179 research outputs found
Frustration driven lattice distortion; an NMR investigation of Y2Mo2O7
We have investigated the 89Y NMR spectrum and spin lattice relaxation, T1, in
the magnetically frustrated pyrochlore Y2Mo2O7. We find that upon cooling the
spectrum shifts, and broadens asymmetrically. A detailed examination of the low
T spectrum reveals that it is constructed from multiple peaks, each shifted by
a different amount. We argue that this spectrum is due to discrete lattice
distortions, and speculate that these distortions relieve the frustration and
reduce the system's energy.Comment: To be published in Phys. Rev. Let
Magneto-thermodynamics of the spin-1/2 Kagome antiferromagnet
In this paper, we use a new hybrid method to compute the thermodynamic
behavior of the spin-1/2 Kagome antiferromagnet under the influence of a large
external magnetic field. We find a T^2 low-temperature behavior and a very low
sensitivity of the specific heat to a strong external magnetic field. We
display clear evidence that this low temperature magneto-thermal effect is
associated to the existence of low-lying fluctuating singlets, but also that
the whole picture (T^2 behavior of Cv and thermally activated spin
susceptibility) implies contribution of both non magnetic and magnetic
excitations. Comparison with experiments is made.Comment: 4 pages, LaTeX 2.09 and RevTeX with 3 figures embedded in the text.
Version to appear in Phys. Rev. Let
The Kagome-staircase lattice: Magnetic ordering in Ni3V2O8 and Co3V2O8
Ni3V2O8 and Co3V2O8 have spin-1 and spin-3/2 magnetic lattices that are a new
anisotropic variant of the Kagome net, wherein edge-sharing MO6 octahedra form
the rises and rungs of a "Kagome staircase". The anisotropy largely relieves
the geometric frustration, but results in rich magnetic behavior.
Characterization of the magnetization of polycrystalline samples reveals that
the compounds are ferrimagnetic in character. Heat capacity measurements show
the presence of four magnetic phase transitions below 9 K for Ni3V2O8 and two
below 11 K for Co3V2O8. Comparison to the low temperature heat capacity of
isostructural nonmagnetic Zn3V2O8 provides an estimate of the magnetic entropy
involved with the phase transitions. The results suggest that Co3V2O8 may
display magnetic transitions below 2 K.Comment: 9 pages, 5 figure
Functional behavior of the anomalous magnetic relaxation observed in melt-textured YBa2Cu3O7-δ samples showing the paramagnetic Meissner effect
We have studied the functional behavior of the field-cooled (FC) magnetic relaxation observed in melt-textured YBa2Cu3O7-δ (Y123) samples with 30 wt% of Y2Ba1Cu1O5 (Y211) phase, in order to investigate anomalous paramagnetic moments observed during the experiments. FC magnetic relaxation experiments were performed under controlled conditions, such as cooling rate and temperature. Magnetic fields up to 5T were applied parallel to the ab plane and along the c-axis. Our results are associated with the paramagnetic Meissner effect (PME), characterized by positive moments during FC experiments, and related to the magnetic flux compression into the samples. After different attempts our experimental data could be adequately fitted by an exponential decay function with different relaxation times. We discuss our results suggesting the existence of different and preferential flux dynamics governing the anomalous FC paramagnetic relaxation in different time intervals. This work is one of the first attempts to interpret this controversial effect in a simple analysis of the pinning mechanisms and flux dynamics acting during the time evolution of the magnetic moment. However, the results may be useful to develop models to explain this interesting and still misunderstood feature of the paramagnetic Meissner effect
Nanoscale magnetic structure and properties of solution-derived self-assembled La0.7Sr0.3MnO3 islands
The following article appeared in Journal of Applied Physics 111.2 (2012): 024307 and may be found at http://scitation.aip.org/content/aip/journal/jap/111/2/10.1063/1.3677985Strain-induced self-assembled La0.7Sr0.3MnO 3 nanoislands of lateral size 50-150 nm and height 10-40 nm have been grown on yttria-stabilized zirconia (001)-substrates from ultradiluted chemical solutions based on metal propionates. The nanoislands grow highly relaxed withstanding the epitaxial relation (001)LSMO[110]//(001) Ysz[010] and show bulk-like average magnetic properties in terms of Curie temperature and saturation magnetization. The interplay of the magnetocrystalline and shape anisotropy within the nanoisland ensemble results in an in-plane magnetic anisotropy with a magnetocrystalline constant K 1(150K) = -(5±1) kJ/m3 and in-plane easy axis along the [110] -La0.7Sr0.3MnO3 direction as measured, for the first time, through ferromagnetic resonance experiments. Magnetic force microscopy studies reveal the correlation between nanoisland size and its magnetic domain structure in agreement with micromagnetic simulations. In particular, we have established the required geometric conditions for single domain, multidomain, and vortex configurations.We acknowledge the financial support from MEC (MAT2008-01022, Consolider NANOSELECT and FPU), Comunidad Autónoma de Madrid (CAM S2009/MAT-1467),
Generalitat de Catalunya (Catalan Pla de Recerca 2009-SGR- 770 and XaRMAE), and EU (NESPA). R. D. Zysler and C. A. Ramos acknowledge support from PIP-1333(2007) CONICET and PICT 829 (2006) and PICT 832(2006) ANPCyT of Argentina. Serveis Científic-Tècnics from Universitat de Barcelona and Servei de Micròscopia from Universitat Auto`noma de Barcelona are acknowledged for TEM facilities
Less than 50% sublattice polarization in an insulating S=3/2 kagome' antiferromagnet at low T
We have found weak long range antiferromagnetic order in the
quasi-two-dimensional insulating oxide which contains
Cr S=3/2 ions on a kagom\'{e} lattice. In a sample with 76%
occupancy of the chromium sites the ordered moment is 1.1(3) per
chromium ion which is only one third of the N\'{e}el value .
The magnetic unit cell equals the chemical unit cell, a situation which is
favored by inter-plane interactions. Gapless quantum spin-fluctuations
(T_NS(Q,\omega)$ in the ordered
phase.Comment: 18 pages, RevTex/Latex, with 6 figure
Freezing and large time scales induced by geometrical frustration
We investigate the properties of an effective Hamiltonian with competing
interactions involving spin and chirality variables, relevant for the
description of the {\it trimerized} version of the spin-1/2 {\it kagome}
antiferromagnet. Using classical Monte Carlo simulations, we show that
remarkable behaviors develop at very low temperatures. Through an {\it order by
disorder} mechanism, the low-energy states are characterized by a dynamical
freezing of the chiralities, which decouples the lattice into ``dimers'' and
``triangles'' of antiferromagnetically coupled spins. Under the presence of an
external magnetic field, the particular topology of the chiralities induces a
very slow spin dynamics, reminiscent of what happens in ordinary spin glasses.Comment: 12 pages, 13 figure
Low-temperature properties of classical, geometrically frustrated antiferromagnets
We study the ground-state and low-energy properties of classical vector spin
models with nearest-neighbour antiferromagnetic interactions on a class of
geometrically frustrated lattices which includes the kagome and pyrochlore
lattices. We explore the behaviour of these magnets that results from their
large ground-state degeneracies, emphasising universal features and systematic
differences between individual models. We investigate the circumstances under
which thermal fluctuations select a particular subset of the ground states, and
find that this happens only for the models with the smallest ground-state
degeneracies. For the pyrochlore magnets, we give an explicit construction of
all ground states, and show that they are not separated by internal energy
barriers. We study the precessional spin dynamics of the Heisenberg pyrochlore
antiferromagnet. There is no freezing transition or selection of preferred
states. Instead, the relaxation time at low temperature, T, is of order
hbar/(k_B T). We argue that this behaviour can also be expected in some other
systems, including the Heisenberg model for the compound SrCr_8Ga_4O_{19}.Comment: to appear in Phys. Rev.
Susceptibility and dilution effects of the kagome bi-layer geometrically frustrated network. A Ga-NMR study of SrCr_(9p)Ga_(12-9p)O_(19)
We present an extensive gallium NMR study of the geometrically frustrated
kagome bi-layer compound SrCr_(9p)Ga_(12-9p)O_(19) (Cr^3+, S=3/2) over a broad
Cr-concentration range (.72<p<.95). This allows us to probe locally the kagome
bi-layer susceptibility and separate the intrinsic properties due to the
geometric frustration from those related to the site dilution. Our major
findings are: 1) The intrinsic kagome bi-layer susceptibility exhibits a
maximum in temperature at 40-50 K and is robust to a dilution as high as ~20%.
The maximum reveals the development of short range antiferromagnetic
correlations; 2) At low-T, a highly dynamical state induces a strong wipe-out
of the NMR intensity, regardless of dilution; 3) The low-T upturn observed in
the macroscopic susceptibility is associated to paramagnetic defects which stem
from the dilution of the kagome bi-layer. The low-T analysis of the NMR
lineshape suggests that the defect can be associated with a staggered
spin-response to the vacancies on the kagome bi-layer. This, altogether with
the maximum in the kagome bi-layer susceptibility, is very similar to what is
observed in most low-dimensional antiferromagnetic correlated systems; 4) The
spin glass-like freezing observed at T_g=2-4 K is not driven by the
dilution-induced defects.Comment: 19 pages, 19 figures, revised version resubmitted to PRB Minor
modifications: Fig.11 and discussion in Sec.V on the NMR shif
Band-width control in a perovskite-type 3d^1 correlated metal Ca_{1-x}Sr_xVO_3. I. Evolution of the electronic properties and effective mass
Single crystals of the perovskite-type metallic alloy system
CaSrVO were synthesized in order to investigate metallic
properties near the Mott transition. The substitution of a Ca ion for a
Sr ion reduces the band width due to a buckling of the V-O-V bond
angle from for SrVO to for CaVO. Thus,
the value of can be systematically controlled without changing the number
of electrons making CaSrVO: one of the most ideal systems for
studying band-width effects. The Sommerfeld-Wilson's ratio (), the
Kadowaki-Woods ratio (in the same region as heavy Fermion systems), and a large
term in the electric resistivity, even at 300 K, substantiate a large
electron correlation in this system, though the effective mass, obtained by
thermodynamic and magnetic measurements, shows only a systematic but moderate
increase in going from SrVO to CaVO, in contrast to the critical
enhancement expected from the Brinkmann-Rice picture. It is proposed that the
metallic properties observed in this system near the Mott transition can be
explained by considering the effect of a non-local electron correlation.Comment: 14 pages in a Phys. Rev. B camera-ready format with 10 EPS figures
embedded. LaTeX 2.09 source file using "camera.sty" and "prbplug.sty"
provided by N. Shirakawa. For OzTeX (Macintosh), use "ozfig.sty" instead of
"psfig.sty". "ozfig.sty" can be also obtained by e-mail request to N.
Shirakawa: . Submitted to Phys. Rev.
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