Magnetotransport studies in La<inf>0.7</inf>Ca<inf>0.3</inf>Mn<inf>1-x</inf>M<inf>x</inf>O<inf>3</inf> (M=Co and Ga)

The effect of doping a magnetic ion like Co and a non-magnetic ion like Ga at the Mn-site in La0.7Ca0.3MnO3 was studied. ac magnetic susceptibility (χ) measurements show a large suppression in the ferromagnetic transition temperature (TC) for the Ga substituted compounds compared to that of the Co substituted compounds indicating that the decrease in ferromagnetic coupling in the former is due to smaller electronic bandwidth. The appearance of a prominent cusp in the χ-T curves for the Ga-compounds compared to the Co-compounds, even at low doping and also the deviation from the Curie-Weiss behaviour at low temperatures indicates a cluster glass kind of behaviour which is less pronounced in identical compositions of the Co-compounds. It has been seen that clustering of spins takes place either when the number of Mn4+ ions [J. Appl. Phys. 85 (1999) 1663] or when the number of mobile charge carriers is reduced [J. Magn. Magn. Mater. 214 (2000) L149]. In this case the number of Mn4+ ions is the same for identical compositions of Co and Ga-substituted compounds and hence the observed changes in TC can be attributed to the differences in the active electronic bandwidth. Resistivity measurements clearly confirm the presence of smaller electronic bandwidth in the Ga-compounds. © 2001 Elsevier Science B.V. All rights reserved.