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Neutron diffraction study of the ferromagnetic to antiferromagnetic transition in La0.3Y0.7Mn2Ge2: phenomenological description of the magnetic behaviour of Mn in ThCr2Si2 silicides and germanides

1995; Elsevier BV; Volume: 223; Issue: 1 Linguagem: Inglês

10.1016/0925-8388(94)01496-5

1873-4669

G. Venturini, R. Welter, E. Ressouche, B. Malaman,

Inorganic Chemistry and Materials

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 (μMnF = 1.52(14) μB) together with an antiferromagnetic component (μMnAF = 1.75(9) μB) 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 (μMnF = 2.10(16) μB) is antiferromagnetically coupled to the adjacent planes whereas the antiferromagnetic component (μMnAF = 1.42(17) μB) 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 μMn = 2.54(16) μB. A Mössbauer study of a 57Fe 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 μMn = 2.23(14) μB. These results are discussed and it is suggested that the properties of La0.3Y0.7Mn2Ge2 might account for the magnetic behaviour of SmMn2Ge2 above the ordering point of the Sm moments. A phenomenological description of the magnetic behaviour of the Mn moment within the RMn2Si2 and RMn2Ge2 series (R is alkaline earth, rare earth and actinides) is developed.