Magnetic properties of Co/Ni grain boundaries after annealing

Magnetic and microstructural properties of <111> textured Cu/Nx[Co/Ni] films are studied as a function of the number of bilayer repeats N and annealing temperature. M(H) loop measurements show that coercivity, H_c, increases with annealing temperature and that the slope of the saturation curve at H_c has a larger reduction for smaller N. An increase of the magnetic anisotropy (K_u) to saturation magnetization (M_s) ratio after annealing Nx[Co/Ni] with N < 15 only partially describes the increase to H_c. Energy-dispersive X-ray spectroscopy analyses performed in scanning transmission electron microscopy mode across cross-sections of as-deposited and annealed Cu/16x[Co/Ni] films show that Cu diffuses from the seed layer into grain boundaries of Co/Ni. Diffusion of Cu reduces exchange coupling (H_ex) between the magnetic grains and explains the increase in H_c. Additionally, the difference in the slope of the M(H) curves at H_c between the thick (N = 16) and thin (N = 4) magnetic multilayers is due to Cu diffusion more effectively decoupling magnetic grains in the thinner multilayer.