Spin phonon coupling in Mn doped HoFeO₃ compounds exhibiting spin reorientation behaviour

An investigation has been carried out on the spin phonon coupling in a series of isostructural polycrystalline orthorhombic (Space group Pnma) compounds HoFe_1-X Mn _X O₃ (x  ⩽  0.6) exhibiting spin reorientation below Néel temperature (T _N), using magnetization, neutron diffraction, and Raman scattering techniques. Mn doping leads to an anomalous increase in the spin reorientation temperature (T _SR), shifting it close to room temperature from T _SR ~ 60 K for x  =  0 sample, and concomitant lowering of T _N. The T _SR is absent in samples for x  ⩾  0.5. The magnetic structure undergoes a transition at T _SR from Γ₄  →  Γ₁ in the Mn doped compounds as against Γ₄  →  Γ₂ observed in HoFeO₃ sample. In the region T  <  T _N an anomalous softening of Raman phonon modes viz., B _2g(5) and B _3g(3), identified with the stretching motion and breathing mode, respectively, of Fe/Mn-O₆ octahedra, is observed in compounds exhibiting spin-reorientation behaviour, indicating a spin-phonon coupling in these compounds. A quadratic correlation between the deviation of phonon frequency and variation of antiferromagnetic moment (Δω [Formula: see text] M ²) is observed in these compounds. The temperature evolution of the M2+  mode obtained from the analysis of neutron diffraction data based on symmetry adapted mode decomposition of the Pnma structure further corroborates the mode softening observation.