Multiferroicity with high-Tc in ceramics of the YBaCuFeO5 ordered perovskite

A dielectric anomaly has been found near the incommensurate to commensurate antiferromagnetic phase transition (TN2=230 K) in YBaCuFeO5 ceramics, a compound which crystallizes in an ordered perovskite structure. The existence of electric polarization below TN2 suggests the magnetism induced charge polarization effect that is also confirmed by its strong magnetic field dependence below TN2. Accordingly, the peak near TN2 of the magnetodielectric effect indicates a maximum of magnetodielectric susceptibility near the spin reorientation transition. Considering the abundance of magnetic compounds which structures derive from the perovskite, these results might open up the way toward the control of electric polarization near room temperature.Comment: 3 pages, 5 figures, journal pape

Magnetocapacitance effect in perovskite-superlattice based multiferroics

We report the structural and magnetoelectrical properties of La$_{0.7}$Ca$_{0.3}$MnO$_3$/BaTiO$_3$ perovskite superlattices grown on (001)-oriented SrTiO$_3$ by the pulsed laser deposition technique. Magnetic hysteresis loops together with temperature dependent magnetic properties exhibit well-defined coercivity and magnetic transition temperature (T$_C$) \symbol{126}140 K. $DC$ electrical studies of films show that the magnetoresistance (MR) is dependent on the BaTiO$_3$ thickness and negative $MR$ as high as 30% at 100K are observed. The $AC$ electrical studies reveal that the impedance and capacitance in these films vary with the applied magnetic field due to the magnetoelectrical coupling in these structures - a key feature of multiferroics. A negative magnetocapacitance value in the film as high as 3% per tesla at 1kHz and 100K is demonstrated, opening the route for designing novel functional materials.Comment: To be published in Applied Physics Letter

Current-induced metallic behavior in Pr0.5_{0.5}Ca0.5_{0.5}MnO3_3 thin films: competition between Joule heating and nonlinear conduction mechanism

Thin films of Pr0.5Ca0.5MnO3 manganites exhibiting charge/orbital-ordered properties with colossal magnetoresistance have been synthesized by the pulsed laser deposition technique on both (100)-SrTiO3 and (100)-LaAlO3 substrates. The effects of current-induced metallic-behavior of the films are investigated as a function of the temperature and the magnetic field. Calculations based on a heat transfer model across the substrate, and our resistivity measurements reveal effects of Joule heating on charge transport over certain ranges of temperatures and magnetic fields. Our results also indicate that a nonlinear conduction, which cannot be explained by homogeneous Joule heating of the film, is observed when the material is less resistive (10-2 W.cm). The origin of this behavior is explained with a model based on local thermal instabilities associated with phase-separation mechanism and a change in the long range charge-ordered state.Comment: To be published in Phys. Rev.

Current-driven magnetization decrease in single crystalline ferromagnetic manganese oxide

The electrical and magnetic response to a bias current has been investigated in a singlecrystalline ferromagnetic manganese oxide $\Pr_{0.8}$Ca$_{0.2}$MnO$_3$ . A significant decrease of the magnetization is observed at the same threshold current where a non-linearity of V-I characteristics appears. Such a behavior cannot be understood in the framework of the filamentary picture usually invoked for the non linearity of the other manganese oxides. Instead, an analogy with spintronic features might be useful and experimental signatures seem to be in agreement with excitations of spin waves by an electric current. This provides an example of a bulk system in which the spin polarized current induces a macroscopic change in the magnetization.Comment: 3 pages, 4 figure

The role of ferroelectric-ferromagnetic layers on the properties of superlattice-based multiferroics

A series of superlattices and trilayers composed of ferromagnetic and ferroelectric or paraelectric layers were grown on (100) SrTiO3 by the pulsed laser deposition technique. Their structural and magneto-electric properties were examined. The superlattices made of ferromagnetic Pr0.85Ca0.15MnO3 (PCMO) and a ferroelectric, namely Ba0.6Sr0.4TiO3 (BST) or BaTiO3, showed enhanced magnetoresistance (MR) at high applied magnetic field, whereas such enhancement was absent in Pr0.85Ca0.15MnO3/SrTiO3 superlattices, which clearly demonstrates the preponderant role of the ferroelectric layers in this enhanced MR. Furthermore, the absence of enhanced MR in trilayers of PCMO/BST indicates that the magneto-electric coupling which is responsible for MR in these systems is stronger in multilayers than in their trilayer counterparts.Comment: to be published in J. Appl. Phy

Experimental study of the correlation length of critical-current fluctuations in the presence of surface disorder: Probing vortex long-range interactions

We report on critical currents and voltage noise measurements in Niobium strips in the superconducting state, in the presence of a bulk vortex lattice ($B < B_{C2}$) and in the surface superconducting state ($B_{c2}< B < B_{C3}$). For homogeneous surfaces, the correlation length of the current fluctuations can be associated with the electromagnetic skin depth of vortex superficial instabilities. The modification of the surface state by means of low energy irradiation induces a strong modification of the critical current and of the noise. The appearance of a corner frequency in the spectral domain can be linked with the low wave-vectors of the artificial corrugation. Since this latter occurs only for $B < B_{C2}$, we propose that the long-range interactions allow the correlation length to extend up to values imposed by the surface topography.Comment: accepted for publication in PR

Control of the colossal magnetoresistance by strain effect in Nd0.5_{0.5}Ca0.5_{0.5}MnO3_{3} thin films

Thin films of Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ manganites with colossal magnetoresistance (CMR) properties have been synthesized by the Pulsed Laser Deposition technique on (100)-SrTiO$_{3}$. The lattice parameters of these manganites and correlatively their CMR properties can be controlled by the substrate temperature $T_{S}$. The maximum CMR effect at 75K, calculated as the ratio $\rho (H=0T)/\rho (H=7T)$ is 10$^4$ for a deposition temperature of $T_{S}=680$ degC. Structural studies show that the Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ film is single phase, [010]-oriented and has a pseudocubic symmetry of the perovskite subcell with a=3.77$\AA$ at room temperature. We suggest that correlation between lattice parameters, CMR and substrate temperature $T_{S}$ result mainly from substrate-induced strains which can weaken the charge-ordered state at low temperature.Comment: 9 pages, 4 figures. To be published in Applied Physics Letter

Longitudinal and transverse noise in a moving Vortex Lattice

We have studied the longitudinal and the transverse velocity fluctuations of a moving vortex lattice (VL) driven by a transport current. They exhibit both the same broad spectrum and the same order of magnitude. These two components are insensitive to the velocity and to a small bulk perturbation. This means that no bulk averaging over the disorder and no VL crystallization are observed. This is consistently explained referring to a previously proposed noisy flow of surface current whose elementary fluctuator is measured isotropic.Comment: accepted for publication in Phys Rev