Pinning Sites With Tilted Magnetization for Domain Wall Motion Control in Racetrack Memory

Domain wall based devices such as racetrack memory have been proposed as promising candidates for high capacity, non-volatile information storage [1, 2]. In these devices, multiple domain walls can be propagated through nanowires at speeds of many hundreds m/s using spin transfer torque, thus allowing data to be written, read and processed. One of the main challenges towards the commercial realization for domain wall memory is the stochasticity of domain wall motion. In thin film systems, where domain wall motion is the dominant reversal mechanism, the magnetization reversal does not happen in a controlled fashion but instead forms zig-zag domains extending over several micrometers in length to form. Such stochastic behavior limits the maximum data density. To overcome this issue, researchers have used lithographically fabricated notches to act as domain wall pinning centers [3]. The other method to form pinning centers is based on exchange bias [4]. In our previous study, we proposed the use of non-magnetic metal diffusion or ions implanted locally to control and pin domains in nanowires [5]. In this report, we demonstrate the formation of pinning centers by using exchange interaction between films with perpendicular magnetic anisotropy and in-plane magnetization to create locally tilted magnetization. The perpendicular magnetic anisotropy (PMA) material Ta (1)/Pt (5)/[Co (0.3)/Ni (0.4)]
₃
(0.4)]
₃
(all thickness values are in nm) multilayers were deposited on Si (SiO
₂
) substrate and the in-plane magnetic (IMA) anisotropy material Co
₁₀
Ni
₉₀
(2.5 nm) alloy was grown on top of the PMA layer. The thickness of a Pt spacer layer between the PMA layer and IMA layer was varied to tune the angle of magnetization. Figure 1(a) shows film stack studied in this report. The hysteresis loops of films with various Pt spacer layer thickness in the in-plane direction and out-of-plane direction are shown in (b) and (c) respectively. At Pt layer thickness below 2 nm, the switching behavior of the PMA and IMA layer suggests the presence of exchange interaction between the layers. While a thicker Pt layer of 5 nm causes the switching behavior to be independent of each other and thus increasing the coercivity [6]. Mumax3 simulation was performed to study the domain wall pinning effect and depinning current with various tilt angles. Figure 2(a) shows the domain wall propagation without pinning. Figure 2(b) shows domain wall propagation with a pinning center, where the magnetization is tilted at 5 degrees from the perpendicular axis. The result shows that the domain wall is pinned at the edge of the pinning center. Our results further indicate that the magnetization tilt angle can be increased to decrease the barrier energy, which reduces the depinning current as demonstrated in figure 2(c).