Magnetic field effect on static antiferromagnetic order and spin excitations in the underdoped iron arsenide superconductor BaFe1.92Ni0.08As2

We use neutron scattering to study the magnetic-field effect on the static antiferromagnetic (AF) order and low-energy spin excitations in the underdoped iron arsenide superconductor BaFe1.92Ni0.08As2. At zero field, superconductivity that occurs below a critical temperature of T-c = 17 K coincides with the appearance of a neutron spin resonance and reduction in the static ordered moment. Upon application of a similar to 10-T magnetic field in the FeAs plane, the intensity of the resonance is reduced, accompanied by decreasing T-c and enhanced static AF scattering. These results are similar to those for some copper oxide superconductors, and demonstrate that the static AF order is a competing phase to superconductivity in BaFe1.92Ni0.08As2.