T. J. Hayward

Pinning induced by inter-domain wall interactions in planar magnetic nanowires

Pinning Induced by Inter-Domain Wall Interactions in Planar Magnetic Nanowires T.J. Hayward 1 , M.T. Bryan 1 , P.W. Fry 2 , P.M. Fundi 1 , M.R.J. Gibbs 1 , D.A. Allwood 1 , M.-Y. Im 3 and P. Fischer 3 Department of Engineering Materials, University of Sheffield, Sheffield, UK Nanoscience and Technology Centre, University of Sheffield, Sheffield UK Center for X-ray Optics, Lawrence Berkeley Natl Lab, Berkeley, CA, USA PACS: 07.85.Tt, 75.60.Ch, 75.75.+a, 85.70.Kh We have investigated pinning potentials created by inter-domain wall magnetostatic interactions in planar magnetic nanowires. We show that these potentials can take the form of an energy barrier or an energy well depending on the walls’ relative monopole moments, and that the applied magnetic fields required to overcome these potentials are significant. Both transverse and vortex wall pairs are investigated and it is found that transverse walls interact more strongly due to dipolar coupling between their magnetization structures. Simple analytical models which allow the effects of inter- domain wall interactions to be estimated are also presented. There is great interest in developing memory [1] and logic [2] devices based upon the controlled motion and interaction of domain walls (DWs) in ferromagnetic planar nanowires. Such domain walls have particle-like properties which allow them to be propagated around complex circuits using rotating magnetic fields [3,4] or short electric current pulses [5], and hence they may be used to represent binary data in a similar way to electric charge in conventional microelectronics. DWs in planar magnetic nanowires have head-to-head (H2H) or tail-to-tail (T2T) character (Fig 1(a)), and consequently they carry a net monopole moment (i.e. a localised excess of north (H2H) or south (T2T) magnetic poles). Therefore, to a first approximation DWs in adjacent nanowires will interact via a Coulomb-like potential: if the DWs have like monopole moments there will be a repulsive interaction, whereas if they have opposite monopole moments their interaction will be attractive. Understanding these effects and how they affect DW propagation is likely to be important to the development of DW based devices, where large nanowire densities will be desirable. So far there have been relatively few investigations into these effects, with studies characterizing attractive coupling between walls with opposite monopole moments for a limited range of nanowire geometries and DW structures [6,7]. We have also previously demonstrated that DW interaction energies are dependent to some degree on the DWs magnetization structure and chirality [8].

Realization of the Manipulation of Ultracold Atoms with a Reconfigurable Nanomagnetic System of Domain Walls

Planar magnetic nanowires have been vital to the development of spintronic technology. They provide an unparalleled combination of magnetic reconfigurability, controllability, and scalability, which has helped to realize such applications as racetrack memory and novel logic gates. Microfabricated atom optics benefit from all of these properties, and we present the first demonstration of the amalgamation of spintronic technology with ultracold atoms. A magnetic interaction is exhibited through the reflection of a cloud of (87)Rb atoms at a temperature of 10 μK, from a 2 mm × 2 mm array of nanomagnetic domain walls. In turn, the incident atoms approach the array at heights of the order of 100 nm and are thus used to probe magnetic fields at this distance.

Design and fabrication of SU8 encapsulated digital magnetic carriers for high throughput biological assays

A design of a biological molecule carrier is presented for the application of high throughput multiplexing biological assays. This carrier contains a bit addressable “magnetic barcode” made of either Permalloy or cobalt thin films, sandwiched between two planar SU8 protective layers. We describe how the design of the magnetic carriers is optimized by engineering the coercivity of each barcode element, allowing the number of available signatures to be increased. Fully encapsulated digital magnetic carriers which carry a 5 bit addressable barcode were also fabricated and are presented. Writing and reading of digital carriers were both performed after releasing in dried solution.

A simple model for calculating magnetic nanowire domain wall fringing fields

We present a new approach to calculating magnetic fringing fields from head-to-head type domain walls in planar magnetic nanowires. In contrast to calculations based on micromagnetically simulated structures the descriptions of the fields are for the most part analytic and thus significantly less time and resource intensive. The models presented begin with an intuitive picture of domain walls, which is built upon in a phenomenological manner. Comparisons with fields calculated using micromagnetic methods show good quantitative agreement.

Controlling domain walls velocities in ferromagnetic ring-shaped nanowires

We demonstrate a method by which domain walls (DWs) in planar magnetic ring-shaped nanowires can be propagated controllably at arbitrarily low velocities by confining them to geometrically defined energy minima. Using this technique, we propagate domain walls around a ring-shaped nanowire at velocities as small as 0.6 mm/s, low enough to allow ultra-cold atoms to be transported in magnetic “traps” formed by the domain wall’s stray field. We also show how the frequency of an external applied rotating field can be used to determine the domain walls’ velocity and that the thermally activated depinning of the walls from defects ultimately limits the precision to which their motion can be controlled.

Nanomagnetic engineering of the properties of domain wall atom traps

We have used the results of micromagnetic simulations to investigate the effects of nanowire geometry and domain wall magnetization structure on the characteristic parameters of magnetic atom traps formed by domain walls in planar ferromagnetic nanowires. It is found that when traps are formed in the near-field of a domain wall both nanowire geometry and wall structure have a substantial effect on trap frequency (how tightly atoms are spatially confined) and adiabaticity (how closely the atoms’ magnetic moments track the applied field direction within the trap). We also show that in certain regimes a trap’s depth depends only on the amplitude of an externally applied rotating magnetic field, thus allowing it to be tuned independently of the trap’s other critical parameters.

Design and characterization of a field-switchable nanomagnetic atom mirror

We present a design for a switchable nanomagnetic atom mirror formed by an array of 180° domain walls confined within Ni80Fe20 planar nanowires. A simple analytical model is developed which allows the magnetic field produced by the domain wall array to be calculated. This model is then used to optimize the geometry of the nanowires so as to maximize the reflectivity of the atom mirror. We then describe the fabrication of a nanowire array and characterize its magnetic behavior using magneto-optic Kerr effect magnetometry, scanning Hall probe microscopy, and micromagnetic simulations, demonstrating how the mobility of the domain walls allow the atom mirror to be switched “on” and “off” in a manner which would be impossible for conventional designs. Finally, we model the reflection of R87b atoms from the atom mirror’s surface, showing that our design is well suited for investigating interactions between domain walls and cold atoms.

Ballistic rectification of vortex domain wall chirality at nanowire corners

The interactions of vortex domain walls with corners in planar magnetic nanowires are probed using magnetic soft X-ray transmission microscopy. We show that when the domain walls are propagated into sharp corners using applied magnetic fields above a critical value, their chiralities are rectified to either clockwise or anticlockwise circulation depending on whether the corners turn left or right. Single-shot focused magneto-optic Kerr effect measurements are then used to demonstrate how, when combined with modes of domain propagation that conserve vortex chirality, this allows us to dramatically reduce the stochasticity of domain pinning at artificial defect sites. Our results provide a tool for controlling domain wall chirality and pinning behavior both in further experimental studies and in future domain wall-based memory, logic and sensor technologies.

Suppression of stochastic pinning in magnetic nanowire devices using “virtual” domain walls

We have investigated the pinning and depinning of “virtual” domain walls in planar magnetic nanowires. Such virtual walls are created when a conventional domain wall becomes annihilated at a narrow gap between two segments of a discontinuous nanowire. By using focused magneto-optical Kerr effect magnetometry to study the repeatability of their depinning, we show that virtual walls exhibit single-mode depinning distributions, characterized by remarkably low, sub-Oersted standard deviations. This is in stark contrast to the depinning of domain walls from conventional notch-shaped defects, which typically exhibit multi-mode depinning field distributions spanning tens to hundreds of Oersteds. High-resolution magnetic soft x-ray microscopy measurements are used to reveal that this high level of repeatability is the result of a simple mediated-nucleation process, which decouples the depinning mechanism from structure of the initially injected DWs. Our work serves as an example of how the complex and dynamical stochastic behaviors exhibited by domain walls in nanowires can be controlled.

Domain walls in ring-shaped nanowires under rotating applied fields

We present a study of the motion of domain walls confined to 1D propagating energy minima in ferromagnetic nanowires. The energy minima are defined by the combination of the geometry of a ring-shaped planar nanowire and the influence of an external magnetic field, and may be controllably propagated via rotation of this field. Focused magneto-optic Kerr effect measurements are used to characterize the walls behavior at a range of field amplitudes and frequencies. Combining these measurements with simple models allows us to demonstrate that the domain walls propagate by thermally assisted “hopping” between defect sites and that the relative smoothness of their motion can be controlled by variation of the applied field strength. Frequency-domain analysis indicates that the nanowires retain domain wall structure, rather than form quasi-saturated states, over a large range of applied magnetic fields and including fields that result in smooth wall motion. Our results are important to applications where tight c...