Spectroscopic and magnetic investigations of a spin-frustrated Mn-doped CoAl₂O₄ spinel

Single-phase polycrystalline spin-frustrated spinel oxides Co1-xMnxAl2O4 (0 ≤ x ≤ 0.3) have been prepared to investigate the optical and magnetic properties. Linear variation of the lattice parameter along with the characteristic hyperfine electron paramagnetic resonance (EPR) signal establish the fact that the Mn2+ ions are incorporated at Co2+ sites of the CoAl2O4 lattice. Optical absorption spectra reveal three absorption features in wavelength regions: 250-400 nm, 500-700 nm and 1000-1700 nm. The optical band gap associated with the d-d transition increases from 1.84 eV to 1.88 eV with 30% Mn substitution. Temperature dependent magnetization measurements indicate a clear transformation of the magnetic ground state from the collinear antiferromagnetic state (for x = 0) to the spin-glass-like state (for x = 0.1) to the cluster-glass-like state (for x = 0.2 and 0.3) with the increase of Mn concentration. In addition, our time dependent isothermal remanent magnetization (IRM) study further fortifies the above transformation of the magnetic ground state. The value of the magnetic frustration parameter moderately decreases with Mn substitution, but the compositional variation is not monotonous.