New Masters Student Starts
This month, Aurèle Kamber started work in our group as a Masters student working with Dr. Floris Braakman. Aurèle received his B.S. in Nanoscience with focus in Physics in 2019 from the University of Basel. Welcome!
New Post-doc Joins Group
On the 15th, Dr. Francesco Fogliano started work in our group as a post-doctoral researcher. Francesco earned his Bachelor of Science in Physics in 2012 and his Masters of Science in Physics in 2015, both from the University of Pisa. In 2019, he received his Ph.D. in Physics from the Néel Institute in Grenoble. There he worked on ultrasensitive nanowire force sensors at very low temperatures under the supervision of Dr. Olivier Arcizet, Dr. Benjamin Pigeau, and Dr. Jean-Philippe Poizat. Welcome Francesco!
Paper published on Stability of Néel-type Skyrmions
On the 3rd, Phys. Rev. B published our paper entitled, Stability of Néel-type skyrmion lattice against oblique magnetic fields in GaV4S8 and GaV4Se8.
The discovery of magnetic skyrmions has spurred renewed interest in non-centrosymmetric magnets. Their topologically protected spin-texture, which can be stable even at room temperature, their nanometer-scale size, and their easy manipulation via electric currents and fields make skyrmions a promising platform for information storage and processing applications.
Until recently, most investigations in bulk crystals have focused on Bloch-type skyrmions, in which the local magnetization rotates perpendicular to the skyrmion’s radial direction. Recently, however, Néel-type skyrmions, in which the local magnetization rotates parallel to its radial direction, have been observed in bulk GaV4S8, GaV4Se8, and GaMo4S8. These materials, which crystallize in the cubic lacunar spinel structure, are multiferroic and could enable nearly dissipation free manipulation of the skyrmions magnetic order by electric fields. In addition to having different spin textures and phase diagrams, the lack of a competing conical phase makes Néel-type skyrmions more robust than their Bloch-type counterparts.
The paper presents experiments, in which we use dynamic cantilever magnetometry (DCM) to map the magnetic phase boundaries in GaV4S8 and GaV4Se8 as a function of the strength and orientation of magnetic field. We determine the corresponding phase diagrams, which reproduce the major features predicted by a recent theoretical model. The measurements constitute a direct experimental confirmation of the robustness of Néel-type skyrmions to oblique magnetic fields in two materials with uniaxial magnetic anisotropy of opposite signs. In addition to magnetic transitions between the cycloidal, skyrmion lattice, and field-polarized ferromagnetic states, in GaV4Se8, we also observe sharp anomalies in the DCM, which we assign to field-driven transformations of magnetic states confined to polar domain walls (DWs).
Our work represents a new and original application of DCM to the study of magnetic skyrmions. It serves both as a template for future studies of skyrmion-hosting materials and provides new information about lacunar spinels and Néel-type skyrmions. In particular, the measured magnetic phase diagrams for GaV4S8 and GaV4Se8 are in good qualitative agreement with the theoretical predictions and provide insight for how to improve these initial models. Furthermore, the DCM measurements yield new evidence for distinct magnetic states confined to polar structural DWs and their transition from the Cyc to FM state.
The project represents a tight collaboration between our group, the group of István Kézsmárki from the University of Augsburg, Sándor Bordács from Budapest Univeristy, and Andrey Leonov from Hiroshima University. Our group contributed by making sensitive magnetic torque measurements at low temperature as a function of magnetic field magnitude and direction. Combined with information gained by the other experimental techniques, we were able to construct a complete picture of the sample’s magnetic states. Former Ph.D. student Dr. Andrea Mehlin did much of the early experiments and analysis along with Dr. Boris Groß. Ph.D. student Simon Philipp contributed to experiments in the later part of the project. In the final months of the project, Boris worked extensively on the data analysis and on creating a model with our collaborators that was consistent with all of the measurements.