Magnetic and transport properties of layered La1.2Sr1.8Mn2-xTxO7 (T = Fe, Co, Cr)

The magnetic and transport properties of layered manganese oxides La1.2Sr1. 8Mn2-x TxO7 (T = Fe, Co, Cr) are reported. The undoped oxide La1.2Sr1.8Mn2O 7 is ferromagnetic (FM) below 125 K where an insulating-metallic (I-M) tran sition occurs. Fe and Co doping have similar effects on the properties of L a1.2Sr1.8Mn2O7, resulting in quick weakening of FM ordering, occurrence of a cluster-glass or spin-glass state for x > 0.07, immediate disappearance o f the I-M transition and enhancement of low-temperature magnetoresistance. But a different effect is observed for Cr doping. In La1.2Sr1.8Mn2-x Cr-x O -7, although the I-M transition also disappears quickly, FM ordering is mai ntained even for x = 0.5. These doping effects could be understood by consi dering the different exchange interactions between Mn and T (Fe, Co, Cr) an d the corresponding change of the double-exchange interaction in La1.2Sr1.8 Mn2-xTxO7.