NMR and Specific Heat of YCo12B6 and LaCo12B6

NMR measurements were made for YCo 12 B 6 and LaCo 12 B 6 mainly by the steady-state method in zero external field as a function of temperature. Two 59 Co domain signals with quadrupole splittings and two wall signals with single lines were observed. With decreasing temperature, a low-frequency domain signal in LaCo 12 B 6 vanished below about 25 K but another domain signal emerged below about 40 K, revealing a discontinuous change in frequency and in spacing between the quadrupole-split components. The specific heat of LaCo 12 B 6 did not show any anomaly in this temperature range. These suggest that the observed change in NMR in LaCo 12 B 6 around 35 K is not of first order but is presumably associated with a change in magnetic states. In YCo 12 B 6 there was no such a discontinuous change in the NMR spectrum.