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

We report a detailed study on the composition (x) dependence of structural, electronic, magnetic, and optical studies of nickel chromate spinel (NiCr2O4) at various levels of Mn substitution at B sites. No significant structural distortion from cubic symmetry Fd-3m was noticed for all the compositions in the range 0 <= x <= 1 of Ni(Cr1-xMnx)(2)O-4. However, there is significant alteration in the bond angles angle B-O-B (90.51 degrees-93.86 degrees) and angle A-O-B (122.48 degrees-124.90 degrees) (both of which follow completely opposite trend with increasing x) and bond lengths A-O (1.82-1.94 angstrom) and B-O (2.02-2.08 angstrom). The corresponding lattice parameter (a) follows Vegard's law (8.32 +/- 0.001 angstrom <= a <= 8.45 +/- 0.001 angstrom). The electronic structure determined from the x-ray photoelectron spectroscopy reveals the divalent nature of Ni (with spin-orbit splitting energy Delta similar to 17.62 eV). While the Cr and Mn are stable with trivalent electronic states having Delta = 8 and 11.7 eV, respectively. These results are in consonance with the cationic distribution (Ni)(A)[(Cr1-xMnx)(2)](B)O-4 obtained from the Rietveld refinement analysis. Interestingly, the current series shows a direct bandgap (E-G) semiconducting nature in which E-G varies from 1.16 to 2.40 eV within the range of x = 0.85-0. Such variation of E-G (x) is consistent with the compositional variation of the crystal structure data with anomalous change between x = 0.25 and 0.6. Beyond this range, the E-g mode (140 cm(-1)) 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 Neel temperature, T-FN due to the unequal magnetic moments of the cations. However, both the ordering temperature T-FN and saturation magnetization (M-S) increases progressively from 73.3 K (1500 emu mol(-1)) to 116 K (3600 emu mol(-1)) with increasing the Mn content from 0 to 1, yet the maximum anisotropy (H-K similar to 4.5 kOe, K-1 similar to 2.5 x 10(4) erg cc(-1)) shows an opposite trend with x. Such variation is ascribed to the altered magnetic superexchange interactions between the cations located at A and B sites following the trend J(BB) > J(AB) > J(AA), (J(BB)/k(B) =13.36 K).