  We predict a two-dimensional (2D) antiferromagnetic (AFM) boron (designated
as M-boron) by using ab initio evolutionary methodology. M-boron is entirely
composed of B20 clusters in a hexagonal arrangement. Most strikingly, the
highest valence band of M-boron is isolated, strongly localized, and quite
flat, which induces spin polarization on each cap of the B20 cluster. This flat
band originates from the unpaired electrons of the capping atoms, and is
responsible for magnetism. M-boron is thermodynamically metastable and is the
first cluster-based 2D magnetic material in the elemental boron system.
