Neutron diffraction study of the ferromagnetic to antiferromagnetic transition in La 0.3Y 0.7Mn 2Ge 2: phenomenological description of the magnetic behaviour of Mn in ThCr 2Si 2 silicides and germanides

Abstract

The magnetic structures of La 0.3Y 0.7Mn 2Ge 2 were determinated by powder neutron study and the magnetic structure of YMn 2Ge 2 was reinvestigated. La 0.3Y 0.7Mn 2Ge 2 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 ( μ MnF = 1.52(14) μ B) together with an antiferromagnetic component ( μ MnAF = 1.75(9) μ 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 ( μ MnF = 2.10(16) μ B) is antiferromagnetically coupled to the adjacent planes whereas the antiferromagnetic component ( μ MnAF = 1.42(17) μ 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 μ Mn = 2.54(16) μ B. A Mössbauer study of a 57Fe doped sample indicates that the manganese moments are still ordered above the Curie point. The magnetic structure of YMn 2Ge 2 is a collinear antiferromagnet. The (001) Mn planes are purely ferromagnetic. The Mn moment at 2 K is μ Mn = 2.23(14) μ B. These results are discussed and it is suggested that the properties of La 0.3Y 0.7Mn 2Ge 2 might account for the magnetic behaviour of SmMn 2Ge 2 above the ordering point of the Sm moments. A phenomenological description of the magnetic behaviour of the Mn moment within the RMn 2Si 2 and RMn 2Ge 2 series (R is alkaline earth, rare earth and actinides) is developed.