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

Abstract

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 symmetryFd-3m was noticed for all the compositions in the range 0 ⩽x⩽ 1 of Ni(Cr_1-xMn_x)₂O₄. 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 increasingx) and bond lengths A-O (1.82-1.94 Å) and B-O (2.02-2.08 Å). The corresponding lattice parameter (a) follows Vegard's law (8.32 ± 0.001 Å ⩽a⩽ 8.45 ± 0.001Å). The electronic structure determined from thex-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)_A[(Cr_1-xMn_x)₂]_BO₄obtained from the Rietveld refinement analysis. Interestingly, the current series shows a direct bandgap (E_G) semiconducting nature in whichE_Gvaries from 1.16 to 2.40 eV within the range ofx= 0.85-0. Such variation ofE_G(x) is consistent with the compositional variation of the crystal structure data with anomalous change betweenx= 0.25 and 0.6. Beyond this range, theE_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,T_FNdue to the unequal magnetic moments of the cations. However, both the ordering temperatureT_FNand 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~4.5 kOe,K₁~2.5 × 10⁴erg cc^-1) shows an opposite trend withx. Such variation is ascribed to the altered magnetic superexchange interactions between the cations located at A and B sites following the trendJ_BB>J_AB>J_AA, (J_BB/k_B=13.36 K).