168 research outputs found
Low energy magnetic excitation spectrum of the unconventional ferromagnet CeRhB
The magnetic excitation spectrum of the unconventional ferromagnet
CeRhB was measured by inelastic neutron scattering on single
crystal sample in the magnetically ordered and paramagnetic phases. The
spin-wave excitation spectrum evidences high exchange interaction along the
c-axis about two orders of magnitude higher than the ones in the basal plane of
the hexagonal structure. Both strong out of plane and small in plane
anisotropies are found. This latter point confirms that considering the =5/2
multiplet alone is not adequate for describing the ground state of
CeRhB. Quasielastic scattering measured above is also
strongly anisotropic between the basal plane and the c-axis and suggests
localized magnetism.Comment: 8 Figure
Crystal-fields in YbInNi4 determined with magnetic form factor and inelastic neutron scattering
The magnetic form factor of YbInNi4 has been determined via the flipping
ratios R with polarized neutron diffraction and the scattering function S(Q,w)
was measured in an inelastic neutron scattering experiment. Both experiments
were performed with the aim to determine the crystal-field scheme. The magnetic
form factor clearly excludes the possibility of a \Gamma7 doublet as the ground
state. The inelastic neutron data exhibit two, almost equally strong peaks at
3.2 meV and 4.4 meV which points, in agreement with earlier neutron data,
towards a \Gamma8 quartet ground state. Further possibilities like a
quasi-quartet ground state are discussed.Comment: 7 pages, 5 figures, 2 tables, submitted to PR
Theoretical Analysis of the "Double-q" Magnetic Structure of CeAl2
A model involving competing short-range isotropic Heisenberg interactions is
developed to explain the "double-q" magnetic structure of CeAl. For
suitably chosen interactions, terms in the Landau expansion quadratic in the
order parameters explain the condensation of incommensurate order at
wavevectors in the star of (1/2 , 1/2 , 1/2), where
is the cubic lattice constant. We show that the fourth order terms in the
Landau expansion lead to the formation of the so-called "double-q" magnetic
structure in which long-range order develops simultaneously at two
symmetry-related wavevectors, in striking agreement with the magnetic structure
determinations. Based on the value of the ordering temperature and of the
Curie-Weiss of the susceptibility, we estimate that the nearest
neighbor interaction is ferromagnetic, with K and the
next-nearest neighbor interaction is antiferromagnetic with K.
We also briefly comment on the analogous phenomenon seen in the similar system
TmS.Comment: 22 pages, 6 figure
Observation of a Griffiths-like phase in the paramagnetic regime of ErCo_2
A systematic x-ray magnetic circular dichroism study of the paramagnetic
phase of ErCo2 has recently allowed to identify the inversion of the net
magnetization of the Co net moment with respect to the applied field well above
the ferrimagnetic ordering temperature, Tc. The study of small angle neutron
scattering measurements has also shown the presence of short range order
correlations in the same temperature region. This phenomenon, which we have
denoted parimagnetism, may be related with the onset of a Griffiths-like phase
in paramagnetic ErCo2. We have measured ac susceptibility on ErCo2 as a
function of temperature, applied field, and excitation frequency. Several
characteristics shared by systems showing a Griffiths phase are present in
ErCo2, namely the formation of ferromagnetic clusters in the disordered phase,
the loss of analyticity of the magnetic susceptibility and its extreme
sensitivity to an applied magnetic field. The paramagnetic susceptibility
allows to establish that the magnetic clusters are only formed by Co moments as
well as the intrinsic nature of those Co moments
Detection of Neutron Scattering from Phase IV of Ce0.7La0.3B6: A Confirmation of the Octupole Order
We have performed a single crystal neutron scattering experiment on
Ce0.7La0.3B6 to investigate the order parameter of phase IV microscopically.
Below the phase transition temperature 1.5 K of phase IV, weak but distinct
superlattice reflections at the scattering vector (h/2,h/2,l/2) (h, l = odd
number) have been observed by neutron scattering for the first time. The
intensity of the superlattice reflections is stronger for high scattering
vectors, which is quite different from the usual magnetic form factor of
magnetic dipoles. This result directly evidences that the order parameter of
phase IV has a complex magnetization density, consistent with the recent
experimental and theoretical prediction in which the order parameter is the
magnetic octupoles Tbeta with Gamma5 symmetry of point group Oh. Neutron
scattering experiments using short wavelength neutrons, as done in this study,
could become a general method to study the high-rank multipoles in f electron
systems.Comment: 4 pages, 4 figure
Near-Zero Moment Ferromagnetism in the Semiconductor SmN
The magnetic behaviour of SmN has been investigated in stoichiometric
polycrystalline films. All samples show ferromagnetic order with Curie
temperature (T_c) of 27 +/- 3 K, evidenced by the occurrence of hysteresis
below T_c. The ferromagnetic state is characterised by a very small moment and
a large coercive field, exceeding even the maximum applied field of 6 T below
about 15 K. The residual magnetisation at 2 K, measured after cooling in the
maximum field, is 0.035 mu_B per Sm. Such a remarkably small moment results
from a near cancellation of the spin and orbital contributions for Sm3+ in SmN.
Coupling to an applied field is therefore weak, explaining the huge coercive
field . The susceptibility in the paramagnetic phase shows
temperature-independent Van Vleck and Curie-Weiss contributions. The Van Vleck
contribution is in quantitative agreement with the field-induced admixture of
the J=7/2 excited state and the 5/2 ground state. The Curie-Weiss contribution
returns a Curie temperature that agrees with the onset of ferromagnetic
hysteresis, and a conventional paramagnetic moment with an effective moment of
0.4 mu_B per Sm ion, in agreement with expectations for the crystal-field
modified effective moment on the Sm3+ ions.Comment: 5 pages, 3 figure
Preparation, magnetic properties and microstructure of lean rare-earth permanent magnetic materials
Abstract Nanocrystalline, lean rare-earth composite alloys around the nominal composition Nd Tb Fe Co B were prepared by various techniques, such as melt-spinning, melt-extraction, splat cooling and mechanically alloying. The Tb and Co content have been found to be crucial in these alloys for achieving high coercivities, up to 500 kA/m. A typical value for the remanence is 1.05}1.10 T. No crystallographic texture was observed, thus, the remanence enhancement was obtained by the spring magnet behavior. The best results have been found when using amorphous precursors and fairly high heating rates during the annealing treatment in an infrared furnace. The amount of Tb could be reduced which yields a lower coercivity but a higher remanence was obtained. TEM and MoK ssbauer analysis was carried out to determine the volume fraction of soft and hard phases. Optimized magnets contained typically a homogeneous nanocrystalline microstructure of about 50 vol% hard magnetic phase and about 50 vol% soft magnetic -Fe plus (Fe,Co) B with a narrow grain size distribution. The addition of Si or Nb and Cu improved the microstructure and the magnetic properties. Bonded magnets produced from optimized lean rare-earth magnetic powders show an improved corrosion resistance compared to magnets with higher rare-earth content. 2000 Elsevier Science B.V. All rights reserved. PACS: 61.16.Bg; 61.10.Eq; 61.18.Fs; 75.50.Ww; 81.05.Ys; 81.4
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