Magnetic skyrmions have been the subject of extensive research in recent years due to their potential applications in spintronics. These topologically protected quasiparticles exhibit unique particle-like properties and have shown promise in spintronic storage devices. However, their limited stability and the need for an external magnetic field have hindered their wider applications. In a new study published in Science Advances, Yuzhu Song and a team of researchers have overcome this limitation by forming high-density, spontaneous magnetic biskyrmions in ferrimagnets without the need for an external magnetic field. Through their experiments, the researchers explored the role of negative thermal expansion in the generation and stabilization of biskyrmions within a rare-earth magnet.
Magnetic skyrmions are nanoscale magnetic domain structures that possess topological protection. Since their discovery in 2009, researchers have made significant progress in understanding these structures and their properties. The competition between magnetic dipole interactions and uniaxial magnetic anisotropy typically determines the generation of biskyrmions. In this study, Song and the research team focused on the stabilization of high-density, spontaneous magnetic biskyrmions across a wide temperature range by investigating the negative thermal expansion of a lattice.
To investigate the stabilization of biskyrmions, the researchers studied a rare-earth magnet composed of a holmium-cobalt system (Ho(Co,Fe)3). They characterized the compound by performing variable-temperature dependent neutron-powder diffraction measurements and Lorentz transmission electron microscopy measurements. These techniques allowed them to observe the atomic-scale ferrimagnetic structure and nanoscale magnetic domains.
One of the key findings of the study was the strong connection between the atomic-scale ferrimagnetic structure and the nanoscale magnetic domains in the ferrimagnet compound. The researchers proposed that negative thermal expansion played a critical role in generating the spontaneous biskyrmions. The magneto-elastic coupling effects induced by the negative thermal expansion were responsible for the creation and stability of the biskyrmions. This finding has significant implications for the design and development of spintronic storage devices.
To gain a deeper understanding of the magnetic and crystal structures of the compound, the research team performed further analyses. They obtained the crystal and magnetic structures of the compound through variable-temperature dependent neutron-powder diffraction measurements. They also explored the behavior of the magnetic moments of the rare earth element holmium (Ho) and the transition metal atom cobalt (Co) at different temperatures. The magnetic structure of the compound exhibited a phenomenon known as spin reorientation, which allowed the researchers to measure the temperature-dependence of the magnetization process.
The researchers observed that the unit cell of the magnetic compound expanded with increasing temperature due to anharmonic lattice vibrations. This expansion was attributed to negative thermal expansion, which is a unique characteristic of the lattice. By calculating the band structures and density states of the compound, the research team demonstrated that the complex magnetic ordering in the ferrimagnetic holmium-cobalt system was governed by Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions.
Under zero magnetic field, the researchers imaged the magnetic domain structures of the ferrimagnet across a wide temperature range. They observed varying magnetic biskyrmions in the compound, which represented very high densities with stability over a broad temperature range. The spin texture of the biskyrmions was found to be composed of two skyrmions with opposite helices. This observation indicated the significance of negative thermal expansion and its correlation with the formation and stabilization of biskyrmions.
To further support their findings, the research team compared the outcomes with another compound containing iron, which exhibited positive thermal expansion. They did not observe any skyrmions in this ferrous-integrated compound, indicating the importance of negative thermal expansion in stabilizing the biskyrmions. The results of this study provide valuable insights into the role of lattice expansion and the increasing formation of biskyrmions at lower temperatures.
Yuzhu Song and the research team have demonstrated the formation and stabilization of high-density, spontaneous magnetic biskyrmions in ferrimagnets without the need for an external magnetic field. Through their experiments, they identified the critical role of negative thermal expansion in generating biskyrmions and explored the complex magnetic and crystal structures of the compound. These findings contribute to the understanding of quasiparticles and open up new possibilities for the design and development of spintronic storage devices. The study highlights the importance of considering lattice expansion and magneto-elastic coupling effects in the exploration of topologically protected quasiparticles.
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