Effect of Ti doping on magnetic properties and magnetoresistance in LaSr2Mn2O7

The effect of Ti substitution for Mn on magnetic and transport properties has been investigated for layered manganese oxides LaSr2Mn2−xTixO7. Titanium doping hampered the canted antiferromagnetic (AFM) exchange at low temperature and their Neel temperature (TN) decreased from 138 K (x = 0) to 106 K (x = 0.1). Meanwhile, spin glass, charge ordering and metal-insulator transition are suppressed by Ti addition. This can be attributed to Mn-site disorder caused by random substitution of Ti4+. The suppression of charge ordering leads to magntetoresistance (MR) ratio increase and MR reaches maximum at x = 0.3. The resistivity increases obviously with x increasing because of double exchange interaction channel broken by Ti4+ addition. The resistivity of all samples in low temperature range fits to the Mott's variable range hopping (VRH) model, while it fits to nearest neighbor hopping of small polarons model in high temperature range. We also found that both disorder and distortion in A-site and B-site will induce the similar effect to electrical and magnetic properties.