Ferromagnetic exchange-spring nanocomposites of A1 + L10 CoPt

"Natural" ferromagnetic exchange-spring nanocomposite thin films have been synthesized from sputter-deposited CoPt. Depending upon the details of the annealing treatment, the films consist of two phases in varying proportions: the chemically disordered A1 phase with low coercivity and the chemically ordered L1₀ phase with high coercivity. Transmission electron microscopy studies reveal <111> fiber texture formation accompanying grain growth in the annealed films and further show that each grain is comprised, on average, of six chemically ordered domains. Magnetic studies show single-phase magnetic character, signaling robust interphase exchange coupling, and indicate that the coercivity development becomes more pinning controlled as the volume fraction of high-coercivity L1₀ phase increases. The increase of domain wall pinning with increased L1₀ content is attributed to the microstructure of the L1₀-ordered phase, which contains nanoscale defects such as antiphase, c-axis variant, and grain boundaries that are a source of effective pinning sites for the narrow (∼5 nm) domain walls found in L1₀ CoPt.