THE X-T MAGNETIC PHASE-DIAGRAM OF THE LAMN2-XFEXCE2 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1) SYSTEM BY NEUTRON-DIFFRACTION STUDY

The ThCr2Si2-type structure LaMn2-xFexGe2 (x = 0.125, 0.25, 0.375, 1.0 ) compounds were studied by neutron diffraction. LaMn1.875Fe0.125Ge2 a ntiferromagnetic below T-N = 435 K with antiferromagnetic (001) Mn pla nes. Below T-C = 280 K, the moments are canted and hence display a fer romagnetic component on the (001) Mn planes. Simultaneously, the inter plane coupling of the in-plane antiferromagnetic components becomes in commensurate (T-ic= 280 K). LaMn1.75Fe0.25Ge2 displays identical magne tic phenomena (T-N = 420 K, T-C = T-ic = 225 K). At 2 K, the antiferro magnetic component (mu(Mn)AF = 2.65(9) mu(B)) lies in the (001) plane whilst the ferromagnetic component (mu(Mn)F = 1.34(39) mu(B)) is align ed along the c axis. The LaMn1.625Fe0.375Ge2 compound is antiferromagn etic at room temperature and contains antiferromagnetic (001) Mn plane s with the moments aligned along the c axis. Below T-C = 200 K, the Mn moments become canted and, as before, display a ferromagnetic compone nt in the (001) plane. Below T-SF = 175 K, a rotation of the moment ta kes place. At 2 K, the antiferromagnetic component (mu(Mn)AF = 2.61(7) mu(B)) makes an angle of 55(11)degrees with the c axis while the ferr omagnetic component (mu(Mn)F = 1.77(24) mu(B)) makes an angle of 35(11 )degrees with the c axis. The antiferromagnetic component gives rise t o a commensurate interplane arrangement down to 2 K. The LaMnFeGe2 com pound is antiferromagnetic below T-N = 230 K and is also made up of an tiferromagnetic (001) Mn planes. This magnetic behaviour remains uncha nged down to 2 K. The moment (mu(Mn) = 2.76 mu(B)) is aligned along th e c axis. The x-T magnetic phase diagram is depicted and compared with those of the LaMn2-xCuxGe2 and NdMn2-xFexGe2 systems.