High-quality CoFe2O4 thin films with large coercivity grown via a wet chemical route

Chengxi Zhao,Kathy A Walsh,Anming Gao,Cheng Tu,Songbin Gong,Ankita Bhutani,Yansong Yang,Daniel P Shoemaker

doi:10.1063/1.5085232

Abstract

In permanent magnet applications, response often scales with volume or dimension in power-conversion and magnetostrictive applications, even in film form. In microelectromechanical devices it is necessary to explore versatile methods of dense film deposition with film thicknesses approaching one micron. In this study, we present a wet chemical route to hard magnetic cobalt ferrite (CoFe2O4) films to produce films with large coercivity, controllable thickness, saturation approaching that of the bulk, and smoother morphology than state-of-the art sputtered or pulsed-laser-deposited films. The development of etching and releasing processes demonstrates how these films are suitable for precise engineering in a variety of form factors and applications.

Highlights

Cobalt ferrite (CoFe2O4) is a ubiquitous hard magnet that possesses outstanding magnetoelastic, magnetooptical, and photomagnetic, properties.1–3 Most notably, its large saturated moment and coercivity drive its use in biomedical devices, high-density magnetic recording media, and noncontact force and torque sensors.4–10 The coercivity11–16 of cobalt ferrite is often a crucial property in materials selection, regardless of film or particulate form of the material.12–16 The large magnetocrystalline anisotropy of CoFe2O4 makes it a candidate for fabricating devices require strong directional magnetostrictive behavior
The X-ray diffractometer (XRD) pattern of a cobalt ferrite film with thickness of 480 nm (8 layers) deposited onto (100) Si is shown in Figure 1(a) in the supplementary material, with all peaks fit to the known cubic spinel structure
This pattern indicates the same structure of the XRD pattern in Figure 1(b) in the supplementary material of a powder sample prepared from the same solution precursor and the same annealing route using Rietveld refinement

Summary

INTRODUCTION

Cobalt ferrite (CoFe2O4) is a ubiquitous hard magnet that possesses outstanding magnetoelastic, magnetooptical, and photomagnetic, properties. Most notably, its large saturated moment and coercivity drive its use in biomedical devices, high-density magnetic recording media, and noncontact force and torque sensors. The coercivity (as high as 4 kOe for nanocrystals, compared to ∼250 Oe for Fe3O4 at room temperature) of cobalt ferrite is often a crucial property in materials selection, regardless of film or particulate form of the material. The large magnetocrystalline anisotropy of CoFe2O4 makes it a candidate for fabricating devices require strong directional magnetostrictive behavior. To that end, coupling it with piezoelectric materials has been a staple of microelectromechanical devices.17,18 When such a device must achieve a highquality factor through resonance, the acoustic resonance frequency depends on the precise geometry of the film and the thickness of the film must is typically hundreds of nanometers or more, and should be rigorously controlled.. Existing reports of sol-gel-derived CoFe2O4 suffer from ubiquitous pinholes, and correspondingly show roughness consistent with the grain size.. Existing reports of sol-gel-derived CoFe2O4 suffer from ubiquitous pinholes, and correspondingly show roughness consistent with the grain size.22–24 These limitations, combined with the brittleness of ferrites, have hindered efforts to pattern microscale devices on high-quality CoFe2O4 thin films. Scitation.org/journal/adv films that surpass the thicknesses achievable by sputtering and PLD, a dense microstructure, magnetic saturation near the bulk value, and coercivity over 4 kOe

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSION