Magnetic skyrmions have garnered significant attention due to their potential as topologically protected quasiparticles with applications in spintronics. These skyrmions are small, swirling magnetic excitations with particle-like characteristics. However, their limited stability confines them to a narrow temperature range and necessitates an external magnetic field, restricting their broader use.
In a recent report published in Science Advances, Yuzhu Song and a team of researchers achieved the formation of high-density, spontaneous magnetic biskyrmions in ferrimagnets without the need for a magnetic field. They accomplished this through the thermal expansion of the lattice. The team established a strong connection between the atomic-scale ferrimagnetic structure and nanoscale magnetic domains in a ferrimagnetic compound by employing neutron powder diffraction and Lorentz transmission electron microscopy.
Song and the research team delved into the pivotal role of negative thermal expansion in generating biskyrmions within the ferrimagnet compound. They compared this behavior with materials exhibiting positive thermal expansion.
Magnetic skyrmions and biskyrmions are nanoscale magnetic domain structures with topological protection. Their unique properties, diminutive size, and low energy consumption in response to electric currents make them promising candidates for applications in spintronic storage devices.
Since their initial discovery in 2009, magnetic skyrmions have undergone rapid development. Researchers have identified various topological spin structures, including skyrmions, biskyrmions, anti-skyrmions, merons, and antimerons. The emergence of biskyrmions is typically determined by the interplay between magnetic dipole interactions and uniaxial magnetic anisotropy.
In this study, the research team proposed a method to stabilize high-density, spontaneous magnetic biskyrmions across a broad range of temperatures. They achieved this by investigating the negative thermal expansion of the lattice, comparing it to a bulk metallic ferrimagnetic compound composed of a holmium-cobalt system [Ho(Co,Fe)3].
To understand the stability of magnetic biskyrmions within the rare earth magnet (HoCo3), the team conducted extensive experiments. They initially obtained crystal and magnetic structures of the magnetic compound through variable-temperature dependent neutron-powder diffraction measurements, revealing complex magnetic structural changes at different temperatures.
The researchers determined the crystal structure of the material and explored the magnetic moments of holmium (Ho) and cobalt (Co), the constituent elements of the ferrimagnetic compound. The magnetic moment of the ferrimagnet rotated as temperatures varied, leading to a phenomenon known as spin reorientation. Beyond a certain temperature (~425 K), the magnetic structure transitioned into a disordered paramagnetic state, and this transition was well-supported by the neutron powder diffraction data.
The team summarized the temperature-dependent evolution of the magnetic and structural parameters of the ferrimagnet across the entire temperature range. They noted that the unit cell of the magnetic compound expanded with increasing temperature due to anharmonic lattice vibrations. Additionally, they conducted further neutron powder diffraction studies to calculate the magnetic components and total magnetic moments of the holmium and cobalt atoms.
To elucidate the complex magnetism of the ferrimagnetic holmium-cobalt system, the team analyzed band structures and density states of the compound through first principles. Like many rare-earth systems, the Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions played a crucial role in shaping the complex magnetism of the ferrimagnet.
In another set of experiments, Song and the team conducted neutron powder diffraction analysis to observe the rotating magnetic moment of the holmium-cobalt system with negative thermal expansion during cooling. Without applying an external magnetic field, they visualized the magnetic domain structures of the ferrimagnet over a range of temperatures, revealing varying magnetic biskyrmions within the compound. These biskyrmions were composed of two skyrmions with opposite helices.
These spontaneous skyrmions exhibited high densities and remained stable over a wide temperature range. The researchers compared the negative thermal expansion of the holmium-cobalt system with the positive thermal expansion observed in another compound containing iron. Notably, they did not observe any skyrmions in the iron-containing compound at the same temperature range where biskyrmions appeared in the holmium-cobalt system.
In summary, Yuzhu Song and the research team investigated the correlation between lattice expansion and the emergence of biskyrmions as temperatures decreased. They confirmed the occurrence of negative thermal expansion as a key factor in stabilizing biskyrmions within a rare earth magnet. By avoiding the need for an external magnetic field, the team achieved the formation of high-density, spontaneous magnetic biskyrmions across a broad temperature range in bulk holmium-cobalt systems. This work sheds light on a novel mechanism for generating spontaneous, high-density skyrmions in rare earth metal systems.