High-field magnetoresistance oscillations in α-[bis(ethylenedithio)tetrathiafulvalene]2KHg(SCN)4: The effects of magnetic breakdown, exchange interactions, and Fermi-surface reordering

High-field magnetoresistance measurements have been performed on single-crystal samples of the charge-transfer salt \ensuremath{\alpha}-${\mathrm{ET}}_{2}$KHg (SCN${)}_{4}$ [where ET is bis(ethylenedithio)tetrathiafulvalene] using fields of up to 30 T. We examine the character of the Shubnikov--de Haas oscillations for a range of samples, temperatures, and field regimes to understand the role played by spin splitting and magnetic breakdown effects. Angle-dependent magnetoresistance oscillations (AMRO) in magnetic fields below the kink transition (22 T) indicate that the low-magnetic-field band structure is dominated by a one-dimensional (1D) open section of Fermi surface (FS) inclined at \ensuremath{\sim}21\ifmmode^\circ\else\textdegree\fi{} to the crystallographic ${\mathit{b}}^{\mathrm{*}}$c plane; above the kink, the AMRO change in character, indicating that the FS now possesses a 2D closed section in the form of a distorted cylinder. The data are discussed in terms of a recent model of the low-field band structure and give definitive proof that the kink is the signature of a magnetic-field-induced change in the FS.