824 research outputs found
An insight into spin-chain magnetism through Moessbauer spectroscopic investigations in Eu-doped Ca3Co2O6 and Ca3CoRhO6
We report the results of 151Eu Moessbauer effect and magnetization
measurements in the Eu-doped Ca3Co2O6 and Ca3CoRhO6, which are of great current
interest in the fields of spin-chain magnetism and geometrical frustration. We
find that there is a pronounced increase in the line-width of the Moessbauer
spectra below a certain characteristic temperature which is well-above the one
at which three-diensional ordering features set in. This unusual broadening of
the spectra indicates the existence of a characteristic temperature in these
'exotic' magnetic systems, attributable to the onset of incipient
one-dimensional magnetic order. This is inferred from an intriguing correlation
of this characteristic temperature with the paramagnetic Curie temperature (a
measure of intrachain coupling strength in these cases)
Magnetic structure and phase diagram in a spin-chain system: CaCoO
The low-temperature structure of the frustrated spin-chain compound
CaCoO is determined by the ground state of the 2D Ising model on
the triangular lattice. At high-temperatures it transforms to the honeycomb
magnetic structure. It is shown that the crossover between the two magnetic
structures at 12 K arises from the entropy accumulated in the disordered
chains. This interpretation is in an agreement with the experimental data.
General rules for for the phase diagram of frustrated Ising chain compounds are
formulated.Comment: 4 pages, 2 figure
Magnetization reversal in mixed ferrite-chromite perovskites with non magnetic cation on the A-site
In this work, we have performed Monte Carlo simulations in a classical model
for RFeCrO with R=Y and Lu, comparing the numerical simulations
with experiments and mean field calculations. In the analyzed compounds, the
antisymmetric exchange or Dzyaloshinskii-Moriya (DM) interaction induced a weak
ferromagnetism due to a canting of the antiferromagnetically ordered spins.
This model is able to reproduce the magnetization reversal (MR) observed
experimentally in a field cooling process for intermediate values and the
dependence with of the critical temperatures. We also analyzed the
conditions for the existence of MR in terms of the strength of DM interactions
between Fe and Cr ions with the x values variations.Comment: 8 pages, 7 figure
Magneto-elastic coupling and unconventional magnetic ordering in triangular multiferroic AgCrS2
The temperature evolution of the crystal and magnetic structures of
ferroelectric sulfide AgCrS2 have been investigated by means of neutron
scattering. AgCrS2 undergoes at TN = 41.6 K a first-order phase transition,
from a paramagnetic rhombohedral R3m to an antiferromagnetic monoclinic
structure with a polar Cm space group. In addition to being ferroelectric below
TN, the low temperature phase of AgCrS2 exhibits an unconventional collinear
magnetic structure that can be described as double ferromagnetic stripes
coupled antiferromagnetically, with the magnetic moment of Cr+3 oriented along
b within the anisotropic triangular plane. The magnetic couplings stabilizing
this structure are discussed using inelastic neutron scattering results.
Ferroelectricity below TN in AgCrS2 can possibly be explained in terms of
atomic displacements at the magneto-elastic induced structural distortion.
These results contrast with the behavior of the parent frustrated
antiferromagnet and spin-driven ferroelectric AgCrO2
Stability of geometrically frustrated magnetic state of Ca3CoRhO6 to applications of positive and negative pressure
The influence of negative chemical pressure induced by gradual replacement of
Ca by Sr as well as of external pressure (up to 10 kbar) on the magnetism of
Ca3CoRhO6 has been investigated by magnetization studies. It is found that the
solid solution, Ca(3-x)Sr(x)CoRhO6, exists at least till about x= 1.0 without
any change in the crystal structure. Apart from insensitivity of the spin-chain
feature to volume expansion, the characteristic features of geometrical
frustration interestingly appear at the same temperatures for all compositions,
in sharp contrast to the response to Y substitution for Ca (reported
previously). Interestingly, huge frequency dependence of ac susceptibility
known for the parent compound persists for all compositions. We do not find a
change in the properties under external pressure. The stability of the magnetic
anomalies of this compound to the volume change (about 4%) is puzzling
The New Fe4a2o9 (A=Nb Or Ta) Magnetoelectric Oxides: on the Peculiar Role of Divalent Iron in the "429" Corundum Derivatives
The enhancement of phase separation aspect in electron doped manganite Ca0.8Sm0.16Nd0.04MnO3
The complex lanthanide doping of electron manganites results in enhancement
of various phase separation effects in physical properties of these compounds.
Selecting Ca0.8Sm0.16Nd0.04MnO3 as a model case we show that the first order
structural phase transition from paramagnetic semi-metallic phase into
anti-ferromagnetic semi-metallic phase at TS ~ 158 +- 4 K is marked by an
abrupt decrease in magnetization, a step like anomaly DL/L = 10-4 in thermal
expansion and large latent heat DQ = 610 J/mol. In a certain temperature range
below TS, the high field magnetization exhibits hysteretic metamagnetic
behavior due to field-induced first order transformation. ac-susceptibility,
magnetization and resistivity data suggest rather a non-uniform state in
Ca0.8Sm0.16Nd0.04MnO3 at low temperatures. The metal - insulator transition
occurs at TMI ~112 +- 3 K, accompanied by a step-like increase in
magnetization. These features could be ascribed to "sponging" of electrons from
neighboring anti-ferromagnetic matrix by clusters undergoing the ferromagnetic
ordering.Comment: submitted to J.Phys. Cond. Matte
Negative chemical pressure effects induced by Y substitution for Ca on the exotic magnetic behavior of spin-chain compound, Ca3Co2O6
The magnetic behavior of a solid solution, Ca3-xYxCo2O6, based on an exotic
spin-chain compound, Ca3Co2O6, crystallizing in K4CdCl6-derived rhombohedral
structure is investigated. Among the compositions investigated (x= 0.0, 0.3,
0.5, 0.75 and 1.0), single phase formation persists up to x= 0.75, with the
elongation of the c-axis. The present investigations reveal that the
temperature at which the partially disordered antiferromagnetic structure sets
in (which occurs at 24 K for the parent compound, x= 0.0) undergoes gradual
reduction with the substitution of Y for Ca, attaining the value of about 2.2 K
for the nominal x = 1.0. The trend observed in this characteristic temperature
is opposite to that reported under external pressure, thereby establishing that
Y substitution exerts negative chemical pressure. Anomalous steps observed in
the isothermal magnetization at very low temperatures (around 2 K) for x= 0.0,
which have been proposed to arise from quantum tunneling effects are found to
vanish by a small substitution (x = 0.3) of Y for Ca. Systematics in ac and dc
magnetic susceptibility behavior with Y substitution for Ca have also been
probed. We believe that the present results involving the expansion of chain
length without disrupting the magnetic chain may be useful to the overall
understanding of the novel magnetism of the parent compound.Comment: Pramana - J. Phys. 65 (2005) 49
Inhomogeneous Magnetism in La-doped CaMnO3. (II) Mesoscopic Phase Separation due to Lattice-coupled FM Interactions
A detailed investigation of mesoscopic magnetic and crystallographic phase
separation in Ca(1-x)La(x)MnO3, 0.00<=x<=0.20, is reported. Neutron powder
diffraction and DC-magnetization techniques have been used to isolate the
different roles played by electrons doped into the eg level as a function of
their concentration x. The presence of multiple low-temperature magnetic and
crystallographic phases within individual polycrystalline samples is argued to
be an intrinsic feature of the system that follows from the shifting balance
between competing FM and AFM interactions as a function of temperature. FM
double-exchange interactions associated with doped eg electrons are favored
over competing AFM interactions at higher temperatures, and couple more
strongly with the lattice via orbital polarization. These FM interactions
thereby play a privileged role, even at low eg electron concentrations, by
virtue of structural modifications induced above the AFM transition
temperatures.Comment: 8 pages, 7 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
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