Hexagonal DyMn₆Ge₆ with HfFe₆Ge₆ structure is a prototypical rare-earth transition-metal compound with complex magnetic couplings. Rare-earth and transition metal ions order simultaneously in a ferrimagnetic helix with locally anti-parallel moments of the sublattices at a high transition temperature (T_C = 423 K). This layered magnetic system is a suitable experimental model for studies on the relation between magnetization processes and magnetoresistance in complex metallic alloys. Field and temperature-dependent spin-reorientation in this compound cause anomalies of the electrical resistivity and sizeable magnetoresistance effects.

We report results from measurements of the magnetoresistance and magnetization in static and pulsed fields up to 50 T for polycrystalline samples of pure and diluted Dy(Mn₆Ge₆)_1-x(Fe₆Al₆)_x. The samples were prepared by melt-spinning and subsequent annealing. Annealed pure and low-doped samples x<0.05 have the HfFe₆Ge₆-structure. At room-temperature, the pure compound shows negative magnetoresistance MR(H) = (R(H) - R(0)) / R(0), that reaches about 7% at 4 T and displays a marked change of slope at the helix-fan-transition near 5 T. Below 100 K, positive magnetoresistance is found with a non-monotonic dependence MR(H) in high fields above 20 T. The anomalous decrease of MR(H) is explained by a metamagnetic behaviour related to a transition from the ferrimagnetic fan into a spin-flop state. For low substitutions of MnGe by FeAl (x = 0.01) an increase of the specific resistivity and reduced magnetic ordering-temperatures are found. Near the helix-magnetic ordering temperature, an anomaly of the resistivity occurs indicating a strong influence of magnetic impurity-scattering on electronic transport. As-quenched and more strongly alloyed samples show different crystalline phases, derived from the hexagonal TbCu₇-type structure, but have similar magnetoresistive behaviour.

This work is supported by BMBF project: 03SC5DRE.

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