Stable room temperature magnetic ordering and excellent catalytic activity of mechanically activated high surface area nanosized Ni0.45Zn0.55Fe2O4

Abstract

Herein we report the structural, microstructural, dc magnetic and hyperfine properties along with catalytic activity of mechanosynthesized nanosized Ni_0.45Zn_0.55Fe₂O₄ (∼12 nm). The Rietveld refinement of the powder X-ray diffraction data, high resolution transmission electron microscopy and the infield Mössbauer study suggest that the sample is nanosized pure single phase cubic spinel of Fdm symmetry with good crystallinity and it possesses equilibrium cation distribution ((Fe³⁺_0.45Zn²⁺_0.55)_A[Fe³⁺_1.55Ni²⁺_0.45]_BO₄). The sample exhibits ferrimagnetic ordering with high saturation magnetization (M_SAT = 53, 72 and 76 emu g⁻¹ at 300, 100 and 10 K, respectively), coercivity (H_C = 280, 1200 and 2800 Oe at 300, 100 and 10 K, respectively), collective magnetic excitations, spin canting and memory effect in dc magnetization. The infield Mössbauer study suggests that the interior region of the particles is perfectly ferrimagnetic in nature, while the spins at the surface region are ferrimagnetically coupled but noncollinearly aligned. Despite its nanometric size, the sample does not show superparamagnetic behavior but rather retains stable magnetic order at room temperature due to enhancement of stress and surface anisotropy energy caused by high energy ball milling. We have shown that the presence of collective magnetic state along with surface spin disorder are the underlying reason for having slow dynamics and memory effect in the sample. Further, the BET (Brunauer–Emmett–Teller) surface area and the pore volume of the sample are 233 m² g⁻¹ and 0.475 cm³ g⁻¹, respectively. The temperature programmed desorption (TPD) of ammonia suggests that the surface of this porous material is highly acidic (1.516 mmol g⁻¹). Because of its high surface acidity and BET surface area the material acts as an efficient heterogeneous catalyst in the one pot synthesis of 3,4-dihydropyrimidine-2(1H)-ones (DHPMs) by Biginelli condensation reaction. This sample can be used in magnetic data storage devices, coding, storing and retrieving of binary numbers through magnetic field change and also as a very efficient heterogeneous, magnetically separable and recyclable catalyst.