Dynamics of the internal field in RB12 (R = Er, Yb, Lu)

The results of muSR studies on isostructural LuB12, YbB12 and ErB12 between 1.8 K and 300 K are described. LuB12 is non-magnetic. YbB12 is a Kondo insulator in which a previous muSR study reported the formation of dynamically correlated Yb moments of less than 10(-2) mu(B) below 20 K. ErB12 is an antiferromagnet (AFM) with T-N = 6.7 K. We found the muSR spectra of LuB12 and YbB12 to be quite alike at all temperatures which excludes any contribution from Yb magnetic moments. Spectral shape is determined by the field of the B-11 nuclear moments. These fields show dynamic behavior, but nuclear spin relaxation can be excluded since its rate is below the muSR time window. It is suggested that the dynamical features arise from atomic motions within the B-12, clusters and perhaps (at higher temperatures) from muon motion. In ErB12, long-range magnetic order (LRO) exists up to 6.6 K, Between 6.7 and 9 K a mixture of LRO and paramagnetic phases is present. This, together with the fact that below 6.5 K the muSR spectral parameter show little change with temperature, indicates a transition of first-order type. The local field in the LRO state is broadly distributed pointing towards a complex spin structure. At temperatures above T-N the spectra are dominated by the dynamics of the Er spins. (C) 2002 Elsevier Science B.V. All rights reserved.