Ferromagnetism and superconductivity in CeFeAs${}_{1-x}$$P_x$O ($0\le x\le 40\%$)

We report on superconductivity in CeFeAs${}_{1-x}$P${}_x$O and the possible coexistence with Ce ferromagnetism (FM) in a small homogeneity range around $x=30\%$ with ordering temperatures of $T_{\text{SC}}\cong T_{\text{C}}\cong 4$ K. The antiferromagnetic (AFM) ordering temperature of Fe at this critical concentration is suppressed to $T_{\mathrm{N}}^{\mathrm{Fe}}\approx 40$ K and does not shift to lower temperatures with a further increase of the P concentration. Therefore, a quantum-critical-point scenario with $T_{\mathrm{N}}^{\mathrm{Fe}}\to 0$ K which is widely discussed for the iron based superconductors can be excluded for this alloy series. Surprisingly, thermal expansion and x-ray powder diffraction indicate the absence of an orthorhombic distortion despite clear evidence for short-range AFM Fe ordering from muon-spin-rotation measurements. Furthermore, we discovered the formation of a sharp electron spin resonance signal unambiguously connected with the emergence of FM ordering.

Figure 1

Electrical resistivity along the $ab$ plane, normalized to room temperature (RT), of CeFeAs${}_{1-x}$P${}_x$O single crystals for $0⩽x⩽40\%$ at (a) ambient pressure and (b) for $x=22\%$ as a function of pressure. Above a critical concentration of $x\approx 27\%$ and a critical pressure of $p\approx 2$ GPa, $T_{\mathrm{N}}^{\mathrm{Fe}}$ does not shift to lower temperatures anymore; instead, the signature fades away. CeFeAs${}_{0.70}$P${}_{0.30}$O (red lines) shows clear signatures of the Fe ordering at $T\approx 40$ K and the onset of superconductivity at $T_{\mathit{SC}}=4$ K. Insets: Derivatives showing clear anomalies at $T_{\mathrm{N}}^{\mathrm{Fe}}$.

Figure 2

(a) Electrical resistivity of single crystalline CeFeAs${}_{1-x}$P${}_x$O normalized to RT. Zero resistivity is observed for $x=30\%$ (plotted for four different samples of two different batches) whereas other samples show only weak ($x=27\%,35\%$) or absolutely no indications ($x=22\%,40\%$) for SC. (b) Anisotropic magnetization of CeFeAs${}_{0.70}$P${}_{0.30}$O showing FM behavior with the spontaneous moment along the $c$ axis. Note: The same single crystal shows zero resistivity (lower curve of batch 2 in the left panel). The single crystal used for the measurements (upper left) and a corresponding Laue back reflection pattern (lower right) are displayed.

Figure 3

(a) Anisotropic thermal expansion coefficients of CeFeAs${}_{0.70}$P${}_{0.30}$O showing no indication for an orthorhombic distortion [results for two different directions in the $ab$ plane, $\sphericalangle (\alpha ,{\alpha }^{\prime })\approx {60}^{\circ }$] and a pronounced negative thermal expansion along the $c$ axis. The sharp peaks below 15 K are connected to the magnetic ordering of Ce. Inset: Synchrotron XRD pattern close to the 220 reflection—no indications for splitting or broadening are observable. (b) Magnetic volume fraction determined from $\mu$SR measurements. Static magnetic ordering sets in below $T=70$ K. Upper inset: The strong relaxation of the muon-spin polarization evidences short-range magnetic ordering. Lower inset: Longitudinal field $\mu$SR measurements proving the predominantly static character of the magnetism.

Figure 4

(a) Sharp ESR signal for $x=35\%$ compared to a weak signal for $x=22\%$ (multiplied by a factor of 20 for visibility). (b) Observability range of the ESR signal as a function of $x$ indicated by vertical lines, with the strongest signals indicated by circles. No ESR line was observed for $x<22\%$ and $x>90\%$. (c) A strong increase of the absorption at small fields indicates SC with a lower critical field of $H_{c1}=70$ Oe, which was solely observed for $x=30\%$ and $T<4$ K. (d) Characteristic temperatures as a function of $x$ determined by several techniques. The inset shows $T_{\mathrm{N}}^{\mathrm{Fe}}$ for $x=22\%$ as a function of pressure.