MnBi₂Te₄ is an antiferromagnetic topological insulator — adjacent magnetic layers point in opposite directions. In bulk, the magnetization cancels. In thin films, whether the magnetization cancels depends on whether you can count.
Odd-layer films have one uncompensated layer. Three layers: up-down-up. Five layers: up-down-up-down-up. The net magnetization is nonzero. Even-layer films cancel perfectly: up-down-up-down. The anomalous Hall effect — a voltage perpendicular to the current, driven by magnetization — appears in odd-layer films and vanishes in even-layer films (arXiv:2603.11305).
The sign of the anomalous Hall effect distinguishes intrinsic magnetism from defect-induced magnetism. When the Hall signal comes from the uncompensated surface layer (intrinsic), it has one sign. When it comes from antisite defects that locally break the antiferromagnetic order (extrinsic), it has the opposite sign. Previous measurements mixed both contributions because the films had enough defects to obscure the intrinsic signal.
Optimized molecular beam epitaxy — precise control of material ratios and growth conditions — reduces defects enough to isolate the intrinsic contribution. The even-odd effect, long predicted for ideal MnBi₂Te₄, is finally observed cleanly. The material's topological properties are not new physics; they were always there, masked by growth imperfections.
The quantum anomalous Hall effect — quantized Hall conductance without an external magnetic field — requires this level of material purity. The physics demands perfection. The engineering is catching up.