Aggregated Synthesis of Superparamagnetic Nickel Ferrite Nanostructures

Spinel ferrites (AB₂O₄, A = transition metal ion; B = Fe) are promising candidates as the magnetic core of RF inductors, to enable the next generation 5G wireless systems, with a low form factor and high Q factor. If the particles prepared are nanosized, the superparamagnetic (SPM) behavior of spinel ferrites can be suitably tuned by controlling the size and composition of the nanoparticles. Thus, nanocrystalline spinel ferrites with crystallite size less than the SPM limit can be a good choice to upshift the ferromagnetic resonance (FMR) frequency and stem the magnetic loss in RF inductors. Such functionalities are usually achieved by inducing the far-from-equilibrium distribution of cations on tetrahedrally coordinated sites (A-site) and octahedrally coordinated sites (B-sites) in a spinel structure. Typically, high-temperature annealing followed by quenching is needed to achieve such non-thermodynamic conditions. However, any temperature above 400 °C is prohibited for on-chip integration. Recently, a low temperature (< 200 °C) kinetically driven, solution-based, microwave-assisted solvothermal (MAS) technique has been explored as a CMOS-compatible process to produce spinel ferrite powders and films of high quality with desired crystallite size (< 20 nm) and intriguing magnetic properties.
In this work, we report the single-step synthesis of nickel ferrite (NiFe₂O₄) nanostructures through the “aggregation” of β-ketoester complexes in a microwave-assisted process, at a temperature < 200 °C and pressure < 150 psi, without the aid of any surfactants or catalysts. We show that as-synthesized nanostructures comprise very small crystallites (~6 nm) and are homogeneous on a macroscopic scale with robust magnetic properties. The isothermal field-dependent magnetization plots show the presence of superparamagnetic interactions in the ferrite nanostructures with a saturation magnetization (M_S) of ~30 emu/g and negligible coercivity at 300 K. We show that the M_S value of the as-prepared sample is significantly lower than that of their respective bulk values due to the surface-spin cantering effect in the nanostructures. Temperature-dependent magnetization data indicates a very low blocking temperature (T_B) of 60 K, with no significant differences in the magnetization between the zero-field cooled (ZFC) and field-cooled (FC) curves above T_B. A narrow ZFC curve implies that the crystallite size distribution is not broad, indicating the superparamagnetic interactions in the samples. We envisage that the low-temperature growth of nickel ferrite with tunable magnetic properties opens avenues for new applications in novel magnetic devices and sensors.