Research Highlights from the Miao Group

Three-dimensional topological magnetic monopoles and their interactions in a ferromagnetic meta-lattice

Topological magnetic monopoles (TMMs), also known as hedgehogs or Bloch points, are three-dimensional nonlocal spin textures that are robust to thermal and quantum fluctuations. 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, Miao and collaborators report 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, a coherent diffractive imaging technique, to determine the magnetization vector and emergent magnetic field of the TMMs with a 3D spatial resolution of 10 nm [A. Rana et al., Nat. Nanotechnol. 18, 227–232 (2023)]. This spatial resolution is comparable to the magnetic exchange length of transition metals, enabling them to probe monopole-monopole interactions. They 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.

Fig. 1. Visualizations of positive (red) and negative (blue) topological magnetic monopoles (TMMs) and their spin configurations. Scale bar: 10 nm

 Revealing 3D atomic packing in amorphous solids with liquid-like structure

Liquids and solids are two fundamental states of matter. Although the structure of crystalline solids has long been solved by crystallography, our understanding of the 3D atomic structure of liquids and amorphous materials remained speculative due to the lack of direct experimental determination. Recently, Miao led a team from UCLA, LBL and Brown University that advanced atomic electron tomography to determine the 3D atomic positions in monatomic amorphous materials. Despite different chemical composition and synthesis methods, they observed that pentagonal bipyramids are the most abundant atomic motifs in these amorphous materials. Contrary to the traditional understanding, most pentagonal bipyramids do not assemble icosahedra, but are closely connected to form networks extending to medium-range scale [Y. Yuan et al., Nat. Mater. 21, 95–102 (2022)].

Their molecular dynamic simulations further revealed that the 3D atomic structure of monatomic liquids is similar to these experimental amorphous materials. Additionally, they found that the pentagonal bipyramid networks, prevalent in monatomic liquids, rapidly grow in size and form icosahedra during the quench from a liquid to a metallic glass state, providing a possible new picture of structural evolution through the glass transition.

Fig. 2. Experimental 3D atomic image of a tiny, noncrystalline palladium particle (left), in which the pentagonal bipyramid (right) is the most prevalent motif for how atoms pack together; the orange lines represent the pentagonal bipyramid shape. The inset shows the amorphous halo of the sample.