Skyrmions, topologically-protected spin structures, are the focus of intense research for their potential applications in spintronics: next-generation electronic devices that exploit the spin degree of freedom. Skyrmions are curious magnetic systems, and offer the possibility of high-density, low-power magnetic storage devices such as racetrack memory. The formation of skyrmions requires the presence of a Dzyaloshinskii-Moriya interaction (DMI) and previous research has shown that this is present in the bulk forms of B20 compounds, which naturally possess the necessary inversion symmetry breaking in their crystal structure. In research recently published in Physical Review B (editor's choice), an international team of researchers used polarised neutron reflectometry (PNR) on the POLREF beamline to investigate the preliminary helical magnetic phase in B20 thin films required for the development of electronic devices based on skyrmions and the Topological Hall Effect (THE).
The Hall Effect (now often called the ordinary Hall effect, OHE) describes the production of a voltage across an electrical conductor when a magnetic field is applied in a direction perpendicular to the flow of current. The underlying cause is the Lorentz force on the moving charge carriers causing them to move to either side of the conductor, depending on their polarity, hence the induced voltage. However in certain materials that have topological magnetic textures (such as skyrmions), the topological Hall effect (THE) is also present. This is due to the accumulation of a Berry phase as an electron spin tracks the spatially varying magnetic texture, which can be represented as a fictitious 'emergent' magnetic field. A skyrmion produces enough Berry phase to correspond to exactly one flux quantum of this field, and so the THE can be used for electrically detecting the presence of skyrmions in potential device applications.
The B20 compounds, which - in their bulk form - display promising skyrmion characteristics, have been the topic of much research. Here, an international team of researchers has used Molecular Beam Epitaxy (MBE) to grow thin films, necessary for devices, of the B20 Fe1−yCoyGe onto a silicon substrate. Their pioneering use of Polarised Neutron Reflectometry (PNR) on POLREF allowed them to study the underlying helical magnetic structure, by determining the magnetic depth profile of the film. They were able to measure the helix pitch of the spiral - essentially the distance in which it completes one 'twist' - which is intrinsically linked to the DMI and THE.
The researchers were able to observe the changes in the helix pitch and the Hall effects with increasing concentrations of cobalt, and demonstrated that it is possible to control the helix pitch a crucial step towards developing working devices. Their results provide a comprehensive data set for the magnetic and magnetotransport properties of Fe1−yCoyGe epitaxial B20 films. This is backed up by Density Functional Theory (DFT) calculations which provide a good description of how the magnetisation and DMI vary with composition, in agreement with the experimental results.
An intriguing feature of the results, which is not currently completely understood, is the large discrepancy between experimental and theoretical values of THE for intermediate concentrations of cobalt. This discrepancy remains unexplained and presents a challenge for the future that can be resolved by gaining an even more detailed knowledge of the spin texture in the skyrmion phase.
Spencer CS et al. Helical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 Fe1−yCoyGe films. Phys. Rev. B 97, 214406 (2018). DOI: 10.1103/PhysRevB.97.214406.