NiO structure–exchange anisotropy relation in the Ni81Fe19/NiO films and thermal stability of its NiO film

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Abstract
We studied an antiferromagnetic (AF) NiO film for an exchange‐biased layer, focusing especially on the relationships between the exchange coupling properties of the Ni81Fe19(top)/NiO(bottom) films and the character of its NiO film. Among the variable sputtering conditions, our experimental data showed that the dominant factor determining the exchange coupling properties was the Ar pressure during the NiO film preparation. Better exchange coupling properties resulted when the NiO film was deposited at low Ar pressure which was attributed to: (i) the smooth surface of the NiO film and (ii) the presence of relatively large particle sizes within it. The former was thought to bring about not only an increase in the number of unidirectional exchange coupled Ni81Fe19/NiO spins, but also the appearance of exchange paths having large local exchange anisotropies. The latter was thought to produce an increase in the AF clusters with a particle volume larger than KeiA/KAFi, where Kei, A, and KAFi are local unidirectional anisotropy, interfacial area of the NiO cluster in contact with the Ni81Fe19 film, and local magnetocrystalline anisotropy of the NiO cluster, respectively. Moreover, the NiO film was thermally stable up to 250 °C, although the AF anisotropy of the NiO film was weakened on increasing the annealing temperature above 250 °C.