Electronic origin of half-metal to semiconductor transition and colossal magnetoresistance in spinel <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Hg</mml:mi><mml:msub><mml:mi>Cr</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Se</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>

Half-metals are ferromagnets hosting spin-polarized conducting carriers and crucial for spintronics applications. The chromium spinel HgCr2Se4 represents a unique type of half-metal, which features half-metal to semiconductor transition (HMST) exhibits colossal magnetoresistance (CMR) across the ferromagnetic-paramagnetic (FM-PM) transition. Using angle-resolved photoemission spectroscopy (ARPES), we find that Fermi surface n-type (n-HgCr2Se4) consists single electron pocket moves above level (EF) upon FM-PM transition, leading HMST. Such Lifshitz manifests giant band splitting originates from exchange interaction unveiled with specific chemical nonstoichiometry. nonstoichiometry two key ingredients HMST CMR, consistent our ab-initio calculation. Our findings provide spectroscopic evidences electronic origin anomalous properties HgCr2Se4, address phase in half-metals.