Magnetic properties of small multi-layered rings

Abstract

Thin film rings can be an alternative geometry of magnetic memory cells, in which data bits are stored by the chirality of the flux-closed or 'vortex' state of the ring. The absence of the stray field in the vortex state is advantageous of high density data storage. Elliptical rings with 3 / 2 pm major / minor diameter and widths of 300 nm above were fabricated from multi-layer thin film structures such as the ferromagnetic-antiferromagnetic exchange bias bilayer or giant magnetoresistance (GMR) spin valve structure, and their magnetic and magnetoelectric properties were investigated. Exchange-biased elliptical rings show an interplay between shape anisotropy and exchange anisotropy. When both the exchange bias and applied field are oriented along the major axis, an elliptical ring shows a shifted hysteresis loop and strong in-plane anisotropy. The switching behavior and vortex state stability of the rings are strongly dependent on the pinning direction and applied field direction relative to the major axis of the ellipse. It has proven difficult to control the vortex chirality in a simple manner. A model is described that predicts the vortex chirality of an elliptical magnetic ring as a function of the direction of the applied field and of the exchange bias, based on the change in the energy of the system as the domain walls move. Experimental measurements of the chirality in Co and Co / IrMn magnetic rings with a 3.2 pm major axis are in excellent agreement with the model.

(cont.) The vortex circulation direction can therefore be tailored with an appropriate combination of the applied field direction and exchange bias direction with respect to the major axis. NiFe / Cu / Co / IrMn spin valve elliptical rings with 3.2 / 1.9 pm major / minor diameter and the width of 340 - 370 nm were fabricated and the magnetoresistance (MR) of the rings were measured with applying an in-plane field. Spin valve rings show asymmetric MR curves with three different MR states. Minor loop MR measurements, which give rise to switching of only the free layer of the spin valve ring, demonstrate that an individual control of the vortex chirality in each ferromagnetic layer is possible in a ring-shaped multilayered structure, such as a spin valve ring.