NEUTRON-DIFFRACTION EVIDENCE FOR AN ANTIFERROMAGNETIC ORDERING IN THECMR MANGANITES PR0.7CA0.3-XSRXMNO3

A powder neutron diffraction study on the giant magnetoresistive oxide s 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 paramag netic to ferromagnetic state as the temperature decreases (T-c = 170 K ). The second sample, Pr0.70Ca0.25Sr0.05MnO3, that exhibits an extreme ly 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 eff ect. The determination of the nuclear structure of these oxides confir ms 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 t he magnetic transition.