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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
In a study published in Scientific Reports, which is part of the Nature Publishing Group, scientists from MIPT and GPI RAS succeeded in measuring the resistivity of solid manganese monosilicide to a very high degree of accuracy (~ 10-9 Ohm*cm) depending on the temperature and direction of the magnetic field. As noted by one of the authors of the paper, Prof. Vladimir Glushkov, “in magnetic metals, carrier scattering depends on the orientation of the magnetic structure in relation to the crystal lattice and it is normally strongly anisotropic. However, the connection between the magnetic and crystal structures may be lost if the moments in the vortex are rotated, due to their high number (a cross-section of a vortex has more than 200 magnetic moments) and their change in direction. Therefore, an experiment to measure the angular dependence of magnetic resistance allows us to obtain information on the anisotropy of the system, which is not possible in direct structural studies”.
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