Influence of interface and microstructure on magnetization of epitaxial Fe4N thin film

TL;DR

This study investigates how interface quality and microstructure influence the magnetic properties of epitaxial Fe4N thin films grown on LaAlO3 substrates, highlighting the benefits of HiPIMS in reducing interdiffusion and enhancing magnetization.

Contribution

It introduces the use of HiPIMS for growing Fe4N films, demonstrating improved interface sharpness and magnetic properties compared to conventional sputtering methods.

Findings

HiPIMS yields smoother, denser Fe4N films with sharper interfaces.

Interdiffusion at the substrate-film interface reduces magnetic moment.

HiPIMS-grown films exhibit higher magnetic moments and distinct anisotropies.

Abstract

Epitaxial Fe4N thin films grown on lattice-matched LaAlO3 (LAO) substrate using sputtering and molecular beam epitaxy techniques have been studied in this work. Within the sputtering process, films were grown with conventional direct current magnetron sputtering (dcMS) and for the first time, using a high power impulse magnetron sputtering (HiPIMS) process. Surface morphology and depth profile reveal that HiPIMS deposited film has the lowest roughness, the highest packing density and the sharpest interface. La from the LAO substrate and Fe from the film interdiffuse and forms an undesired interface spreading to an extent of about 10-20 nm. In the HiPIMS process, layer by layer type growth leads to a globular microstructure which restricts the extent of the interdiffused interface. Such substrate-film interactions and microstructure play a vital role in affecting the electronic hybridization and magnetic properties of Fe4N films. The magnetic moment (Ms) was compared using bulk, element-specific and magnetic depth profiling techniques. We found that Ms was the highest when the thickness of the interdiffused layer was lowest and only be achieved in the HiPIMS grown samples. Presence of small moment at the N site was also evidenced by element-specific x-ray circular dichroism measurement in HiPIMS grown sample. A large variation in the Ms values of Fe4N found in the experimental works carried out so far could be due to such interdiffused layer which is generally not expected to form in otherwise stable oxide substrate. In addition, a consequence of substrate-film interdiffusion and microstructure results in different kinds of different kind of magnetic anisotropies in films grown using different techniques.