  We report on lasing at visible wavelengths in arrays of ferromagnetic Ni
nanodisks overlaid with an organic gain medium. We demonstrate that by placing
an organic gain material within the mode volume of the plasmonic nanoparticles
both the radiative and, in particular, the high ohmic losses of Ni nanodisk
resonances can be compensated. Under increasing pump fluence, the systems
exhibit a transition from lattice-modified spontaneous emission to lasing, the
latter being characterized by highly directional and sub-nanometer linewidth
emission. By breaking the symmetry of the array, we observe tunable multimode
lasing at two wavelengths corresponding to the particle periodicity along the
two principal directions of the lattice. Our results pave the way for
loss-compensated magnetoplasmonic devices and topological photonics.
