Electronic transport, magnetism and colossal magneto resistance of (La0.7-3xTbx)(Ca0.3Sr2x,)MnO3 (0.025≤x≤0.125) compounds

Structural, electrical, magnetization and magnetoresistance measurements have been performed on (La0.7-3x,Tb-x)(Ca0.3Sr2x)MnO3 (0.025 less than or equal toxless than or equal to 0.125) compounds. Simultaneous substitution of Tb3+ and Sr2+ 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 Puma). 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 (p) of x = 0.025 and 0.05 samples follows the two-magnon scattering law (pproportional to T-4.5). However, in a field of 5 Tesla, the resistivity of x = 0.025 sample obeys unconventional one-magnon scattering law (p cc 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 (pproportional to 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 searegation. which is also evident from deviation of magnetic susceptibility from the Curie-Weiss behavior. Magrietoresistance measurements up to a field of 9 Tesla reveal that, at low temperatures, magneto resistance increases with increasing carrier density and size disorder. These results are discussed in the light of inter-grain tunneling maganetoresistance. (C) 2003 Elsevier B.V. All rights reserved.