Probing Room-temperature Skyrmions at the Nanoscale using Diamond Quantum Sensors
Abdelghani Laraoui (Mechanical & Materials Engineering)
Magnetic skyrmions are nanoscale spin textures characterized by topological charge and are proposed for the next generation of ultradense magnetic memories. Skyrmions can occur as two-dimensional ground states in magnetic systems where strong spin orbit coupling and broken inversion symmetry lead to Dzyaloshinskii-Moriya interaction (DMI). Of particular interest are room-temperature skyrmions, recently observed in magnetic stacks composed of ultrathin ferromagnetic layers sandwiched between heavy metal thin films. Also, they have been proposed to explain unusual topological Hall effects in ferrimagnetic spinel NiCo2O4 (NCO) thin films. The effect of interfaces, pinning sites, strength of the DMI and its role in stabilizing the skyrmions is not well explored, and imaging their full spin structure is challenging owing to the need for nonperturbative magnetic probes with spatial resolution below 10 nm that work over a wide range of magnetic fields and temperatures.
Here we employ a scanning spin probe microscope, based on nitrogen vacancy (NV) color centers in diamond tips (Fig 1.), to map room-temperature skyrmions in ferromagnetic layers, composed of ultrathin Co layers sandwiched between 5d transition metal (such Ir, Pt, W) layers, and in NCO thin films. The electronic spins of the NV centers are optically polarized and coherently manipulated with microwave pulses to resolve spatially maps of local magnetic fields. The straight fields generated from the skyrmions become encoded in the NV fluorescence signal, and by scanning the diamond tip across the magnetic substrate we create magnetic images with spatial resolution below 10 nm. By taking a series of images at various magnetic field strengths and directions the complete spatial structures can be determined. These measurements will be correlated with magnetic and magneto-transport bulk measurements and compared with theoretical models.
The proposed work involves an interdisciplinary research team, including: NV microscopy (Laraoui), sputtering deposition and epitaxial thin film growth (Adenwalla, Binek, Hong), magneto-transport (Hong), and theory (Kovalev). If successful, this technique will be extended to map spin textures at low temperature in complex oxide heterostructures and recently discovered two dimensional magnetic films.
Fig. 1: Diamond nitrogen vacancy-based scanning probe microscope for imaging room temperature skyrmions in ferromagnetic multilayers and ferrimagnetic spinels