Strong anomalous temperature dependence of the contact hyperfine contribution to the muon Knight shift in HoB2C2

Tetragonal HoB2C2 is one of the few compounds which show antiferroquadrupolar ordering below the onset of antiferromagnetic order (TQ=4.5 K, TN=5.9 K). We have started the study of this system with the ultimate aim to learn more about the interaction of the magnetic and quadrupolar degrees of freedom in this context. In the contribution we present results of Knight-shift measurements on a single crystal above TN. The results, firstly, allowed to determine the muon site, which is identified to be the 8i interstitial site, and secondly, revealed an unusual temperature dependence of the contact hyperfine constant Ac