Magnetism and ferroelectricity
Effect of the oxygen excess on the properties of weakly doped La$_{1-x}$Ca$_x$MnO$_{3+\delta}$ lanthanum manganites
N. I. Solin,
S. V. Naumov,
N. M. Chebotaev,
A. V. Korolev Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg
Abstract:
The effect of an oxygen excess
$\delta$ on the magnetic and electrical properties of La
$_{1-x}$Ca
$_x$MnO
$_{3+\delta}$ (
$x$ = 0.10–0.15) has been studied over wide ranges of temperatures and magnetic fields. As
$\delta$ increases, the magnetic ordering temperature Tcdecreases by 70–90 K, the magnetoresistance increases (the electrical resistivity decreases by a factor of up to 10
$^4$ in a field of 9 T), and the effective moment
$\mu_{\mathrm{eff}}$ of the paramagnetic susceptibility substantially exceeds the theoretical value at temperatures two to four times higher than
$T_c$ and undergoes a jump, just as the activation energy of electrical resistivity, at
$T\approx$ 270 K. These results are attributed to the formation of cation vacancies, the localization of electrons in their vicinity with the subsequent formation of magnetic clusters, tunneling (or hopping) of carriers among them, changes in the sizes of clusters with variations in the temperature and magnetic field strength, the onset of frustrations initiated by the competition among different types of exchange interaction, and the dependence of the cluster parameters on the annealing conditions. Annealing of the oxygen-excess samples at high temperatures in vacuum (above 1100
$^\circ$C) restores the samples to a nearly initial state with the magnetic and magnetotransport properties characteristic of weakly doped manganites, as a result of the removal of cation vacancies.
Received: 05.05.2009
