Topological magnetic monopoles (TMMs), also known as hedgehogs or Bloch points, are three-dimensional (3D) nonlocal spin textures that are robust to thermal and quantum fluctuations due to their topology. Understanding their properties is of fundamental interest and practical applications. However, it has been difficult to directly observe the 3D magnetization vector field of TMMs and probe their interactions at the nanoscale. Now, a STROBE team from UCLA, CU Boulder, UC Berkeley and LBNL collaborated with the Penn State MRSEC reports the creation of 138 stable TMMs at the specific sites of a ferromagnetic meta-lattice at room temperature. They developed 3D soft x-ray vector ptychography to determine the magnetization vector and emergent magnetic field of the TMMs with a 3D spatial resolution of 10 nm. This spatial resolution is comparable to the magnetic exchange length of transition metals, enabling them to probe monopole-monopole interactions. The team found that the TMM and anti-TMM pairs are separated by 18.3±1.6 nm, while the TMM and TMM, anti-TMM and anti-TMM pairs are stabilized at comparatively longer distances of 36.1±2.4 nm and 43.1±2.0 nm, respectively. They also observed virtual TMMs created by magnetic voids in the meta-lattice. This work demonstrates that ferromagnetic meta-lattices could be used as a new platform to create and investigate the interactions and dynamics of TMMs. Furthermore, it is expected that soft x-ray vector ptychography can be broadly applied to quantitatively image 3D vector fields in magnetic and anisotropic materials at the nanoscale.
