Increase in Tc and spin glass transition in La0.9Ca0.1Mn1−yCryO3 (y=0, 0.1 and 0.2)

We report on the structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La0.9Ca0.1Mn1−yCryO3 (y=0, 0.1 and 0.2) prepared by sol–gel technique. For y=0, a paramagnetic to ferromagnetic transition is observed at Tc=136 K. Both for y=0.1 and 0.2, Tc increases from 136 to 180 K. For y=0, the imaginary part of the ac susceptibility shows a broad transition at Tf<Tc which does not depend very much on the frequency. However, for y=0.1 and 0.2, the frequency dependence resembles that of a spin glass. Though all the three samples show a semi-conducting behavior between 300 and 5 K, a negative MR is observed corresponding to Tc and Tf. The value of MR decreases for the Cr substituted samples.