Effects of Transition Metal Substitutions on the Incommensurability and Spin Fluctuations in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ by Elastic and Inelastic Neutron Scattering

The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ are compared quantitatively by inelastic neutron scattering measurements and are found to be indistinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensurate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a significant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior.

Figure 1

Inelastic neutron scattering from the Co and Cu substituted samples. (a) Normalized dynamic magnetic susceptibility at 25 K determined from constant-$\mathbf{Q}$ $E$-scans at the (1 0 1) magnetic Bragg point. (b–i) $\mathbf{Q}$-scans at several fixed values of the energy loss. The feature at (1.3 0 1) results from spurious scattering not related to spin excitations.

Figure 2

Scattering near the (1 0 3) magnetic Bragg point for $\mathrm{Ba}({\mathrm{Fe}}_{1\mathrm{\text{−}}x}{M}_x{)}_2{\mathrm{As}}_2$ where $M$ is (a) Ni and (b) Cu. (c) Temperature dependence of the scattering near the (1 0 3) magnetic Bragg point for $\mathrm{Ba}({\mathrm{Fe}}_{0.963}{\mathrm{Ni}}_{0.037}{)}_2{\mathrm{As}}_2$. Intensities are normalized by mass of the samples to facilitate comparisons. Lines are fits to the data, as described in the text.

Figure 3

Trends in the FWHM and maximum ordered moment for $M$ substitution in $\mathrm{Ba}(\mathrm{F}{\mathrm{e}}_{1-x}{\mathrm{M}}_x{)}_2{\mathrm{As}}_2$. (a) Evolution of the FWHM of the magnetic peaks vs. concentration. The solid (open) circles represent the FWHM of the C-AFM (IC-AFM) peaks. (b) Measured ordered moment derived from the integrated intensity of the magnetic Bragg peaks as a function of the extra electron count, assuming that Co donates 1, Ni 2, and Cu 3, extra electrons to the $d$-band. The data for $\mathrm{Ba}({\mathrm{Fe}}_{1\mathrm{\text{−}}x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ are taken from Refs. 17, 26.

Figure 4

For $\mathrm{Ba}({\mathrm{Fe}}_{1\mathrm{\text{−}}x}{M}_x{)}_2{\mathrm{As}}_2$, the KKR-CPA (a) site-projected DOS versus $E\mathrm{\text{−}}{E}_F$ at 6% Co, 3% Ni (fixed $e^{-}/\mathrm{Fe}$), and 2% Cu (the Fe DOS changes negligibly with $M$); and Bloch spectral functions, $A(\mathbf{k};E_F)$, along specific $\mathbf{k}$-directions versus at. % $M$ for (b) electrons, and (c) holes. Insets: $\mathbf{k}$-direction of the cut across electron (centered at $X$) and hole (centered at $Z$) states. Peak locations of electron/hole states are compared to the “rigid-band” expectations (vertical dashed lines) from parent compound at fixed $e^{-}/\mathrm{Fe}$ and three at. % Cu values.