YB-170 MOSSBAUER AND NEUTRON-DIFFRACTION INVESTIGATION OF THE ANTIFERROMAGNETIC PHASE IN THE KONDO LATTICES YBP AND YBN

The Kondo lattices YbP and YbN (nonstoichiometric samples) have been i nvestigated by Yb-170 Mossbauer spectroscopy, in the temperature range 0.085 K to 80 K in zero magnetic field in YbN and with an applied mag netic field at T = 0.045 K in YbP and YbN, and by neutron diffraction in YbN. In both compounds, the antiferromagnetic transition at T(N) co ngruent-to 0.7 K is first order and occurs through a progressive growt h of paramagnetic domains as the temperature increases. The measured e xchange interaction is much larger than k(B)T(N) in both compounds and it is found to be isotropic in YbP and anisotropic in YbN. The satura ted magnetic moment in both compounds is reduced by 50% with respect t o the value calculated from the crystal field and exchange interaction s. These properties are interpreted in terms of the competition betwee n the Kondo effect and the RKKY exchange interaction; using a model me an field NCA calculation to describe the magnetic phase, the Kondo tem peratures T(K) are found to be near 5-10 K.