Cu Substitution Effects on the Local Magnetic Properties of ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x)}_2{\mathrm{As}}_2$: A Site-Selective ${}^{75}\mathrm{As}$ and ${}^{63}\mathrm{Cu}$ NMR Study

We take advantage of the site-selective nature of the ${}^{75}\mathrm{As}$ and ${}^{63}\mathrm{Cu}$ NMR techniques to probe the Cu substitution effects on the local magnetic properties of the FeAs planes in ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x)}_2{\mathrm{As}}_2$. We show that the suppression of antiferromagnetic Fe spin fluctuations induced by Cu substitution is weaker than a naive expectation based on a simple rigid band picture, in which each Cu atom would donate three electrons to the FeAs planes. Comparison between ${}^{63}\mathrm{Cu}$ and ${}^{75}\mathrm{As}$ NMR data indicates that spin fluctuations are suppressed at the Cu and their neighboring Fe sites in the tetragonal phase, suggesting the strongly local nature of the Cu substitution effects. We attribute the absence of a large superconducting dome in the phase diagram of ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x)}_2{\mathrm{As}}_2$ to the emergence of a nearly magnetically ordered FeAs plane under the presence of orthorhombic distortion.

Figure 1

The phase diagram of ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x)}_2{\mathrm{As}}_2$ for the structural ($T_s$) and spin density wave ($T_{\mathrm{SDW}}$) transitions reported by N. Ni et al. [8], together with the results for our crystals with $x=0.023$ and $x=0.04$. Also shown is the onset temperature of the NMR signal intensity wipeout (cross symbol) as determined from Fig. 3; notice the clear correlation with $T_s$.

Figure 2

(a) Fourier-transformed ${}^{75}\mathrm{As}$ NMR spectra of the center lines for ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x)}_2{\mathrm{As}}_2$ at 250 K with the external magnetic field $H=7.8076\mathrm{T}$ applied along the $c$ axis. The inset shows the satellite lines. (b) A schematic picture of the environment around the ${\mathrm{As}}_{\text{host}}$ (cross symbol), ${\mathrm{As}}_{\mathrm{NNN}}$ (square), and ${\mathrm{As}}_{\mathrm{NN}}$ (upper triangle), whose nearest neighbor Fe sites are substituted by Cu sites.

Figure 3

(a) ${}^{75}K$, (b) FWHM for the center line, and (c) the integrated NMR intensity $I$ (normalized by the Boltzmann factor) for ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x)}_2{\mathrm{As}}_2$.

Figure 4

${}^{75}(1/T_{1}T)$ observed for ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x)}_2{\mathrm{As}}_2$ and ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x)}_2{\mathrm{As}}_2$ (from [13, 16]).

Figure 5

X-ray diffraction reciprocal space maps within the $H–K$ plane near (1,1,6) observed for ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x)}_2{\mathrm{As}}_2$ with $x=0.04$. The splitting of the intensity in the $H$ direction below 50 K indicates the tetragonal-to-orthorhombic phase transition near $T_s=47\pm 2\mathrm{K}$.

Figure 6

(a) Temperature dependence of ${}^{63}\mathrm{Cu}$ NMR spectra observed for ${\mathrm{Ba}({\mathrm{Fe}}_{0.96}{\mathrm{Cu}}_{0.04})}_2{\mathrm{As}}_2$ in $H=8.9976\mathrm{T}$ applied along the $c$ axis. The signal intensity is normalized for the Boltzmann factor. (b) ${}^{63}K$ and ${}^{75}K$. (c) ${}^{63}(1/T_{1}T)$ and ${}^{75}(1/T_{1}T)$. ${}^{75}(1/T_{1}T)$ at the ${\mathrm{As}}_{\text{host}}$ site was measured using the center peak. ${}^{75}(1/T_{1}T)$ at the ${\mathrm{As}}_{\mathrm{NNN}}$ was measured using the satellite peak to avoid contamination by the strong signal from the ${\mathrm{As}}_{\text{host}}$ site.