The vortex state of a magnetic nanoring has special attributes of no magnetic poles nor stray fields. The circulatory magnetization can have two chiralities:, left-handed or right-handed, for storing "0" and "1", as shown in Fig. 1.
Fig. 1: Two chiralities of a magnetic nanorings for storing "1" and "0".
We have developed a new method for the fabrication of 100-nm magnetic nanorings using colloidal lithography. We first form a monolayer of polystyrene nanospheres (e.g., 100 nm) on a substrate, followed by deposition of a thin film (e.g., Co) as shown in Fig. 2(a). After normal-incident ion-beam etching (a = 0), which removes all materials except those protected by the nanospheres, we obtain symmetric nanorings as shown in Fig. 2(b). The size of the nanospheres dictates that of the nanorings, which have widths of only 20 nm.
Fig. 2: Fabrication of nanorings with colloidal lithography. After deposition of Co (a), normal incident etching (b) leads to symmetric ring (d), whereas oblique etching leads to asymmetric rings (e).
There are two switching processes of magnetic nanoring: the vortex process (V-Process), with the annihilation of the two poles (or domain walls), and the rotating onion process (O-Process), where the two poles move in unison in opposite direction. For the 100-nm symmetric nanoring, the fractions are 40% for V-Process and 60% for O-Process. For symmetric nanorings with a uniform crosssection, the two chiralities of magnetization cannot be controlled by magnetic field.
Very recently, using oblique angle etching with an angle a [Fig. 2(c)], we have obtained asymmetrical nanorings, in which the cross-section is continuously varying across the circumference [Fig. 2(e)]. In the asymmetrical nanorings, the domain walls preferentially moves towards the narrower sections of the asymmetrical nanorings as shown by our theoretical model. As a result, the fraction of the V-Process increases with theta, and reaching 100% at theta = 90 degrees.
Furthermore, we not only can achieve exclusively the vortex state, but also control the chirality of the vortex state being left-handed or right-handed by using the magnetic field applied along the asymmetric axis as shown in Fig. 3. These results indicate that nanorings provide a very favorable geometrical shape for the memory units such as those in MRAM. Magnetic nanorings can store information through its chirality but without generating stray field.
Fig. 3: Vortex state with a definite chirality dictated by the direction of the magnetic field.
