Condensed Spin Excitation of Quantized Dirac Fermions in the Quasi-Two-Dimensional Semimetal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>BaMnBi</mml:mi> </mml:mrow> <mml:mrow> <m

Dirac semimetals provide a new platform for the quantum Hall effect at low magnetic fields. In the presence of strong spin-orbit coupling, a spin-split Landau level is expected to enhance the bulk quasiparticle excitation. Here we report NMR spectroscopy that site selectively probes dynamic spin susceptibility on the magnetic semimetal BaMnBi_{2}. We find that spontaneous staggered fields from antiferromagnetic Mn moments are completely canceled at the Bi layer hosting Dirac fermions. The nuclear spin-lattice relaxation rate 1/T_{1} follows the cubic temperature dependence down to low temperatures under the in-plane field, manifesting the chemical potential close to the Dirac point. 1/T_{1} becomes constant below 20 K under the out-of-plane field, where the well-separated Laudau level appears. The strong anisotropy of 1/T_{1} exceeding 100 suggests spin-split Landau levels in the quantum Hall regime.