Evolution of tunable energy bandgap and magnetic anisotropy in Mn substituted ferrimagnetic nickel-chromates

We report a detailed study on the composition ( ) dependence of structural, electronic, magnetic, and optical studies of nickel chromate spinel (NiCr O ) at various levels of Mn substitution at B sites. No significant structural distortion from cubic symmetry 3m was noticed for all the compositions in the range 0 ⩽ ⩽ 1 of Ni(Cr Mn ) O . However, there is significant alteration in the bond angles B O B (90.51°-93.86°) and A O B (122.48°-124.90°) (both of which follow completely opposite trend with increasing ) and bond lengths A O (1.82-1.94 Å) and B O (2.02-2.08 Å). The corresponding lattice parameter ( ) follows Vegard's law (8.32 ± 0.001 Å ⩽ ⩽ 8.45 ± 0.001Å). The electronic structure determined from the ray photoelectron spectroscopy reveals the divalent nature of Ni (with spin-orbit splitting energy Δ ∼ 17.62 eV). While the Cr and Mn are stable with trivalent electronic states having Δ 8 and 11.7 eV, respectively. These results are in consonance with the cationic distribution (Ni) [(Cr Mn ) ] O obtained from the Rietveld refinement analysis. Interestingly, the current series shows a direct bandgap ( ) semiconducting nature in which varies from 1.16 to 2.40 eV within the range of = 0.85-0. Such variation of ( ) is consistent with the compositional variation of the crystal structure data with anomalous change between = 0.25 and 0.6. Beyond this range, the mode (140 cm ) in Raman spectra arising from Mn-O octahedral decreases continuously and vanishes at higher Mn concentrations. Our analysis shows that all the investigated compounds show long-range ferrimagnetic ordering below the Néel temperature, due to the unequal magnetic moments of the cations. However, both the ordering temperature and saturation magnetization ( ) increases progressively from 73.3 K (1500 emu mol ) to 116 K (3600 emu mol ) with increasing the Mn content from 0 to 1, yet the maximum anisotropy ( ~4.5 kOe, ~2.5 × 10 erg cc ) shows an opposite trend with . Such variation is ascribed to the altered magnetic superexchange interactions between the cations located at A and B sites following the trend > > , ( / =13.36 K).