Integrating the exotic world of topological magnetism with practical semiconductor technology has been a grand challenge. Two key obstacles are the difficulty in stabilizing topological spin textures like skyrmions and bimerons without external energy-intensive magnets, and the lack of an efficient mechanism to harness these textures for controlling electrons in semiconductors. A research team has now surmounted these challenges through a single, sophisticated material design that introduces two interconnected innovations.
Reporting their study in Science Bulletin, the team from Xiamen University and Beihang University presents a novel multi-periodic (Pd/Fe/FeO/MgO)4 spin tunnel junction. This architecture is not merely a spin injector but an integrated platform that simultaneously addresses stability, filtering, and control.
The first innovation is the creation of a robust host for coexisting topological spin textures at zero field. The carefully engineered repetition of the magnetic and tunneling layers, combined with a high magnetic field during growth, creates an ideal environment for stabilizing a unique mixture of Bloch-type skyrmions (circular, particle-like spin whirls) and bimerons (in-plane, stripe-derived counterparts). This coexistence of skyrmions and bimerons is a valuable state, as it indicates a finely balanced magnetic energy landscape, crucial for low-power device operation.
The second, pivotal innovation is the junction's function as a "cascaded spin filter." The multi-periodic design forces electrons to undergo sequential spin-selective tunneling through four identical MgO barrier units. "Think of it as an assembly line for spin polarization," explains Prof. Yaping Wu, a corresponding author. "With each tunneling event through a MgO/Fe interface, the electron current becomes purer in its spin orientation. This cascaded filtering mechanism amplifies the spin polarization to a level unattainable with single-barrier junctions."
Beyond filtering, the research reveals a profound effect: the coexisting topological textures actively guide and manipulate the electrons themselves. The team's analysis, supported by finite-element simulations, shows that the non-trivial spin configuration of a skyrmion creates a nanoscale gradient magnetic field. This field acts as a lens for electron spins: electrons injected vertically are deflected by this gradient, with their trajectories bent towards the skyrmion core.
"This topology-guided trajectory control is crucial." Prof. Wu explains. "The spin texture acts like natural electron directors. Most whirlpools channel electrons to their centers to align spins. Meanwhile, the other type of magnetic texture (bimerons) works in harmony, adding to this spin-alignment effect. This synergistic effect, where the textures' topology directly manipulates electron paths, ensures that a vast majority of electrons arriving at the semiconductor have the desired spin."
When this advanced spin injector, combining cascaded filtering and topology-guided trajectory control, was integrated into a GaN-based light-emitting diode (LED), the result was a dramatic leap in performance. The electroluminescence exhibited a record-high degree of circular polarization of 25.3% at zero external magnetic field, a value that substantially surpasses all previous reports for GaN-based spin-LEDs.
These results have moved beyond simply observing spin textures to actively employing them as functional components that directly participate in device operation. The multi-periodic tunnel junction is the enabling platform, but it is the synergy between the topology-guided control of electron flow and cascaded filtering that delivers the breakthrough performance. This provides a strategy for the next generation of spin-based logic and optoelectronic devices.
The study establishes a comprehensive and scalable pathway for merging topological spintronics with semiconductor technology, pointing the way toward highly efficient, integrated systems for computing, memory, and communication.
