Ferromagnetic insulating and spin glass behavior in Cr substituted La0.85Ag0.15MnO3 compounds

Single phase samples of polycrystalline La0.85Ag0.15Mn1-yCryO3 ( y = 0, 0.05, 0.10, 0.15, 0.20) were prepared by the solid-state route. These samples were studied by recording x-ray diffraction patterns to investigate their crystal structure, by measuring temperature and frequency variations of ac susceptibility and high temperature magnetization to investigate their magnetic properties and by measuring magneto-resistivity. X-ray diffraction patterns could be refined by using the R (3) over barc space group. The lattice parameters and Mn-O bond lengths were found to decrease with Cr doping. Paramagnetic to ferromagnetic transitions followed by low temperature spin glass like behavior have been observed from ac susceptibility results. The above transitions could be understood on the basis of double exchange ferromagnetic interaction in Mn3+ -O2- -Mn4+, super-exchange ferromagnetic interaction in Cr3+ -O2- -Mn3+ and super-exchange antiferromagnetic interaction in Mn4+ -O2- -Mn4+ networks. Even though a strong ferromagnetic signal was observed in all the Cr doped samples, no metal -insulator transition has been observed. Thus Cr doping gives rise to a ferromagnetic insulating state and the Cr might take part in super-exchange ferromagnetic interactions. Colossal magneto-resistivity has been observed up to 20% of Cr doping in a wide temperature range down to low temperatures.