42 research outputs found
Heat capacity of the site-diluted spin dimer system Ba3(Mn1-xVx)2O8
Heat capacity and susceptibility measurements have been performed on the
diluted spin dimer compound Ba3(Mn1-xVx)2O8. The parent compound Ba3Mn2O8 is a
spin dimer system based on pairs of antiferromagnetically coupled S = 1, 3d2
Mn5+ ions such that the zero field groundstate is a product of singlets.
Substitution of non-magnetic S = 0, 3d0 V5+ ions leads to an interacting
network of unpaired Mn moments, the low temperature properties of which are
explored in the limit of small concentrations, 0<x<0.05. The zero-field heat
capacity of this diluted system reveals a progressive removal of magnetic
entropy over an extended range of temperatures, with no evidence for a phase
transition. The concentration dependence does not conform to expectations for a
spin glass state. Rather, the data suggest a low temperature random singlet
phase, reflecting the hierarchy of exchange energies found in this system.Comment: Full Publication Citation Include
Onboard image processing
The possibility of onboard geometric correction of Thematic Mapper type imagery to make possible image registration is considered. Typically, image registration is performed by processing raw image data on the ground. The geometric distortion (e.g., due to variation in spacecraft location and viewing angle) is estimated by using a Kalman filter updated by correlating the received data with a small reference subimage, which has known location. Onboard image processing dictates minimizing the complexity of the distortion estimation while offering the advantages of a real time environment. In keeping with this, the distortion estimation can be replaced by information obtained from the Global Positioning System and from advanced star trackers. Although not as accurate as the conventional ground control point technique, this approach is capable of achieving subpixel registration. Appropriate attitude commands can be used in conjunction with image processing to achieve exact overlap of image frames. The magnitude of the various distortion contributions, the accuracy with which they can be measured in real time, and approaches to onboard correction are investigated
Singlet-triplet dispersion reveals additional frustration in the triangular dimer compound BaMnO
We present single crystal inelastic neutron scattering measurements of the
S=1 dimerized quasi-two-dimensional antiferromagnet BaMnO. The
singlet-triplet dispersion reveals nearest-neighbor and next-nearest-neighbor
ferromagnetic interactions between adjacent bilayers that compete against each
other. Although the inter-bilayer exchange is comparable to the intra-bilayer
exchange, this additional frustration reduces the effective coupling along the
c-axis and leads to a quasi-two dimensional behavior. In addition, the obtained
exchange values are able to reproduce the four critical fields in the phase
diagram.Comment: 4 pages, 3 color figures, submitted to an APS physical review journa
Dispersive magnetic excitations in the S=1 antiferromagnet BaMnO
We present powder inelastic neutron scattering measurements of the S=1
dimerized antiferromagnet BaMnO. The K magnetic spectrum
exhibits a spin-gap of meV and a dispersive spectrum with
a bandwidth of approximately 1.5 meV. Comparison to coupled dimer models
describe the dispersion and scattering intensity accurately and determine the
exchange constants in BaMnO. The wave vector dependent scattering
intensity confirms the proposed S=1 dimer bond. Temperature dependent
measurements of the magnetic excitations indicate the presence of both
singlet-triplet and thermally activated triplet-quintet excitations.Comment: 8 pages, 8 figures, Submitted to Physical Review B, Resubmited
versio
Low temperature structural phase transition and incommensurate lattice modulation in the spin gap compound BaCuSi2O6
Results of high resolution x-ray diffraction experiments are presented for
single crystals of the spin gap compound BaCuSiO in the temperature
range from 16 to 300 K. The data show clear evidence of a transition from the
room temperature tetragonal phase into an incommensurately modulated
orthorhombic structure below 100 K. This lattice modulation is
characterized by a resolution limited wave vector {\bf
q}=(0,0.13,0) and its 2 and 3 harmonics. The phase
transition is first order and exhibits considerable hysteresis. This
observation implies that the spin Hamiltonian representing the system is more
complex than originally thought.Comment: 4 pages, 4 figure
Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet
We present inelastic neutron scattering and thermodynamic measurements
characterizing the magnetic excitations in a disordered non-magnetic
substituted spin-liquid antiferromagnet. The parent compound Ba3Mn2O8 is a
dimerized, quasi-two-dimensional geometrically frustrated quantum disordered
antiferromagnet. We substitute this compound with non-magnetic vanadium for the
S = 1 manganese atoms, Ba3(Mn1-xVx)2O8, and find that the singlet-triplet
excitations which dominate the spectrum of the parent compound persist for the
full range of substitution examined, x = 0.02 to 0.3. We also observe
additional low-energy magnetic fluctuations which are enhanced at the greatest
substitution values. These excitations may be a precursor to a low-temperature
random singlet phase which may exist in Ba3(Mn1-xVx)2O8Comment: 30 pages, 9 figure
Magnetocaloric effect and magnetic cooling near a field-induced quantum-critical point
The presence of a quantum critical point (QCP) can significantly affect the
thermodynamic properties of a material at finite temperatures T. This is
reflected, e.g., in the entropy landscape S(T, r) in the vicinity of a QCP,
yielding particularly strong variations for varying the tuning parameter r such
as pressure or magnetic field B. Here we report on the determination of the
critical enhancement of near a B-induced QCP via
absolute measurements of the magnetocaloric effect (MCE), , and demonstrate that the accumulation of entropy around the QCP can be
used for efficient low-temperature magnetic cooling. Our proof of principle is
based on measurements and theoretical calculations of the MCE and the cooling
performance for a Cu-containing coordination polymer, which is a very
good realization of a spin-1/2 antiferromagnetic Heisenberg chain - one of the
simplest quantum-critical systems.Comment: 21 pages, 4 figure
Finite-Temperature Transition in the Spin-Dimer Antiferromagnet BaCuSi2O6
We consider a classical XY-like Hamiltonian on a body-centered tetragonal
lattice, focusing on the role of interlayer frustration. A three-dimensional
(3D) ordered phase is realized via thermal fluctuations, breaking the
mirror-image reflection symmetry in addition to the XY symmetry. A heuristic
field-theoretical model of the transition has a decoupled fixed point in the 3D
XY universality, and our Monte Carlo simulation suggests that there is such a
temperature region where long-wavelength fluctuations can be described by this
fixed point. However, it is shown using scaling arguments that the decoupled
fixed point is unstable against a fluctuation-induced biquadratic interaction,
indicating that a crossover to nontrivial critical phenomena with different
exponents appears as one approaches the critical point beyond the transient
temperature region. This new scenario clearly contradicts the previous notion
of the 3D XY universality.Comment: 16 pages, 7 figure
Ordered magnetic phases of the frustrated spin-dimer compound Ba3Mn2O8
Ba3Mn2O8 is a spin-dimer compound based on pairs of S=1, 3d^2, Mn^{5+} ions
arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads
to a singlet ground state in zero-field. Here we present the first results of
thermodynamic measurements for single crystals probing the high-field ordered
states of this material associated with closing the spin gap to the excited
triplet states. Specific heat, magnetocaloric effect, and torque magnetometry
measurements were performed in magnetic fields up to 32 T and temperatures down
to 20 mK. For fields above H_{c1} ~ 8.7 T, these measurements reveal a single
magnetic phase for H parallel to c, but two distinct phases (approximately
symmetric about the center of the phase diagram) for H perpendicular to c.
Analysis of the simplest possible spin Hamiltonian describing this system
yields candidates for these ordered states corresponding to a simple spiral
structure for H parallel to c, and to two distinct modulated phases for H
perpendicular to c. Both single-ion anisotropy and geometric frustration play
crucial roles in defining the phase diagram.Comment: 13 pages, 11 figure