Magnetic skyrmions have gained significant attention in recent years due to their potential applications in spintronics. These small, swirling magnetic excitations have unique particle-like properties and are considered topologically protected quasiparticles. However, magnetic skyrmions are limited by their low stability, narrow temperature range, and low density, requiring the presence of an external magnetic field. To overcome these limitations, a team of researchers led by Yuzhu Song has successfully formed high-density, spontaneous magnetic biskyrmions without the need for a magnetic field in ferrimagnets.
The research team explored the connection between the atomic-scale ferrimagnetic structure and nanoscale magnetic domains by utilizing neutron powder diffraction and Lorentz transmission electron microscopy measurements. They investigated the behavior of a ferrimagnet compound composed of a holmium-cobalt system (Ho(Co,Fe)3) and its response to negative thermal expansion compared to positive thermal expansion.
By performing variable-temperature dependent neutron powder diffraction measurements, the team obtained the crystal and magnetic structures of the ferrimagnet compound. The magnetic moments of the rare earth element holmium (Ho) and the transition metal atom cobalt (Co) were explored, revealing a temperature-dependent rotation of the ferrimagnet’s magnetic moment known as spin reorientation. At temperatures exceeding ~425 K, the magnetic structure transitioned into a disordered paramagnetic state.
The expansion of the magnetic compound’s unit cell with increasing temperature was observed, attributed to anharmonic lattice vibrations. Through additional neutron powder diffraction studies, the team calculated the magnetic components and total magnetic moments of the holmium and cobalt atoms. Complex magnetic ordering in the ferrimagnetic system was further explored using first principles calculations and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions commonly found in rare-earth systems.
Under zero magnetic field, the researchers imaged the magnetic domain structures of the ferrimagnet and observed the presence of magnetic biskyrmions across a wide temperature range. These biskyrmions were composed of two skyrmions with opposite helices. Interestingly, the density of spontaneous skyrmions was extremely high, indicating their stability and potential for practical applications. Comparisons with another compound containing iron revealed the absence of skyrmions, emphasizing the importance of negative thermal expansion in the generation of biskyrmions.
Yuzhu Song and the research team successfully generated high-density, spontaneous magnetic biskyrmions in a ferrimagnet compound without the need for an external magnetic field. By investigating the negative thermal expansion of the lattice, the stability of these biskyrmions was achieved across a broad temperature range. The team’s findings provide valuable insights into the generation of unique topological magnetic domain structures and open up new possibilities for their practical utilization in spintronic storage devices. The study highlights the significance of thermal expansion effects and magneto-elastic coupling in the creation of stable magnetic quasiparticles. Future research in this field will undoubtedly focus on further optimizing the control, manipulation, and application of these fascinating magnetic skyrmions and biskyrmions.