Novel physical properties of non-trivial magnetic materials: Cr1/3NbS2, MnSi and Fe3GeTe2

Helimagnets and quasi-two-dimensional layered materials have attracted much recent interest due to their rich physical properties and potential for spintronics applications. In this dissertation, the helimagnets Cr_1/3NbS₂ [chromium one-third niobium disulfide] and thin-film MnSi [manganese silicide] as well as the layered magnetic material Fe₃GeTe₂ [iron three germanium ditelluride] were extensively and carefully studied. Cr_1/3NbS₂ was previously reported to be helimagnet with a transition temperature of approximately 120 K. Cr_1/3NbS₂ has a layered structure with Cr³⁺ ions intercalating between NbS­₂ sheets, and is easily cleaved. Using in-situ STM [scanning tunneling microscopy] to study the cleaved Cr_1/3NbS₂ single crystal sample, we observed a surface consisting of two different terminations with different electronic states that are tunable under certain applied condition. More excitingly, we discovered localized quantum states on Cr_1/3NbS₂ surface. Both cleaved thin layers of Cr_1/3NbS₂and MnSi thin films were found to have an enlarged region of skyrmion phase; in bulk, MnSi is a helimagnet with a small region of skyrmion phase just below the transition temperature of about 30 K. To study the helimagnetic behavior under confinement, we successfully grew MnSi thin films of thickness less than MnSi helix pitch (48nm) by MBE [molecular beam epitaxy] method under ultra-high vacuum. The magnetic phase diagram of our MnSi thin films was constructed. Interesting topography and surface electronic states were also observed using STM on ultra-thin films of MnSi; Fe₃GeTe₂ was recently discovered to be a layered itinerant ferromagnetic material with a transition temperature of about 220 K. From magnetic measurements as well as theoretical calculations, we determined that the ground state is in fact antiferromagnetically ordered below 152 K. A magnetic structure model describing the evolution of the ferromagnetic and antiferromagnetic ordering states for Fe₃GeTe₂was proposed.