443 research outputs found
Orbital structure and magnetic ordering in stoichiometric and doped crednerite CuMnO2
The exchange interactions and magnetic structure in layered system CuMnO2
(mineral crednerite) and in nonstoichiometric system Cu1.04Mn0.96O2, with
triangular layers distorted due to orbital ordering of the Mn3+ ions, are
studied by ab-initio band-structure calculations, which were performed within
the GGA+U approximation. The exchange interaction parameters for the Heisenberg
model within the Mn-planes and between the Mn-planes were estimated. We explain
the observed in-plane magnetic structure by the dominant mechanism of the
direct d-d exchange between neighboring Mn ions. The superexchange via O ions,
with 90 degree Mn-O-Mn bonds, plays less important role for the in-plane
exchange. The interlayer coupling is largely dominated by one exchange path
between the half-filled 3z^2-r^2 orbitals of Mn3+. The change of interlayer
coupling from antiferromagnetic in pure CuMnO2 to ferromagnetic in doped
material is also explained by our calculations
Electronic structure of VO: charge ordering, metal-insulator transition and magnetism
The low and high-temperature phases of VO have been studied by
\textit{ab initio} calculations. At high temperature, all V atoms are
electronically equivalent and the material is metallic. Charge and orbital
ordering, associated with the distortions in the V pseudo-rutile chains, occur
below the metal-insulator transition. Orbital ordering in the low-temperature
phase, different in V and V chains, allows to explain the
distortion pattern in the insulating phase of VO. The in-chain magnetic
couplings in the low-temperature phase turn out to be antiferromagnetic, but
very different in the various V and V bonds. The V dimers
formed below the transition temperature form spin singlets, but V ions,
despite dimerization, apparently participate in magnetic ordering.Comment: 10 pages, 6 figures, 2 table
Role of local geometry in spin and orbital structure of transition metal compounds
We analyze the role of local geometry in the spin and orbital interaction in
transition metal compounds with orbital degeneracy. We stress that the tendency
observed for the most studied case (transition metals in O octahedra with
one common oxygen -- common corner of neighboring octahedra and with metal--oxygen--metal bonds), that ferro-orbital ordering renders
antiferro-spin coupling, and, {\it vice versa}, antiferro-orbitals give
ferro-spin ordering, is not valid in general case, in particular for octahedra
with common edge and with M--O--M bonds. Special attention is
paid to the ``third case'', neighboring octahedra with common face (three
common oxygens) -- the case practically not considered until now, although
there are many real systems with this geometry. Interestingly enough, the
spin--orbital exchange in this case turns out to be to be simpler and more
symmetric than in the first two cases. We also consider, which form the
effective exchange takes for different geometries in case of strong spin--orbit
coupling.Comment: 31 pages, 9 figures, submitted to JET
Rhodium Doped Manganites : Ferromagnetism and Metallicity
The possibility to induce ferromagnetism and insulator to metal transitions
in small A site cation manganites Ln_{1-x}Ca_xMnO_3 by rhodium doping is shown
for the first time. Colossal magnetoresistance (CMR) properties are evidenced
for a large compositional range (0.35 \leq x < 0.60). The ability of rhodium to
induce such properties is compared to the results obtained by chromium and
ruthenium doping. Models are proposed to explain this behavior.Comment: 11 pages, 8 figure
Spin-state transition in LaCoO3: direct neutron spectroscopic evidence of excited magnetic states
A gradual spin-state transition occurs in LaCoO3 around T~80-120 K, whose
detailed nature remains controversial. We studied this transition by means of
inelastic neutron scattering (INS), and found that with increasing temperature
an excitation at ~0.6 meV appears, whose intensity increases with temperature,
following the bulk magnetization. Within a model including crystal field
interaction and spin-orbit coupling we interpret this excitation as originating
from a transition between thermally excited states located about 120 K above
the ground state. We further discuss the nature of the magnetic excited state
in terms of intermediate-spin (IS, S=1) vs. high-spin (HS, S=2) states. Since
the g-factor obtained from the field dependence of the INS is g~3, the second
interpretation looks more plausible.Comment: 10 pages, 4 figure
Structural transition in AuAgTe4 under pressure
Gold is inert and forms very few compounds. One of the most interesting of
those is calaverite AuTe2, which has incommensurate structure and which becomes
superconducting when doped or under pressure. There exist a "sibling" of AuTe2
the mineral sylvanite AuAgTe4, which properties are almost unknown. In
sylvanite Au and Ag ions are ordered in stripes, and Te6 octahedra around
metals are distorted in such a way that Ag becomes linearly coordinated, what
is typical for Ag^{1+}, whereas Au is square coordinated - it is typical for
d^8 configurations, i.e. one can assign to Au the valence 3+. Our theoretical
study shows that at pressure P_C ~ 5 GPa there should occur in it a structural
transition such that above this critical pressure Te6 octahedra around Au and
Ag become regular and practically identical. Simultaneously Te-Te dimers,
existing at P = 0 GPa, disappear, and material from a bad metal becomes a usual
metal with predominantly Te 5p states at the Fermi energy. We expect that,
similar to AuTe2, AuAgTe4 should become superconducting above P_C.Comment: 8 pages, 4 figure
Microphase separation in Pr0.67Ca0.33MnO3 by small angle neutron scattering
We have evidenced by small angle neutron scattering at low temperature the
coexistence of ferromagnetism (F) and antiferromagnetism (AF) in
Pr0.67Ca0.33MnO3. The results are compared to those obtained in
Pr0.80Ca0.20MnO3 and Pr0.63Ca0.37MnO3, which are F and AF respectively.
Quantitative analysis shows that the small angle scattering is not due to a
mesoscopic mixing but to a nanoscopic electronic and magnetic ''red cabbage''
structure, in which the ferromagnetic phase exists in form of thin layers in
the AF matrix (stripes or 2D ''sheets'').Comment: 4 figure
Giant positive magnetoresistance in metallic VOx thin films
We report on giant positive magnetoresistance effect observed in VOx thin
films, epitaxially grown on SrTiO3 substrate. The MR effect depends strongly on
temperature and oxygen content and is anisotropic. At low temperatures its
magnitude reaches 70% in a magnetic field of 5 T. Strong electron-electron
interactions in the presence of strong disorder may qualitatively explain the
results. An alternative explanation, related to a possible magnetic
instability, is also discussed.Comment: 4 pages, 5 figures included in the text, references update
Crystal field splitting in correlated systems with the negative charge-transfer gap
Special features of the crystal field splitting of levels in the
transition metal compounds with the small or negative charge-transfer gap
are considered. We show that in this case the Coulomb term and
the covalent contribution to the splitting have different signs.
In order to check the theoretical predictions we carried out the ab-initio band
structure calculations for CsAuCl, in which the charge-transfer gap
is negative, so that the electrons predominantly occupy low-lying bonding
states. For these states the -levels lie below ones, which
demonstrates that at least in this case the influence of the covalency on
the total value of the crystal field splitting is stronger than the Coulomb
interaction (which would lead to the opposite level order). We also show that
the states in conduction band are made predominantly of states of ligands
(Cl), with small admixture of states of Au
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