NEUTRON-DIFFRACTION STUDY OF THE FERROMAGNETIC TO ANTIFERROMAGNETIC TRANSITION IN LA0.3Y0.7MN2GE2 - PHENOMENOLOGICAL DESCRIPTION OF THE MAGNETIC-BEHAVIOR OF MN IN THCR2SI2 SILICIDES AND GERMANIDES

The magnetic structures of La0.3Y0.7Mn2Ge2 were determinated by powder neutron study and the magnetic structure of YMn2Ge2 was reinvestigated. La0.3Y0.7Mn2Ge2 is a ferromagnetic compound above about 150 K and an antiferromagnetic compound at lower temperature. The magnetic structure in the ferromagnetic state is canted. At 200 K the Mn moments exhibit a ferromagnetic component (mu(MnF) = 1.52(14) mu(B)) together with an antiferromagnetic component (mu(MnAF) = 1.75(9) mu(B),) within the (001) Mn layers. The ferromagnetic component is ferromagnetically coupled to the ferromagnetic components of the adjacent planes and the antiferromagnetic component leads to a commensurate arrangement (I-centered mode). At 2 K, both components persist and the magnetic structure is a double-cone type. The ferromagnetic component (mu(MnF) = 2.10(16) mu(B)) is antiferromagnetically coupled to the adjacent planes whereas the antiferromagnetic component (mu(MnAF) = 1.42(17) mu(B)) gives rise to an incommensurate arrangement (helical type). In the whole temperature range, the ferromagnetic component is aligned along the c axis and the antiferromagnetic component lies in the a-b plane. The total Mn moment at 2 K is mu M = 2.54(16) mu(B). A Mossbauer study of a Fe-57 doped sample indicates that the manganese moments are still ordered above the Curie point. The magnetic structure of YMn2Ge2 is a collinear antiferromagnet. The (001) Mn planes are purely ferromagnetic. The Mn moment at 2 K is mu(Mn) = 2.23(14) mu(B). These results are discussed and it is suggested that the properties of L(0.3)Y(0.7)Mn(2)Ge-2 might account for the magnetic behaviour of SmMn,Ge, above the ordering point of the Sm moments. A phenomenological description of the magnetic behaviour of the Mn moment within the RMn(2)Si(2) and RMn(2)Ge(2) series (R is alkaline earth, rare earth and actinides) is developed.