Large magnetocapacitance in electronic ferroelectric manganite systems

We have observed a sizable positive magnetocapacitance ($\sim$$5-90\%$) in perovskite Pr$_{0.55}$Ca$_{0.45}$MnO$_3$ and bilayer Pr(Sr$_{0.1}$Ca$_{0.9}$)$_2$Mn$_2$O$_7$ system under 5T magnetic field across 20-100 K below the magnetic transition point T$_N$. The magnetodielectric effect, on the other hand, exhibits a crossover: (a) from positive to negative for the perovskite system and (b) from negative to positive for the bilayer system over the same temperature range. The bilayer Pr(Sr$_{0.1}$Ca$_{0.9}$)$_2$Mn$_2$O$_7$ system exhibits a sizable anisotropy as well. We have also noticed the influence of magnetic field on the dielectric relaxation characteristics of these systems. These systems belong to a class of improper ferroelectrics and are expected to exhibit charge/orbital order driven ferroelectric polarization below the transition point T$_{CO}$. Large magnetocapacitance in these systems shows typical multiferroic behavior even though the ferroelectric polarization is small in comparison to that of other ferroelectrics.Comment: 6 pages with 5 embedded figures; accepted for publication in J. Appl. Phy

Confined spin waves reveal an assembly of nanosize domains in ferromagnetic La(1-x)CaxMnO3 (x=0.17,0.2)

We report a study of spin-waves in ferromagnetic La$_{1-x}$Ca$_{x}$MnO$_3$, at concentrations x=0.17 and x=0.2 very close to the metallic transition (x=0.225). Below T$_C$, in the quasi-metallic state (T=150K), nearly q-independent energy levels are observed. They are characteristic of standing spin waves confined into finite-size ferromagnetic domains, defined in {\bf a, b) plane for x=0.17 and in all q-directions for x=0.2. They allow an estimation of the domain size, a few lattice spacings, and of the magnetic coupling constants inside the domains. These constants, anisotropic, are typical of an orbital-ordered state, allowing to characterize the domains as "hole-poor". The precursor state of the CMR metallic phase appears, therefore, as an assembly of small orbital-ordered domains.Comment: 4 pages, 5 figure

Spin dynamics of strongly-doped La_{1-x}Sr_xMnO_3

Cold neutron triple-axis measurements have been used to investigate the nature of the long-wavelength spin dynamics in strongly-doped La$_{1-x}$Sr$_{x}$MnO$_3$ single crystals with $x$=0.2 and 0.3. Both systems behave like isotropic ferromagnets at low T, with a gapless ($E_0 < 0.02$ meV) quadratic dispersion relation $E = E_0 + Dq^2$. The values of the spin-wave stiffness constant $D$ are large ($D_{T=0}$ = 166.77 meV$\AA^2$ for $x$=0.2 and D$_{T=0}$ = 175.87 meV$\AA^2$ for $x$=0.3), which directly shows that the electron transfer energy for the $d$ band is large. $D$ exhibits a power law behavior as a function of temperature, and appears to collapse as T -> T_C. Nevertheless, an anomalously strong quasielastic central component develops and dominates the fluctuation spectrum as T -> T_C. Bragg scattering indicates that the magnetization near $T_C$ exhibits power law behavior, with $\beta \simeq 0.30$ for both systems, as expected for a three-dimensional ferromagnet.Comment: 4 pages (RevTex), 3 figures (encapsulated postscript

Nonequilibrium 1/f Noise in Low-doped Manganite Single Crystals

1/f noise in current biased La0.82Ca0.18MnO3 crystals has been investigated. The temperature dependence of the noise follows the resistivity changes with temperature suggesting that resistivity fluctuations constitute a fixed fraction of the total resistivity, independently of the dissipation mechanism and magnetic state of the system. The noise scales as a square of the current as expected for equilibrium resistivity fluctuations. However, at 77 K at bias exceeding some threshold, the noise intensity starts to decrease with increasing bias. The appearance of nonequilibrium noise is interpreted in terms of bias dependent multi-step indirect tunneling.Comment: 4pages, 3figures,APL accepte

Quantized spin waves in the metallic state of magnetoresistive manganites

High resolution spin waves measurements have been carried out in ferromagnetic (F) La(1-x)(Sr,Ca)xMnO3 with x(Sr)=0.15, 0.175, 0.2, 0.3 and x(Ca)=0.3. In all q-directions, close to the zone boundary, the spin wave spectra consist of several energy levels, with the same values in the metallic and the x\approx 1/8 ranges. Mainly the intensity varies, jumping from the lower energy levels determined in the x\approx 1/8 range to the higher energy ones observed in the metallic state. On the basis of a quantitative agreement found for x(Sr)=0.15 in a model of ordered 2D clusters, the spin wave anomalies of the metallic state can be interpreted in terms of quantized spin waves within the same 2D clusters, embedded in a 3D matrix.Comment: 4 pages, 5 figure

Monoclinic La1-xBaxMnO3 (x=0.185) at 160 K

Single-crystal X-ray diffraction has shown that lanthanum barium manganese trioxide, La0.815Ba0.185MnO3, is monoclinic (I2/c) below a first-order phase transition at 187.1 (3) K. This result differs from the Pbnm symmetry usually assigned to colossal magnetoresistance oxides, A1-xA\u2032xMnO3 with x 43 0.2, which adopt a distorted perovskite-type crystal structure. The Mn atom lies on an inversion center, the disordered Li/Ba site is on a twofold axis and one of the two independent O atoms also lies on a twofold axis