Layered Quasi-two-dimensionalMagnetism
Study of the Magnetic Phase Transition
Time: Wed 2026-11-25 09.00
Location: Pärlan, Albano Hus 1, Albanovägen 26
Language: English
Doctoral student: Yuqing Ge , Ljus och materiens fysik
Opponent: Catherine Pappas, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629JB Delft, The Netherlands
Supervisor: Martin Månsson, Ljus och materiens fysik; Yasmine Sassa, Ljus och materiens fysik
QC 2026-11-02
Abstract
This thesis investigates phase tuning in several quasi-two-dimensional magnetic systems using neutron diffraction, 𝜇+SR, and other bulk magnetic measurements.
The motivations and the basic background of magnetic interactions are given in Chapter 1. A background and existing knowledge of these systems are presented in Chapter 2. The techniques, experimental methods, and mathematical tools are explained in Chapter 3. Main experimental results and interpretations are presented and discussed in Chapter 4.
Two van der Waals magnets and one magnetic multilayer thin-film system are studied in their ground states and under tuning parameters, including pressure and depth within the film. At ambient pressure, CrCl3 is shown to have a distinct 2D spin-spin correlation below the transition temperature. Under pressure up to 23 kbar, pressure suppresses both its transition temperature and its ambient-pressure magnetic phase, while inducing a new magnetic ground state. Measurements of VI3 at ambient pressure confirm the presence of multiple magnetic phases and weak spin canting.
Persistent spin fluctuations have been measured in SmI3 down to a temperature of 40 mK at ambient pressure. A moderate pressure below 20 kbar changes the nuclear structure, resulting in quasi-static short-range magnetic order. This provides a solid example of the effect of hydrostatic and chemical pressure on the crystal field. High-pressure 𝜇+SR measurements of the Shastry-Sutherland compound SrCu2(BO3)2 confirm the absence of long-range magnetic order up to 24 kbar and indicate changes in the Cu spin fluctuations above 21 kbar, consistent with the formation of a plaquette phase. Topological skyrmion tubes in CoFeB/Ru/Pt are investigated using low-energy 𝜇+SR to study potentially diffusive or mobile skyrmion dynamics. Further depth-dependent experiments probe the variation across the thin film of the skyrmion tubes and stripe tubes resulting from the long-range dipole field.
Finally, conclusions and outlook are discussed in Chapter 5.