The propagation of domain walls in a ferromagnetic film is largely determined by domain wall pinning at defects in the material. In this letter we map the effective potential landscape for domain wall pinning in Permalloy films by raster scanning a single ferromagnetic vortex and monitoring the hysteretic vortex displacement vs. applied magnetic field. The measurement is carried out using a differential magneto-optical microscopy technique which yields spatial sensitivity ∼ 10 nm. We present a simple algorithm for extracting an effective pinning potential from the measurement of vortex displacement vs. applied field. The resulting maps of the pinning potential reveal distinct types of pinning sites, which we attribute to quasi-zero-, one-, and two-dimensional defects in the Permalloy film.Studying the pinning of domain walls by defects in ferromagnetic films is critical for understanding and optimizing domain wall motion and magnetization switching. Pinning occurs when the domain wall, which is a quasi-linear region of high magnetostatic, exchange, and/or anisotropy energy density, encounters a defect which gives rise to a local minimum of the total energy[1-3]. Here we study a simpler situation -the pinning of a ferromagnetic vortex core. The ground state configuration of a thin, micron-scale ferromagnetic disk with negligible magnetocrystalline anisotropy is a vortex. This state is characterized by a curl of in-plane magnetization circulating about a central core [4][5][6]. The vortex core, with half-width ∼ 10 nm and magnetization oriented largely out of plane [7], is a point-like region of high energy density. As it is scanned across the sample via an applied magnetic field, the vortex core acts like a nanoscale probe for imaging the pinning potential [6,8]. The pinning effects studied here in ferromagnetic disks are likely similar to those in vortex domain walls in ferromagnetic nanowires [9], with proposed applications in spintronic memory [10]. More generally, the vortex pinning maps that we obtain give insight into how pinning of extended domain walls would arise in these films, as the effects rely on similar energetic considerations.

The core of a ferromagnetic vortex domain creates a strong, localized magnetic field, which can be manipulated on nanosecond timescales, providing a platform for addressing and controlling individual nitrogen-vacancy centre spins in diamond at room temperature, with nanometre-scale resolution. Here, we show that the ferromagnetic vortex can be driven into proximity with a nitrogen-vacancy defect using small applied magnetic fields, inducing significant nitrogen-vacancy spin splitting. We also find that the magnetic field gradient produced by the vortex is sufficient to address spins separated by nanometre-length scales. By applying a microwave-frequency magnetic field, we drive both the vortex and the nitrogen-vacancy spins, resulting in enhanced coherent rotation of the spin state. Finally, we demonstrate that by driving the vortex on fast timescales, sequential addressing and coherent manipulation of spins is possible on ∼100 ns timescales.

We demonstrate the use of individual magnetic nanowires (NWs), grown by focused electron beam induced deposition (FEBID), as scanning magnetic force sensors. Measurements of their mechanical susceptibility, thermal motion, and magnetic response show that the NWs posses high-quality flexural mechanical modes and a strong remanent magnetization pointing along their long axis. Together, these properties make the NWs excellent sensors of weak magnetic field patterns, as confirmed by calibration measurements on a micron-sized current-carrying wire and magnetic scanning probe images of a permalloy disk. The flexibility of FEBID in terms of the composition, geometry, and growth location of the resulting NWs, makes it ideal for fabricating scanning probes specifically designed for imaging subtle patterns of magnetization or current density. arXiv:1911.00912v1 [cond-mat.mes-hall] 3 Nov 2019Recently, a form of magnetic force microscopy (MFM) based on a transducer made from a magnet-tipped nanowire (NW) demonstrated a high sensitivity to magnetic field gradients of 11 mT/(m √ Hz) with a similar spatial resolution 9 . The high force sensitivity of NW cantilevers coupled together with a small magnetic tip size could allow such sensors to work both close to a sample, maximizing spatial resolution, and in a regime of weak interaction, remaining noninvasive.Here, we demonstrate the use of individual magnetic NWs, patterned by focused electron beam induced deposition (FEBID), as MFM transducers for mapping magnetic fields with high sensitivity and resolution. The monopole-like magnetic charge distribution of their tips makes these transducers directly sensitive to magnetic fields rather

a b s t r a c tWe use a sensitive magneto-optical microscopy technique to image the magnetization response of micron-scale ferromagnetic disks to changes in applied magnetic field. This differential technique relies on a modulated applied magnetic field which allows us to measure changes in magnetization 1% < with submicron resolution. The disks are magnetized in single vortex domains, with defects in the material serving to pin the vortex core at particular positions. By applying a small AC magnetic field, we measure the deformation of the magnetization while the core remains pinned. We can also characterize the strength of the pinning site by increasing the AC magnetic field to unpin the vortex core. While pinned, we find that the magnetization away from the core reorients slightly to better align with an applied field. Additionally, an applied field causes the pinned core itself to tilt in the direction of the field. Once the field is large enough to unpin the core, this tilt disappears, and the core instead translates across the disk.

The strong, localized magnetic field produced by the core of a ferromagnetic vortex provides a platform for addressing and controlling individual nitrogen-vacancy (NV) center spins in diamond. Translation of a vortex state in a thin ferromagnetic disk or wire can be understood as motion through an effective pinning potential, arising from the defects in the material. Coupling an NV spin to a vortex state in a proximal ferromagnet imprints the pinning landscape onto the spin transitions. Quantitative characterization of the pinning potential is necessary to control the spin-vortex system. First, we map the effective pinning potential by raster scanning the vortex core through a permalloy disk and measuring the hysteretic vortex displacement vs. magnetic field using differential magneto-optical microscopy. Second, we demonstrate that the interaction between the vortex and a nearby NV spin can be characterized using the pinning map and the path taken by the vortex core through the landscape. Finally, we identify locations of bistability in the pinning landscape, and use them to manipulate the nitrogen vacancy spin in a controlled bimodal fashion by switching the spin on and off resonance with a driving field on a ∼ 10 ns timescale at room temperature.