Magnetic skyrmions, known as small swirling topological magnetic excitations, have gained significant attention in the field of spintronics. These quasiparticles possess particle-like properties and are topologically protected. However, their limited stability and dependence on a narrow temperature range and low density, as well as external magnetic fields, have restricted their wider applications. In a recent report published in Science Advances, Yuzhu Song and a research team have made a breakthrough by generating high-density, spontaneous magnetic biskyrmions without the need for a magnetic field in ferrimagnets, through thermal lattice expansion.
The research team employed neutron powder diffraction and Lorentz transmission electron microscopy measurements to investigate the link between the atomic-scale ferrimagnetic structure and nanoscale magnetic domains in a ferrimagnet compound. By comparing the magneto-elastic coupling effects and the behavior of the material with positive thermal expansion, the team demonstrated the critical role of negative thermal expansion in the generation of biskyrmions. This unique phenomenon was observed in a rare earth magnet compound called holmium-cobalt (Ho(Co,Fe)3), which exhibited negative thermal expansion due to anharmonic lattice vibrations when compared to the positive thermal expansion of a compound containing iron.
Magnetic skyrmions are nanoscale magnetic domain structures with topological protection. Their small size, unique features, and lower energy consumption make them ideal candidates for spintronic storage devices. Skyrmions come in various forms, including biskyrmions, anti-skyrmions, merons, and antimerons, depending on the competition between magnetic dipole interactions and uniaxial magnetic anisotropy. The research team aimed to stabilize high-density magnetic biskyrmions across a wide range of temperatures by harnessing the negative thermal expansion of the lattice in the holmium-cobalt system.
The team conducted variable-temperature dependent neutron-powder diffraction measurements to obtain the crystal and magnetic structures of the HoCo3 compound. By analyzing the material’s profile intensity at different temperature ranges, the researchers observed complex magnetic structural changes and the phenomenon of spin reorientation as a result of the magnetic moment rotation of holmium (Ho) and cobalt (Co). When the temperature exceeded approximately 425 K, the magnetic structure transitioned into a disordered paramagnetic state. The team successfully fit the magnetic structure data to the neutron powder diffraction data at all temperatures, providing a comprehensive understanding of the material’s magnetic and structural parameters.
To further investigate the complex magnetic ordering in the holmium-cobalt system, the team calculated the band structures and density states of the compound using first principles. The Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, common in rare-earth systems, played a significant role in the ferrimagnet’s magnetism. The researchers also conducted neutron powder diffraction analysis, revealing the rotating magnetic moment of the holmium-cobalt system with lattice negative thermal expansion during cooling. This analysis allowed them to image the magnetic domain structures of the ferrimagnet at various temperatures and observe the presence of magnetic biskyrmions.
The team discovered that the lattice negative thermal expansion of the holmium-cobalt system was closely linked to the stable formation of magnetic biskyrmions. Through characterization and comparison with a compound containing iron, which exhibited positive thermal expansion, the researchers demonstrated the consistency of negative thermal expansion and the gradual increase of biskyrmions as temperatures decreased. Notably, the biskyrmions appeared in the holmium-cobalt system but not in the ferrous-integrated compound, emphasizing the importance of negative thermal expansion for the stabilization of high-density, spontaneous magnetic biskyrmions.
Yuzhu Song and the research team have expanded the mechanism for generating high-density magnetic biskyrmions in the absence of a magnetic field. Through a thorough exploration of the ferrimagnetic structures and the utilization of negative thermal expansion, the researchers successfully stabilized biskyrmions in the holmium-cobalt system across a wide temperature range. This breakthrough opens up new possibilities for the practical use of magnetic skyrmions in spintronic storage devices and highlights the role of lattice properties in the formation of topological magnetic quasiparticles.