Evolution of bulk superconductivity in SrFe₂As₂ with Ni substitution

Single crystals of the Ni-doped FeAs-based superconductor SrFe_2−xNi_xAs₂ were grown using a self-flux solution method and characterized via x-ray measurements and low-temperature transport, magnetization, and specific heat studies. A doping phase diagram has been established where the antiferromagnetic order associated with the magnetostructural transition of the parent compound SrFe₂As₂ is gradually suppressed with increasing Ni concentration, giving way to bulk-phase superconductivity with a maximum transition temperature of 9.8 K. The superconducting phase exists through a finite range of Ni concentrations centered at x=0.15 with full diamagnetic screening observed over a narrow range of x coinciding with a sharpening of the superconducting transition and an absence of magnetic order. An enhancement of bulk superconducting transition temperatures of up to 20% was found to occur upon high-temperature annealing of samples.