Structural and magnetic studies of CuO-TeO2\mathsf{_2} and CuO-TeO2\mathsf{_2}-B2\mathsf{_2}O3\mathsf{_3} glasses

The glass systems $x$CuO$\cdot$(1$-x$)TeO$_2$ and $x$CuO$\cdot$(1$-x$)[ 75TeO$_2$ $\cdot$25B$_2$O$_3$] with 0 < x \leq 50 mol% were investigated by means of X-ray diffraction, electron paramagnetic resonance (EPR) and a.c. magnetic susceptibility ($\chi_{\rm a.c}$) measurements, the principal aim of the investigation being the study of the structural modifications in the tellurite glasses introduced by the addition of boron oxide. In the case of first glass system, i.e. without B$_2$O$_3$, EPR spectra of Cu$^{2+}$ ions undergo changes with the increasing concentration of CuO. At very low concentrations, spectra are due to isolated Cu$^{2+}$ ions in axially distorted octahedral sites. The EPR signal for samples with $3\leq x\leq 20$ mol% can be explained as being the superposition of two EPR absorptions, one showing the hyperfine structure typical for isolated Cu$^{2+}$ ions and the other consisting of a symmetric line typical for clustered ions. The broadening of the absorption band is due to dipolar as well exchange interaction. The susceptibility data show that for $x > 20$ mol% , the Cu$^{2+}$ ions are predominantly clustered and are coupled through antiferromagnetic exchange interaction. A comparative study of amorphous X-ray diffraction pattern of the glasses indicates a structural modification in the TeO$_2$ network with increasing CuO concentration; the effect is quite visible in the samples with CuO concentrations higher than 20 mole percent. Measurements of density corroborate the conclusions drawn from the X-ray diffraction. Additionally, we show that our data validates a model in which CuO rich regions are surrounded by a buffer boundary which separates them from the tellurite glassy network; effect of introducing B$_2$O$_3$ can be best described as breaking these regions into smaller size regions

Magnetic and electric properties of La_1-&delta;MnO₃

The magnetic phase diagram of $La_{1-\delta}MnO_{3}$ powdered samples have been studied as a function of $\delta$ in the low doping range. $La_{0.97}MnO_{3}$ has a canted magnetic structure at low temperature (\theta \simeq 130\QTR{group}{{}^{\circ}}). Above $T_{C}=118K$, it becomes a paramagnet with a huge effective magnetic moment, $\mu_{eff}=6.0\mu_{B}$, reflecting the presence of magnetoelastic polarons which are not affected by the magnetic field (up to 20T) nor the temperature $(1.2T_{C}<T<2.5T_{C})$. When $\delta$ is increased to $\delta =0.07$, polarons are still presen at high temperature, with a smaller size: $\mu_{eff}=5.8\mu_{B}$. The system becomes fully ferromagnetic below 170K but remains insulating down to the lowest temperature.Comment: 8pages; 6 figures as EPS files adde