Magnetic Exchange-Coupling In Copt/Co Bilayer Thin Films

Abstract

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.

Interphase exchange effects in CoPt/Co bilayer thin films

“…This speckled appearance is a result of diffraction contrast in the fine-grained layer of Co. The size of the speckles is a few nanometres, which is in the same range as the grain size of 12 nm found for the pure Co film deposited directly onto silicon dioxide [8,9]. Selected area electron diffraction patterns (not shown) of the as-deposited bilayers may be selfconsistently indexed with XRD traces to show the Co to be hexagonal close-packed (hcp, A3) and 0001 fibre-textured.…”

“…TEM studies of the 700˚C, 120 min annealed, single-layer CoPt confirm that it possesses the L1 0 structure. The film is fine-grained polycrystalline, with an average grain diameter of 43 nm and a 111 fibre texture [6][7][8]10]. Each grain contains a number of ordered L1 0 domains and the associated antiphase and c-axis variant domain boundaries.…”

“…Structural and magnetic studies were performed on polycrystalline CoPt films capped with a thin Co layer [6][7][8][9][10]. In this study, thin-film bilayers of fully ordered L1 0 -type CoPt with near-equiatomic composition and a thickness of 25 nm were fabricated on oxidized Si(100) substrates and capped by a single layer of cobalt with a thickness of 16.7 nm, which corresponds to 40% of the total bilayer thickness.…”

“…The annealing ambient was again Ar-4%H 2 to avoid film oxidation. Additional details regarding the preparation of the CoPt layer and the bilayers are provided in [6][7][8][9][10]. To investigate the effects of annealing on the interphase exchange coupling in these bilayers, the samples were annealed at 300˚C and 550˚C, both procedures carried out for 20 min.…”

“…To further delineate the interdiffusion and the structural changes that occur upon annealing, bilayers with 100 nm L1 0 -CoPt and 25 nm Co were investigated using in-plane and out-of-plane x-ray diffraction (XRD). The results of these studies are given in detail in [7][8][9].…”