Exchange interaction, spin cluster and transport behaviour in perovskites La_0.67Sr_0.33(Mn_1-xNi_x)O₃ (x ⩽ 0.2)

A systematic investigation of the magnetic and transport properties of perovskites La_0.67 Sr_0.33 (Mn_1-x Ni_x )O₃ (x ⩽ 0.2) is reported. The replacement of the Mn ion by a Ni ion results in a reduction of ferromagnetism and metallic conduction. For the sample with x= 0.1, in addition to a metal-insulator (M-I) transition at 240 K, there is a weak insulating behaviour at low temperatures. As the magnetization drop below the Curie temperature (T_C ) in the thermomagnetic curve appears for the dosage of x= 0.15, the insulating behaviour at low temperatures becomes pronounced. For the sample with x= 0.2, a cluster glass-like state with no M-I transition near T_C 194 K is shown. The ferromagnetic superexchange interaction between the Mn³⁺⁽⁴⁺⁾ion and the Ni²⁺ion is found to account for the formation of the cluster glass-like state rather than a spin glass state. All samples have a colossal magnetoresistance (CMR). In particular, with the suppression of the ferromagnetism, the MR effect shifts towards low temperatures and becomes considerably enhanced. Besides the depletion of hopping electrons and the destroyed metallic paths due to the foreign Ni ion terminating the double exchange interaction in the Mn-O network, the randomly frozen spin cluster at low temperatures has been revealed as another crucial factor for localizing charge carriers.