Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase nanocomposite permanent magnets. The bilayers were prepared by magnetron sputter deposition of near-equiatomic CoPt with a thickness of 25 nm onto oxidized Si wafers. In the as-deposited state, CoPt had the A1 (fcc) structure and was magnetically soft. Before reinsertion into the sputtering chamber for the deposition of 2.8–16.7 nm thick Co layers, the CoPt films were annealed at 700 °C for 120 min to produce the magnetically hard, fully ordered L10 phase. The presence of exchange coupling in the bilayers was verified by magnetic hysteresis and recoil measurements and showed that only for Co thicknesses below 6.3 nm was this layer (in its as-deposited state) coupled through its full thickness to the CoPt layer. Annealing the bilayer samples at 300 and 550 °C for 20 min resulted in improvement of the interlayer magnetic coupling and produced clear differences in the magnetic reversal coherency and the recoil curves. However, for some samples, the improved coupling resulted in a decrease in coercivity, indicating that there is an optimum in the coupling strength for the attainment of high coercivity. Transmission electron microscopy studies of the bilayers in plan view showed that the increased interlayer coupling with annealing was a result of improved granular epitaxy of Co to CoPt.
Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase hardlsoft composite magnets. CoPt films, with a thickness of 25 mu, were sputter-deposited from a nearly equkkomic alloy target onto oxidized Si wafers. The films were subsequently annealed at 700"C and fully transformed fi-om the FCC phase to the magnetically hard, ordered L 10 phase. The coercivity of the films increased rapidly with rumealing time until it reached a plateau at approximately 9.5 kOe. Fully-ordered CoPt films were then used as substrates for deposition of Co layers, with thicknesses in the range of 2.8-225 mn, in order to produce the hardlsoft composite bilayers. As predicted by theory, the magnetic coherency between the soft Co phase and the had, ordered CoPt phase decreased as the thickness of the sofl phase increased. This decrease in coupling was clearly seen in the magnetic hysteresis loops of the bilayers. At small thicknesses of Co (a few nanometers), the shape of the loop was one of a uniform material showing no indication of the presence of two phases with extremely different coercivities. At larger Co thicknesses, constricted loops, i.e., ones showing the presence of a mixture of two ferromagnetic phases of different harnesses, were obtained, The magnetic exchange present in the bilayer samples was qualitatively analyzed using magnetic recoil curves and the dependence of exchange coupling on the soft phase dimension in the bilayer hard/soft composite magnet films is discussed. mm!wonlufclnolvThe development of high-performance permanent magnet materials for energy-related applications is motivated by the need for increased maximum energy product, which describes the ability to store magnetostatic energy. The energy product increases with coercivity and remanence but can never exceed the intrinsic theoretical limit of ,uOM~/4. This limit corresponds to an ideal rectangular hysteresis loop, where the saturation magnetization (M,) is equal to the remanence (M,) and~0 is the permeability in vacuum [1]. Thus, permanent magnet development has been focused on achieving this theoretical limit by increasing both coercivity and remanence. However, finther increase of the energy product in single phase permanent magnets such as SmCo5, and Nd2Fe14B is intrinsically limited by their relatively low saturation magnetization.The "exchange-spring magnets", which are nano-dispersed composites of hard and soft magnets, have been explored in attempts to approach the theoretical limit of the energy product [1,2]. It is expected that a significantly improved energy product maybe obtained by exchangecoupling a soft phase with a high saturation magnetization to a hard phase with a high coercivity. However, appropriate control of the nano-scale microstructure is required to take fall advantage of the attributes of both phases, since the exchange coupling depends strongly on microstructural factors.
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