Exchange anisotropy in cobalt/cobalt monoxide bilayers studied with anisotropic magnetoresistance

Exchange-biased samples, in particular sputtered Co/CoO bilayers, were investigated with anisotropic magnetoresistance (AMR). A large number of exchange bias induced properties are probed with various methods in the standard system of Co/CoO. Reversible methods are employed with the advantage of probing the antiferromagnet (AF), since the ferromagnet remains in a single domain state. In-plane angular magnetic field dependence in small fields perturbs the pinned ferromagnet and measures a reversible exchange anisotropy, in large fields it probes irreversible changes in the antiferromagnet. Systematic studies on AF thickness reveal different behavior for CoO thickness near the critical thickness of the onset of exchange bias and for thick CoO films. A model to measure a rotatable anisotropy in thin CoO films is presented. It allows to measure instabilities in the AF material and explain the training effect and asymmetry in the magnetization reversal. The temperature dependence of unidirectional, uniaxial and rotatable anisotropy were studied with AMR. The rotational hysteresis is largest near the blocking temperature and induces training due to the magnetic aftereffect. A technique using anisotropic magnetoresistance to identify different magnetization reversals is demonstrated. Three different temperature regimes with separate magnetization reversals are found in Co/CoO samples. An asymmetry in the magnetization reversal is explained with a phenomenological model based on independent AF grains, which interact with ferromagnetic domains. Evidence that interfacial magnetic frustration leads to increased coercivity is shown near the blocking temperature, where an effective positive exchange bias was discovered. A transition from rounded to square hysteresis loops was found in a 5 K temperature window. It is explained by a unidirectional coercivity enhancement. The magnetization reversal in exchange-biased micro-magnetic rectangles is measured both with AMR and magnetic force microscopy.