141 research outputs found
Superparamagnetic-like ac susceptibility behavior in a "partially disordered antiferromagnetic" compound, CaCoRhO
We report the results of dc and ac magnetization measurements as a function
of temperature (1.8 - 300 K) for the spin chain compound, CaCoRhO,
which has been recently reported to exhibit a partially disordered
antiferromagnetic (PDAF) structure in the range 30 - 90 K and spin-glass
freezing below 30 K. We observe an unexpectedly large frequency dependence of
ac susceptibility in the T range 30 - 90 K, typical of superparamagnets. In
addition, we find that there is no difference in the isothermal remanent
magnetization behavior for the two regimes below 90 K. These findings call for
more investigations to understand the magnetism of this compound.Comment: 4 pages, 3 figure
Dimers on the Triangular Kagome Lattice
We derive exact results for close-packed dimers on the triangular kagome
lattice (TKL), formed by inserting triangles into the triangles of the kagome
lattice. Because the TKL is a non-bipartite lattice, dimer-dimer correlations
are short-ranged, so that the ground state at the Rokhsar-Kivelson (RK) point
of the corresponding quantum dimer model on the same lattice is a short-ranged
spin liquid. Using the Pfaffian method, we derive an exact form for the free
energy, and we find that the entropy is 1/3 ln2 per site, regardless of the
weights of the bonds. The occupation probability of every bond is 1/4 in the
case of equal weights on every bond. Similar to the case of lattices formed by
corner-sharing triangles (such as the kagome and squagome lattices), we find
that the dimer-dimer correlation function is identically zero beyond a certain
(short) distance. We find in addition that monomers are deconfined on the TKL,
indicating that there is a short-ranged spin liquid phase at the RK point. We
also find exact results for the ground state energy of the classical Heisenberg
model. The ground state can be ferromagnetic, ferrimagnetic, locally coplanar,
or locally canted, depending on the couplings. From the dimer model and the
classical spin model, we derive upper bounds on the ground state energy of the
quantum Heisenberg model on the TKL.Comment: 9 pages, 7 figures, http://www.physics.purdue.edu/~dyao
Two dimensionality in quasi one-dimensional cobalt oxides
By means of muon spin rotation and relaxation (SR) techniques, we have
investigated the magnetism of quasi one-dimensional (1D) cobalt oxides
CoO (=Ca, Sr and Ba, =1, 2, 3, 5 and
), in which the 1D CoO chain is surrounded by six equally spaced
chains forming a triangular lattice in the -plane, using polycrystalline
samples, from room temperature down to 1.8 K. For the compounds with =1 - 5,
transverse field SR experiments showed the existence of a magnetic
transition below 100 K. The onset temperature of the transition () was found to decrease with ; from 100 K for =1 to 60 K for
=5. A damped muon spin oscillation was observed only in the sample with
=1 (CaCoO), whereas only a fast relaxation obtained even at 1.8
K in the other three samples. In combination with the results of susceptibility
measurements, this indicates that a two-dimensional short-range
antiferromagnetic (AF) order appears below for all
compounds with =1 - 5; but quasi-static long-range AF order formed only in
CaCoO, below 25 K. For BaCoO (=), as decreased
from 300 K, 1D ferromagnetic (F) order appeared below 53 K, and a sharp 2D AF
transition occurred at 15 K.Comment: 12 pages, 14 figures, and 2 table
XXZ and Ising Spins on the Triangular Kagome Lattice
The recently fabricated two-dimensional magnetic materials Cu9X2(cpa)6.xH2O
(cpa=2-carboxypentonic acid; X=F,Cl,Br) have copper sites which form a
triangular kagome lattice (TKL), formed by introducing small triangles
(``a-trimers'') inside of each kagome triangle (``b-trimer''). We show that in
the limit where spins residing on b-trimers have Ising character, quantum
fluctuations of XXZ spins residing on the a-trimers can be exactly accounted
for in the absence of applied field. This is accomplished through a mapping to
the kagome Ising model, for which exact analytic solutions exist. We derive the
complete finite temperature phase diagram for this XXZ-Ising model, including
the residual zero temperature entropies of the seven ground state phases.
Whereas the disordered (spin liquid) ground state of the pure Ising TKL model
has macroscopic residual entropy ln72=4.2767... per unit cell, the introduction
of transverse(quantum) couplings between neighboring -spins reduces this
entropy to 2.5258... per unit cell. In the presence of applied magnetic field,
we map the TKL XXZ-Ising model to the kagome Ising model with three-spin
interactions, and derive the ground state phase diagram. A small (or even
infinitesimal) field leads to a new phase that corresponds to a
non-intersecting loop gas on the kagome lattice, with entropy 1.4053... per
unit cell and a mean magnetization for the b-spins of 0.12(1) per site. In
addition, we find that for moderate applied field, there is a critical spin
liquid phase which maps to close-packed dimers on the honeycomb lattice, which
survives even when the a-spins are in the Heisenberg limit.Comment: 12 pages, 12 figure
Geometrically frustrated magnetic behavior of Sr3NiRhO6 and Sr3NiPtO6
The results of ac and dc magnetic susceptibility isothermal magnetization and
heat-capacity measurements as a function of temperature (T) are reported for
Sr3NiRhO6 and Sr3NiPtO6 containing magnetic chains arranged in a triangular
fashion in the basal plane and crystallizing in K4CdCl6-derived rhombohedral
structure. The results establish that both the compounds are magnetically
frustrated, however in different ways. In the case of the Rh compound, the
susceptibility data reveal that there are two magnetic transitions, one in the
range 10 -15 K and the other appearing as a smooth crossover near 45 K, with a
large frequency dependence of ac susceptibility in the range 10 to 40 K; in
addition, the features in C(T) are smeared out at these temperatures. The
magnetic properties are comparable to those of previously known few compounds
with partially disordered antiferromagnetic structure. On the other hand, for
Sr3NiPtO6, there is no evidence for long-range magnetic ordering down to 1.8 K
despite large value of paramagnetic Curie temperature.Comment: A slightly different version from that of PRB version (that is to
appear in print). Phys, Rev, B, in pres
JSPS-7 Bovine Respiratory Syncytial Virus Infection Enhances Pasteurella multocida Adherence on Respiratory Epithelial Cells
Bovine respiratory syncytial virus (BRSV) is a single negative-stranded RNA virus belonging to the Paramyxoviridae family and shows a close genetic relationship with human respiratory syncytial virus (HRSV). BRSV is the primary etiological agent of respiratory disease in calves aged up to 12 months [1]; beef and dairy cattle worldwide [2, 3]. Initial infection by BRSV alter bovine immune system and facilitates secondary infection of the lower respiratory tract by bacteria [1, 4]. Therefore, BRSV is considered to be a causative agent of bovine respiratory disease complex, which results in economic losses to farmers because of the morbidity and mortality in cattle [2, 3]. According to our preliminary findings based on the gene detection from respiratory samples, paired virus and bacteria were detected; Pasteurella multocida (PM) was the most common bacterial agent (unpublished data). PM is common in the nasopharynx of cattle [5, 6], although PM appears to be part of the normal flora, it can contribute to pneumonia when cattle stressed and/or infected by a respiratory virus [7]. However, the interactions between multiple agents associated with BRDC are not clear. Therefore, the aim of this study was to investigate the effect of BRSV infection on PM adherence to respiratory epithelial cells
Nature of phase transition(s) in striped phase of triangular-lattice Ising antiferromagnet
Different scenarios of the fluctuation-induced disordering of the striped
phase which is formed at low temperatures in the triangular-lattice Ising model
with the antiferromagnetic interaction of nearest and next-to-nearest neighbors
are analyzed and compared. The dominant mechanism of the disordering is related
to the formation of a network of domain walls, which is characterized by an
extensive number of zero modes and has to appear via the first-order phase
transition. In principle, this first-order transition can be preceded by a
continuous one, related to the spontaneous formation of double domain walls and
a partial restoration of the broken symmetry, but the realization of such a
scenario requires the fulfillment of rather special relations between the
coupling constants.Comment: 10 pages, 7 figures, ReVTeX
Magnetic frustration in a stoichiometric spin-chain compound, CaCoIrO
The temperature dependent ac and dc magnetization and heat capacity data of
CaCoIrO, a spin-chain compound crystallizing in a KCdCl-derived
rhombohedral structure, show the features due to magnetic ordering of a
frustrated-type below about 30 K, however without exhibiting the signatures of
the so-called "partially disordered antiferromagnetic structure" encountered in
the isostructural compounds, CaCoO and CaCoRhO. This class
of compounds thus provides a variety for probing the consequences of magnetic
frustration due to topological reasons in stoichiometric spin-chain materials,
presumably arising from subtle differences in the interchain and intrachain
magnetic coupling strengths. This compound presents additional interesting
situations in the sense that, ac susceptibility exhibits a large frequency
dependence in the vicinity of 30 K uncharacteristic of conventional
spin-glasses, with this frustrated magnetic state being robust to the
application of external magnetic fields.Comment: Physical Review (Rapid Communications), in pres
Anomalous magnetic phase in an undistorted pyrochlore oxide Cd2Os2O7 induced by geometrical frustration
We report on the muon spin rotation/relaxation study of a pyrochlore oxide,
Cd2Os2O7, which exhibits a metal-insulator (MI) transition at T_{MI}~225 K
without structural phase transition. It reveals strong spin fluctuation
(>10^8/s) below the MI transition, suggesting a predominant role of geometrical
spin frustration amongst Os^{5+} ions. Meanwhile, upon further cooling, a
static spin density wave discontinuously develops below T_{SDW}~150 K. These
observations strongly suggest the occurrence of an anomalous magnetic
transition and associated change in the local spin dynamics in undistorted
pyrochlore antiferromagnet.Comment: 5 pages, 4 figure
Long range magnetic ordering in a spin-chain compound, CaCuMnO, with multiple bond distances
The results of ac and dc magnetization and heat capacity measurements as a
function of temperature (T = 1.8 to 300 K) are reported for a
quasi-one-dimensional compound, CaCuMnO, crystallizing in a
triclinically distorted KCdCl-type structure. The results reveal that
this compound undergoes antiferromagnetic ordering close to 5.5 K. In addition,
there is another magnetic transition below 3.6 K. Existence of two long-range
magnetic transitions is uncommon among quasi-one-dimensional systems. It is
interesting to note that both the magnetic transitions are of long-range type,
instead of spin-glass type, in spite of the fact that the Cu-O and Mn-O bond
distances are multiplied due to this crystallographic distortion. In view of
this, this compound could serve as a nice example for studying
"order-in-disorder" phenomena.Comment: Physical Review (in press
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