| Magnetic skyrmions,nanometric vortex-like spin textures,are interesting for both their fundamental physics and their potential applications.Unlike the conventional spin shapes,such as helical or conical,that are usually founding chiral magnets,a magnetic skyrmion has a particle-like swirling-spin configuration characterized by a topological index called the skyrmion number.The nontrivial topology of magnetic skyrmions results in a number of intriguing electromagnetic phenomena,including the topological Hall effect,skyrmion magnetic resonance and thermally induced ratchet motion.These topologically protected properties,together with their nanoscale dimensions(10 ~100nm)and ultralow threshold for current-driven motion(105~106Am-2),suggest that skyrmions are promising materials for technological applications in spintronic devices.However,to date,studies of skyrmions have largely been limited to a few rare,cubic,non-centrosymmetric chiral materials in which magnetic skyrmions can survive only at very low temperatures and in a very narrow temperature range.In Chapter 3 of this thesis,we report the discovery of the non-cubic and non-collinear magnets MnNiGa by the consideration of chemical bonding and electronic structure.Magnetic skyrmions are the first time observed in a centrosymmetric hexagonal MnNiGa with a board temperature range which can also survive beyond the room temperature.In addition,the biskyrmion states in MnNiGa thin plates,as determined by the combination of in situ Lorentz transmission electron microscopy images,magnetoresistivity,and topological Hall effect measurements,are surprisingly stable over a broad temperature range of 100-340 KFurthermore,in Chapter 4,we report a systematic study of the magnetic and transport properties of magnetron sputtered MnNiGa films on the different temperature thermally oxidized Si substrates.A large topological Hall effect(THE)is observed in a very wide temperature range of 10K-300 K and a magnetic field region where the field-dependent Hall resistivity largely deviates from the magnetization.Interestingly,the measurements on the cubic MnNiGa films do not show any evidence of the existence of THE.These results indicate that the non-collinear spin configurations from geometrical frustration of Mn moments are the likely origin of the observed large THE in hexagonal MnNiGa films. |
