Microstructure of antiferromagnetic layer affecting on magnetic exchange coupling in trilayered Ni-Fe/25 at% Ni-Mn/Ni-Fe films

The correlation between the microstructure of antiferromagnetic (AF) Ni-Mn layer and exchange coupling performed at the ferromagnetic (F)/AF layer interface was discussed in the trilayered Si(1 0 0)/Ni-Fe/25 at% Ni-Mn/Ni-Fe films with various thickness combinations fabricated by a facing-targets sputtering method. Unidirectional anisotropy constant, J(k) performed at the lower F/AF interface became large with increasing underlaid F layer thickness d(F) up to 200 Angstrom and showed constant value about 0.06-0.07 erg/cm(2) beyond it when the AF layer thickness (d(AF)) > 200 Angstrom. On the other hand, J(k) at the upper interface rose to peak with increasing d(AF) at 100-200 Angstrom and then fell down to zero at 1000 Angstrom in contrast to the J(k) at the lower interface. Following the field cooling study of trilayered films in a wide temperature range (10-473 K), it was concluded that the blocking temperature of gamma-Ni-Mn grains at the lower interface were distributed more widely down to the cryogenic temperature when d(F) = 50 Angstrom in comparison with d(F) = 200 Angstrom. This difference was supposed to be caused by the difference of the in-plane diameter of gamma-Ni-Mn grains epitaxially grown on the underlaid Ni-Fe grains growing with increasing d(F). The cross-sectional TEM observation for the trilayer with d(AF) = 1000 Angstrom made it clear that the crystal structure of Ni-Mn layer had separated from gamma-Ni-Mn phase into alpha-Mn + theta-NiMn phase with increasing d(AF), and resulted in the vanishing of J(k) at the upper interface.