Dissertation: Thomas Gredig

Download PDF: Exchange Bias Investigated with Anisotropic Magnetoresistance (AMR) (4.5 MB).

This is my dissertation (Ph.D. thesis) that I finished at the University of Minnesota in 2002. Most commonly, exchange bias has been measured with hysteresis loops. In my dissertation, I demonstrate and show how exchange bias can be investigated with other means, including anisotropic magnetoresistance.

Abstract: Exchange Bias Investigated with Anisotropic Magnetoresistance (AMR)

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 anitferromagnet (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. The temperature dependence of unidirectional, uniaxial and rotatable anisotropy were studied with AMR. The rotational hysteresis is largest near the blocking temperature due to a distribution in grain sizes in the AF. 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 demonstrated near the blocking temperature, where a unidirectional coercivity enhancement was discovered. Furthermore, exchange bias samples with non-mangetic spacer layers, such as Co/Ag/CoO trilayers were investigated. These systems exhibit giant magnetoresistance depending on the content of oxygen in the CoO layer. Its temperature dependence was correlated to the exchange bias effect.