MAGNETIC-PROPERTIES OF THE LAMN2-XFEXGE2 SOLID-SOLUTION 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1) AND MAGNETIC-STRUCTURES OF LAMN1.5FE0.5GE2 FROM NEUTRON-DIFFRACTION STUDY

The magnetic properties of the ThCr2Si2-type structure LaMn2-xFexGe2 s olid solution have been studied in the composition range 0 less than o r equal to x less than or equal to 1. For x less than or equal to 0.4, all samples exhibit an easy-axis ferromagnetic behaviour while an eas y plane occurs when 0.4 less than or equal to x less than or equal to 0.7. The Curie temperatures and the saturation magnetization values (4 .2 K, H-appl. = 1.5 T) decrease continuously with the Mn substitution rate from 325 K and 1.45 mu B per Mn atom for x = 0, to 65 K and 0.62 mu B for x = 0.7. The Fe rich compounds (0.8 less than or equal to x l ess than or equal to 1) do not exhibit ferromagnetism. At 2 K, LaMn1.5 Fe0.5Ge2 (T-c = 162 K) exhibits a canted ferromagnetic structure chara cterized by an antiferromagnetic component (mu(Mn)(AF)similar to 2.7 m u(B)) aligned along the c-axis and a ferromagnetic component (mu(Mn)(F )similar to 1.1 mu(B)) lying in the (001) plane. Moreover, at 300 K, t he neutron diffraction study clearly shows that LaMn1.5Fe0.5Ge2 is a p urely easy-plane antiferromagnet (not detected by bulk magnetic measur ements) characterized by a stacking of antiferromagnetic (001) Mn plan es. The thermal dependence of the magnetic moment yields an ordering t emperature T-N similar to 385 K. These results are discussed and compa red with those obtained for the RMn(2)Ge(2) (R = Ca, Ba, La-Nd) series .