Spin reorientation behaviour and dielectric properties of Fe-dopedh-HoMnO3.

We have studied the magnetic structure, spin reorientation behaviour and dielectric properties of polycrystalline HoMn1-xFexO3(0.0 ⩽x⩽ 0.25) compounds using magnetization, neutron diffraction and dielectric measurements. These compounds crystallize predominantly in the hexagonal phase (P63cm) with a small phase fraction of the orthorhombic phase (Pnma) which increases with increase in dopant concentration and a total suppression of the hexagonal phase is observed atx= 0.25. Doping Fe at the Mn site leads to an increase in the spin reorientation temperature (TSR) from 33 K (x= 0) to 55 K (x= 0.1) while theTNremains nearly constant at 72 K. The magnetic structure of the hexagonal phase was found to be Γ4(P63'c'm) belowTNand Γ3(P63'cm') belowTSR. The magnetic ordering temperature of Ho3+ions at 2(a) site appears to coincide with theTSRonly in the case ofx= 0 sample. The Ho ions at 4(b) site are found to magnetically order below 8 K. TheTNof the Ho ions at both 4(b) and 2(a) sites do not appear to be affected by doping at the Mn site. The temperature variation of the Mn and Ho moments follow the Brillioun function dependence albeit with differing values of the molecular field constantλ0andλ1. Short range magnetic order alone was found for the completely orthorhombic sample (x= 0.25). An anomalous suppression of the dielectric constant (ε) atTNis observed in the case of hexagonal samples. Further, a linear correlation between Δε(=ε(T) -ε(0)) and the square of the antiferromagnetic momentM, is observed in these compounds.

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 HoFe1-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.