Temperature dependence of exchange field and coercivity in polycrystallineNiO/NiFe film with thin antiferromagnetic layer: Role of antiferromagnet grain size distribution

The temperature dependence of the exchange bias H-eb and coercivity H-c, ha s been measured for a NiO(6.5 nm)/Ni81Fe19(12 nm) bilayer which demonstrate d at room temperature zero exchange bias, but an enhanced coercivity (H-c=6 5 Oe). Upon cooling the sample in a magnetic field (H=300 Oe), the exchange bias remains zero down to T=250 K, whereas the coercivity increases roughl y linearly with decreasing temperature. Below this critical temperature an exchange shift of the hysteresis loop occurs that is accompanied by a decre ase in the coercivity. Decreasing the temperature further results in an inc rease in both H-eb and H-c. A thermal fluctuation model, where both fluctua ting and stable AF grains contribute to the coercivity, with an additional assumption of increasing the density of NiO interfacial uncompensated spins at low temperature, predicts the correct temperature behavior of H-eb and H-c. (C) 2000 American Institute of Physics. [S0021-8979(00)36308-3].