Magnetic structure and giant magnetoresistance of ferromagnetic La_1−δMn_1−δO₃- an example of double-exchange striction?

Abstract

A neutron powder diffraction study of the structure of the rhombohedral phase of La_1−δMn_1−δO₃(δ=0.04) as a function of temperature has been carried out. The crystal structure is well described at all temperatures by a model in R- 3c(a=5.52264(5) and c=13.3309(5) at 300 K) and the low temperature data contain magnetic contributions to the Bragg scattering due to ferromagnetic ordering below T_c~ 200 K. The spin is canted at approximately 45° from [001]. The Mn-O-Mn angle decreases on cooling to T_c, but shows a slight increase on further cooling, presumably due to the onset of a double-exchange striction effect. Resistivity and magnetization data corroborate the neutron findings and reveal an insulator to metal transition close to T_c with a negative giant magnetoresistance value of ~50% at this temperature. The results confirm the striking similarity between the structural, magnetic, and conducting properties of rhombohedral La_1−δMn_1−δO₃ and those of the solid solution La_1−xA_xMnO₃(x~0.3).