ABSTRACT
We observe a seemingly complex magnetic field dependence of the dielectric constant of hexagonal YbMnO(3) near the spin ordering temperature. After rescaling, the data taken at different temperatures and magnetic fields collapse on a single curve describing the sharp anomaly in nonlinear magnetoelectric response at the magnetic transition. We show that this anomaly is a result of the competition between two magnetic phases. The scaling and the shape of the anomaly are explained using the phenomenological Landau description of the competing phases in hexagonal manganites.
ABSTRACT
The search for multifunctional materials as multiferroics to be applied in microelectronic or for new, chemically stable and nontoxic, thermoelectric materials to recover waste heat is showing a common interest in the oxides whose structures contain a triangular network of transition-metal cations. To illustrate this point, two ternary systems, Ba-Co-O and Ca-Co-O, have been chosen. It is shown that new phases with a complex triangular structure can be discovered, for instance, by introduction of Ga (3+) into the Ba-Co-O system to stabilize Ba 6Ga 2Co 11O 26 and Ba 2GaCo 8O 14, which both belong to a large family of compounds with formula [Ba(Co,Ga)O 3-delta] n [BaCo 8O 11]. In the latter, both sublattices contain triangular networks derived from the hexagonal perovskite and the spinel structure. Among the hexagonal perovskite, the Ca 3Co 2O 6 crystals give clear evidence where the coupling of charges and spins is at the origin of a magnetocapacitance effect. In particular, the ferrimagnetic to ferromagnetic transition, with a one-third plateau on the M( H) curve characteristic of triangular magnetism, is accompanied by a peak in the dielectric constant. A second class of cobaltites is the focus of much interest. Their 2D structure, containing CoO 2 planes isostructural to a CdI 2 slice that are stacked in an incommensurate way with rock salt type layers, is referred to misfit cobaltite. The 2D triangular network of edge-shared CoO 6 octahedra is believed to be responsible for large values of the Seebeck coefficient and low electrical resistivity. A clear relationship between the structuresincommensurability ratiosand the electronic properties is evidenced, showing that the charge carrier concentration can be tuned via the control of the ionic radius of the cations in the separating layers.
ABSTRACT
The magnetic, structural and electronic properties of Bi(0.75)Ca(0.25)MnO(3) have been investigated in comparison with those of Bi(0.75)Sr(0.25)MnO(3). Magnetometry, diffraction and muon spin relaxation (µSR) data confirm different structural, magnetic and electronic transitions in the two compounds. The anisotropic changes of cell parameters across the structural transition in Bi(0.75)Ca(0.25)MnO(3) (275 K) differ markedly from the lattice anomalies in Bi(0.75)Sr(0.25)MnO(3) (600 K) and also from those in Bi(0.50)Ca(0.50)MnO(3) (325 K). The ground state of Bi(0.75)Ca(0.25)MnO(3) is characterized by a high degree of spin disorder and frustrated interactions. There is no evidence of a ferromagnetic component in the ground state of Bi(0.75)Ca(0.25)MnO(3). However, the application of a magnetic field (even of a few gauss) produces a continuous progressive polarization of the Mn moments (≈2 µ(B)/Mn at 5 T, ZFC, 5 K). Differences between Ca and Sr perovskites with x = 1/4 are greater than for the x = 1/2 counterparts, and point to distinct ground states and charge/orbital configurations.
