PAPER 360 ° domain walls : stability , magnetic fi eld and electric current effects

The formation of 360°magnetic domainwalls (360DWs) inCo andNi80Fe20 thinfilmwires was demonstrated experimentally for different wirewidths, by successively injecting two 180° domain walls (180DWs) into thewire. For narrowwires („50 nmwide for Co), edge roughness prevented the combination of the 180DWs into a 360DW, and forwidewires (200 nm forCo) the 360DWwas unstable and annihilated spontaneously, but over an intermediate range of wirewidths, reproducible 360DW formation occurred. The annihilation and dissociation of 360DWswas demonstrated by applying amagneticfield parallel to thewire, showing that annihilation fieldswere several times higher than dissociation fields in agreementwithmicromagneticmodeling. The annihilation of a 360DWby current pulsingwas demonstrated. Magnetic domainwalls (DWs) in narrowwires provide a data token for devices such as racetrackmemory and logic gates [1–3]. DWdevicesmaintain the traditionalmerits ofmagnetic data storage including non-volatility and high density, but offer new functionality including fully electrical operation by using spin torque transfer to manipulate theDWs andmagnetoresistance to detect them. This can in principle enable faster switching speeds and lower energy consumption compared toDRAMs and other semiconductor devices [4].Many proposedDW devices aremade fromnanowires containing 180DWs, where the orientation ofmagnetization rotates through 180° [5, 6]. An essential requirement forDWdevices is to be able to translateDWswithin the device, which can be accomplished using amagnetic field or a current pulse due to spin transfer torque [7, 8]. The 180DW in awirewith in-planemagnetization adopts a vortex or a transverse configuration, with transverseDWs favored in narrow or thinwires [6] as shown in the upper panel of figure 1(a). Closely spaced 180DWs in a nanowire interactmagnetostatically, and the attraction between two 180DWs of opposite core magnetization can lead to the formation of ametastable 360° domainwall (360DW), as shown in the lower panel offigure 1(a). 360DWs are also known as 1D skyrmions [9], an example of a class of topologically protected structures which are under intense study due to their stability and device applications [10]. 360DWshave been observed both in continuous ferromagnetic films [11] and in ferromagnetic nanostructures such as thinfilm rings or ellipses [12–14]. The orientation ofmagnetization rotates through 360°, and due to the opposite sense of coremagnetization in the two component 180DWs,magnetic flux closure reduces the stray field around the 360DW in a thinfilmwire compared to that of a 180DW [15]. 360DWs are not expected to be translated by an appliedfield, but instead can be dissociated or annihilated.However, micromagnetic simulations predict that a current can translate the 360DWvia spin torque transfer [15]. Moreover, instead ofWalker Breakdown as observed in 180DWs, a 360DW is predicted to undergo annihilation at a sufficiently high spin current density. Simulations predict that 360DWs of different chirality can befiltered OPEN ACCESS