  Graphene, due to its exceptional properties, is a promising material for
nanotechnology applications. In this context, the ability to tune the
properties of graphene-based materials and devices with the incorporation of
defects and impurities can be of extraordinary importance. Here we investigate
the effect of uniaxial tensile strain on the electronic and magnetic properties
of graphene doped with substitutional Ni impurities (Ni_sub). We have found
that, although Ni_sub defects are non-magnetic in the relaxed layer, uniaxial
strain induces a spin moment in the system. The spin moment increases with the
applied strain up to values of 0.3-0.4 \mu_B per Ni_sub, until a critical
strain of ~6.5% is reached. At this point, a sharp transition to a high-spin
state (~1.9 \mu_B) is observed. This magnetoelastic effect could be utilized to
design strain-tunable spin devices based on Ni-doped graphene.
