Magnetic Breakdown and the Spin-Split Conduction Band in HgSe andHg1−xMnxSe

New measurements of the anomalous magnetic field dependence in the amplitude of the oscillatory magnetoresistance (Shubnikov-de Haas effect) of $n$-type HgSe and ${\mathrm{Hg}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}$ are presented. These data are interpreted with use of a closed-orbit magnetic breakdown model which permits Fermi-surface parameters of interest to be determined. It is found that the addition of 0.1-at.% Mn to HgSe results in a Fermi surface whose shape is temperature dependent.