Avoided Ferromagnetic Quantum Critical Point: Unusual Short-Range Ordered State in CeFePO

Cerium $4f$ electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac susceptibility, specific heat, and muon-spin relaxation ($\mu \mathrm{SR}$). Short-range static magnetism occurs below the freezing temperature $T_g\approx 0.7\mathrm{K}$, which prevents the system from accessing a putative ferromagnetic quantum critical point. In the $\mu \mathrm{SR}$, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms.

Figure 1

(a) ac susceptibility ${\chi }^{\prime }(T)$ of the PC sample with its imaginary part ${\chi }^{\prime \prime }(T)$ (inset). (b) ${\chi }^{\prime }(T)$ vs $T$ for the SC sample with $B\perp c$. (c) ${\chi }^{\prime }(T)$ vs $T$ for the SC sample with $B\parallel c$. (d) Specific heat of the PC sample plotted as $C/T$ vs $T$.

Figure 2

Frequency dependence of ${\chi }^{\prime }(T)$ for the PC sample. Inset: Arrhenius plot for the PC sample and for the SC sample with $B\perp c$.

Figure 3

(a) Normalized muon-spin asymmetry function $G(t,B)$ at an applied field $B=0.01\mathrm{T}$ for representative temperatures above and below $T_g$. Solid lines are fits according to Eq. (1). For clarity, the different curves are shifted subsequently by 0.33. (b) $T$ dependence of the dynamic $\mu \mathrm{SR}$ rate ${\lambda }_L$. The arrow marks the magnetic transition from dynamic to static magnetism at $T_g=0.70(3)\mathrm{K}$. (c) $T$ dependence of the static $\mu \mathrm{SR}$ rate ${\lambda }_T$. (d) $T$ dependence of the exponent $\beta$ in Eq. (1). The dotted line denotes the value $\beta =0.5$.

Figure 4

(a) Field dependence of the muon-spin asymmetry function $G(t,B)$ in CeFePO at $T=0.02\mathrm{K}$ and (b) $T=0.9\mathrm{K}$. (c) The same data as in (b), plotted as a function of the scaling variable $t/B^{0.5}$.