The nucleation field of perpendicularly magnetized nanowires can be controlled by changing their width, so that below a critical width the nucleation field decreases as the width decreases. Placing pads at the ends of the nanowires prevents any reduction in coercivity with width, demonstrating that at small widths domain walls nucleate from the ends of the wires. Using this technique, we are able to create asymmetric nanowires with controlled nucleation at a defined point. We also show how dipole fields from a neighboring wire in close proximity can be used to shift the hysteresis loop of the asymmetric nanowire, creating a simple NOT gate. These results show how control of the in-plane shape of perpendicularly magnetized nanoscale elements can directly lead to device functionality.

Considerable difficulties exist in generating appreciable magnetic fields, localized on nanometer length scales for future experiments and technologies. Here we experimentally demonstrate selective reversal of a ferromagnetic nanowire by the stray field from a domain wall. The use of a domain wall as a persistent, mobile source of magnetic field is an alternative to localized Oersted fields and current induced switching, with possible use in future domain wall based data storage schemes and magnetic random access memory applications. V C 2012 American Institute of Physics.Many emerging nano-scale experiments and technologies require appreciable magnetic fields, localized on nanometer length scales, including spintronic devices, magnetic random access memories (MRAM), racetrack memories, and quantum information processing devices. 1-4 Large permanent-or electro-magnets can supply considerable fields but lack spatial confinement. Using the Oersted field from a patterned current-carrying strip offers localized fields, however is incompatible with future device scaling. To this aim, recent research has demonstrated the use of patterned nanoscale ferromagnetic elements as a source of tailorable, highly localized magnetic field. 5 The stray field from a domain wall (DW), H DW , in a planar ferromagnetic nanowire has also been identified as a source of localized magnetic field. 6 These DWs have the added advantage that they may be propagated (e.g., under the application of a magnetic field) within the nanowires and so may be considered as a highly mobile source of localized field. This mobile field source underpins data writing in proposed DW-based storage devices, 3 may manipulate magnetic beads for biological sensing, 7 and has seen theoretical interest in atom trapping. 8 Furthermore, DWs may be moved using electrical currents 9,10 allowing an all electrical control of magnetic data writing and manipulation. In this letter we experimentally demonstrate the use of a DW as a mobile source of magnetic field, localized on the nano-meter scale. We show using such a source of field, a magnetic element may be directly and selectively reverseda mechanism potentially highly useful for data writing in magnetic DW data storage and MRAM applications.In ferromagnetic nanowires (NWs) patterned from magnetically soft material [e.g., permalloy (Py)], reversal of the magnetization is mediated by the nucleation of a DW and occurs when the field exceeds a critical nucleation field threshold (NFT). The magnitude of this field depends on the angle the field makes with the nanowire axes. The nucleation mechanism is well described by the thermally activated reversal of a small magnetic volume, on the order of the characteristic volume of the DW ($10 À22 m 3 in our devices). 11,12 This reversal typically occurs at the end of the nanowire and is highly sensitive to the relative angle of the applied field as well as the temperature and experimental wait time for reversal. Once nucleated, it is possible to propagate a DW under a substantially ...

Simultaneous anisotropic magnetoresistance and magneto-optical Kerr effect measurements have been performed on L-shaped Permalloy nanowires. It is shown that through magnetoresistance measurements at the corner of the device, the switching information of both arms in a single loop can be accessed. This is a very efficient method that allows for the characterization of the pinning properties in such a device as well as the understanding of the fundamental physics behind the nucleation and propagation processes in such a system. Experimental results are in good agreement with micromagnetic simulations.

The interaction of two domain walls (DWs) at a cross-shaped vertex fabricated from two ferromagnetic nanowires has been experimentally investigated. Both magnetostatically repulsive and attractive interactions have been probed. It is found that in the repulsive case, a passing DW may directly induce the depinning of another that is already pinned at a vertex. This effect can be qualitatively described by considering only simple, magnetostatic-charge-based arguments. In the attractive case, however, asymmetric pinning is found, with complete suppression of depinning possible. This observed effect is contrary to simple charge-based arguments and highlights the need for full micromagnetic characterization of the DW interactions in more complex systems.