Study of the influence of edge profiles of permalloy (Ni80Fe20) nanowires on the magnetic behavior of domain walls using micromagnetic simulations

Abstract

Understanding and controlling of domain wall (DW) behavior in patterned ferromagnetic nanowires are crucial for their potential applications in proposed digital logic and memory devices. For the operation of a memory, logic or sensing device, firstly well-defined domain walls are necessary and secondly a precise control in the domain walls movement is required. Previous investigations demonstrated that trapping sites such as notch or anti-notch of different geometries allow control of the position of DWs in ferromagnetic nanowires. Magneto-optic Kerr effect magnetometry, off-axis electron holography and magnetic imaging techniques have contributed useful information for a greater understanding of the behavior of DWs in nanowires. Since many years, micromagnetic simulations have also been playing a significant role to understand the controlled behavior of DWs in ferromagnetic nanowires. So far to simulate the behavior of the DWs along nanowires, a standard rectangular cross-section of the wire edge (referred as rectangular/vertical wire edge) was considered. But patterning of nanowires by using advanced nanofabrication techniques like electron-beam lithography and focused ion beam milling and their cross-sectional images recorded by using transmission electron microscope demonstrated that the wire edge of the patterned nanowires is either sloped or tapered rather than rectangular (vertical). Therefore, in the present investigation by using micromagnetic simulations we have studied the influence of standard rectangular and experimentally observed edge profiles of Permalloy (Ni80Fe20) nanowires on the magnetic behavior of DWs. We have observed the energy minimization in nanowires which were modeled based on experimentally observed sloped and tapered edge profiles compared to that of rectangular edge profiles. The domain wall depinning field from anti-notch which actually indicates the strength of the pinning potential was found to increase if the edge profile of the nanowires is also sloped or tapered rather than rectangular. The sloped or tapered edges certainly have an affect due to the effective variation of nanowire width. In the case of vortex domain wall, we also believe that nucleation of wall also affects for example the vortices also changes significantly with the sloped or tapered edge compared to the flat rectangular (vertical) edge. The domain wall structure was found to extend significantly prior to depin from the anti-notch if the edge profile of the modeled nanowires is rectangular. Notably, such an extension of the wall structure is either absent or reduced if the edge profiles are modeled following the experimentally observed profiles of the patterned nanowires. The unmodified wall structure prior to depin from the anti-notch is highly expected to realize the proposed DW based nanowire devices. This is due to the fact that the size of the domain walls governs the achievable miniaturization and thus the data storage density. The total energy as a function of normalized magnetization, depinning field strength, interaction mechanism of DWs with anti-notch and most importantly the wall structure prior to depin from the anti-notch were reported in this investigation. Finally the consequences of our findings were noted and a definition of the experimentally observed edge profiles of the nanowires was highlighted for a reliable operation of the future logic and memory devices.