintaining an energy
resolution of less than 1mueV. Foremost among these is a novel phase-space
transformation chopper that significantly reduces the mismatch between the beam
divergences of the primary and secondary parts of the instrument. This resolves
a long-standing problem of backscattering spectrometers, and produces a
relative gain in neutron flux of 4.2. A high-speed Doppler-driven monochromator
system has been built that is capable of achieving energy transfers of up to
+-50mueV, thereby extending the dynamic range of this type of spectrometer by
more than a factor of two over that of other reactor-based backscattering
instruments
Feasibility of study magnetic proximity effects in bilayer "superconductor/ferromagnet" using waveguide-enhanced Polarized Neutron Reflectometry
A resonant enhancement of the neutron standing waves is proposed to use in
order to increase the magnetic neutron scattering from a
"superconductor/ferromagnet"(S/F) bilayer. The model calculations show that
usage of this effect allows to increase the magnetic scattering intensity by
factor of hundreds. Aspects related to the growth procedure (order of
deposition, roughness of the layers etc) as well as experimental conditions
(resolution, polarization of the neutron beam, background etc) are also
discussed.
Collected experimental data for the S/F heterostructure
Cu(32nm)/V(40nm)/Fe(1nm)/MgO confirmed the presence of a resonant 60-fold
amplification of the magnetic scattering.Comment: The manuscript of the article submitted to Crysstalography Reports.
23 pages, 5 figure
Ferromagnetic Domain Distribution in Thin Films During Magnetization Reversal
We have shown that polarized neutron reflectometry can determine in a
model-free way not only the mean magnetization of a ferromagnetic thin film at
any point of a hysteresis cycle, but also the mean square dispersion of the
magnetization vectors of its lateral domains. This technique is applied to
elucidate the mechanism of the magnetization reversal of an exchange-biased
Co/CoO bilayer. The reversal process above the blocking temperature is governed
by uniaxial domain switching, while below the blocking temperature the reversal
of magnetization for the trained sample takes place with substantial domain
rotation
Crossover between aperiodic and homogeneous semi-infinite critical behaviors in multilayered two-dimensional Ising models
We investigate the surface critical behavior of two-dimensional multilayered
aperiodic Ising models in the extreme anisotropic limit. The system under
consideration is obtained by piling up two types of layers with respectively
and spin rows coupled via nearest neighbor interactions and
, where the succession of layers follows an aperiodic sequence. Far
away from the critical regime, the correlation length is smaller
than the first layer width and the system exhibits the usual behavior of an
ordinary surface transition. In the other limit, in the neighborhood of the
critical point, diverges and the fluctuations are sensitive to the
non-periodic structure of the system so that the critical behavior is governed
by a new fixed point. We determine the critical exponent associated to the
surface magnetization at the aperiodic critical point and show that the
expected crossover between the two regimes is well described by a scaling
function. From numerical calculations, the parallel correlation length
is then found to behave with an anisotropy exponent which
depends on the aperiodic modulation and the layer widths.Comment: LaTeX file, 9 pages, 8 eps figures, to appear in Phys. Rev.
Anomalous Diffusion in Aperiodic Environments
We study the Brownian motion of a classical particle in one-dimensional
inhomogeneous environments where the transition probabilities follow
quasiperiodic or aperiodic distributions. Exploiting an exact correspondence
with the transverse-field Ising model with inhomogeneous couplings we obtain
many new analytical results for the random walk problem. In the absence of
global bias the qualitative behavior of the diffusive motion of the particle
and the corresponding persistence probability strongly depend on the
fluctuation properties of the environment. In environments with bounded
fluctuations the particle shows normal diffusive motion and the diffusion
constant is simply related to the persistence probability. On the other hand in
a medium with unbounded fluctuations the diffusion is ultra-slow, the
displacement of the particle grows on logarithmic time scales. For the
borderline situation with marginal fluctuations both the diffusion exponent and
the persistence exponent are continuously varying functions of the
aperiodicity. Extensions of the results to disordered media and to higher
dimensions are also discussed.Comment: 11 pages, RevTe
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