Effect of Cu doping on the magnetic and electrical properties of n=2 Ruddlesden--Popper manganates

A series of polycrystalline Cu-doped n=2 Ruddlesden--Popper manganates La_1.2Sr_1.8Cu_xMn_(2-x)O_7 (x=0, 0.04, 0.13) were synthesized by the solid state reaction method. The effect of Cu doping on the magnetic and transport properties has been studied. It is found that Cu substitution for Mn greatly affects the magnetic and electrical properties of the parent phase La_1.2Sr_1.8Mn_2O_7. With the increase of Cu content, the system undergoes a transition from long-range ferromagnetic order to the spin glass state and further to an antiferromagnetic order. A little of Cu dopant can lead to the samples showing semiconductor or insulator behaviour in the whole observed temperature range while the parent phase has a metal--insulator transition.These samples show colossal magnetoresistance at low temperatures and the value of it decreases with increasing Cu content.