Magnetic and structural characteristics of exchange biasing systems based on NiMn antiferromagnetic films

We have investigated the magnetic and the structural characteristics of bi-layer exchange biasing (EB) systems NiMn-Co as function of the antiferromagnetic (AFM) film deposition parameters and of the post-deposition annealing and field-cooling procedures. The effects of sputtering pressure, growth rate, seed layer and of the annealing parameters were studied by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Magneto-Optical Kerr (MOKE) effect measurements. The results yielded optimised deposition conditions and permitted us to establish efficient annealing and field-cooling procedures. The best obtained EB systems with 5 nm Co film reveal coercive field Hc≥35 Oe and exchange bias field HEB≥95 Oe and have good stability in air up to at least 200 deg. C. The obtained magnetic characteristics are closely correlated with the crystalline structure of the as-deposited samples and with the phase transformation effects. According to our results, the as-deposited samples are a mixture of a non-equilibrium FCC NiMn phase (quenched by the sputtering deposition process) and a variable amount of the equilibrium face centred tetragonal (FCT) phase. The ratio of these two phases depends on the deposition conditions--the amount of the FCT NiMn phase is larger if deposition conditions closer to equilibrium have been used, especially at higher sputtering gas pressure and higher deposition rates. The annealed samples contain dominantly the FCT AFM phase as confirmed by XRD and TEM-diffraction analyses. The transition to this AFM state depends on the initial structure of the as-grown samples and, respectively, on their deposition conditions