Magnetic field effect on static antiferromagnetic order and spin excitations in the underdoped iron arsenide superconductor BaFe${}_{1.92}$Ni${}_{0.08}$As${}_2$

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 BaFe${}_{1.92}$Ni${}_{0.08}$As${}_2$. 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 $~$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 BaFe${}_{1.92}$Ni${}_{0.08}$As${}_2$.

Figure 1

(a) The antiferromagnetic spin structure of the undoped parent compound BaFe${}_2$As${}_2$ and the direction of applied field. (b) The reciprocal space probed in the present experiment and the direction of applied field. (c) Temperature dependence of the AF Bragg peak at $(0.5,0.5,3)$ at zero field and at the 10-T in-plane field. The data were taken on BT-7 and showed $T_N=44$ K. The background scattering has no temperature or field dependence. (d) Temperature dependence of the Meissner and shielding signals on thin slabs of BaFe${}_{1.92}$Ni${}_{0.08}$As${}_2$. These measurements were taken in zero-field cooling (ZFC) with a 5-Oe applied field along the thin slab’s direction. (e) The field-on subtract field-off rocking-curve scan through the $(0.5,0.5,3)$ AF Bragg peak at 6 K. The positive scattering centered at the correct $\theta$ angle indicates that the field-induced effect occurs at the $(0.5,0.5,3)$ AF Bragg position. (f) Identical rocking-curve scans at 20 K, clearly indicating that the applied field has no observable effect on the static AF order below $T_N$ and above $T_c$. (g) Black squares indicate a high-resolution scan along the $[H,H,3]$ direction in the AF-ordered state. The red circles show the identical scan above $T_N$ without the cold Be filter, which gives $\lambda /2$ scattering from the lattice structural Bragg peak $(1,1,6)$. (h) Similar scans along the $[0.5,0.5,L]$ direction.

Figure 2

Temperature dependence of the imaginary part of the dynamic susceptibility, ${\chi }^{″}(Q,\omega )$, after subtracting the background scattering and correcting for the Bose population factor. (a) $Q$-scans at $E=1.5$ meV along the $[0.5,0.5,L]$ direction above and below $T_c$. The data show clear $L$-direction sinusoidal modulation. (b) Similar scans at an energy just below the resonance ($E=3$ meV). (c) $Q$-scans at the resonance energy of $E=6$ meV. The intensity gain below $T_c$ is clearly not uniform at different $L$ values.

Figure 3

Temperature dependence of ${\chi }^{″}(Q,\omega )$ along the $[H,H,0]$ and $[H,H,1]$ directions. (a) Constant-energy scans at $E=1.5$ meV along the $[H,H,0]$ direction across $T_c$. A clean spin gap is seen at this energy. (b) Identical scans along the $[H,H,1]$ direction, which show clear magnetic scattering centered at $Q=(0.5,0.5,1)$ in the superconducting state. (c) Constant-energy scans at $E=3$ meV show no change in ${\chi }^{″}$ below and above $T_c$ at $Q=(0.5,0.5,0)$. (d) The scattering clearly increases at $Q=(0.5,0.5,1)$ below $T_c$. (e) Constant-energy scans at $E=6$ meV. The peak at $Q=(0.3,0.3,0)$ is spurious. (f) Similar scans at $E=6$ meV across $Q=(0.5,0.5,1)$. Superconductivity clearly induces more magnetic scattering at $Q=(0.5,0.5,1)$ than at $Q=(0.5,0.5,0)$.

Figure 4

Energy dependence of the dynamic susceptibility ${\chi }^{″}(\omega )$ above and below $T_c$ for wave vectors $Q=(0.5,0.5,L)$ with $L=0,0.4,1,1.4,2$. In the normal state, ${\chi }^{″}(\omega )$ increases with increasing energy at all $L$ values. On entering into the superconducting state, the neutron spin resonance develops and exhibits dispersion, occuring at different energies for different $L$ values as marked by the vertical arrows.

Figure 5

Energy and wave-vector dependence of the spin excitations as a function of an applied magnetic field in the FeAs plane. (a) Constant-$Q$ scans at $Q=(-0.5,-0.5,1)$ below and above $T_c$ at zero field and at the 11-T in-plane field. Inspection of the raw data clearly reveals the reduction of the resonance intensity under the field at $T=4.5$ K. (b) The field-off–field-on difference plot at $T=4.5$ K shows that the magnetic scattering near the resonance energy is affected most by the applied field. (c) Constant-energy scans at the resonance energy for the zero and the 11-T fields. The field-induced reduction in magnetic scattering occurs at the AF wave vector $Q=(-0.5,-0.5,1)$.

Figure 6

Temperature dependence of the resonance and elastic scattering at zero field and at the 11-T in-plane field. (a) Temperature dependence of the $E=6$ meV scattering in zero field at $Q=(0.5,0.5,1)$ for BaFe${}_{1.92}$${\mathrm{Ni}}_{0.08}$${\mathrm{As}}_2$. The scattering increases in intensity below the $T_c$ of 17 K. (b) Identical temperature dependence of the $E=6$ meV scattering under the 11-T field. The field-induced $T_c$ has now shifted to 15 K. (c) Expanded plot of the elastic magnetic scattering in zero field and in the 11-T field. The data confirm the shift in $T_c$ with a nonzero field.