Density functional determination of the magnetic state of $\beta$-MnAs

Ferromagnetic hexagonal $\alpha$-MnAs has a first order phase transition to $\beta$-MnAs at about 40 $^{\circ}$ C. Because it shows no long range antiferromagnetic order, most workers assume that it is paramagnetic in spite of the fact that it does not have a Curie-Weiss magnetic susceptibility. With the aid of density functional calculations, we show that it is antiferromagnetic and explain the lack of observed long range order.