68 research outputs found
31P NMR study of Na2CuP2O7: a S=1/2 two-dimensional Heisenberg antiferromagnetic system
The magnetic properties of Na2CuP2O7 were investigated by means of 31P
nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity
measurements. We report the 31P NMR shift, the spin-lattice 1/T1, and spin-spin
1/T2 relaxation-rate data as a function of temperature T.
The temperature dependence of the NMR shift K(T) is well described by the
S=1/2 square lattice Heisenberg antiferromagnetic (HAF) model with an
intraplanar exchange of J/k_B \simeq 18\pm2 K and a hyperfine coupling A =
(3533\pm185) Oe/mu_B. The 31P NMR spectrum was found to broaden abruptly below
T \sim 10 K signifying some kind of transition. However, no anomaly was noticed
in the bulk susceptibility data down to 1.8 K. The heat capacity appears to
have a weak maximum around 10 K. With decrease in temperatures, the
spin-lattice relaxation rate 1/T1 decreases monotonically and appears to agree
well with the high temperature series expansion expression for a S = 1/2 2D
square lattice.Comment: 12 pages, 8 figures, submitted to J. Phys.: Cond. Ma
Heavy-Fermions in LiV2O4: Kondo-Compensation vs. Spin-Liquid Behavior?
7Li NMR measurements were performed in the metallic spinel LiV2O4. The
temperature dependencies of the line width, the Knight shift and the
spin-lattice relaxation rate were investigated in the temperature range 30 mK <
T < 280 K. For temperatures T < 1 K we observe a spin-lattice relaxation rate
which slows down exponentially. The NMR results can be explained by a
spin-liquid behavior and the opening of a spin gap of the order 0.6 K
Unconventional magnetism in the 4d based () honeycomb system AgLiRuO
We have investigated the thermodynamic and local magnetic properties of the
Mott insulating system AgLiRuO containing Ru
(4) for novel magnetism. The material crystallizes in a monoclinic
structure with RuO octahedra forming an edge-shared
two-dimensional honeycomb lattice with limited stacking order along the
-direction. The large negative Curie-Weiss temperature ( = -57
K) suggests antiferromagnetic interactions among Ru ions though magnetic
susceptibility and heat capacity show no indication of magnetic long-range
order down to 1.8 K and 0.4 K, respectively. Li nuclear magnetic
resonance (NMR) shift follows the bulk susceptibility between 120-300 K and
levels off below 120 K. Together with a power-law behavior in the temperature
dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic
spin correlations with gapless excitations. Electronic structure calculations
suggest an description of the Ru-moments and the possible importance of
further neighbour interactions as also bi-quadratic and ring-exchange terms in
determining the magnetic properties. Analysis of our SR data indicates
spin freezing below 5 K but the spins remain on the borderline between static
and dynamic magnetism even at 20 mK.Comment: 10 pages, 11 figures. accepted in Phys. Rev.
Spin and orbital frustration in MnSc_2S_4 and FeSc_2S_4
Crystal structure, magnetic susceptibility, and specific heat were measured
in the normal cubic spinel compounds MnSc_2S_4 and FeSc_2S_4. Down to the
lowest temperatures, both compounds remain cubic and reveal strong magnetic
frustration. Specifically the Fe compound is characterized by a Curie-Weiss
temperature \Theta_{CW}= -45 K and does not show any indications of order down
to 50 mK. In addition, the Jahn-Teller ion Fe^{2+} is orbitally frustrated.
Hence, FeSc_2S_4 belongs to the rare class of spin-orbital liquids. MnSc_2S_4
is a spin liquid for temperatures T > T_N \approx 2 K.Comment: 4 pages, to be published in Physical Review Letter
Short-range repulsion and isospin dependence in the KN system
The short-range properties of the KN interaction are studied within the
meson-exchange model of the Juelich group. Specifically, dynamical explanations
for the phenomenological short-range repulsion, required in this model for
achieving agreement with the empirical KN data, are explored. Evidence is found
that contributions from the exchange of a heavy scalar-isovector meson
(a_0(980)) as well as from genuine quark-gluon exchange processes are needed.
Taking both mechanisms into account a satisfactory description of the KN phase
shifts can be obtained without resorting to phenomenological pieces.Comment: 26 pages, 5 figure
Structural, thermodynamic, and local probe investigations of a honeycomb material AgLiMnO
The system Ag[LiMn]O belongs to a quaternary
3R-delafossite family and crystallizes in a monoclinic symmetry with space
group and the magnetic Mn() ions form a honeycomb
network in the -plane. An anomaly around 50 K and the presence of
antiferromagnetic (AFM) coupling (Curie-Weiss temperature
K) were inferred from our magnetic susceptibility data. The magnetic specific
heat clearly manifests the onset of magnetic ordering in the vicinity of 48\,K
and the recovered magnetic entropy, above the ordering temperature, falls short
of the expected value, implying the presence of short-range magnetic
correlations. The (ESR) line broadening on approaching the ordering temperature
could be described in terms of a Berezinski-Kosterlitz-Thouless
(BKT) scenario with K. Li NMR line-shift probed as a
function of temperature tracks the static susceptibility (K) of
magnetically coupled Mn ions. The Li spin-lattice relaxation rate
(1/) exhibits a sharp decrease below about 50 K. Combining our bulk
and local probe measurements, we establish the presence of an ordered ground
state for the honeycomb system AgLiMnO.Our ab-initio
electronic structure calculations suggest that in the -plane, the nearest
neighbor (NN) exchange interaction is strong and AFM, while the next NN and the
third NN exchange interactions are FM and AFM respectively. In the absence of
any frustration the system is expected to exhibit long-range, AFM order, in
agreement with experiment.Comment: 11 pages, 13 figures, accepted in Phys Rev
Switching the Ferroelectric Polarization by External Magnetic Fields in the Spin = 1/2 Chain Cuprate LiCuVO4
We present a detailed study of complex dielectric constant and ferroelectric
polarization in multiferroic LiCuVO4 as function of temperature and external
magnetic field. In zero external magnetic field, spiral spin order with an ab
helix and a propagation vector along the crystallographic b direction is
established, which induces ferroelectric order with spontaneous polarization
parallel to a. The direction of the helix can be reoriented by an external
magnetic field and allows switching of the spontaneous polarization. We find a
strong dependence of the absolute value of the polarization for different
orientations of the spiral plane. Above 7.5 T, LiCuVO4 reveals collinear spin
order and remains paraelectric for all field directions. Thus this system is
ideally suited to check the symmetry relations for spiral magnets as predicted
theoretically. The strong coupling of ferroelectric and magnetic order is
documented and the complex (B,T) phase diagram is fully explored.Comment: 6 pages, 5 figure
New high magnetic field phase of the frustrated chain compound LiCuVO
Magnetization of the frustrated chain compound LiCuVO, focusing
on high magnetic field phases, is reported. Besides a spin-flop transition and
the transition from a planar spiral to a spin modulated structure observed
recently, an additional transition was observed just below the saturation
field. This newly observed magnetic phase is considered as a spin nematic
phase, which was predicted theoretically but was not observed experimentally.
The critical fields of this phase and its dM/dH curve are in good agreement
with calculations performed in a microscopic model (M. E. Zhitomirsky and H.
Tsunetsugu, preprint, arXiv:1003.4096v2).Comment: 5 pages, 4 figure
Magnetic field driven 2D-3D crossover in the S=12 frustrated chain magnet LiCuVO4
We report on a heat-capacity study of high-quality single-crystal samples of LiCuVO4 — a frustrated spin S = 1/2 chain system—in a magnetic field amounting to 3/4 of the saturation field. A detailed examination of magnetic phase transitions observed in this field range shows that although the low-field helical state clearly has three-dimensional properties, the field-induced spin-modulated phase turns out to be quasi-two-dimensional. The model proposed in this paper allows one to qualitatively understand this crossover, thus eliminating the presently existing contradictions in the interpretations of NMR and neutron-scattering measurements
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