  To elucidate the physics underling magnetism observed in nominally
nonmagnetic materials with only $sp$-electrons, we built an extreme model to
simulate H-adsorption (in a straight-line form) on graphene. Our first
principles calculations for the model produce a ferromagnetic ground state with
a magnetic moment of one Bohr magneton per H atom and an estimated Curie
temperature above 250~K. The removal of the $p_z$-orbitals from sublattice B of
graphene introduces $p_z$-vacancies. The $p_z$-vacancy-induced states are not
created from changes in interatomic interactions but are created because of a
$p_z$-orbital imbalance between two sublattices (A and B) of a conjugated
$p_z$-orbital network. Therefore, there are critical requirements for the
creation of these states (denoted as $p_z^{\rm imbalance}$) to avoid further
imbalances and minimize the effects on the conjugated $p_z$-orbital network.
The requirements on the creation of $p_z^{\rm imbalance}$ are as follows: 1)
$p_z^{\rm imbalance}$ consists of $p_z$-orbitals of only the atoms in
sublattice A, 2) the spatial wavefunction of $p_z^{\rm imbalance}$ is
antisymmetric, and 3) in principle, $p_z^{\rm imbalance}$ extends over the
entire crystal without decaying, unless other $p_z$-vacancies are crossed. Both
the origin of spin polarization and the magnetic ordering of the model arise
from the aforementioned requirements.
