Quenching of spin fluctuations in the 3d and 4f aluminides YFe₂Al₁₀ and YbFe₂Al₁₀: A comparative ²⁷Al NMR and specific heat study

We present investigations on the magnetic susceptibility χ(T), the specific heat coefficient γ(T)=C(T)/T, and the spin-lattice relaxation (SLR) rate on the two isostructural iron aluminides YFe₂Al₁₀ and YbFe₂Al₁₀. Below 10K, both systems display a monotonous field-dependent increase with decreasing temperature in χ(T), γ(T) and the SLR coefficient ²⁷R(T)=²⁷(1/T₁T). These divergences are associated with spin fluctuations near a quantum critical point (QCP). Most probably, the QCP is of ferromagnetic (FM) nature, but so far no magnetic order is observed in either system down to 0.35K. The application of moderate fields of a few Tesla suppresses the low-T upturn in the properties. The comparison between zero-field and high field measurements allow for the determination of enhancement factors in χ(T), the γ(T), and the spin-lattice relaxation coefficient (SLRC) ²⁷R(T). Surprisingly, the relative enhancement in γ(T) and the T-dependence [∝ ln(1/T)] are very similar for both systems. In the SLRC, the enhancement is larger for the Yb-system but both show ln(1/T) behavior for T→0. The divergences are very similar because the Yb ions in YbFe₂Al₁₀ are in an intermediate valence state at low temperatures and contribute only very little towards the observed spin fluctuations. This indicates strongly the same underlying physics and the same energy scales for both systems. We propose the presence of dominant q=0 low-energy FM spin fluctuations from weak itinerant Fe as the origin for the observed logarithmic divergences.