RELATIONSHIP BETWEEN SPIN ORDER AND TRANSPORT AND MAGNETOTRANSPORT PROPERTIES IN LA0.67CA0.33MN1-XALXOY COMPOUNDS

We analyzed the magnetic, transport, and magnetotransport properties o f La0.67Ca0.33Mn1-xAlxOy as a function of aluminum concentration, temp erature, and magnetic field and we found that when the aluminum concen tration is increased, magnetization and T-c decrease while resistivity and magnetoresistance increase, respectively. Such results are in agr eement with the literature dealing with differently doped La-Mn perovs kites and from this it can be hypothesized that magnetic and transport properties are correlated. Based on the analysis of our data and the available literature, we have formulated a phenomenological descriptio n which accounts for the general relationships between magnetic and tr ansport characteristics. In the case of fully ferromagnetically ordere d materials the magnetoresistance effect was attributed to the spin al ignment induced by the magnetic field which opposes the thermal motion . The comparison between the experimental resistivity and magnetizatio n data, and those calculated on the basis of our model, proves to be r easonably consistent, even on quantitative levels. In mixed ferro- and antiferromagnetically ordered materials (T-c < RT), the model we prop ose does not account for the experimental data. In this case we sugges t a different phenomenological approach based on a field-induced spin reorientation transition, which qualitatively justifies the observed d ata.