  The efficiency of ultrafast excitation of spins in antiferromagnetic
$\mathrm{\alpha-Fe_{2}O_{3}}$ using nearly single-cycle THz pulse is studied as
a function of the polarization of the THz pulse and the sample temperature.
Above the Morin point the most efficient excitation is achieved when the
magnetic field of the THz pulse is perpendicular to the antiferromagnetically
coupled spins. Using the experimental results and equations of motion for
spins, we show that the mechanism of the spin excitation above and below the
Morin point relies on magnetic-dipole interaction of the THz magnetic field
with spins and the efficiency of the coupling is proportional to the time
derivative of the magnetic field.
