MAGNETIC-PROPERTIES OF NDMN6GE6 AND SMMN6GE6 COMPOUNDS FROM SUSCEPTIBILITY MEASUREMENTS AND NEUTRON-DIFFRACTION STUDY

Investigations by susceptibility measurements and neutron diffraction experiments have been performed on the ternary germanides NdMn6Ge6 and SmMn6Ge6 of the YCo6Ge6-type structure (P6/mmm). This structure, clos ely related to the HfFe6Ge6 type, can be described as a disordered fil led derivative of the CoSn B35-type structure. The Mn atoms build Kago me layers stacked along the c axis, in between which the rare earth at oms are randomly distributed. Owing to the Mn atom coordination of the rare earth, this structure appears closely related to the CaCu5- and ThMn12-type structures. NdMn6Ge6 and SmMn6Ge6 are ferromagnetic below- 417 and 441 K respectively. At 35 K a second magnetic transition relat ed to a spin reorientation process takes place in NdMn6Ge6. Neutron di ffraction study of NdMn6Ge6 confirms that both the rare earth and Mn s ublattices order simultaneously above room temperature. In the whole t emperature range 2-300 K NdMn6Ge6 is a collinear ferromagnet (i.e. pos itive Nd-Mn interaction). In the temperature range 300-35 K the moment direction deviates by Phi approximate to 40 degrees from the c axis, whereas an easy plane occurs at 2 K (mu(Nd)=2.87(43) mu(B) and mu(Mn)= 2.26(14) mu(B)) The results are discussed and compared with those prev iously obtained for the parent YMn6Ge6, GdMn6Ge6 and RMn(6)Sn(6) (R=Tb -Er, Lu) compounds.