Spontaneous and field-induced magnetic phase transitions in Dy 2 Co 3 Al 9 : Effects of exchange frustration

Due to the long-range oscillatory character of RKKY exchange interactions, for ${\mathrm{Dy}}_{2}{\mathrm{Co}}_{3}{\mathrm{Al}}_{9}$ there exist positive and negative couplings between the Dy magnetic moments that lead to magnetic frustration. As a result, the ground state can be easily perturbed, and the system displays a number of spontaneous and field-induced phase transitions. We performed magnetization, magnetic-susceptibility, specific-heat, and electrical-resistivity measurements as well as neutron-diffraction experiments on a single crystal. We find two transitions to distinct incommensurate antiferromagnetic phases at 6.2 and 5.2 K that evolve to a commensurate phase at 3.7 K. In applied magnetic field, new phases emerge. Field-dependent magnetization exhibits a multistep metamagnetic process with sharp transitions accompanied by pronounced magnetoresistance changes. The large number of phases and their complex magnetic structures suggest that the physical properties of ${\mathrm{Dy}}_{2}{\mathrm{Co}}_{3}{\mathrm{Al}}_{9}$ are ruled by exchange frustration in the presence of a large magnetocrystalline anisotropy.