Interplay between structure and magnetism in Ho_xPr_1-x alloys. 1. Neutron scattering

The structural and the magnetic ordering in thin-film HoxPr1-x alloys have been studied using neutron-and x-ray-diffraction techniques. As the concentration of Ho decreases the alloys adopt hexagonal-close-packed (hcp), Sm, and double hexagonal-close-packed (dhcp) crystal structures. The results show enhanced occupation of the cubic sites by Pr in the Sm and dhcp phases.

The magnetic behavior is found to be very different in the three crystalline phases. The hcp samples form basal-plane spirals and the alloys with the Sm structure form a commensurate magnetic structure with the same periodicity as the magnetic order on the hexagonal sites in Sm metal, but the moments are confined to the basal plane.

At low temperatures both Ho and Pr are found to adopt their full saturation moments in these phases. A Pr thin film is found to order with a similar magnetic structure to bulk Pr. However, no long-range magnetic order is observed in the dhcp alloys down to a temperature T similar to 23 mK, suggesting that Ho and Pr have singlet ground states and that the ordering due to the hyperfine interaction is suppressed.

The results are discussed in terms of mean-field theory and the concentration dependence of the Neel temperature is found to be anomalous.