Phase formation in the 1-6-6 pseudobinary system Dy(Mn6Ge6)1-x(Fe6Al6)x

The formation of phases has been studied for the structural transition between two types of 1-6-6 rare earth-transition metal-metal crystalline phases in the pseudo-binary alloy series Dy(Mn6Ge6)1-x(Fe6Al6)x. In equilibrium, the limiting composition DyMn6Ge6 (x=0) is known as a helimagnet with hexagonal HfFe6Ge6-type structure (P6/mmm). DyFe6Al6 is a ferrimagnetic compound with partially disordered CeMn4Al8-type structure (I4/mmm). Both compounds show high magnetic transition temperatures of 423 and 340 K, respectively. Samples with compositions x=0-1 were prepared by arc melting, subsequent melt spinning, and annealing in Ar. The structure of the samples was checked by X-ray diffraction (XRD) both in as-spun and annealed state. After quenching, the composition DyMn6Ge6 (x=0) yields a distorted crystalline structure derived from the hexagonal TbCu7-type. For x≥1/2, annealing has little effect on the crystal structure determined by XRD. Here as-spun samples are crystalline with a structure derived from the equilibrium tetragonal CeMn4Al8-type and may contain an additional crystalline constituent. Magnetoresistive and magnetic properties of these systems are used to characterise the different structures. In particular, annealing modifies the magnetism in DyFe6Al6 (x=1) samples, and (FeAl)-substitution deteriorates the remarkable room temperature magnetoresistance properties of DyMn6Ge6