753 research outputs found
The magnetic behavior of Li2MO3 (M=Mn, Ru and Ir) and Li2(Mn1-xRux)O3
The present study summerizes magnetic and Mossbauer measurements on ceramic
Li2MO3 M= Mn, Ru and Ir and the mixed Li2(Mn1-xRux)O3 materials, which show
many of the features reflecting to antiferromagnetic ordering or to existence
of paramagnetic states. Li2IrO3 and Li2RuO3 are paramagnetic down to 5 K.
Li2(Mn1-xRux)O3 compounds are antiferromagnetically ordered at TN = 48 K for
x=0. TN decreases as the Ru content increases and, for x=0.8, TN =34 K.Comment: accepted to Physica
Competition between ferromagnetism and spin glass: the key for large magnetoresistance in oxygen deficient perovskites SrCo1-xMxO3-d (M = Nb, Ru)
The magnetic and magnetotransport properties of the oxygen deficient
perovskites, SrCo1-xMxO3-d with M = Nb and Ru, were investigated. Both Nb- and
Ru-substituted cobaltites are weak ferromagnets, with transition temperatures
Tm of 130-150 K and 130-180 K, respectively, and both exhibit a spin glass
behavior at temperatures below Tf = 80-90 K. It is demonstrated that there
exists a strong competition between ferromagnetism and spin glass state, where
Co4+ induces ferromagnetism, whereas Nb or Ru substitution at the cobalt sites
induces magnetic disorder, and this particular magnetic behavior is the origin
of large negative magnetoresistance of these oxides, reaching up to 30% at 5 K
in 7 T. The differences between Nb- and Ru-substituted cobaltites are discussed
on the basis of the different electronic configuration of niobium and ruthenium
cations.Comment: 32 pages, 9 figures, to appear in Phys. Rev.
The New Fe4a2o9 (A=Nb Or Ta) Magnetoelectric Oxides: on the Peculiar Role of Divalent Iron in the "429" Corundum Derivatives
Ising magnetism and ferroelectricity in CaCoMnO
The origin of both the Ising chain magnetism and ferroelectricity in
CaCoMnO is studied by electronic structure calculations
and x-ray absorption spectroscopy. We find that CaCoMnO has the
alternate trigonal prismatic Co and octahedral Mn sites in the
spin chain. Both the Co and Mn are in the high spin state. In
addition, the Co has a huge orbital moment of 1.7 which is
responsible for the significant Ising magnetism. The centrosymmetric crystal
structure known so far is calculated to be unstable with respect to exchange
striction in the experimentally observed
antiferromagnetic structure for the Ising chain. The calculated inequivalence
of the Co-Mn distances accounts for the ferroelectricity.Comment: 4 pages, 3 figures, PRL in press (changes made upon referees
comments
Theory of Transition Temperature of Magnetic Double Perovskites
We formulate a theory of double perovskite coumpounds such as SrFeReO
and SrFeMoO which have attracted recent attention for their possible
uses as spin valves and sources of spin polarized electrons. We solve the
theory in the dynamical mean field approximation to find the magnetic
transition temperature . We find that is determined by a subtle
interplay between carrier density and the Fe-Mo/Re site energy difference, and
that the non-Fe same-sublattice hopping acts to reduce . Our results
suggest that presently existing materials do not optimize
Double-exchange via degenerate orbitals
We consider the double-exchange for systems in which doped electrons occupy
degenerate orbitals, treating the realistic situation with double degenerate
orbitals. We show that the orbital degeneracy leads in general to
formation of anisotropic magnetic structures and that in particular, depending
on the doping concentration, the layered magnetic structures of the A-type and
chain-like structures of the C-type are stabilized. The phase-diagram that we
obtain provides an explanation for the experimentally observed magnetic
structures of some over-doped (electron-doped) manganites of the type
NdSrMnO, PrSrMnO and SmCaMnO
with .Comment: 4 pages, 1 figur
Charge and Orbital Ordering and Spin State Transition Driven by Structural Distortion in YBaCo_2O_5
We have investigated electronic structures of antiferromagnetic YBaCo_2O_5
using the local spin-density approximation (LSDA) + U method. The charge and
orbital ordered insulating ground state is correctly obtained with the strong
on-site Coulomb interaction. Co^{2+} and Co^{3+} ions are found to be in the
high spin (HS) and intermediate spin (IS) state, respectively. It is considered
that the tetragonal to orthorhombic structural transition is responsible for
the ordering phenomena and the spin states of Co ions. The large contribution
of the orbital moment to the total magnetic moment indicates that the
spin-orbit coupling is also important in YBaCo_2O_5.Comment: 4 pages including 4 figures, Submitted to Phys. Rev. Let
Room-temperature tuning of magnetic anisotropy in samarium-thulium orthoferrites
Rare-earth orthoferrites (RFeO3) provide a flexible playground for magnetic materials design, combining the magnetic properties arising from complex interactions between R3+ and Fe3+ cations within the robust framework of the perovskite structure. The most important magnetic property common to most orthoferrites is a spin reorientation transition in which the magnetic moments of Fe3+ cations rotate with respect to a crystallographic axis. SmFeO3 is unique among orthoferrites due to its high-temperature spin reorientation. It is possible to tune the spin reorientation transition to occur at room temperature by replacing Sm with Tm in the Sm0.70Tm0.30FeO3 perovskite. In this study, we show how small changes in composition in the Sm1-xTmxFeO3 (x=0.30-0.50) series provide a high degree of control over the magnetic properties. This work also offers a rather unusual look into the magnetic structure of a samarium-based perovskite by means of neutron powder diffraction, which was made possible by using Sm152. The combination of these results and magnetization measurements allowed the construction of the magnetic phase diagram of the series.Fil: Bolletta, Juan Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Cuello, Gabriel Julio. Institut Laue Langevin; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Nassif, Vivian. Universite Grenoble Alpes; FranciaFil: Suard, Emmanuelle. Institut Laue Langevin; FranciaFil: Kurbakov, Alexander I.. No especifíca;Fil: Maignan, Antoine. No especifíca;Fil: Martin, Christine. No especifíca;Fil: Carbonio, Raul Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentin
Instability of metal-insulator transition against thermal cycling in phase separated Cr-doped manganites
We show that metal-insulator transition in Pr0.5Ca0.5Mn1-xCrxO3 (x =
0.015-0.025) is unstable against thermal cycling. Insulator-metal transition
shifts down and low temperature resistivity increases each time when the sample
is cycled between a starting temperature TS and a final temperature TF. The
effect is dramatic lower is x. Insulator-metal transition in x = 0.015 can be
completely destroyed by thermal cycling in absence of magnetic field as well as
under H = 2 T. Magnetic measurements suggest that ferromagnetic phase fraction
decreases with thermal cycling. We suggest that increase in strains in
ferromagnetic- charge ordered interface could be a possible origin of the
observed effect.Comment: 14 pages, 5 figures and 2 tables (revised
Magnetic studies of Ca1-xMxRuO3 (M=La and Sr)
CaRuO3 is a perovskite with an orthorhombic distortion and shows the
characteristics of spin-glass behavior below TC=87 K. The La3+ substitution for
Ca2+ in Ca1-xLaxRuO3 samples, induces a disorder in the Ca site (the A site)
and the system becomes antiferromagnetically (AFM) ordered with TN = 58 and 19
K for x=0.1 x=0.5 respectively. In the Ca1-xSrxRuO3 system, the Ca0.8Sr0.2RuO3
sample is canted-AFM ordered at 107 K. The compounds with higher Sr
concentration display ferromagnetic behavior and the saturation moment
increases with Sr. Huge magnetic hysteresis loops are obtained at low
temperatures. The coercive field (HC) decreases with Sr. For x=0.4 HC =9.5 kOe
(at 5 K) whereas for SrRuO3 HC =2.4 kOe. For x=0.4 and 0.6, HC decreases
sharply with temperature and than increases again with a peak at 95 and 115 K,
respectively. On the other hand, for SrRuO3, HC remains practically unchanged
up to 50 K and shows a peak at 90 K and than decrease sharply up to TC =165 K.Comment: This paper was submitted to JMM
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