Magnetic moment at highly frustrated sites of antiferromagnetic Laves phase structures

The antiferromagnetic, intermetallic compounds TiFe₂ and (Hf_0.80Ta_0.20)Fe₂ with the C14 Laves phase crystalline structure have been investigated with Mössbauer spectroscopy under applied magnetic fields in order to determine whether Fe atoms at the 2a site carry a paramagnetic moment. TiFe₂ was measured at T=4.2 K in various fields up to 12 T. While the Fe(6h) effective field behaved in the expected way for an antiferromagnet in an external field, the Fe(2a) effective field was identical to the applied field, thus evidencing the absence of a local moment. (Hf_0.80Ta_0.20)Fe₂ is antiferromagnetic only in a limited temperature range, so the experiments were carried out at or above T=220 K. For this sample the measured Fe(2a) effective field was smaller than the applied field, revealing a hyperfine contribution from which an Fe moment of ∼0.6 μ_B was estimated. The occurrence of stronger spin fluctuations in (Hf_0.80Ta_0.20)Fe₂ than in TiFe₂, as revealed by electronic-specific heat data, is proposed as an explanation for the different magnetic behaviors of these two isostructural compounds.