Control of Grain Shape and Size in FePt–TiO2 Nanocomposite Thin Films via Artificial Nucleation Layers

We report a general approach to the control of grain shape and size in FePt–TiO₂ nanocomposite thin films with high anisotropy energy and (001) orientation. By introducing an artificial nucleation layer of different materials (crystalline or amorphous) with different surface free energies (surface effect) and different thicknesses (ultrathin of 0–1 nm) and by separating the film growth process from the nucleation process, it is possible to have individual control over both the nucleation and growth processes. It is found that using crystalline materials of thickness 0–1 nm, such as FePt, having surface free energy of about 2000–3000 erg/cm \(^{2}\) , as the artificial nucleation layer, square-shape FePt grains were observed. The grain size decreases with the increase of sputter power for the artificial nucleation layer. Furthermore, the chemical ordering degree is improved using crystalline nucleation layer, through which more TiO₂ can be doped into FePt layer, resulting in even smaller grain size without deteriorating the magnetic properties. While, with amorphous materials of thickness 0–1 nm, such as C, TiO₂, and others, having a much smaller surface free energy of 50–100 erg/cm \(^{2}\) , as the artificial nucleation layer, smaller and circular-shape FePt grains were seen. When the thickness of artificial nucleation layer increases, a clear trend of grain size reduction and grain-shape evolution from square/rectangle shape to circular shape were observed. The mechanism for the grain-shape evolution and size control by using an artificial nucleation layer of different materials is discussed with regard to surface/interface effects. The findings offer a general way to the control of grain size and shape in nanostructured magnetic thin film for various applications.

I. Introduction

The recording density of conventional granular media is limited by the available writing field and the achievable size of magnetic grains which are thermally stable and well isolated [1]. The scaling law of recording density versus grain size demands continuous reduction in grain size that necessitates materials with high anisotropy energy, , to maintain sufficient thermal stability. (001)-textured FePt medium is the most promising candidate that has sufficient thermal stability at ultrasmall grain size (down to 3 nm) [2]–[5]. However, it remains a big challenge to fabricate FePt thin films with well-controlled size, shape, and anisotropy energy. In this paper, we report a general approach to the control of grain size and shape in nanostructured FePt thin films with high anisotropy energy and good (001) orientation by using artificial nucleation layers.