Theory of thermodynamic states and hysteresis of chiral helimagnets

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 加藤 雄介, 東京大学教授 前田 京剛, 東京大学教授 福島 孝治, 東京大学准教授 堀田 知佐, 東京大学准教授 小野瀬 佳文University of Tokyo(東京大学

Order and disorder in the magnetisation of the chiral crystal CrNb3S6

Competing magnetic anisotropies in chiral crystals with Dzyaloshinskii-Moriya exchange interactions can give rise to nontrivial chiral topological magnetization configurations with new and interesting properties. One such configuration is the magnetic soliton, where the moment continuously rotates about an axis. This magnetic system can be considered to be one dimensional and, because of this, it supports a macroscale coherent magnetization, giving rise to a tunable chiral soliton lattice (CSL) that is of potential use in a number of applications in nanomagnetism and spintronics. In this paper, we characterize the transitions between the forced-ferromagnetic (F-FM) phase and the CSL one in CrNb3S6 using differential phase contrast imaging in a scanning transmission electron microscope, conventional Fresnel imaging, ferromagnetic resonance spectroscopy, and mean-field modeling. We find that the formation and movement of dislocations mediate the formation of CSL and F-FM regions and that these strongly influence the highly hysteretic static and dynamic properties of the system. Sample size and morphology can be used to tailor the properties of the system and, with the application of magnetic field, to locate and stabilize normally unstable dislocations and modify their dimensions and magnetic configurations in ways beyond that predicted to occur in uniform films

Geometrical protection of topological magnetic solitons in microprocessed chiral magnets

A chiral soliton lattice stabilized in a monoaxial chiral magnet CrNb3S6 is a magnetic superlattice consisting of magnetic kinks with a ferromagnetic background. The magnetic kinks are considered to be topological magnetic solitons (TMSs). Changes in the TMS number yield discretized responses in magnetization and electrical conductivity, and this effect is more prominent in smaller crystals. We demonstrate that, in microprocessed CrNb3S6 crystals, TMSs are geometrically protected through element-selected micromagnetometry using soft x-ray magnetic circular dichroism (MCD). A series of x-ray MCD data is supported by mean-field and micromagnetic analyses. By designing the microcrystal geometry, TMS numbers can be successfully changed and fixed over a wide range of magnetic fields

Order and Disorder in the Magnetisation of the Chiral Crystal CrNb3S6

Source differential phase contrast data showing the chiral soliton lattice dislocation magnetic configuration at low and high applied fields