362 research outputs found
Crystal Field, Magnetic Anisotropy and Excitations in Rare-Earth Hexaborides
We clarify the role of crystalline electric field (CEF) induced magnetic
anisotropy in the ground state and spin-wave spectrum of cubic rare-earth
materials with dominating isotropic magnetic exchange interactions. In
particular we study the hexaboride NdB_6 which is shown to exhibit strong
spin-quadrupolar coupling. The CEF scheme is analyzed and a non-collinear
magnetization response is found. The spin orientation in the
antiferromagnetically ordered ground-state is identified. Moreover, the spin
excitations are evaluated and in agreement with inelastic neutron scattering a
suppression of one of the two magnetic modes in the strong-coupling regime is
predicted.Comment: 4 pages, 1 eps-figur
Effects of Uniaxial Stress on Antiferromagnetic Moment in the Heavy Electron Compound URu_2Si_2
We have performed the elastic neutron scattering experiments under uniaxial
stress \sigma along the tetragonal [100], [110] and [001] directions for
URu2Si2. For \sigma // [100] and [110], the antiferromagnetic moment \mu_o is
strongly enhanced from 0.02 \mu_B (\sigma=0) to 0.22 \mu_B (\sigma=2.5 kbar) at
1.5 K. The rate of increase d\mu_o/d\sigma is roughly estimated to be ~ 0.1
\mu_B/kbar, which is much larger than that for the hydrostatic pressure (~
0.025 \mu_B/kbar). Above 2.5 kbar, \mu_o shows a tendency to saturate similar
to the behavior in the hydrostatic pressure. For \sigma // [001], on the other
hand, \mu_o shows only a slight increase to 0.028 \mu_B (\sigma = 4.6 kbar)
with a rate of ~ 0.002 \mu_B/kbar. The observed anisotropy suggests that the
competition between the hidden order and the antiferromagnetic state in URu2Si2
is strongly coupled with the tetragonal four-fold symmetry and the c/a ratio,
or both.Comment: 3 pages, 3 eps figures, Proceedings of Int. Conf. on Strongly
Correlated Electrons with Orbital Degrees of Freedom (Sendai, Japan,
September 11-14, 2001
Neutron scattering study of transverse magnetism
In order to clarify the nature of the additional phase transition at H1 (T) \u3c Hc (T) of the layered antiferromagnetic (AF) insulator FeBr2 as found by Aruga Katori et al. (1996) we measured the intensity of different Bragg-peaks in different scattering geometries. Transverse AF ordering is observed in both AF phases, AFI and AFII. Its order parameter exhibits a peak at T1 = T (H1) in temperature scans and does not vanish in zero field. Possible origins of the step-like increase of the transverse ferromagnetic ordering induced by a weak in-plane field component when entering AFI below T1 are discussed
Non-Collinear Magnetism due to Orbital Degeneracy and Multipolar Interactions
The origin of non-collinear magnetism under quadrupolar ordering is
investigated with CeB6 taken as a target system. The mode-mixing effect among
15 multipoles is analyzed based on the Ginzburg-Landau free energy. Then the
lower magnetic transition temperature and the order parameters are derived
within the mean-field approximation. In the presence of pseudo-dipole-type
interactions for the next-nearest neighbors, the observed pattern of
non-collinear ordering is indeed stabilized for certain set of interaction
parameters. The stability of the phase III' in the magnetic field is also
explained, which points to the importance of the next-nearest-neighbor
octupole-octupole interaction. Concerning the phase IV in CexLa1-xB6 with x ~
0.75, a possibility of pure octupole ordering is discussed based on slight
modifications of the strength of interactions.Comment: 12 pages, 7 figures, 3 tables, to appear in J. Phys. Soc. Jpn. 70 (6)
(2001
Neutron Scattering Study on Competition between Hidden Order and Antiferromagnetism in U(Ru_{1-x}Rh_x)_2Si_2 (x <= 0.05)
We have performed elastic and inelastic neutron scattering experiments on the
solid solutions U(Ru_{1-x}Rh_x)_2Si_2 for the Ru rich concentrations: x=0,
0.01, 0.02, 0.025, 0.03, 0.04 and 0.05. Hidden order is suppressed with
increasing x, and correspondingly the onset temperature T_m (~ 17.5 K at x=0)
of weak antiferromagnetic (AF) Bragg reflection decreases. For x=0.04 and 0.05,
no magnetic order is detected in the investigated temperature range down to 1.4
K. In the middle range, 0.02 <= x <= 0.03, we found that the AF Bragg
reflection is strongly enhanced. At x=0.02, this takes place at ~ 7.7 K (=T_M),
which is significantly lower than T_m (~ 13.7 K). T_M increases with increasing
x, and seems to merge with T_m at x=0.03. If the AF state is assumed to be
homogeneous, the staggered moment \mu_o estimated at 1.4 K increases from
0.02(2) \mu_B/U (x=0) to 0.24(1) \mu_B/U (x=0.02). The behavior is similar to
that observed under hydrostatic pressure (\mu_o increases to ~ 0.25 \mu_B/U at
1.0 GPa), suggesting that the AF evolution induced by Rh doping is due to an
increase in the AF volume fraction. We also found that the magnetic excitation
observed at Q=(1,0,0) below T_m disappears as T is lowered below T_M.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Concerning Order and Disorder in the Ensemble of Cu-O Chain Fragments in Oxygen Deficient Planes of Y-Ba-Cu-O
In connection with numerous X-ray and neutron investigations of some high
temperature superconductors (YBaCuO and related compounds) a
non-trivial part of the structure factor, coming from partly disordered
Cu-O--O-Cu chain fragments, situated within basal planes, CuO, can
be a subject of theoretical interest. Closely connected to such a diffusive
part of the structure factor are the correlation lengths, which are also
available in neutron and X-ray diffraction studies and depend on a degree of
oxygen disorder in a basal plane. The quantitative measure of such a disorder
can be associated with temperature of a sample anneal, , at which oxygen
in a basal plane remains frozen-in high temperature equilibrium after a fast
quench of a sample to room or lower temperature. The structure factor evolution
with is vizualized in figures after the numerical calculations. The
theoretical approach employed in the paper has been developed for the
orthorhombic state of YBCO.Comment: Revtex, 27 pages, 14 PostScript figures upon request, ITP/GU/94/0
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