Electronic transport, magnetism and colossal magnetoresistance of (La 0.7−3 xTb x)(Ca 0.3Sr 2 x)MnO 3 (0.025⩽ x⩽0.125) compounds

Abstract

Structural, electrical, magnetization and magnetoresistance measurements have been performed on (La 0.7−3 x Tb x )(Ca 0.3Sr 2 x )MnO 3 (0.025⩽ x⩽0.125) compounds. Simultaneous substitution of Tb 3+ and Sr 2+ has been made to study the effect of increasing carrier density and size disorder on the electronic and magnetic properties. These samples crystallize in a distorted orthorhombic structure (space group Pnma). Insulator–metal (I–M) transition temperature ( T p) decreases from 193 K for x=0.025 to 100 K for x=0.075 and then shows a small rise to 118 K for x=0.10 sample. In metallic region and in zero magnetic field, the temperature ( T) dependence of the resistivity ( ρ) of x=0.025 and 0.05 samples follows the two-magnon scattering law ( ρ∝ T 4.5). However, in a field of 5 Tesla, the resistivity of x=0.025 sample obeys unconventional one-magnon scattering law ( ρ∝ T 3), which points towards field-induced suppression of spin fluctuations. In semiconducting region, the resistivity of all the samples obeys Mott's variable range hopping model ( ρ∝ T −0.25). The Curie temperature ( T C) falls less rapidly (200 K for x=0.025 to 130 K for x=0.10) than the I–M transition temperature. This disparity between T C and T p may be attributed to the phase segregation, which is also evident from deviation of magnetic susceptibility from the Curie–Weiss behavior. Magnetoresistance measurements up to a field of 9 Tesla reveal that, at low temperatures, magnetoresistance increases with increasing carrier density and size disorder. These results are discussed in the light of inter-grain tunneling magnetoresistance.