Bit Storage by 360$^{\circ}$ Domain Walls in Ferromagnetic Nanorings

Abstract

We propose a design for the magnetic memory cell which allows an efficient storage, recording, and readout of information on the basis of thin film ferromagnetic nanorings. The information bit is represented by the polarity of a stable 360• domain wall introduced into the ring. Switching between the two magnetization states is achieved by the current applied to a wire passing through the ring, whereby the 360• domain wall splits into two charged 180• walls, which then move to the opposite extreme of the ring to recombine into a 360• wall of the opposite polarity.

Energy barriers for bit-encoding states based on 360° domain walls in ultrathin ferromagnetic nanorings

“…This yields the exchange length l ' 3.37 nm, the Bloch wall thickness L ' 12 nm, and the quality factor Q ' 0.08, as in Ref. 9. Then the parameters used in our simulations would correspond to a film of thickness d ' 5 nm, inner radius R 1 ' 120 nm, and outer radius R 2 ' 180 nm.…”

“…The latter represents the original proposal of Ref. 9 for the considered design. We found that the 360 domain wall states have the lowest energy among all the metastable configurations and thus represent the ground states of the system within the considered topological class.…”

“…[1][2][3][4][5][6][7][8][9] In the simplest device concept, one bit of information is encoded by the polarity of the vortex state in a ring. It has been realized that switching between the two vortex directions may present difficulties due to high current densities that may be required.…”

“…14 The considerations above motivated a MRAM design concept, in which a bit is encoded by the magnetization configurations containing a 360 domain wall. 3,9 The presence of a 360 domain wall in an otherwise vortex-like configuration ensures that the configuration has topological degree zero. 9 In this concept, the magnetization vector always remains in the film plane, and the configurations are manipulated by the circular Oersted field created by a current passing through the ring center 6,9 (see Fig.…”

Polarization dependent switching of asymmetric nanorings with a circular field

“…The vortex state can have either clockwise (CW) or counterclockwise (CCW) circulation, which have degenerate energies in uniform applied fields. Nanorings have been proposed as data storage elements, [1][2][3][4][5][6][7][8][9] taking advantage of the stable equilibrium CW and CCW configurations with no stray field. Understanding and controlling the switching of nanorings is of fundamental interest and critical to progress towards data storage applications.…”