406 research outputs found
Ni3TeO6 - a collinear antiferromagnet with ferromagnetic honeycomb planes
We report a comprehensive study of magnetic properties of Ni3TeO6. The system
crystallizes in a noncentrosymmetric rhombohedral lattice, space group R3.
There are three differently coordinated Ni atoms in the unit cell. Two of them
form an almost planar honeycomb lattice, while the third one is placed between
the layers. Magnetization and specific heat measurements revealed a single
magnetic ordering at TN = 52 K. Below TN the susceptibility with the magnetic
field parallel to the c-axis drops towards zero while the perpendicular
susceptibility remains constant, a characteristic of antiferromagnetic
materials. Neutron diffraction confirmed that the system is antiferromagnet
below TN with ferromagnetic ab-planes stacked antiferromagnetically along the
c-axis. All Ni moments are in the S = 1 spin state and point along the c-axis.Comment: accepted for publication in Journal of Physics Condensed Matte
Incommensurate magnetic ordering in Cu2Te2O5X2 (X=Cl, Br) studied by single crystal neutron diffraction
Polarized and unpolarized neutron diffraction studies have been carried out
on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl,
Br). A model of the magnetic structure associated with the propagation vectors
k'Cl ~ -0.150,0.422,1/2 and k'Br ~ -0.172,0.356,1/2 and stable below TN=18 K
for X=Cl and TN=11 K for X=Br is proposed. A feature of the model, common to
both the bromide and chloride, is a canted coplanar motif for the 4 Cu2+ spins
on each tetrahedron which rotates on a helix from cell to cell following the
propagation vector. The Cu2+magnetic moment determined for X=Br, 0.395(5)muB,
is significantly less than for X=Cl, 0.88(1)muB at 2K. The magnetic structure
of the chloride associated with the wave-vector k' differs from that determined
previously for the wave vector k~0.150,0.422,1/2 [O. Zaharko et.al. Phys. Rev.
Lett. 93, 217206 (2004)]
Incommensurate magnetic ordering in (X=Cl,Br) studied by neutron diffraction
We present the results of the first neutron powder and single crystal
diffraction studies of the coupled spin tetrahedra systems {\CuTeX} (X=Cl,
Br). Incommensurate antiferromagnetic order with the propagation vectors
{\bf{k}_{Cl}}\approx[0.150,0.422,\half],
{\bf{k}_{Br}}\approx[0.158,0.354,\half] sets in below =18 K for X=Cl
and 11 K for X=Br. No simple collinear antiferromagnetic or ferromagnetic
arrangements of moments within Cu tetrahedra fit these observations.
Fitting the diffraction data to more complex but physically reasonable models
with multiple helices leads to a moment of 0.67(1)/Cu at 1.5 K
for the Cl-compound. The reason for such a complex ground state may be
geometrical frustration of the spins due to the intra- and inter-tetrahedral
couplings having similar strengths. The magnetic moment in the Br- compound,
calculated assuming it has the same magnetic structure as the Cl compound, is
only 0.51(5)/Cu at 1.5 K. In neither compound has any evidence
for a structural transition accompanying the magnetic ordering been found
Evolution of magnetic states in frustrated diamond lattice antiferromagnetic Co(Al1-xCox)2O4 spinels
Using neutron powder diffraction and Monte-Carlo simulations we show that a
spin-liquid regime emerges at $all compositions in the diamond-lattice
antiferromagnets Co(Al1-xCox)2O4. This spin-liquid regime induced by
frustration due to the second-neighbour exchange coupling J2, is gradually
superseded by antiferromagnetic collinear long-range order (k=0) at low
temperatures. Upon substitution of Al3+ by Co3+ in the octahedral B-site the
temperature range occupied by the spin-liquid regime narrows and TN increases.
To explain the experimental observations we considered magnetic anisotropy D or
third-neighbour exchange coupling J3 as degeneracy-breaking perturbations. We
conclude that Co(Al1-xCox)2O4 is below the theoretical critical point
J2/J1=1/8, and that magnetic anisotropy assists in selecting a collinear
long-range ordered ground state, which becomes more stable with increasing x
due to a higher efficiency of O-Co3+-O as an interaction path compared to
O-Al3+-O
Spin-stripe phase in a frustrated zigzag spin-1/2 chain
Motifs of periodic modulations are encountered in a variety of natural
systems, where at least two rival states are present. In strongly correlated
electron systems such behaviour has typically been associated with competition
between short- and long-range interactions, e.g., between exchange and
dipole-dipole interactions in the case of ferromagnetic thin films. Here we
show that spin-stripe textures may develop also in antiferromagnets, where
long-range dipole-dipole magnetic interactions are absent. A comprehensive
analysis of magnetic susceptibility, high-field magnetization, specific heat,
and neutron diffraction measurements unveils -TeVO as a nearly
perfect realization of a frustrated (zigzag) ferromagnetic spin-1/2 chain.
Strikingly, a narrow spin stripe phase develops at elevated magnetic fields due
to weak frustrated short-range interchain exchange interactions possibly
assisted by the symmetry allowed electric polarization. This concept provides
an alternative route for the stripe formation in strongly correlated electron
systems and may help understanding other widespread, yet still elusive,
stripe-related phenomena.Comment: accapted in Nature Communication
Persistent spin dynamics intrinsic to amplitude-modulated long-range magnetic order
An incommensurate elliptical helical magnetic structure in the frustrated
coupled-spin-chain system FeTe2O5Br is surprisingly found to persist down to
53(3) mK (T/T_N ~ 1/200), according to neutron scattering and muon spin
relaxation. In this state, finite spin fluctuations at T -> 0 are evidenced by
muon depolarization, which is in agreement with specific-heat data indicating
the presence of both gapless and gapped excitations. We thus show that the
amplitude-modulated magnetic order intrinsically accommodates contradictory
persistent spin dynamics and long-range order and can serve as a model
structure to investigate their coexistence.Comment: 5 pages + supplementar
Dynamic spin Jahn-Teller effect in small magnetic clusters
We study the effect of spin-phonon coupling in small magnetic clusters,
concentrating on a S=1/2 ring of 4 spins coupled antiferromagnetically. If the
phonons are treated as classical variables, there is a critical value of the
spin-phonon coupling above which a static distortion occurs. This is a good
approximation if the zero point energy is small compared to the energy gain due
to the distortion, which is true for large exchange interactions compared to
the phonons energy (). In the opposite limit, one can
integrate out the phonon degrees of freedom and get an effective spin
hamiltonian. Using exact diagonalizations to include the quantum nature of both
spins and phonons, we obtain the spectrum in the whole range of parameters and
explicit the crossover between the classical and quantum regimes. We then
establish quantitatively the limits of validity of two widely used approaches
(one in the quantum and one in the classical limits) and show that they are
quite poor for small magnetic clusters. We also show that upon reducing
the first excitation of a 4-site cluster becomes a singlet, a
result that could be relevant for CuTeOBr
Evolution of magnetic and crystal structures in the multiferroic FeTe2O5Br
Neutron diffraction and nuclear quadrupole resonance (NQR) measurements were
employed to investigate magnetic order in the non-ferroelectric phase preceding
the low-temperature multiferroic state in FeTe2O5Br. Refnement of the neutron
diffraction data and simulations of 79,81Br NQR spectra reveal that the
incommensurate magnetic ordering in the non-ferroelectric state comprises
amplitude-modulated magnetic moments, similarly as in the multiferroic state.
The two ordered states differ in the orientation of the magnetic moments and
phase shifts between modulation waves. Surprisingly, all symmetry restrictions
for the electric polarization are absent in both states. The different
ferroelectric responses of the two states are thus argued to arise from the
differences in the phase shifts between certain modulation waves, which cancel
out in the non-ferrolectric state.Comment: 9 pages, 8 figures including appendix, published in PR
МІНІ-ІНВАЗИВНЕ ЛІКУВАННЯ ПАХВИННИХ ГРИЖ У ДІТЕЙ (КЛІНІЧНИЙ ВИПАДОК ТА ОГЛЯД ЛІТЕРАТУРИ)
The article presents a review of the literature on the issue of minimally invasive surgical treatment of inguinal hernias in children. It was found that, despite the undoubted successes in the treatment of this disease is not fully resolved and remains debated a number of issues. The debate continues about the tactics and technique of surgery, possible intraoperative complications, early and long-term consequences, the possibility of recurrence of the disease. Accordingly, all these issues require their further in-depth study.В статье представлен обзор литературы по проблеме мини-инвазивного лечения паховых грыж у детей. Отмечено, что несмотря на достигнутые успехи в лечении этого распространенного заболевания, нерешенным остается ряд контраверсионных вопросов. Продолжаются дискуссии о стратегии и технике операционного лечения, возможности интраоперационных осложнений, ранних и отдаленных последствий, возможных рецидивов заболевания. Итак, все эти вопросы требуют глубокого изучения.У статті представлено огляд літератури з проблеми мін–інвазивного лікування пахвинних гриж у дітей. Зазначено, що незважаючи на досягнуті успіхи в лікуванні цього розповсюдженого захворювання, не вирішеним залишається ряд контраверсійних питань. Тривають дискусії щодо тактики та техніки операційного лікування, можливості інтраопераційних ускладнень, ранніх та віддалених наслідків, можливих рецидивів захворювання. Отже, всі ці питання вимагають глибокого вивчення
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