Transverse domain walls in nanoconstrictions

Abstract

The spin structure of domain walls in constrictions down to 30 nm is investigated both experimentally with electron holography and with simulations using a Heisenberg model. Symmetric and asymmetric transverse domain walls for different constriction sizes are observed, consistent with simulations. The experimentally observed asymmetric transverse walls can be further divided into tilted and buckled walls, the latter being an intermediate state just before the vortex nucleation. As the constriction width decreases, the domain wall width decreases faster than linearly, which leads to very narrow domain walls for narrow constrictions. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2779109͔Control and manipulation of magnetic domain walls ͑DWs͒ are in the focus of interest because of the associated exciting physical phenomena and the potential for applications such as magnetic logic and data storage devices. 1,2Here, reproducible and controlled switching by DW motion, induced either by an external magnetic field or a spinpolarized current, is essential. [3][4][5] The detailed DW spin structure and width play a very important role in determining the DW velocity in current-induced 5 and field-induced motion.6,7For very narrow walls, nonadiabatic contributions to the electron transport are predicted to become significant, [8][9][10] which would increase the current-induced DW velocity, which is important for applications. Reciprocal effects of the spin structure on the magnetotransport, such as DW magnetoresistance, also depend on the detailed wall spin structure. 11,12On reducing the lateral dimensions, it is the geometry rather than the material parameters which determines the DW type and spin structure.13-16 For a one dimensional chain model, it has been predicted that the reduction of the lateral dimensions leads to a reduction in the Bloch DW width. 13For 180°Néel walls in Permalloy ͑Fe 20 Ni 80 ͒ thin film rectangular structures, the reduction of the DW width has been studied using scanning electron microscopy with polarization analysis with a magnetic resolution of 20-30 nm. 17 The types of DWs confined in wires or ring elements are very different from these Néel walls, since they exhibit head-tohead wall structures with two types prevailing: transverse and vortex DWs.14,15 Using transmission electron microscopy techniques, the spin structure of head-to-head DWs in elements down to 200 nm lateral size has been measured. 18Observations of DWs in constrictions have also been carried out to determine the pinning potential, but the detailed spin structures and wall widths have not been ascertained. 19Employing micromagnetic simulations, the geometry dependence of the wall types ͑wall phase diagram͒ has been studied down to 20 nm width. In addition to the transverse and vortex walls, asymmetric transverse walls were predicted. 16 In the conventional micromagnetic approach used in Ref. 16, the exchange energy is approximated by ٌ͑ ជ · m ជ ͒ 2 ͑m ជ is the local magnetization͒, which is the first order Taylor exp...

Mechanism for domain wall pinning and potential landscape modification by artificially patterned traps in ferromagnetic nanowires

“…Furthermore, the ability to control the structure of a domain wall through the geometrical dimensions of the magnetic wire allows the experimental study of fundamental physical properties of these different types of DWs. [9][10][11][12] Although there have been several experimental studies reporting the ability of artificially created constrictions to pin DWs, 10,[13][14][15] and numerous spin-transfer experiments currently use such artificial defects to precisely locate and hold DWs within magnetic nanostructures, 16,17 a complete understanding of how the local DW energy landscape is modified by artificial structural defects is currently lacking.…”

Optimized cobalt nanowires for domain wall manipulation imaged by in situ Lorentz microscopy

“…Therefore, many Py nanostructures of various shapes and sizes have been synthetized using electron beam or UV lithography processes. [1][2][3][4][5][6][7][8][9][10][11][12]15,16 The use of ferromagnetic materials alternative to Py and the development of advanced nanofabrication methods allowing creating magnetic nanostructures of dimensions less than 100 nm are however needed to explore their functionalities and possible applications. In the last years, focused electron beam induced deposition (FEBID) technique has demonstrated a capacity to produce high quality nanostructures based on multiple materials.…”

“…[1][2][3][4][5] These innovative ideas and promising applications have motivated extensive developments of ferromagnetic nanostructures in which DW can be "easily" created and driven either by external magnetic fields and/or spin-polarized currents. [6][7][8][9][10][11][12] DW in magnetic nanostructures have therefore become a major topic for the research community in the field of Nanomagnetism. The specific DW configuration is the result of the balance between the magnetostatic energy, the magnetocrystalline anisotropy, and the exchange coupling.…”

Domain wall spin structures in mesoscopic Fe rings probed by high resolution SEMPA

“…For the imaging of the domain wall spin configurations in rings of other materials, previous work has employed electron holography [21] or Lorentz microscopy, which require that the samples are fabricated on delicate membranes for the transmission measurements [10], photo-emission electron microscopy [17,18,31], which is mainly available at large-scale facilities and can be limited in its resolution, or magnetic force microscopy (MFM) [9,[32][33][34], which can modify the spin configuration of the sample and is however sensitive only to the stray magnetic field from a sample and therefore harder to relate directly to the spin structures obtained from simulations. Therefore, in this paper we chose the imaging technique SEMPA [26], which is a powerful lab-based method with an excellent spatial resolution of less than 20 nm and which can provide quantitative direct information concerning the spin configurations [35].…”

“…While many studies just consider the two types of domain wall mentioned above, it has been predicted [13] and experimentally confirmed [21] that the transverse domain wall can occur in both symmetric and asymmetric configurations, which are expected to have different properties. For example, it has recently been revealed that the symmetry of the transverse domain wall is important in determining the depinning process from domain wall traps [22] and wire kinks [23].…”