Intrinsic inhomogeneity in a (La0.4 Eu0.6) 0.7 Pb0.3 MnO3 single crystal: Magnetization, transport, and electron magnetic resonance studies

Conventional magnetic and transport measurements of the melt-grown mixed-valence manganite (La0.4 Eu0.6) 0.7 Pb0.3 MnO3 have been supplemented by a magnetic resonance study. The experimental data support the model of two magnetic phases coexisting in the crystal volume. At a temperature T*, which is well above Curie temperature TC, ferromagnetic clusters appear in the sample. These ferromagnetic regions possess a higher conductivity in comparison with the paramagnetic background (majority phase). On cooling through TC, the magnetization of the spatially confined ferromagnetic clusters of the minority phase freezes in random directions with respect to the magnetization of the ferromagnetic majority phase due to the difference of the exchange interactions at the phase boundaries from the intraphase interactions in sign and value. Such a mixed state is responsible for the observed magnetic glassylike behavior of the system that is characteristic of inhomogeneous magnets. The fluctuations of the magnetic coupling value and sign in the sample volume are related to strong competition between the ferromagnetic and antiferromagnetic exchange interactions, which, in turn, results from the quenched disorder caused by the random chemical replacement of the perovskite A site of the manganite. A phase-separation state comprised of two different ferromagnetic phases has been used to account for the colossal magnetoresistance phenomenon and the magnetic-field-driven nonlinear conduction found in the crystal.