Magnetic exchange coupling in hard/soft ferromagnetic composite thin films of cobalt platinum/cobalt: Role of processing and structure

The study of magnetic exchange coupling in nanocomposites of magnetically-hard phases combined with magnetically-soft phases is important for the development of high-performance permanent magnets. The present work aimed to elucidate the microstructure-exchange coupling relationship in model hard/soft bilayers of CoPt(L1₀, hard)/Co(soft) in both the as-deposited and annealed states. The CoPt layer (25–100 nm) was deposited at room temperature and annealed at 700°C to develop the magnetically hard, ordered L1 ₀ structure prior to room-temperature deposition of Co (2.8–225 nm). The transformation from FCC to L1₀ in CoPt was accompanied by grain growth and evolution of a ⟨111⟩ fiber texture. The L1 ₀ mean domain area reached approximately one sixth of the mean grain area in nearly-fully and fully ordered CoPt. The coercivity of CoPt and the contribution of a pinning-type mechanism to the coercivity increased with increasing annealing time, concurrent with the increase in L1₀ fraction. In CoPt/Co bilayers, Co exhibited a ⟨0001⟩ fiber texture. For as-deposited bilayers, the exchange length of Co was close to the magnetic domain wall width for CoPt. Annealing the bilayers at 300–450°C resulted in minor microstructural changes, increased the extent or strength of coupling and thereby enhanced the reversal coherency of the bilayers such that a Co layer of twice the CoPt domain wall width was fully coupled to the CoPt. Note that in bilayers with robust coupling both phases are affected, i.e., the magnetically-hard phase is exchange-softened, while the magnetically-soft phase is exchange-hardened. In contrast to lower-temperature anneals, annealing at 500–550°C, or for extended periods at 450°C, resulted in interdiffusion of Co and CoPt, consumption of the latter and formation of two new FCC and HCP Co-Pt solid solutions. These interdiffused bilayers showed an increased reversal coherency and a highly developed out-of-film plane anisotropy. When the magnetic and microstructural data are considered together, this study shows that the properties of hard/soft nanocomposites depend not only on the dimensions of both phases, but also on the physical and magnetic characteristics of each phase. A most significant outcome of this work is the demonstration that exchange coupling may be altered, and perhaps tailored, by processing-induced changes in the interphase interface.