Mapping out the spin fluctuations in Co-doped LaFeAsO single crystals by NMR

We determine the phase diagram of ${\mathrm{LaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{AsO}$ single crystals by using nuclear magnetic resonance (NMR). Up to a nominal doping of $x=0.03$, it follows the phase diagram for F-doped polycrystals. Above $x=0.03$, the F-doped samples become superconducting, whereas for Co doping the structural and magnetic transitions can be observed up to $x=0.042$, and superconductivity occurs only for higher doping levels and with reduced transition temperatures. For dopings up to $x=0.056$, we find evidence for short-range magnetic order. By means of relaxation-rate measurements, we map out the magnetic fluctuations that reveal the interplay of nematicity and magnetism. Above the nematic ordering, the spin fluctuations in ${\mathrm{LaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{AsO}$ are identical to those in $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$, suggesting that nematicity in LaFeAsO is a result of the fluctuating spin density wave as well.

Figure 1

Upper row: ${\left(T_{1}T\right)}^{-1}$ vs temperature with the magnetic field applied along the $ab$ plane. In the last panel, ${\left(T_{1}T\right)}^{-1}$ of $x=0.06$ F doping (open triangles) is included for comparison [35]. The $y$ axis for $x=0.056$ and $x=0.06$ is ten times zoomed compared with the magnetically ordered samples. The green lines are Curie-Weiss fits with $\mathrm{\Theta }$ fixed to $T_N$. Middle row: Stretching parameter $\beta$ of $T_1^{-1}$; open squares, stretching of the spin-spin relaxation rate $T_2^{-1}$. Lower row: Ratio $R=T_{1c}/T_{1ab}$. SC, superconductivity.

Figure 2

NMR spectra for $x=0.03$, 0.042, and 0.056 for ${H\left|\right|\left[100\right]}_{\mathrm{ortho}}$ or ${\left[010\right]}_{\mathrm{ortho}}$. Solid lines mark $T_N$, and dashed lines mark $T_S$. For better visibility, spectra at low temperatures are enlarged. The excitation bandwidth is about 100 kHz for single-frequency scans; broader spectra were measured by a step-and-sum technique.

Figure 3

(a) FWHM for all doping levels for $H\left|\right|\left[100\right]$ or [010]. Open symbols are for $H\left|\right|\left[110\right]$. (b) Spin-spin decay of the nuclear magnetization $M_{xy}$ for $x=0.042$. (c) $M_{xy}$ at $t=0$ s vs temperature for $x=0.042$ and $x=0.056$. (d) Spin-spin decay for $x=0.056$. (e) ${\left(T_{1}T\right)}^{-1}$ for $\mathrm{La}{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}\mathrm{O}$ with $x=0.035$ and $x=0.06$, for $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ with $x=0.04$ and $x=0.08$ [45] and for FeSe [13]. Solid down arrows indicate $T_N$, dashed-down arrows indicate $T_c$, and up arrows indicate $T_S$.

Figure 4

Phase diagram resulting from NMR and SQUID susceptibility on Co-doped LaFeAsO single crystals compared with F-doped polycrystals [35]. Closed symbols are from NMR, and open symbols are from SQUID and dilatometry [23]. $T^{\mathrm{nem}}$ is determined by elastoresistivity [15].