We are investigating the fabrication and magnetic properties of rings (Figure 1) for magnetic logic and memory devices. The magnetic multilayer rings show giant magnetoresistance, in which the resistance is a function of the relative orientation of the magnetization directions in the magnetic layers. These small structures have potential uses in magnetic-random-access memories (MRAM), magnetic logic devices, and other magneto-electronic applications. Unlike that of conventional MRAM devices, the ring-shaped geometry of these devices allows for a complex response with multiple stable resistance states. This capability can be used for multi-bit memory and for programmable, non-volatile memory.

These devices are programmed using either a magnetic field or a current. We have shown that spin-polarized currents can reverse the devices by domain wall motion. We are currently investigating concatenation of multiple devices through stray field interactions and control of the behavior of individual devices through interlayer magnetostatic pinning interactions and manipulation of 360° domain walls with a spin-polarized current.

We are also investigating the effects of repeated cycling on rhombic ring devices. Stray field interactions between layers give rise to complex reversal paths, which depend on the field history. These complex reversal behaviors can be attributed to the formation of 360° domain walls and mirror domains in the structure. Understanding these interactions will facilitate the development of multilayer domain wall devices.

1. C. Nam, M. D. Mascaro, and C. A. Ross, “Magnetostatic control of vortex chirality in Co thin film rings,” Appl. Phys. Lett. vol. 96, pp. 012505:1-3, 2010.
2. Y. Ren, S.  Jain, A. O. Adeyeye, and C. A. Ross, “Magnetization states in coupled Ni80Fe20 bi-ring nanostructures,” New J. Phys. vol. 12, pp. 093003:1-11, 2010.­­­

Many domain wall (DW) devices require the presence of multiple DWs, each of which may need to be independently controlled and moved. As a result, we are investigating the interaction of domain walls in a single ferromagnetic nanowire. In particular, we are focusing on 360^o domain walls (360 DWs) in which the magnetization makes a full in-plane 360° turn in a localized region of the stripe, while the rest of the stripe is magnetized parallel to its edges.

Applied current, utilizing the spin-torque effect, is the most promising method for driving domain wall devices. Our simulations indicate that 360 DWs have a response to a current that is qualitatively different from the behavior of the constituent 180 DWs. The 360 DWs move at a velocity independent of applied magnetic fields and can be destroyed by a burst of applied current. The stability of the domain wall can be controlled by an externally applied field, as shown in Figure 1. These features make the 360 DW a potential candidate as a data token in novel domain wall logic devices. Additionally, we have observed and are currently investigating resonant behavior in the 360 DW, which can be tuned by an applied field, as in Figure 1(b)). Simultaneously, we experimentally demonstrate the controlled generation of 360 DWs in rings and in a nanostructure consisting of a circular pad attached to a curved wire, as shown in Figure 2. The response of a 360 DW to current is being characterized using anisotropic magnetoresistance.

1. M. D. Mascaro, C. Nam, and C. A. Ross, “Interactions between 180 and 360 degree domain walls in magnetic multilayer stripes,” Appl. Phys. Letts. Vol. 96, no. 162501, pp. 1-3, 2010.
2. C. Nam, M. D. Mascaro, and C. A. Ross, “Magnetostatic control of vortex chirality in Co thin film rings,” Appl. Phys. Lett., vol. 96, no. 012505, pp. 1-3, 2010.
3. M. D. Mascaro and C.A. Ross, “AC and DC Current-Induced Motion of a 360˚ Domain Wall,” Phys. Rev. B vol. 82, p. 214411, 2010.