Unequivocal evidence of enhanced magnetodielectric coupling in Gd3+ substituted multiferroic Bi2Fe4O9

Abstract

We report an enhanced magnetodielectric (MD) coupling in antiferromagnetic Bi₂Fe₄O₉ (BFO) lightly substituted by gadolinium (Gd³⁺). Rietveld refined X-ray diffraction and Raman spectra reveal the formation of orthorhombic phase for all the synthesized samples. Gd³⁺ substitution results in a notable increase of antiferromagnetic transition (T_N) from 250 K (x = 0, BFO) to 256 K (x = 0.02, BGFO2). At the same time, deviation from linear M–H behaviour in the substituted samples at T ≤ 30 K implies the onset of weak ferromagnetic ordering, along with a sudden rise in magnetisation around ∼30 K. This result indicates the magnetically active nature of Gd³⁺ ions possessing a large magnetic moment of ∼8 μ_B and this is explained by considering the two sub-lattice mode. The temperature dependence of the dielectric study shows unusual oppression in ε′ and tan loss plots at T ∼ T_N which hints a plausible MD coupling in the above studied samples. A contrasting behaviour in the temperature dependant MD% is seen for BFO and the substituted samples have an enhancement of ∼30% for BGFO2 at 300 K. Further, confirmation to this coupling is drawn by studying frequency and magnetic field dependant MD% at 300 K, where it is found that both obey a power law of the form ∝exp(ω)ⁿ and ∝(H)^m respectively. Interestingly, the values of ‘n’ and ‘m’ obtained from fitting are found to lie in the range 0.45–0.5, thus signifying a similar dependence of MD on frequency as well as magnetic field. Lastly, the effect of Gd³⁺ substitution on P–E loop of BFO and its substituted samples at room temperature have been discussed.