Size-dependent structural, magnetic, and optical properties of MnCo2O4 nanocrystallites

Abstract

Finite-size (d = 5.4–112 nm) and surface effects on the structural, optical, and magnetic properties of ferrimagnetic inverse-spinel MnCo2O4 are reported. For d ≥ 87 nm, partial tetragonal distortion of the inverse spinel-lattice was observed. The Curie temperature TC of MnCo2O4 nanostructures, as determined by dc-magnetic susceptibility (χ) measurements, follows a finite-size scaling relation TC(d) = TC(∞)[1−(ξ0/d)λ] with a shift exponent λ = 0.75 ± 0.15 and microscopic correlation length ξ0 = 1.4 ± 0.3 nm, which is consistent with the mean field theory. For T > TC, χ(T) fits Néel's expression for the two-sublattice model with antiferromagnetic molecular field (exchange) constants NBB ∼ 85.16 (JBB ∼ 2.94 × 10−22 J), NAB ∼ 110.96 (JAB ∼ 1.91 × 10−22 J), and NAA ∼ 43.8 (JAA ∼ 1.13 × 10−22 J) and asymptotic Curie temperature Ta ∼ 717.63 K. The optical energy bandgap Eg, evaluated from the Kubelka-Munk function ([F(R∞)ℏω]2 = C2(ℏω - Eg)) is blueshifted to 2.4 eV (d ∼ 5.4 nm) from 1.73 eV (d ∼ 112 nm) due to the quantum confinement and non-stoichiometry. The role of tetragonal distortion and grain-size-effects in the intensity of crystal field transitions and variation in the magnetic ordering are further discussed and compared with Co3O4 nanostructures.