Nanostructured Zn-Substituted Nickel Ferrite Thin Films: CMOS-Compatible Deposition and Excellent Soft Magnetic Properties

Nanostructured Ni _x Zn _1-x Fe ₂ O ₄ (x = 1, 0.5) films, about 1.5 μm thick on Si (100) substrates, were deposited using a low-temperature ('150 °C) microwave-assisted solvothermal (MAS) technique that is compatible with back-endof-the-line Si-CMOS processing. A nanocrystalline single-phase spinel structure with crystallite sizes of ~4 nm for the nickel ferrite film (NF) and ~6 nm for the zinc-substituted NF (ZNF) was obtained. The films demonstrate excellent surface smoothness and strong adherence to the substrate. Deconvolution of the A _1g vibration mode in Raman spectra of both films reveals a “far-from-equilibrium” crystallographic inversion induced by the MAS process. Its effect on the magnetic characteristics of the films is analyzed here. Both films exhibit in-plane (xy plane) isotropy with very low room-temperature coercivities, 25 Oe for NF and 35 Oe for ZNF, which is essential for high-frequency, soft magnetic applications. The presence of interparticular dipolar interaction in both films is confirmed from temperature-dependent magnetization measurements made under different dc bias fields. The CMOS-compatible ferrite processing and superparamagnetic Ni-ferrite and NiZn-ferrite thin films presented here can meet upcoming technological needs in on-chip integrated passive devices.