Phase diagram of magnetic domain walls in spin valve nano-stripes

Abstract

We investigate numerically the transverse versus vortex phase diagram of head-to-head domain walls in Co/Cu/Py spin valve nano-stripes (Py: Permalloy), in which the Co layer is mostly single domain while the Py layer hosts the domain wall. The range of stability of the transverse wall is shifted towards larger thickness compared to single Py layers, due to a magnetostatic screening effect between the two layers. An approached analytical scaling law is derived, which reproduces faithfully the phase diagram.The fundamental study of magnetic domain walls (DWs) has found a playground in lithographicallydefined nano-stripes. In the range of widths from a few tens to a few hundreds of nanometers their complexity is dramatically reduced with respect to extended thin films, while still retaining a few internal degrees of freedom leaving a rich physics. For stripes with in-plane magnetization, head-to-head DWs are of transverse (TW) or vortex (VW) type (FIG. 1b) 1,2 , and may be characterized by a chirality, asymmetry and/or polarity. Their propagation under the stimulus of a magnetic field or a current of spin-polarized electrons is intrinsically precessional and reveals effects such as DW inertia 3 and a so-called Walker limit beyond which periodic DW transformations occur 4-7 . Due to their small size and fast dynamics, magnetic domain walls (DWs) are promising candidates in the area of information processing and storage 5,7 . Most reports so far have considered DWs in a single layer of magnetically-soft materials, such as Permalloy (Py) or CoFeB alloys. Magnetic trilayers F1/NM/F2 (with Fi a ferromagnet, and NM a non-magnetic material) also deserve attention as they play a key role in spintronics, especially in devices using Giant MagnetoResistance. These stacking are called spin valve for a metal spacer layer, and pseudo spin valve or magnetic tunnel junction for an insulator spacer layer. F1 is a soft layer in which the DW moves, while F2 is a reference layer that is supposed to remain uniformly magnetized. Unusually-large current-driven DW mobilities with speed exceeding 600 m/s have been reported in such stacks 8,9 . Tentative explanations point at either out-of-plane spin accumulation and torque 10,11 , Oersted field 12 or magnetostatic coupling between the layers that may modify the statics or dynamics of the DW 12,13 . While the role of magnetostatic coupling is well established in thin films with the formation of quasi-walls in the supposedly uniformly-magnetized layer 14 , it has been scarcely addressed 13 in stripes. In this Letter we determine the phase diagram of head-to-head DWs in spin valves with in-plane magnetization (TW versus VW), combining numerical simulation and analytical modeling. For this purpose, we consider Co/Cu/Py nano-stripes with various widths w and thickness of the individual layers t Co , t Cu and t Py . The DW is located in the Py layer, while the Co layer is initialized with uniform magnetization. We predict that the stability of TWs is enhanced towards larger Py thickness compared...

Third type of domain wall in soft magnetic nanostrips

“…They carry a magnetic charge, have a chirality or a circulation123 and can show inertia456. The domain wall internal structure depends on the thickness, width and material of the constituting magnetic nanostructure178. This internal structure has a strong influence on the DW dynamics, which is of uttermost importance for devices in which the domain wall position is controlled and manipulated by field910111213 or current pulses141516.…”

Manipulating the magnetization direction of transverse domain walls in Permalloy/Ir strips using nanosecond current pulses

“…In previous measurements on trilayer Py/Cu/Co nanostrips, we observed high current-induced DW velocities for current densities corresponding to transverse Oersted fields of about 3-4 mT [27]. In that case, the Py layer thickness was smaller (5nm), leading to a higher stability of the TW configuration according to the DW phase diagram [17,18], a stability that is further increased by the magnetostatic interaction with the Co layer [28]. The stabilization of a given TW configuration and the consequent shift of the Walker breakdown to higher current densities may contribute to the high maximum domain wall velocities observed in these trilayer nanostrips [27,29].…”

Domain wall dynamics of periodic magnetic domain patterns in Co2MnGe-Heusler microstripes

“…It is well known that DW dynamics is sensitive to the internal DW structure. The DW internal structure depends on the thickness, width and material parameters of the constituting magnetic nanostructure [9,10,36]. According to analytical models [12,14], the DW mobility depends directly on the DW width; since vortex-like DWs exist for larger widths, one could expect higher mobility for a vortex wall.…”