Neutron diffraction evidence for an antiferromagnetic ordering in the CMR manganites Pr0.7Ca0.3-xSrxMnO3

A powder neutron diffraction study on the giant magnetoresistive oxides Pr0.70Ca0.3-xSrxMnO3 has been performed versus temperature for x = 0.1 and 0.05. The first one, Pr0.7Ca0.3Sr0.1MnO3, exhibits the smaller R(0)/R(H) ratio (100 at 100 K) and evidences a transition from paramagnetic to ferromagnetic state as the temperature decreases (T-c = 170 K). The second sample, Pr0.70Ca0.25Sr0.05MnO3, that exhibits an extremely high R(0)/R(H) ratio (2.5 X 10(5) at 88 K), shows a transition from the paramagnetic state to an antiferromagnetic state and finally to a canted ferromagnetic state. The presence of an intermediate AF state explains the M(T) curves and the exceptional high magnetoresistive effect. The determination of the nuclear structure of these oxides confirms their 'O-3' oxygen stoichiometry and evidences a contraction of the lattice parameters at the ferromagnetic transition, The evolution of the Mn-O distances shows a decrease of the Jahn-Teller distortion at the magnetic transition.