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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. Phys

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 figures

The nature of the polarons in the optimally doped colossal magnetoresistive (CMR) materials La0.7Ba0.3MnO3 (LBMO) and La0.7Sr0.3MnO3 (LSMO) is studied by elastic and inelastic neutron scattering. In both materials, dynamic nanoscale polaron correlations develop abruptly in the ferromagnetic state. However, the polarons are not able to lock-in to the lattice and order, in contrast to the behavior of La0.7Ca0.3MnO3. Therefore ferromagnetic order in LBMO and LSMO survives their formation, explaining the conventional second order nature of the ferromagnetic--paramagnetic transition. Nevertheless, the results demonstrate that the fundamental mechanism of polaron formation is a universal feature of these ferromagnetic perovskite manganites., Comment: published in Physical Rev. B 78, 212301 (2008)

We report a series of magnetic and transport measurements on high-quality single crystal samples of colossal magnetoresistive manganites, La_{0.7} Ca_{0.3} Mn O_3 and Pr_{0.7} Sr_{0.3} Mn O_3. 1 % Fe doping allows a Moessbauer spectroscopy study, which shows (i) unusual line broadening within the ferromagnetic phase and (ii) a coexistence of ferro- and paramagnetic contributions in a region, T_1T_2. This shows that phase separation into metallic (ferromagnetic) and insulating (paramagnetic) phases cannot be generally responsible for the resistivity peak (and hence for the associated colossal magnetoresistance). Our results can be understood phenomenologically within the two-fluid approach, which also allows for a difference between T_C and T_{MI}. Our data indeed imply that while magnetic and transport properties of the manganites are closely interrelated, the two transitions at T_C and T_{MI} can be viewed as distinct phenomena., Comment: 6 pages, including 3 figures; revised version; Europhys. Lett. (http://epljournal.edpsciences.org/), in press

Neutron elastic, inelastic and high energy x-ray scattering techniques are used to explore the nature of the polaron order and dynamics in La0.7Ca0.3MnO3. Static polaron correlations develop within a narrow temperature regime as the Curie temperature is approached from low temperatures, with a nanoscale correlation length that is only weakly temperature dependent. The static nature of these short-range polaron correlations indicates the presence of a glass-like state, similar to the observations for the bilayer manganite in the metallic-ferromagnetic doping region. In addition to this elastic component, inelastic scattering measurements reveal dynamic correlations with a comparable correlation length, and with an energy distribution that is quasielastic. The elastic component disappears at a higher temperature T*, above which the correlations are purely dynamic. The overall behavior bears a remarkable resemblance to the magnetic fluctuation spectrum recently observed in underdoped cuprates., Comment: 11 pages, 7 figures, submitted to Phys. Rev. B

The effect of resistive switching in doped manganites being in the ferromagnetic state has been studied using resistive and magneto-optic methods. The visualization of magnetic structure of La0.75Sr0.25MnO3-x single crystals, and its transformation under electric current proposed local superheating of the material above the Curie temperature, which was supported by numerical calculation. The obtained results suggest a significant role of micrometer-scale inhomogeneity of manganites in phase separation, magnetic and transport properties of the material.

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 figures

Neutron scattering has been used to study the nature of the ferromagnetic transition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, and polycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where the naturally occurring O-16 can be replaced with the O-18 isotope. Small angle neutron scattering on the x=0.3 single crystal reveals a discontinuous change in the scattering at the Curie temperature for wave vectors below ~0.065 A^-1. Strong relaxation effects are observed for this domain scattering, for the magnetic order parameter, and for the quasielastic scattering, demonstrating that the transition is not continuous in nature. There is a large oxygen isotope effect observed for the T_C in the polycrystalline samples. For the optimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5 K at 90% O-18 substitution. The temperature dependence of the spin-wave stiffness is found to be identical for the two samples despite changes in T_C. Hence, T_C is not solely determined by the magnetic subsystem, but instead the ferromagnetic phase is truncated by the formation of polarons which cause an abrupt transition to the paramagnetic, insulating state. Application of uniaxial stress in the x=0.3 single crystal sharply enhances the polaron scattering at room temperature. Measurements of the phonon density-of-states show only modest differences above and below T_C and between the two different isotopic samples., Comment: 13 pages, 16 figures, submitted to Phys. Rev. B

The ferromagnetic and insulating state observed in La$_{1-x}$Ca$_{x}$MnO$_3$, 0.125$<$x$<$0.2, is characterized by structural and magnetic anomalies below T$_C$, similar to those observed in the x$_{Sr}$$\approx$1/8. A neutron scattering study of the superlattice {\bf Q$_0$}= (0,0,1+/4)$_{cub}$ peak, and of the magnetic excitations are reported in the x$_{Ca}$=0.2 sample. The occurrence of this superstructure is associated with the observation of a gap in the spin dynamics, at a {\bf q$_0$} wave-vector ({\bf q$_0$}={\bf Q$_0$}-$\tau$) with the same modulus $|${\bf q$_0$}$|$ in all directions, which divides the dispersion into two regimes. For $|${\bf q}$|$$<$$|${\bf q$_0$}$|$ the dispersion is splitted into two or three curves. For $|${\bf q}$|$$>$$|${\bf q$_0$}$|$, magnetic excitations lock on acoustic and optic phonon energies, revealing a new kind of magneto-vibrational coupling. We suggest an analysis in terms of two distinct magnetic couplings, associated with two ferromagnetic media involved into a collective state.

We report on the optical properties of undoped single crystal LaMnO_3, the parent compound of the colossal magneto-resistive manganites. Near-Normal incidence reflectance measurements are reported in the frequency range of 20-50,000 cm-1 and in the temperature range 10-300 K. The optical conductivity, s_1(w), is derived by performing a Kramers-Kronig analysis of the reflectance data. The far-infrared spectrum of s_1(w) displays the infrared active optical phonons. We observe a shift of several of the phonon to high frequencies as the temperature is lowered through the Neel temperature of the sample (T_N = 137 K). The high-frequency s_1(w) is characterized by the onset of absorption near 1.5 eV. This energy has been identified as the threshold for optical transitions across the Jahn-Teller split e_g levels. The spectral weight of this feature increases in the low-temperature state. This implies a transfer of spectral weight from the UV to the visible associated with the paramagnetic to antiferromagnetic state. We discuss the results in terms of the double exchange processes that affect the optical processes in this magnetic material., Comment: 7 pages, 5 figures

We describe the evolution of the static and dynamic spin correlations of La$_{1-x}$Ca$_x$MnO$_3$, for x=0.1, 0.125 and 0.2, where the system evolves from the canted magnetic state towards the insulating ferromagnetic state, approaching the metallic transition (x=0.22). In the x=0.1 sample, the observation of two spin wave branches typical of two distinct types of magnetic coupling, and of a modulation in the elastic diffuse scattering characteristic of ferromagnetic inhomogeneities, confirms the static and dynamic inhomogeneous features previously observed at x$<$0.1. The anisotropic q-dependence of the intensity of the low-energy spin wave suggests a bidimensionnal character for the static inhomogeneities. At x=0.125, which corresponds to the occurence of a ferromagnetic and insulating state, the two spin wave branches reduce to a single one, but anisotropic. At this concentration, an anomaly appears at {\bf q$_0$}=(1.25,1.25,0), that could be related to an underlying periodicity, as arising from (1.5,1.5,0) superstructures. At x=0.2, the spin-wave branch is isotropic. In addition to the anomaly observed at q$_0$, extra magnetic excitations are observed at larger q, forming an optical branch. The two dispersion curves suggest an anti-crossing behavior at some {\bf q$_0$'} value, which could be explained by a folding due to an underlying perodicity involving four cubic lattice spacings.

Neutron scattering has been used to study the nature of the ferromagnetic transition in a single crystal of the perovskite La0.7Ca0.3MnO3. Diffuse scattering from lattice polarons develops as the Curie temperature is approached from below, along with short range polaron correlations that are consistent with stripe formation. Both the scattering due to the polaron correlations and the anomalous quasielastic component in the magnetic fluctuation spectrum maximize very close to T_C, in a manner remarkably similar to the resistivity, indicating that they have a common origin., Comment: 4 pages, 4 eps figures, to be published in Phys. Rev. Lett

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)