Correlation between crystallographic superstructure and magnetic structures in finite magnetic fields: A neutron study on a single crystal of Ho ₂ PdSi₃

We present results of neutron-diffraction experiments on a Ho₂ PdSi₃ single crystal in the magnetically ordered state at T=1.5 K for magnetic fields applied in the easy (0,0,L) direction. Moderate fields of less than 0.8 T are sufficient to wipe out the antiferromagnetic order in the basal plane of the AlB₂ -type hexagonal lattice structure. Concurrently strong magnetic reflections along the (0,0,L) direction develop with increasing fields that are closely related to the nuclear superstructure reflections found in the single crystal at all temperatures. The field dependence of the intensity of these reflections is rather unusual: after a maximum at 0.6 T and a shallow minimum at 2 T a further increase in intensity for fields up to 5 T is observed. A phase transition into a field induced, saturated ferromagnetic state is expected only at very high fields (>13 T) as inferred from magnetization data. For the origin of the high-field magnetic structure two models are proposed and the corresponding model calculations are compared to our experimental results. Furthermore, the experimental data in low fields (<0.8 T) are used to clarify the hitherto unsolved zero-field magnetic structure of Ho₂ PdSi₃. The relevance of our findings with respect to other members of the R₂ PdSi₃ series (R=Tb, Er, Tm, and others) is emphasized.