http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

The exchange bias effect, discovered more than fifty years ago, is a fundamental interfacial property, which occurs between ferromagnetic and antiferromagnetic materials. After intensive experimental and theoretical research over the last ten years, a much clearer picture has emerged about this effect, which is of immense technical importance for magneto-electronic device applications. In this review we start with the discussion of numerical and analytical results of those models which are based on the assumption of coherent rotation of the magnetization. The behavior of the ferromagnetic and antiferromagnetic spins during the magnetization reversal, as well as the dependence of the critical fields on characteristic parameters such as exchange stiffness, magnetic anisotropy, interface disorder etc. are analyzed in detail and the most important models for exchange bias are reviewed. Finally recent experiments in the light of the presented models are discussed.

Exchange Bias Effect of Ferro-/Antiferromagnetic Heterostructures

Training of the exchange bias effect in antiferro-/ferromagnetic heterostructures is considered in the theoretical framework of spin configurational relaxation, which is activated through consecutively cycled hysteresis loops. The corresponding exchange bias fields, ${\ensuremath{\mu}}_{0}{H}_{\text{EB}}(n)$, reveal relaxation from the initial state, $n=1$, of high antiferromagnetic interface magnetization to the equilibrium state of reduced magnetization in the limit of large $n$. The evolution of ${\ensuremath{\mu}}_{0}{H}_{\text{EB}}$ vs $n$ is calculated with the help of a discretized Landau-Khalatnikov equation, where the continuous time parameter is replaced by the loop index $n$. The result reveals the origin of the well established but hitherto unexplained power-law decay of ${\ensuremath{\mu}}_{0}{H}_{\text{EB}}(n)$ for $ng1$. Moreover, in contrast with the breakdown of the power-law behavior at $n=1$, the relaxation approach describes the training effect for $n\ensuremath{\geqslant}1$. The full capability of the theory is explored in comparison with experimental results obtained recently on a $\mathrm{NiO}(001)∕\mathrm{Fe}(110)$ heterostructure.

/pdf/training-of-the-exchange-bias-effect-a-simple-analytic-5bzylwh197.pdf

Training of the exchange-bias effect: A simple analytic approach

Exchange coupled bilayers of soft and hard ferromagnetic thin films show remarkable analogies to conventional antiferromagnetic/ferromagnetic exchange bias heterostructures. Not only do all these ferromagnetic bilayers exhibit a tunable exchange bias effect, they also show a distinct training behavior upon cycling the soft layer through consecutive hysteresis loops. In contrast with conventional exchange bias systems, such all ferromagnetic bilayer structures allow the observation of training induced changes in the bias-setting hardmagnetic layer by means of simple magnetometry. Our experiments show unambiguously that the exchange bias training effect is driven by deviations from equilibrium in the pinning layer. A comparison of our experimental data with predictions from a theory based upon triggered relaxation phenomena shows excellent agreement.

/pdf/exchange-bias-training-effect-in-coupled-all-ferromagnetic-2otmrrd6n9.pdf

Exchange bias training effect in coupled all ferromagnetic bilayer structures.

/pdf/exchange-bias-training-effect-in-coupled-all-ferromagnetic-3v67p955d1.pdf

Exchange bias training effect in coupled all ferromagnetic bilayer structures

Exchange bias of a partially oxidized thin film of ferromagnetic Co was studied by magnetization measurements and polarized neutron reflectivity (PNR). The magnetization curve shows strong effects of training with cycling of the magnetic field. Reflectivity measurements with the field parallel to the cooling field showed the onset of spin-dependent diffuse scattering—off the specular reflection—after a training cycle. Such scattering, of the Yoneda type, is due to misaligned Co domains possibly close to the Co/CoO interface. Subjecting the field cooled Co/CoO pair to a field perpendicular to the cooling field causes a rotation of the magnetization. The PNR measurements confirmed earlier susceptibility studies by indicating that the rotation of the magnetization is reversible in fields up to 400 Oe. The rotation of the magnetization of Co is uniform across the film thickness.

/pdf/training-effects-and-the-microscopic-magnetic-structure-of-14g589li29.pdf

B. K. Hill

Papers

Training effects and the microscopic magnetic structure of exchange biased Co/CoO bilayers