Comparative study of ordered and disordered Y1-xSrxCoO3-d

We have succeeded in preparing A-site ordered- and disordered-Y1/4Sr3/4CoO3-d with various oxygen deficiencies delta, and have made comparative study of their structural and physical properties. In the A-site ordered structure, oxygen vacancies order, and d = 0.34 sample shows a weak ferromagnetic transition beyond 300 K. On the other hand, in the A-site disordered structure, no oxygen vacancy ordering is observed, and d = 0.16 sample shows a ferromagnetic metallic transition around 150 K. A-site disordering destroys the orderings of oxygen-vacancies and orbitals, leading to the strong modification of the electronic phases.Comment: 3 pages, 3 figures, proceeding of 52nd Mangetism and Magnetic Materials Conference (MMM 2007), published in Journal of Applied Physic

Reduction of critical field for magnetic and orbital-ordering phase transition in impurity-substituted Nd0.45_{0.45}Sr0.55_{0.55}MnO3_3 crystal

We have investigated the Mn-site substitution effect in Nd$_{0.45}$Sr$_{0.55}$MnO$_3$ single crystal, which has an $A$-type layered antiferromagnetic ($A$-AFM) phase with the 3$d_{x^2-y^2}$-type orbital-order. Substitution of Fe or Ga for Mn-site suppresses both the $A$-AFM order and competing ferromagnetic (FM) correlation whereas Cr substitution suppresses only the $A$-AFM order but reactivates the underlying FM correlation via double-exchange mechanism along the AFM coupled $c$-direction. In Nd$_{0.45}$Sr$_{0.55}$Mn$_{0.95}$Cr$_{0.05}$O$_3$, the $A$-AFM state with the orbital-order is changed into the orbital-disordered three-dimensional FM metallic state by applying magnetic field of $\mu_0 H = 12$ T, which is much smaller than that of the parent compound Nd$_{0.45}$Sr$_{0.55}$MnO$_3$.Comment: 5 pages, 4 figures, to appear in APL Material

Refinement Type Inference via Horn Constraint Optimization

We propose a novel method for inferring refinement types of higher-order functional programs. The main advantage of the proposed method is that it can infer maximally preferred (i.e., Pareto optimal) refinement types with respect to a user-specified preference order. The flexible optimization of refinement types enabled by the proposed method paves the way for interesting applications, such as inferring most-general characterization of inputs for which a given program satisfies (or violates) a given safety (or termination) property. Our method reduces such a type optimization problem to a Horn constraint optimization problem by using a new refinement type system that can flexibly reason about non-determinism in programs. Our method then solves the constraint optimization problem by repeatedly improving a current solution until convergence via template-based invariant generation. We have implemented a prototype inference system based on our method, and obtained promising results in preliminary experiments.Comment: 19 page

Cr-doping effect on the orbital fluctuation of heavily doped Nd1-xSrxMnO3 (x ~ 0.625)

We have investigated the Cr-doping effect of Nd0.375Sr0.625MnO3 near the phase boundary between the x2-y2 and 3z2-r2 orbital ordered states, where a ferromagnetic correlation and concomitant large magnetoresistance are observed owing to orbital fluctuation. Cr-doping steeply suppresses the ferromagnetic correlation and magnetoresistance in Nd0.375Sr0.625Mn1-yCryO3 with 0 < y < 0.05, while they reappear in 0.05 < y < 0.10. Such a reentrant behavior implies that a phase boundary is located at y = 0.05, or a phase crossover occurs across y = 0.05.Comment: 3 pages, 3 figures, to be published in Journal of Applied Physic

Multiferroic properties of an \aa kermanite Sr2_2CoSi2_2O7_7 single crystal in high magnetic fields

The magnetic and dielectric properties of \aa kermanite Sr$_2$CoSi$_2$O$_7$ single crystals in high magnetic fields were investigated. We have observed finite induced electric polarization along the c axis in high fields, wherein all Co spins were forcibly aligned to the magnetic field direction. Existence of the induced polarization in the spin-polarized state accompanied with the finite slope in the magnetization curve suggests the possible role of the orbital angular momenta in the excited states as its microscopic origin. The emergence of the field-induced polarization without particular magnetic order can be regarded as the magnetoelectric effects of the second order from the symmetry point of view. A low magnetic field-driven electric polarization flip induced by a rotating field, even at room temperature, has been successfully demonstrated.Comment: 12 pages, 4 figure

Magnetoelectric Properties of (Ca1−x_{1-x}Srx_{x})2_2CoSi2_2O7_7 Crystals

We have investigated the magnetoelectric properties of (Ca$_{1-x}$Sr$_{x}$)$_2$CoSi$_2$O$_7$ ($0\leq x\leq 1$) crystals with a quasi-two-dimensional structure. In Ca$_2$CoSi$_2$O$_7$ ($x=0$), a canted antiferromagnetic transition occurs at 5.6 K. The transition temperature $T_{\rm N}$ is increasing with increasing Sr concentration, and the rises of the magnetization and dielectric constant become larger. Since the dielectric constant shows large change at $T_{\rm N}$ and the magnetocapacitance effect is observed below $T_{\rm N}$, a coupling between the magnetism and dielectricity is strong in (Ca$_{1-x}$Sr$_{x}$)$_2$CoSi$_2$O$_7$. The positive magnetocapacitance is reduced by Sr substitution, and is not observed in $x\geq 0.5$. Namely, the compound of $x\geq 0.5$ does not show the magnetic-field-induced electric polarization. On the other hand, the negative magnetocapacitance is enhanced by Sr substitution.Comment: 4 pages, 2figures, proceeding of International Conference on Magnetism 200

Competition between ferromagnetic and charge-orbital ordered phases in Pr1−x_{1-x}Cax_{x}MnO3_3 for xx=1/4, 3/8, and 1/2

Spin, charge, and orbital structures in models for doped manganites are studied by a combination of analytic mean-field and numerical relaxation techniques. At realistic values for the electron-phonon and antiferromagnetic $t_{2g}$ spin couplings, a competition between a ferromagnetic (FM) phase and a charge-orbital ordered (COO) insulating state is found for $x$=1/4, 3/8, and 1/2, as experimentally observed in Pr$_{1-x}$Ca$_{x}$MnO$_3$ for $x$=0.3$\sim$0.5. The theoretical predictions for the spin-charge-orbital ordering pattern are compared with experiments. The FM-COO energy difference is surprisingly small for the densities studied, result compatible with the presence of a robust colossal-magnetoresistive effect in Pr$_{1-x}$Ca$_{x}$MnO$_3$ in a large density interval.Comment: 4 pages, Revtex, with 2 figures embedded in the text. Submitted to Phys. Rev.