Size dependence of the exchange bias field in NiO/Ni nanostructures

NiO/Ni wires have been investigated as a function of their width in order to investigate the size dependence of exchange bias. The samples have been prepared by e-beam lithography and ion milling of ion beam sputtered thin films. For NiO/Ni wires narrower than 3 μm, the exchange bias field significantly depends on the wire width. A NiO/Ni film shows an exchange bias field of −78 Oe whereas the exchange bias field of wires narrower than 200 nm is reduced to approximately −40 Oe. The coercive field of the NiO/Ni film is 28 Oe and increases to 210 Oe for the narrowest wires. The decrease of the exchange bias field for the narrowest wires is consistent with a recent microscopic model of exchange bias where the appearance of a unidirectional anisotropy in ferromagnet/antiferromagnet bilayers has been attributed to the presence of antiferromagnetic domains in the bulk of the antiferromagnet. A possible onset of a transition from a multidomain to a single-domain state of the antiferromagnet as a function of the NiO/Ni wire width seems to be the origin for the observed decrease of the exchange bias field for narrow wires.