  The magnetic properties of black phosphorene nanoribbons are investigated
using static and dynamical mean-field theory. Besides confirming the existence
of ferromagnetic/antiferromagnetic edge magnetism, our detailed calculations
using large unit-cells find a phase-transition at weak interaction strength to
an incommensurate (IC) magnetic phase. A detailed Fourier analysis of the
magnetization patterns in the IC phase shows the existence of a second critical
interaction strength, where the incommensurate phase changes to an
antiferromagnetic (AFM) or ferromagnetic (FM) phase. We demonstrate that the
difference of the ground state energies of the AFM and FM phase is
exponentially small, making it possible to switch between both states by a
small external field. Finally, we analyze the influence of strain and disorder
on the magnetic properties and show that while the IC phase is robust to
Anderson type disorder, it is fragile against strain.

Strongly Correlated Electrons