Abstract
Results of a detailed investigation of the structure and electron-transport properties of (A=Ca, Sr) over a wide range of compositions are presented along with those of parent containing different percentages of . The electrical resistivity (ρ) and magnetoresistance (MR) of polycrystalline pellets have been measured in the 4.2–400 K range in magnetic fields up to 6 T and the Seebeck coefficient (S) from 100 to 400 K. The electrical measurements were supplemented by ac susceptibility and magnetization measurements. MR is large and negative over a substantial range of compositions and peaks around temperatures close to the ferromagnetic transition temperatures (). An insulator to metal-like transition occurs near the and the temperature dependence of ρ below is related to the magnetization although ρ in the metallic state is generally much larger than the Mott’s maximum metallic resistivity. The occurrence of giant magnetoresistance is linked to the presence of an optimal proportion of ions and is found in the rhombohedral and the cubic structures where the Mn-O distance is less than 1.97 Å and the Mn-O-Mn angle is 170°±10°. The field dependence of MR shows the presence of two distinct regimes. The thermopower S shows a positive peak in the composition range at a temperature where MR also peaks; S becomes more negative with increase in . © 1996 The American Physical Society.
- Received 5 September 1995
DOI:https://doi.org/10.1103/PhysRevB.53.3348
©1996 American Physical Society