Effect of Al substitution at the Ga site on the structural and magnetic properties of GaFeO3

Abstract

GaFeO3 compound figures among those orthoferrites that are potential candidates for multiferroics. However, GaFeO3 suffers from a relatively low Nèel temperature, TN ≈ 200 K, and magnetic moment. Based on the results of a detailed investigation of Raman-active modes, structural and magnetic properties of the Ga1-xAlxFeO3 (x = 0.0, 0.05, 0.10, 0.20, 0.30) samples, we show that Al substitution at the Ga sites remedies these shortcomings to a large extent. This work brings out clearly that Al preferentially occupies the tetrahedral (Ga1) sites rather than the octahedral (Ga2, Fe1 and Fe2) sites. The effect of Al substitution in Ga1-xAlxFeO3 is to (i) decrease the lattice parameters and cation-O bond lengths which results in a shift in the positions of Raman peaks towards higher wavenumbers, and (ii) increase (decrease) the cation-O-cation bond-angles such that the antiferromagnetic (ferromagnetic) superexchange interactions grow in strength. An enhancement in these interactions accounts for the observed increase in the Nèel temperature, TN, as well as in the saturation magnetization. The radically different temperature variations of the ‘zero-field-cooled’ and ‘field-cooled’ magnetizations below the irreversibility temperature can be qualitatively understood in terms of the Nèel molecular-field theory for ferrimagnetism. The temperature dependence of inverse susceptibility in the paramagnetic state provides evidence for the non-linear behavior at temperatures well above TN and hence for the persistence of short-range magnetic order to temperatures as high as 1.75 TN.