In manganese monosilicide (MnSi), microscopic magnetic vortices – skyrmions – may behave as “collectivists” or “individuals”, i.e. they are able to create a single structure, or they can also split up individually. These are the findings of scientists from MIPT and Prokhorov General Physics Institute of RAS. Studying the behaviour of skyrmions will help to create unique quantum devices based on new physical principles.
Manganese monosilicide is a model object for spintronics – a branch of quantum electronics to study the possibility of controlling spin-polarized currents (conventional radio and electronic devices use non-polarized charge carriers). Spintronics-based devices, which use stable magnetic states as information bits, will help scientists to develop faster and more compact processors with low levels of power consumption, and fast and reliable non-volatile memory. This is why scientists are carefully studying the electronic and magnetic properties of materials with exotic magnetic structures.
Theorists are not yet able to fully explain the unusual magnetic properties of manganese monosilicide. For example, at very low temperatures (approximately -245C) the external magnetic field inside a manganese monosilicide crystal “rotates” the electron spins into a complex arrangement of tiny magnetic vortices, or skyrmions. The structure formed by the vortices resembles a honeycomb, with cells that are approximately 18 nanometres wide. According to theory, these structures – skyrmion lattices – can only be stable in two dimensions (in thin films); however skyrmion lattices are also observed experimentally in high quality single crystals of MnSi.
In order to use a skyrmion for practical purposes, scientists need to know whether the periodic magnetic structure consists of individual skyrmions (see image) that can be examined independently of one another, or forms a more complex magnetic structure which depends on the direction of the crystal and cannot be divided into separate vortices.
Fig. 1 a) Skyrmions – magnetic vortices named after the British physicist Tony Skyrme. They are special formations of magnetization vectors: in the centre the vector is oriented perpendicular to the surface and at the edges they form a structure which resembles a vortex. The magnetization vector is linked to the mutual arrangement of electron spins (the quantum characteristics of elementary particles) of individual atoms.
b) Periodic vortex magnetic structure in manganese monosilicide MnSi.
Image source: Y. Nii et al. Uniaxial stress control of skyrmion phase. Nature Communications
From a practical point of view, individual skyrmions can be used to transmit and store information and perform various logical operations. Magnetic vortices in existing specially prepared film structures – nanopillars, are significantly larger, and occur as a result of a specific mode of magnetic fluctuations in a limited area. Therefore, spintronics, which is based on the use of individual quasi-particles or skyrmions, will open up new prospects for miniaturizing devices and will reduce control currents. The only thing that physicists need to do now is to find materials similar to high-temperature superconductors, in which tiny magnetic vortices will be stable at room temperatures.
Fig. 2 Types of magnetic structure of manganese monosilicide MnSi depending on magnetic field strength, temperature and crystal orientation. The red area is similar to the mixed state of type II superconductors where the vortex structure is formed as a result of condensation of individual quasi-particles – skyrmions. The pink area shows the region of the anisotropic skyrmion-like phase.