NMR investigation of spin correlations in ${\mathrm{BaCo}}_2{\mathrm{As}}_2$

We use NMR techniques to investigate the magnetic properties of ${\mathrm{BaCo}}_2{\mathrm{As}}_2$ single crystals, the nonsuperconducting end member of the Co-substituted iron-pnictide high-$T_c$ superconductor $\mathrm{Ba}\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right){}_2{\mathrm{As}}_2$ with $x=1$. We present ${}^{75}\mathrm{As}$ NMR evidence for enhancement of low frequency spin fluctuations below $\sim 100$ K. This enhancement is accompanied by that of static uniform spin susceptibility at the wave vector $\mathbf{q}$ = $\mathbf{0}$, suggesting that the primary channel of the spin correlations is ferromagnetic rather than antiferromagnetic. Comparison between the NMR Knight shift ${}^{75}K$ and bulk susceptibility ${\chi }_{\mathrm{bulk}}$ data uncovers the presence of two separate components of spin susceptibility with distinct temperature dependencies, presumably because multiple electronic bands crossing the Fermi energy play different roles in the electronic properties of ${\mathrm{BaCo}}_2{\mathrm{As}}_2$.

Figure 1

Representative ${}^{75}\mathrm{As}$ NMR line shapes observed in a magnetic field $B=7.734$ T applied along the aligned $c$ axis of single crystals. A downward arrow marks the central transition at 290 K, while dashed arrows mark the satellite transitions split from the central transition by ${|}^{75}{\nu }_Q^c|\sim 0.55$ MHz. For clarity, we shifted the origin of the horizontal axis for different temperatures. The vertical dotted line represents the bare Zeeman frequency ${}^{75}{f}_o=56.398$ MHz, where we expect to observe the central peak if the Knight shift ${}^{75}K$ was vanishingly small. Inset: The temperature dependence of the $c$-axis component of the ${}^{75}\mathrm{As}$ nuclear quadrupole frequency ${}^{75}{\nu }_Q^c$. For clarity, we assume ${}^{75}{\nu }_Q^c>0$ at 4.2 K. The solid curve is a guide for the eyes.

Figure 2

(a) Left axis: Comparison of the ${}^{75}\mathrm{As}$ NMR Knight shift ${}^{75}K$ observed for ${\mathrm{BaCo}}_2{\mathrm{As}}_2)•$) and $\mathrm{Ba}\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right){}_2{\mathrm{As}}_2$ for representative compositions: ($\square$) the SDW phase $x=0$ with $T_{SDW}=135$ K, ($\circ$) the superconducting phase $x=0.08$ with $T_c=22$ K, and ($⧫$) the overdoped nonsuperconducting metallic phase with $x=0.26$. Vertical arrows mark the superconducting transition temperature $T_c$ for $x=0.08$ and SDW transition temperature $T_{SDW}$ for $x=0$. Right axis: The bulk susceptibility ${\chi }_{\mathrm{bulk}}$ (measured in 0.1 T) of ${\mathrm{BaCo}}_2{\mathrm{As}}_2$ matched with ${}^{75}K$. The wide horizontal dashed line represents the approximate value of ${}^{75}{K}_{\mathrm{chem}}=0\%–0.2$%, which corresponds to the temperature independent contribution ${\chi }_{\mathrm{spin}}^{\left(2\right)}=2.5–3\times {10}^{-3}$ emu/mol. (b) Temperature dependence of $1/T_{1}T$. The symbols are the same as in (a).

Figure 3

The Knight shift ${}^{75}K$ in ${\mathrm{BaCo}}_2{\mathrm{As}}_2$ plotted as a function of ${\chi }_{\mathrm{bulk}}$ with temperature as the implicit parameter. The linear fit through the data point shows the linear relation between ${}^{75}K$ and ${\chi }_{\mathrm{spin}}^{\left(1\right)}\left(T\right)$. Slight deviation from the linear fit at low temperatures (shown with open circles) is caused by a small impurity contribution in ${\chi }_{\mathrm{bulk}}$ below $\sim 10$ K, as evident in Fig. 2. For comparison, we also show the corresponding result reported for ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ [13].