Competing effects of Mn and Y doping on the low-energy excitations and phase diagram of ${\mathrm{La}}_{1-y}{\mathrm{Y}}_y{\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ iron-based superconductors

Muon spin rotation ($\mu \mathrm{SR}$) and ${}^{19}\mathrm{F}$ nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in ${\mathrm{La}}_{1-y}{\mathrm{Y}}_y{\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors. While for $y=0$ a very low critical concentration of Mn ($x=0.2\%$) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y=20\%$ it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y=0$ and for $y=20\%$. Low-energy spin fluctuations give rise to a peak in ${}^{19}\mathrm{F}$ NMR $1/T_1$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by ${}^{19}\mathrm{F}$ NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground state are discussed in the light of the proximity of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ to a quantum critical point.

Figure 1

Intensity of ${}^{19}\mathrm{F}$ NMR signal at 1.5 T, normalized by the sample mass, at room temperature for selected LaY20 and LaYMn05 samples (see legend).

Figure 2

(a) Zero field $\mu \mathrm{SR}$ signal for the ${\mathrm{La}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ sample with $x=20\%$, measured at different temperatures. (b) ZF $\mu \mathrm{SR}$ signal for LaY20 samples with $x=6\%$, 10%, 15%, and 20%, at $T=2\mathrm{K}$. Solid lines in (a) and (b) represent the best fits to Eq. (1).

Figure 3

(a) The values of $\mathrm{\Delta }B$ at 2 K, for all the samples showing a magnetic order (see Fig. 7), obtained from the fit of the muon asymmetry to Eq. (1). The dashed line is a guide to the eye. (b) The magnetic volume fraction temperature dependence is shown for $y=20\%$ and $x=10\%$, $15\%$, and $20\%$. The solid lines are best fits to Eq. (2).

Figure 4

${}^{19}\mathrm{F}$ nuclear magnetization recovery for ${\mathrm{La}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ at 22 K, around the $1/T_1$ peak, for different values of $x$ (see legend). The solid lines are fits to Eq. (3). Inset: temperature dependence of the stretching exponent $\beta$ used to fit the longitudinal nuclear magnetization recovery for $x=2.5\%$ and $y=20\%$.

Figure 5

Temperature dependence of ${}^{19}\mathrm{F}$ NMR $1/{\mathrm{T}}_1$ for ${\mathrm{La}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ for Mn doping levels up to $x=20\%$. The solid lines are fits of the data according to Eq. (6) in the text. The arrows indicate $T_m$ for the magnetic samples determined from $\mu \mathrm{SR}$ measurements (from left to right: $x=10\%$, $x=15\%$, $x=20\%$).

Figure 6

${}^{19}\mathrm{F}$ NMR full width at half intensity for three representative samples of the LaY20 series. Solid lines are best fits according to a Curie-Weiss law while the arrows indicate $T_c$ of the $x=1\%$ sample (green arrow) and $T_m$ of the $x=15\%$ sample (orange arrow). Inset: a typical ${}^{19}\mathrm{F}$ NMR spectrum ($x=4\%$, $y=20\%$, $T=11$ K).

Figure 7

Phase diagram of ${\mathrm{LaFe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ (top) and of ${\mathrm{La}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ (bottom). The red and blue shaded areas are the superconductive and the magnetic phases, respectively. The magnetic transition temperature (blue diamonds) was determined by ZF-$\mu \mathrm{SR}$ while the superconducting transition temperature $T_c$ (red circles) was determined from SQUID magnetization measurements.

Figure 8

Critical temperatures for the LaYMn05 samples studied in this paper. The dashed line is a guide to the eye. Inset: $T_c\left(x\right)/T_c(x=0)$ versus the Mn content normalized by the critical content causing the vanishing of $T_c$ for LaY20 and LaY0 (Ref. [11]) series (see text).

Figure 9

Parameters extracted from the fit of $1/T_1$ to Eq. (6) as a function of the Mn content for the LaY20 series (filled symbols, bottom horizontal axes) and for the LaY0 series (open symbols, top horizontal axes). Top panel: mean value of the local fluctuating magnetic field $h_{\perp }$ (left) and correlation time ${\tau }_0$ (right). Bottom panel: energy barrier $E_a$ (left) and width of the energy barrier distribution $\mathrm{\Delta }$ (right).