  Manganese monophosphide (MnP) shows complicated magnetic states varying with
both temperature and pressure. We calculate the magnetic and electronic
structures of MnP at different pressures using first-principles methods and
obtain spiral ground states whose propagation vector changes from the c-axis at
low pressure to the b-axis at high pressure. In between, we find a
ferromagnetic state, as observed in the experimental phase diagram. The
propagation vector of the spiral states is found to vary nonmonotonically with
pressure, consistent with neutron measurements. Our results indicate that the
complicated magnetic phase diagram originates from a delicate competition
between neighboring exchange interactions between the Mn-ions. At all
pressures, the electronic structures indicate the existence of
quasi-one-dimensional charge carriers, which appear in the ferromagnetic state
and become gapped in the spiral state, and anisotropic three-dimensional charge
carriers. We argue that this two-fluid behavior originates from the special
crystal structure of MnP and may be relevant for understanding the pairing
mechanism of the superconductivity at the border of the high pressure spiral
phase.
