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

We report a detailed study on the composition (<i>x</i>) 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<i>Fd-</i>3m was noticed for all the compositions in the range 0 ⩽<i>x</i>⩽ 1 of Ni(Cr_1-<i>x</i>Mn<i>_x</i>)₂O₄. However, there is significant alteration in the bond angles<i>∠</i>B<i>-</i>O<i>-</i>B (90.51°-93.86°) and<i>∠</i>A<i>-</i>O<i>-</i>B (122.48°-124.90°) (both of which follow completely opposite trend with increasing<i>x</i>) and bond lengths A<i>-</i>O (1.82-1.94 Å) and B<i>-</i>O (2.02-2.08 Å). The corresponding lattice parameter (<i>a</i>) follows Vegard's law (8.32 ± 0.001 Å ⩽<i>a</i>⩽ 8.45 ± 0.001Å). The electronic structure determined from the<i>x-</i>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<i>=</i>8 and 11.7 eV, respectively. These results are in consonance with the cationic distribution (Ni)_A[(Cr_1-<i>x</i>Mn<i>_x</i>)₂]_BO₄obtained from the Rietveld refinement analysis. Interestingly, the current series shows a direct bandgap (<i>E</i>_G) semiconducting nature in which<i>E</i>_Gvaries from 1.16 to 2.40 eV within the range of<i>x</i>= 0.85-0. Such variation of<i>E</i>_G(<i>x</i>) is consistent with the compositional variation of the crystal structure data with anomalous change between<i>x</i>= 0.25 and 0.6. Beyond this range, the<i>E</i>_gmode (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 Néel temperature,<i>T</i>_FNdue to the unequal magnetic moments of the cations. However, both the ordering temperature<i>T</i>_FNand saturation magnetization (<i>M</i>_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 (<i>H_K</i>~4.5 kOe,<i>K</i>₁~2.5 × 10⁴erg cc^-1) shows an opposite trend with<i>x</i>. Such variation is ascribed to the altered magnetic superexchange interactions between the cations located at A and B sites following the trend<i>J</i>_BB><i>J</i>_AB><i>J</i>_AA, (<i>J</i>_BB/<i>k</i>_B=13.36 K).