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.

Ising magnetism and ferroelectricity in Ca3_3CoMnO6_6

The origin of both the Ising chain magnetism and ferroelectricity in Ca$_3$CoMnO$_6$ is studied by $ab$ $initio$ electronic structure calculations and x-ray absorption spectroscopy. We find that Ca$_3$CoMnO$_6$ has the alternate trigonal prismatic Co$^{2+}$ and octahedral Mn$^{4+}$ sites in the spin chain. Both the Co$^{2+}$ and Mn$^{4+}$ are in the high spin state. In addition, the Co$^{2+}$ has a huge orbital moment of 1.7 $\mu_B$ 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 $\uparrow\uparrow\downarrow\downarrow$ 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 Sr$_2$FeReO$_6$ and Sr$_2$FeMoO$_6$ 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 $T_c$. We find that $T_c$ 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 $T_c$. Our results suggest that presently existing materials do not optimize $T_c$

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 $e_g$ 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 Nd$_{1-x}$Sr$_x$MnO$_3$, Pr$_{1-x}$Sr$_x$MnO$_3$ and Sm$_{1-x}$Ca$_x$MnO$_3$ with $x > 0.5$.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