Spin ordering and hyperfine interactions in langasite-like ferrite Ba₃SbFe₃Si₂O₁₄: ⁵⁷Fe Mössbauer reinvestigation and ESR measurements

The present work reports new analysis and reinterpretation of the ⁵⁷Fe Mössbauer data for the ferrite Ba₃SbFe₃Si₂O₁₄ with langasite-like crystal structure. The Mössbauer measurements show a two-step magnetic ordering process. Relaxation behavior, for temperatures below T_N ≈ 32 K, persisting down to T* ≈ 20 K, indicates the coexistence of the short-range ordering spin clusters within a helically ordering state. Below T*, Mössbauer spectra are consistent with a modulated helical magnetic order, where iron magnetic moments rotate within the (ab) plane with propagation vector along c axis. All experimental spectra at T < T_N were analyzed assuming the modulation of hyperfine interactions and anisotropy of the magnetic hyperfine field B_hf at the ⁵⁷Fe nuclei when Fe³⁺ magnetic moments rotate with respect to the principal axis of the EFG tensor. The broadening of the electron spin resonance lines upon approaching T_N from above was explained in terms of the short-range correlations near magnetic phase transition. The present results evidence a 3D character of magnetic correlations for high-temperatures T ≥ T₀, which changes for rather 2D character of magnetic correlations below T₀ ≈ 60 K. Such spin dynamics in Ba₃SbFe₃Si₂O₁₄ probably triggered by quasi-2D crystal structure and non-trivial helical magnetic structure.