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The magnetocaloric effect (MCE) has been observed in three well-defined temperature intervals (region I: 2–115 K, region II: around charge order transition TCO, and region III: around room temperature) in polycrystalline Nd0.5Ca0.5MnO3 (NCMO) manganite system showing a first order (CO) transition at a temperature, TCO∼250 K. The magnitude of ΔSM(H) increases monotonically with applied magnetic field but does not reach saturation even at fields as high as 60 kOe. In the three given temperature regions, negligible magnetic and thermal hysteresis are found, which satisfies the requirements of using NCMO as an effective magnetocaloric material for magnetic refrigeration. [ABSTRACT FROM AUTHOR]

We report a low field (H=0.5 T) magnetoresistance of colossal magnetoresistive (CMR) composites prepared with ferromagnetic (FM)—antiferromagnetic (AFM) coupled system (100-x) La0.7Pb0.3MnO3(LPMO)+xPr0.63Ca0.37MnO3 with x=0–85 wt %. A large increase (∼16% maximum) of magnetoresistance (MR) (∼10% for pure LPMO) is observed at a relatively higher optimum concentration (x=70% AFM). Interesting double peaks are observed for lower AFM concentrations (x<=35%) in the resistivity-temperature [ρ(T)] curve: one due to the usual intrinsic double exchange interaction (between Mn3+ and Mn4+) mechanism and the other due to the stronger FM-AFM coupling for lower concentration. Unlike the usual ceramic CMR material, thermoelectric power (TEP) in this system is also nonadditive in character, indicating the presence of additional defects and strain in this composites. A two-channel (conducting and semiconductor) model has been used which fit well with the ρ(T) data near the optimum concentration x=70%. Around this highest AFM concentration, showing maximum MR, the corresponding TEP is also higher than those of the individual parent compound, which indicates the presence of maximum grain boundary effect and defects. Positron annihilation lifetime data of the composites also support the presence of such defects and strain in the system with maximum around the critical concentration xC=70%. [ABSTRACT FROM AUTHOR]

From the low-temperature (down to 10 K) dc-magnetization data of the La0.7Sr0.3MnO3 (LSM), La0.7Ca0.3MnO3 (LCM), and Pr0.7Ca0.3MnO3 (PCM) systems, we estimated the critical exponents β, γ, and hence δ from the analysis of the modified Arrot plots. The exponent β estimated for the LCM system is less than that predicted by Heisenberg model and resides within the zone predicted by Ising model while for the LSM sample, β is higher than that predicted from the Heisenberg model which is considered to be due to the presence of dipole-dipole interaction arising from the large spin moment in the LSM system. The magnetization data of the PCM system cannot be fitted to the modified Arrot plots, which suggest highly inhomogeneous ground state even under 5 T magnetic field. Both LSM and LCM have almost equal values of γ. Seebeck coefficient data indicate a crossover from higher-temperature n-type to lower-temperature p-type conductivity behavior in both LSM and LCM systems, while the semiconducting PCM system shows p-type conductivity throughout the temperature range (300–80 K). It is noticed that for LSM system TC (Curie temperature) and TMI (metal-insulator transition temperature) are almost equal (∼360 K), whereas for the LCM system there exist a large difference between TC and TMI (TC∼245 K and TMI∼265 K), which may give some idea regarding the critical behavior of the respective samples. [ABSTRACT FROM AUTHOR]

Pulsed magnetic field (0-4.4 T) was used to study the magnetic field dependent resistivity (12-350 K) and thermoelectric power (0-1.5 T) of Na and K-doped La[SUB0.7]Ca[SUB0.7-y]A[SUBy]MnO[SUB3] (0.0≤y≤0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature T[SUBp]. Na/K-doping increases both conductivity and T[SUBp]. In La[SUB1-x]Ca[SUBx]MnO[SUB3], an increase of T[SUBp] and conductivity with an increase of Ca (for x≤0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y≥0.05, Motts' condition of strong el-ph interaction breaks down in the high temperature (T>T[SUBp]) phase. Increase of conductivity in the Na/K doped system is caused by the)decrease of γ, binding energy (W[SUBp]), hopping energy (W[SUBH]), and effective mass (m[SUBp]) of the polarons leading to the melting (we call it) of polarons in the T>T[SUBp] phase. This melting results in an increase)of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80-300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (T[SUBp]θ[SUBD]/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (TT[SUBp]. [ABSTRACT FROM AUTHOR]

Magnetic-field (B=0-1.5T) dependent thermoelectric power (TEP) of the Na-doped La[sub 0.7]Ca[sub 0.3-y]Na[sub y]MnO[sub 3] (0.0≤y≤0.3) system has been studied in the temperature range 80-300 K. X-ray diffraction studies indicate a rhombohedrally distorted perovskite structure of the samples. The observed sharp peak in the resistivity (ρ) versus temperature (T) curve falls and shifts to higher temperature with increasing Na concentration (y). In the low-temperature ferromagnetic (FM) regime, thermopower (Seebeck coefficient, S) obeys the expression S=S[sub 0] + S[sub 1.5]T[sup 1.5] + S[sub 4]T[sup 4] over the entire range of y. Electron-magnon scattering is found to dominate the low-temperature resistivity and TEP data. High-temperature TEP data can be well fitted with Mott's small polaron hopping model. The activation energy (E[sub S]) and polaron hopping energy (W[sub H]) decrease with increasing Na content. Both E[sub S] and W[sub H] decrease while polaron radius (r[sub p]) increase with the application of a magnetic field. Field-dependent TEP data also indicate the suppression of spin fluctuations in the presence of a magnetic field. In the low-temperature FM region, both magnon drag and phonon drag effects coexist. [ABSTRACT FROM AUTHOR]

A two-phase multifunctional La2CoMnO6 (LCMO)/Poly-vinylidene fluoride 0-3 flexible nanocomposite (NCOMP) thick films have been prepared. Interestingly, this NCOMP shows linear increase of magnetoelectric voltage coefficient (MVC) with applied dc magnetic-bias fields up to 5 kOe, exhibiting no sign of hysteresis. For the typical LCMO concentration, fLCMO = 0.15, the maximum MVC measured is 1.45 mV cm-1 Oe-1 (using 50 kHz resonant signal). This low dielectric loss (∼0.12) NCOMP also showed intriguing high magnetodielectric effect (∼10%) around the ferromagnetic transition temperature (Tc ∼ 230 K) with minimal leakage current. Such results make the present nanocomposite significantly important for the development of magnetic field tuned electronic devices. [ABSTRACT FROM AUTHOR]

Flexible magnetoelectric 0–3 composite films formed with high dielectric (εr ∼ 104) CuO microparticles in a polyvinylidene fluoride (PVDF) polymer matrix showed multifunctional properties. The films prepared by the hot-moulding technique exhibit low-loss high relative permittivity (εeff ∼ 103) with good ferroelectric behaviour (with P–E loop) at the percolation threshold (fCuO = fc ∼ 0.25). Dielectric, ferroelectric, magnetic and direct magnetoelectric properties of the composites depend strongly on the CuO filler concentration (fCuO). Interesting low field magnetoelectric response appears with about 5% decrease in ferroelectric polarization in the insulating composite (fCuO = 0.20) along with 7.5% increase in the magnetodielectric coefficient under a magnetic field of 6 kG in the semiconducting percolative composite at room temperature. At this field, a maximum magnetoelectric voltage coefficient of 1.45 mV cm−1 Oe−1 was obtained. Even though the CuO sample is paramagnetic at ambient temperature, this typical composite film exhibits weakly ferromagnetic (with M–H loop) behaviour indicating multiferroic character. Our result represents an important step towards practical device applications using the high dielectric and magnetoelectric effects. [ABSTRACT FROM AUTHOR]