Abstract
We have performed a detailed characterization of both magnetic and transport properties of (, 0.05, 0.10, and 0.15) compounds. These measurements provide clear evidence for three temperature intervals in which the magnetic and transport properties are altered. The most notable is an intermediate regime called the clustered state, , of phase coexistence involving the presence of both ferromagnetic (FM) and antiferromagnetic (AF) clusters and remnants of the high-temperature paramagnetic (PM) phase. The results further indicate a strong connection between spin cluster formation and the colossal magnetoresistivity effect. This magnetically clustered regime was observed to be altered by both magnetic field and disorder. The metal-insulator transition temperature and the width of the peak of were found to change linearly with increasing disorder (Y content) and application of magnetic field with negative and positive derivatives, respectively. This indicates that disorder and application of magnetic fields would cause the same effect but in an opposite way. We argue that correlated disorder effects influence both the shape and the magnitude of versus curves, as predicted theoretically. We have also inferred, from the computed excess of conductivity (or resistivity decrease) under decreasing and application of , that the electronic conduction process in the clustered state is described by a hopping mechanism. This result suggests two contributions to the electrical conductivity in the clustered state: (1) a semiconductinglike contribution and (2) hopping of carriers between FM metallic clusters.
1 More- Received 4 November 2004
DOI:https://doi.org/10.1103/PhysRevB.71.054404
©2005 American Physical Society