Suppression of charge ordering phenomenon in nanocrystalline Nd0.67Ca0.33MnO3 manganite

A systematic investigation of electrical, magnetic and elastic properties was undertaken in nano and microcrystalline Nd_0.67Ca_0.33MnO₃ manganite, mainly to understand the charge ordering phenomenon. There is a clear and distinct behaviour in the electrical and magnetic properties of nano and microcrystalline samples and the observed behaviour is explained.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

The relation between magnetoresistance and magnetocaloric effect in La0.85Ag0.15MnO3 manganite

We present the temperature dependence of La_0.85Ag_0.15MnO₃ resistivity in the temperature interval between 77 and 340 K and magnetic fields up to 26 kOe. We offer a method of separating tunnel magnetoresistance from total magnetoresistance. A change in both the magnetic entropy, which is caused by the magnetocaloric effect (MCE), and the magnetoresistance are shown to be connected through a simple relationship to La_0.85Ag_0.15MnO₃.     

Study of Sb substitution for Pr in the Pr0.67Ba0.33MnO3 system

We study the effect of Sb substitution for Pr in the hole-doped system Pr_0.67Ba_0.33MnO₃ (PBMO) for different doping levels of Sb. The two electrical resistivity transitions observed in the pristine sample PBMO shift to low temperatures on Sb doping with an overall increase in the electrical resistivity. The significant local lattice distortion and the grain boundary effects caused by the large cation size mismatch between Pr³⁺ and Sb³⁺ suppresses the double-exchange (DE) interaction and enhances the super-exchange (SE) interaction. The compounds show a significant and increasing value of magnetoresistance at temperatures below the Curie temperature, not expected from the DE model. The Curie temperature decreases with increase in Sb content but the saturation magnetization is little affected by the substitution. The spins, however, stay well aligned in the low-temperature regime. Our X-ray near-edge absorption spectra (XANES) and core level photoemission (XPS) data clearly show the Sb cation to be in +3 state and rule out any possibility of e-doping in our compounds.     

Magnetic and magnetocaloric properties of perovskite manganite Pr0.55Sr0.45MnO3

In this paper, we have studied the magnetic and magnetocaloric properties of the perovskite manganite Pr_0.55Sr_0.45MnO₃. It shows a sharp paramagnetic-ferromagnetic phase transition at 291 K and possesses a moderate magnetic entropy change near room temperature. In addition, a large relative cooling power (143.64 J/kg) and a wide temperature range (84 K) have been found in this material. Compare with the Landau model, we find that the itinerant electrons mainly contribute the larger magnetic entropy change at paramagnetic region.     

Effects of ZnO film thickness on electrical and magnetoresistance characteristics of La0.8Sr0.2MnO3/ZnO heterostructures

The La_0.8Sr_0.2MnO₃/ZnO heterostructures with different thicknesses of ZnO films are fabricated by using RF magnetron sputtering technique. The heterojunctions exhibit excellent rectifying properties at 300 K. At low temperatures the temperature dependent junction resistance exhibits a metal-insulator transition like behavior. A magnetic field strongly impacts on electrical characteristics of La_0.8Sr_0.2MnO₃/ZnO p-n junctions, i.e., depressing the junction resistance greatly and driving the metal-insulator transition temperature (T_MI) towards higher temperatures. Large magnetoresistance is observed below T_MI, and it increases with increasing magnetic field and almost saturates at 5 T, i.e., above −90% at 100 K and 5 T.     

Giant magnetoimpedance and permeability change in La2/3Sr1/3MnO3 manganite under low fields

We demonstrated that La_2/3Sr_1/3MnO₃ sintered manganite could exhibit a magnetoreactance ΔX/X₀ of −25.5% at 100 kHz, a giant magnetoimpedance ΔZ/Z₀ of −20% at 1-2 MHz and a giant AC magnetoresistance ΔR/R₀ of −39.3% at 5 MHz under a very low field of 300 Oe at room temperature, whereas the DC magnetoresistance Δρ/ρ₀ was −3.95% under H=10 kOe and only about −0.18% under H=300 Oe. Large field-induced change of real and imaginary circular permeabilities (Δμ′_?/μ′_?(0) and Δμ″_?/μ″_?(0)) were obtained for La_2/3Sr_1/3MnO₃ sintered manganite. The giant magnetoreactance (giant magneto-inductive effect) at very low frequencies originates from the field induced change of transverse permeability. At 100 kHz under H=300 Oe, La_2/3Sr_1/3MnO₃ sintered manganite has Δμ′_?/μ′_?(0)=−25.8% and Δμ″_?/μ″_?(0)=−10.9%. The values of ΔR/R₀ and ΔZ/Z₀ are very small under 300 Oe at 100 kHz. The giant magnetoimpedance at high frequencies mainly originates from the large transverse permeability change induced by DC magnetic fields, via the penetration depth. Under H=300 Oe, La_2/3Sr_1/3MnO₃ sintered manganite presents values of Δμ′_?/μ′_?(0)=−24.9%, Δμ″_?/μ″_?(0)=−49.8% at 1 MHz, and Δμ′_?/μ′_?(0)=−21.2%, Δμ″_?/μ″_?(0)=−58.2% at 5 MHz.     

Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La_0.67Ba_0.33MnO₃ bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (T_IM) and the change in MR around the ferromagnetic transition temperature (T_C). The decrease in MR around T_C at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around T_IM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons.     

Large room-temperature magnetoresistance and temperature-dependent magnetoresistance inversion in La0.67Sr0.33MnO3/Alq3 - Co nanocomposites/Co devices

Large room-temperature (RT) magnetoresistance (MR) and temperature-dependent MR inversion have been observed in tris (8-hydroxyquinoline) aluminum (Alq₃)-cobalt nanocomposites-based organic-inorganic hybrid devices. Negative MR-high resistance for parallel electrodes configuration — due to magnetization reversal of ferromagnetic (FM) electrodes has been observed at low temperatures. As the temperature increases, the MR undergoes a sign change. At room temperature, a positive MR of ∼9.7% with the resistivity dropping monotonously with increasing magnetic fields has been observed. The RT MR is about two orders of magnitude of that in organic-FM nanocomposites measured with nonmagnetic electrodes. The enhancement of RT MR is attributed to the injection of spin polarized carriers into Alq₃-Co nanocomposites.     

Magnetotransport properties of La0.67Ca0.33MnO3//La0.67Sr0.33MnO3 bilayers

The perovskite bilayers La0.67Ca0.33MnO3 （LCMO） （100 nm） / La0.67Sr0.33MnO3（LSMO） （100 nm） and LSMO （100 nm） / LCMO （100 nm） are fabricated by a facing-target sputtering technique. Their transport and magnetic properties are investigated. It is found that the transport properties between them are different obviously due to distinguishable structures, and the different lattice strains in both films result in the difference of metal-to-insulator transition. Only single-step magnetization loop appears in our bilayers from 5K to 320K, and the coercive force of LSMO/LCMO varies irregularly with a minimum ～ 2387A/m which is lower than that of LCMO and LSMO single layer films. The behaviour is explained by some magnetic coupling.     

Low temperature resistivity minimum and its correlation with magnetoresistance in La0.67Ba0.33MnO3 nanomanganites

A systematic investigation of structural, magnetic and electrical properties of nanocrystalline La_0.67Ba_0.33MnO₃ materials, prepared by citrate gel method has been undertaken. The temperature-dependant low-temperature resistivity in ferromagnetic metallic (∼50 K) phase shows upturn behavior and is suppressed with applied magnetic field. The experimental data (<75 K) can be best fitted in the frame work of Kondo-like spin-dependant scattering, electron-electron and electron-phonon interactions. It has been found that upturn behavior may be attributed to weak spin disorder scattering including both spin polarization and grain boundary tunneling effects, which are the characteristic features of extrinsic magnetoresistance behavior, generally found in nanocrystalline manganites. The variation of electrical resistivity with temperature in the high temperature ferromagnetic metallic part of electrical resistivity (75K<T<T_P) has been fitted with grain/domain boundary, electron-electron and magnon scattering mechanisms, while the insulating region (T>T_P) of resistivity data has been explained based on adiabatic small polaron hopping mechanism.     

Magnetotransport and magnetoelastic effects in Co-doped La0.7Sr0.3MnO3 nanocrystalline perovskites

La_0.7Sr_0.3Mn_1−xCo_xO₃ (x=0, 0.05, 0.1) nanoparticles, prepared by sol-gel method, were studied by means of X-ray diffraction, transmission electron microscopy, resistivity, magnetoresistance, thermal expansion and magnetostriction measurements. Results show that partial substitution of Mn by Co leads to a reduction in lattice parameters, enhancement of resistivity and room temperature magnetoresistance MR, decrease of metal-insulator transition temperature T_MI and T_C, an increase in thermal expansion coefficient, volume magnetostriction and anisotropic magnetostriction. The latter increases about one order of magnitude with 10% Co substitution. In comparison with Mn ions, the Co ions possess higher anisotropy energy, larger magnetostriction effect, smaller ionic size and spin state transitions with increase in temperature and magnetic field; this suggests that Co substitution leads to double-exchange interaction weakening, resulting in suppression of ferromagnetic long-range order and metallic state and increase of magnetic anisotropy. Furthermore, our samples have a relatively lower T_MI and T_C, higher resistivity and MR, compared with the reported values for similar compounds with larger particle sizes. This is attributed to the nanometric grain size and spin-polarized tunneling between neighboring grains.     

Superparamagnetic behavior of La0.67Sr0.33MnO3 nanoparticles prepared via sol-gel method

Magnetic nanoparticles of La_0.67Sr_0.33MnO₃ (LSMO) manganite were prepared by sol-gel method. Phase formation and crystal structure of the synthesized powder were examined by the X-ray diffraction (XRD) using the Rietveld analysis. The mean particle size was determined by the transmission electron microscopy (TEM). Infrared transmission spectroscopy revealed that stretching and bending modes are influenced by calcinations temperature. The temperature dependence of the ac magnetic susceptibility was measured at different frequencies and ac magnetic fields in the selected ranges of 40-1000 Hz and 80-800 A/m, respectively. The temperature dependence of ac susceptibility shows a characteristic maxima corresponding to the blocking temperature near room temperature. The frequency dependence of the blocking temperature is well described by the Vogel-Fulcher law. By fitting the experimental data with this law, the relaxation time τ₀=1.7×10⁻¹² s, characteristic temperature T₀=262±3 K, anisotropy energy E_a/k=684±15 K and effective magnetic anisotropy constant k_eff=2.25×10⁴ erg/cm³ have been obtained. dc Magnetization measurement versus magnetic field shows that some of LSMO nanoparticles are blocked at 293 K. The role of magnetic interparticle interactions on the magnetic behavior is also investigated.     

Ultra-sharp metamagnetic transition in manganite Pr0.6Na0.4MnO3

We report the magnetic and electrical transport properties of manganite Pr_0.6Na_0.4MnO₃. At the temperature of 2 K, a field-induced steplike magnetization and resistivity transition are observed. The step transitions of magnetization and resistivity are shifted to higher fields as a result of field cooling, and transformed to a smooth broad one when the cooling field is higher than 20 kOe. Moreover, in a magnetic field slightly below the critical field, the magnetic and resistive relaxation exhibits a spontaneous step after a long incubation time when both the temperature and magnetic field are constant. Such steplike transitions are discussed in terms of a martensiticlike transformation associated with phase separation.     

Probing the magnetic state by linear and non-linear ac magnetic susceptibility measurements in under-doped manganite Nd0.8Sr0.2MnO3

We have thoroughly investigated the entire magnetic states of under-doped ferromagnetic-insulating manganite Nd_0.8Sr_0.2MnO₃ through temperature-dependent linear and non-linear complex ac magnetic susceptibility measurements. This ferromagnetic-insulating manganite is found to have frequency-independent ferromagnetic to paramagnetic transition temperature at around 140 K. At around 90 K (≈T_?) the sample shows a second frequency-dependent re-entrant magnetic transition as explored through complex ac susceptibility measurements. Non-linear ac susceptibility measurements (higher harmonics of ac susceptibility) have also been performed (with and without the superposition of a dc magnetic field) to further investigate the origin of this frequency dependence (dynamic behavior at this re-entrant magnetic transition). Divergence of 3rd harmonic of ac susceptibility in the limit of zero exciting field indicates a spin-glass-like freezing phenomena. However, large value of spin-relaxation time (τ₀=10⁻⁸ s) and small value of coercivity (∼22 Oe) obtained at low temperature (below T_?) from critical slowing down model and dc magnetic measurements, respectively, are in contrast with what generally observed in a canonical spin glass (τ₀=10⁻¹²-10⁻¹⁴ s and very large value of coercivity below freezing temperature). We have attributed our observation to the formation of finite size ferromagnetic clusters which are formed as consequence of intrinsic phase separation and undergo cluster glass-like freezing below certain temperature in this under-doped manganite. The results are supported by the electronic- and magneto-transport data.     

Magnetocaloric effect in La0.75Sr0.25MnO3 manganite

The polycrystalline manganite La_0.75Sr_0.25MnO₃ prepared by an alternative carbonate precipitation route reveals the rhombohedral perovskite structure. Magnetization isotherms measured up to 2 T are used to determine Curie temperature of 332 K by means of Arrott plot. Maximum of magnetic entropy change is found at Curie temperature. The relative cooling power equal to 64 J/kg for 1.5 T magnetic field, is superior as compared to the manganite with the same chemical composition from the sol-gel method.     

Effect of inhomogeneity on magnetic, magnetocaloric, and magnetotransport properties of La0.6Pr0.1Ca0.3MnO3 single crystal

The effect of magnetic inhomogeneity on magnetic, magnetocaloric, and transport properties of the colossal magnetoresistance manganites with first order ferromagnetic-to-paramagnetic phase transition is studied. The experiments were performed on the single-crystalline samples of La_0.6Pr_0.1Ca_0.3MnO₃. The inhomogeneity is described by the Curie temperature distribution function, which is found from the magnetization data. The temperature dependence of the magnetic field induced change in the entropy is shown to be determined by the distribution function and the shift of the transition temperature in a magnetic field. Similarly, magnetoresistance in the transition region is determined by the resistivity at H=0 and the shift of the transition temperature. The maximum entropy change as well as maximum magnetoresistance can be achieved in the magnetic field of order δT_C/B_M where δT_C is the transition width and B_M is the rate of change of the Curie temperature with magnetic field.Our approach to analysis of the effects of inhomogeneity is general and therefore can be used for all compounds with the first order magnetic phase transition.     

Pico-second photoelectric characteristic in manganite oxide La0.67Ca0.33MnO3 films

Ultrafast photoelectric characteristic has been observed in La_0.67Ca_0.33MnO₃ films on tilted SrTiO₃ substrates. A pico-second (ps) open-circuit photovoltage of the perovskite manganese oxide films has been obtained when the films were irradiated by a 1.064μm laser pulse of 25 ps duration. The rise time and full width at half-maximum of the photovoltage pulse are ～300 ps and ～700 ps, respectively. The photovoltaic sensitivity was as large as ～500 mV/mJ.     

Structural,magnetic, magnetocaloric and impedance spectroscopy analysis of Pr0.8Sr0.2MnO3 manganite prepared by modified solid-state route

Orthorhombic Pr_0.8Sr_0.2MnO₃ sample is prepared by solid-state reaction method and quenched in water. The occurrence of a second-order phase transition is confirmed by Arrott plots. The critical exponents determined using different models are in good agreement with the 3D-Ising model. The magnetocaloric results have shown an important maximum of the magnetic entropy change and relative cooling power evaluated at 2.14 J?kg^?1?K^?1 and 101.52 J?kg^?1 under a field change of 2T. The DC conductance measurements revealed that this compound exhibited a semiconductor to metallic behavior at $T_{\mathit{SM}}=400$ K. Moreover, the AC results are described by the Jonscher power law. Concerning the electrical and dielectric properties, they disclosed the behavior-dependency of both temperature and frequency. As for the complex impedance measurements, they showed that the conduction mechanism was governed by grain boundaries. The dielectric results demonstrated important values of the real part of the permittivity, and the Pr_0.8Sr_0.2MnO₃ material can be used for capacitor manufacturing.