MAGNETIC-PROPERTIES OF RARE-EARTH MANGANESE COMPOUNDS OF THE TYPE RMNSI2

We have studied the temperature dependence of the intrinsic coercivity of the compounds RMnSi2 (R = La, Ce, Pr, Nd) from 4.2 to 300 K. The differences in coercivity behaviour observed for these compounds were correlated with the corresponding temperature behaviour of the magnetic structures and with the valence instability of Ce in CeMnSi2. Magnetic isotherms of LaMnSi2 and CeMnSi2 at 4.2 K were studied in field strengths up to 35 T. Neutron diffraction measurements were performed on CeMnSi2 at 8 and 408 K. From the results of neutron diffraction and high field measurements it was concluded that Ce loses its magnetic moment at low temperatures. This is the first example of a strongly ferromagnetic compound in which Ce has an unstable valence.