Magnetic properties of DyMn 6Ge 6 studied by neutron diffraction and magnetic measurements

Abstract

The compound DyMn 6Ge 6 crystallizes in the hexagonal HfFe 6Ge 6-type structure ( P6/ mmm). It orders antiferromagnetically at T N=420 K but there is a second magnetic phase transition near 100 K. Neutron diffraction has shown that the high temperature magnetic structure is a triple flat spiral consisting of ferromagnetic Dy layers and ferromagnetic Mn layers coupled antiparallelly in a three-layer sequence Mn(+), Dy(−), Mn(+). The moment direction is perpendicular to [001] but the direction in the basal plane changes by a constant angle φ s = πq z on going from one unit cell to another. The wavevector length of the incommensurate structure is temperature dependent and equal to q z=0.184 at 293 K, which corresponds to φ s =66°. Below T t=100 K the wavevector length remains constant ( q z=0.163) and the magnetic structure is an incommensurate triple-cone structure in which both the Dy and Mn sublattices have a ferromagnetic component along the c axis. These components are antiparallel and lead to a net moment in the c direction of 2.0(5) μ B per formula unit. The magnetic isotherm at 4.2 K, studied in fields up to 35 T, suggests that below 22 T the cone angle (55° forB=0) gradually decreases. Above 22 T the magnetic isotherm shows a linear behaviour characteristic of bending of the antiparallel rare earth and 3d sublattice moments towards each other. The intersublattice coupling constant derived from the high field slope of the isotherm equals J Dy Mn / k= −9.1 K.