A NEUTRON-DIFFRACTION STUDY OF THE MAGNETIC-ORDERING OF TMMN6GE6

The compound TmMn6Ge6 of HfFe6Ge6-type of structure (P6/mmm) orders an tiferromagnetically below T-N=482 K and undergoes two magnetic transit ions at lower temperatures. Neutron powder diffraction has shown that the high temperature region, T>85 K, consists exclusively of the colli near antiferromagnetic ordering, q(2) = (0,0,1/2), of the Mn sublattic e along the c axis. In the low temperature region, T<T-t=35 K, both th e Mn and the Tm sublattices participate in the magnetic ordering corre sponding to a tilted spiral structure with the wave vector q(l) = (0,0 ,q(z)) and q(z) = 1/4. The intersublattice moment angle Mn-Tm-Mn, +/- (pi-phi[Mn]) = +/- 142 degrees, and the angle of the spiral axis with the c axis theta(s)= 69 degrees are slightly temperature dependent. Fo r T>T-t= 35 K the tilted spiral becomes destabilised and the two subla ttices become weakly coupled. In the intermediate region 35-85 K the T m spiral magnetic moment drops progressively to zero; the ordering of the Mn sublattice corresponds to a distorted skewed spiral with fluctu ating Mn moment values and directions.