Simultaneous structural and magnetic transitions in YFe 4Ge 2 studied by neutron diffraction and magnetic measurements

Abstract

The magnetic behaviour of the tetragonal YFe 4Ge 2 compound has been studied by neutron diffraction and bulk magnetisation measurements. Magnetic ordering occurs below T N =43.5 K simultaneously with a structural transition from tetragonal P4 2/mnm→Pnnm to orthorhombic symmetry. Both transitions are of first order. The symmetry breaking is connected with primary displacive order parameters involving shifts of the Fe atoms. The Fe site splits into two sites in Pnnm, the shifts of the Fe atoms with respect to the tetragonal phase have opposite signs along the a (dilatation) and b (contraction) axes. This induces the symmetry breaking tensile strains. The planar canted antiferromagnetic arrangement with eight sublattices is the ground state associated with a magnetic energy involving isotropic exchange and anisotropic (relativistic) interactions. The magnetic structure is invariant under the magnetic space group P 2 1 n′ 2 1 n′ 2 1 m′ ( Sh 58 399) . The moments of the two orbits have the same value. At 1.5 K the Fe moment equals 0.63(4) μ B/Fe atom. The YFe 4Ge 2 magnetostructural phenomena are compared to the more complex analogue of the ErFe 4Ge 2 compound. The structural and magnetic transitions are described phenomenologically within a Landau–Dzialoshinski approach which assumes a triggering mechanism, in which the spontaneous strain is triggered by the exchange contribution to the magnetic ordering.