Characterizing Interlayer Coupling in Synthetic Ferromagnetic Thin Films

Abstract

The phenomena of interlayer exchange coupling (JIEC) between thin ferromagnetic (FM) layers allows materials with precisely tailored properties to be created, and is key to the future development of spintronics. Synthetic ferromagnets (SFMs) have the same FM/Non-magnetic(NM)/FM structure as the widely studied Synthetic Antiferromagnet (SAF) [1], but describe the case where the magnetisation of the two FM layers are aligned in parallel. The structure of a SFM and its resonant dynamics is shown in Fig.1a. It is known that the formation of a double resonance consisting of an Acoustic Mode (AM) and an Optic Mode (OM) requires a difference in layer magnetisation (MDiff) and a FM JIEC [2] [3]. However, to date, a comprehensive description of the resonant properties as a function of JIEC and difference in layer magnetisation has not been reported despite their widespread use in quasi-static devices [4]. Here, we investigate, both experimentally and via numerical simulations, the hitherto uncharted dependence of the observed double resonance on MDiff and JIEC [5].The samples were fabricated using magnetron sputtering and characterized by X-ray Reflectivity (XRR) to measure layer thickness, vector Vibrating Sample Magnetometry (VSM) to verify AF coupling and a Vector Network Analyser – Ferromagnetic Resonance (VNA-FMR) setup which allows fundamental dynamic properties to be explored. These SFMs exhibited a JIEC consistent with coupling on the 1st FM RKKY coupling peak and have structure Ta(2 nm)/CoFeB(5 nm)/Ru(tRu)/CoFeB(5 nm)/Pt(4 nm) where tRu is the thickness of the ruthenium layer, acting as the non-magnetic spacer layer. Our simulations, performed using MuMax3, show that MDiff and JIEC have orthogonal dependencies on the two resonant modes. We further demonstrate that the conventional understanding of the phases of these modes, described as possessing an in-phase or out-of-phase nature for the AM and OM respectively, does not capture fully the intricacy of the resonant dynamics. Comparison between experimental and simulated data allows MDiff, JIEC and the resonant phases of each mode to be accurately determined. These findings are directly applicable to the creation of tailored SFMs for spintronic devices with applications in STT/SOT-MRAM where control of these parameters is paramount [3] [6]. **