For quantum electronics, the possibility to finely tune the properties of
magnetic topological insulators (TIs) is a key issue. We studied solid
solutions between two isostructural Z2β TIs, magnetic MnBi2βTe4β and
nonmagnetic GeBi2βTe4β, with Z2β invariants of 1;000 and 1;001,
respectively. For high-quality, large mixed crystals of
GexβMn1βxβBi2βTe4β, we observed linear x-dependent magnetic
properties, composition-independent pairwise exchange interactions along with
an easy magnetization axis. The bulk band gap gradually decreases to zero for
x from 0 to 0.4, before reopening for x>0.6, evidencing topological phase
transitions (TPTs) between topologically nontrivial phases and the semimetal
state. The TPTs are driven purely by the variation of orbital contributions. By
tracing the x-dependent 6p contribution to the states near the fundamental
gap, the effective spin-orbit coupling variation is extracted. As x varies,
the maximum of this contribution switches from the valence to the conduction
band, thereby driving two TPTs. The gapless state observed at x=0.42 closely
resembles a Dirac semimetal above the Neel temperature and shows a magnetic gap
below, which is clearly visible in raw photoemission data. The observed
behavior of the GexβMn1βxβBi2βTe4β system thereby demonstrates an
ability to precisely control topological and magnetic properties of TIs