Electron paramagnetic resonance study of (La0.33Sm0.67)0.67Sr0.33−xBaxMnO3 (x<0.1): Griffiths phase

Manganite compounds (La0.33Sm0.67)0.67Sr0.33-xBaxMnO3 with light Ba doping (x = 0.01, 0.03, 0.06, 0.09) have been investigated by EPR over the temperature range 110 – 450 K. It was found that the EPR linewidth behavior changed drastically in samples with these low Ba concentrations. For all the samples there was observed a transition from paramagnetic to ferromagnetic phase below the phase-transition temperature. EPR signals characteristic of Griffiths phase were observed in the samples with x = 0.03, 0.06, 0.09. The temperature dependence of the EPR linewidth in the paramagnetic phase was analyzed on the basis of variable-range-hoping model, which explained well the observed data

EPR and magnetization studies of the manganites La<inf>0.7-x</inf>Eu<inf>x</inf>Sr<inf>0.3</inf>MnO<inf>3</inf> (x = 0.4, 0.5, 0.6, 0.7) and La<inf>0.3</inf>Nd<inf>0.4</inf>Sr<inf>0.3</inf>MnO<inf>3</inf> at different temperatures: Conductivity due to hopping of small polarons

Four Eu-Sr manganites, La0.7-xEuxSr0.3MnO3; x = 0.4, 0.5, 0.6, 0.7, and one Nd-Sr manganite Nd0.4La0.3Sr0.3MnO3, were studied by EPR (electron paramagnetic resonance) at 9.6 GHz at temperatures in the range 175 – 400 K. These studies were further supplemented by magnetization measurements. The positions and widths of the EPR lines of all the Eu-Sr manganite samples, containing Eu3+ ions, changed with temperature in a similar manner. The EPR data showed that these Eu-Sr manganites are ferromagnetic, whose magnetic transition temperature from the ferromagnetic to the paramagnetic state (TC) decreased gradually as the Eu content (x) increased, specifically from TC ~ 111 K (x = 0.5) to TC ~ 57 K (x = 0.7). Furthermore, in these samples the FMR (ferromagnetic resonance) lines appeared significantly above the respective TC, specifically, at 290 K (x = 0.4), 280 K (x = 0.5), 250 K (x = 0.6), together with the EPR lines. As for the sample with x = 0.7 no FMR lines were observed in the temperature range investigated here. The temperature dependence of the EPR linewidth is found to be linear in the various La0.7-xEuxSr0.3MnO3 samples, caused by the presence of conductivity due to small-polaron hopping The peak-to-peak first-derivative EPR linewidths, ΔBpp, of these Eu-doped samples fitted well above the temperature, at which the minimum of ΔBpp occurs, to the expression: ΔBpp (T) = ΔBpp,min + [Formula presented]exp(-Ea/kBT), with the values of the activation energies being Ea = 0.20 eV, 0.17 eV, 0.11 eV, 0.09 eV for x = 0.4, 0.5, 0.6, 0.7, respectively. On the other hand, for the Nd-doped sample, the ΔBpp in Nd0.4La0.3Sr0.3MnO3 decreased monotonically with temperature above TC in accordance with the Curie-Weiss law as ΔBpp (T) = ΔBpp,0 + C/(T-TC). The magnetization measurements were exploited to determine TC for the various samples. The results agreed with those determined by the EPR measurements presented here. In addition, the magnetization data was analyzed to obtain the values of the critical exponents in the phase transition from the paramagnetic to the ferromagnetic state. The critical exponents, as determined for the La0.7-xEuxSr0.3MnO3 sample with x = 0.5, are β = 0.24 ± 0.02, γ = 1.10 ± 0.5, δ = 5.58

Synthesis and characterization of (68-x) CuO – xV<inf>2</inf>O<inf>5</inf> – 32TeO<inf>2</inf> (x = 0–68 mol%) and (35-x) CuO – xV<inf>2</inf>O<inf>5</inf> – 65TeO<inf>2</inf> (x = 0–35 mol%) glasses: Conduction mechanism, structure and EPR study

In this work, two series of glasses, i.e. (68-x) CuO – xV2O5 – 32TeO2 (x = 0–68 mol%, Te32 series) and (35-x) CuO – xV2O5 – 65TeO2 (x = 0–35 mol%, Te65 series), were synthesized by the melt-quenching method and subjected to physical, thermal and electrical characterization. Their vitreous nature was confirmed by X-Ray diffraction and differential scanning calorimetry, while their structural units were determined by Raman spectroscopy. CuO substitution by V2O5 led to a decrease in density and glass-transition temperature, together with a conductivity increase. Conduction mechanism was interpreted as mainly due to small polaron hopping from the lower (V4+) to the higher (V5+) vanadium valence states. Te32 glasses, possessing the highest electronic conductivities (ranging from 2 E−4 to 5 E−7 Ω−1 cm−1), were investigated by the Electron Paramagnetic Resonance technique, in order to more deeply analyze their structure-conductivity correlation. Particularly, the observed signals were determined to consist in a superposition of a first line due to paramagnetic Cu2+ ions and a second line due to exchange-coupled CuO clusters. Differences in the spectra were determined between samples with higher (i.e. 20-30 mol%) Cu2+ concentrations and samples with lower Cu2+ concentrations, suggesting they are located in different local environments. Finally, it was found that the Cu2+ ions are not involved in the process of electron transfer