Non-Fermi-liquid types of behavior associated with a magnetic quantum critical point in Sr ( Co 1 - x Ni x ) 2 As 2 single crystals
The compound SrCo 2 As 2 with the body-centered tetragonal ThCr 2 Si 2 structure is known to remain paramagnetic down to a temperature T = 0.05 K, but inelastic neutron scattering studies have shown that both ferromagnetic (FM) and antiferromagnetic (AFM) fluctuations occur in single crystals. Thus it is of interest to study how the magnetism evolves on doping SrCo 2 As 2 . Previous work on polycrystalline samples of Sr (Co 1-x Ni x ) 2 As 2 indicated the development of AFM order for 0 < x ≲ 0.3. Here we studied single crystals of Sr(Co 1-x Ni x ) 2 As 2 for 0 < x ≤ 1 and confirmed the occurrence of AFM order which we deduce to have a c -axis helix structure. We also find evidence for an unusual composition-induced magnetic quantum critical point at x ≈ 0.3 where non-Fermi-liquid types of behavior were revealed by heat capacity and electrical resisitivity measurements at low T. Electron-doped Sr (Co 1-x Ni x ) 2 As 2 single crystals with compositions x = 0 to 0.9 were grown out of self-flux and SrNi 2 As 2 single crystals out of Bi flux. The crystals were characterized using single-crystal x-ray diffraction (XRD) at room temperature, and magnetic susceptibility χ (H, T), isothermal magnetization M (H, T), heat capacity Cp (H, T), and electrical resistivity ρ (H, T) measurements versus applied magnetic field H and T. The XRD studies show that the system undergoes a continuous structural crossover from the uncollapsed-tetragonal (ucT) structure to the collapsed tetragonal (cT) structure with increasing Ni doping. The χ(T) data show that SrCo 2 As 2 exhibits an AFM ground state almost immediately upon Ni doping on the Co site. Ab initio electronic-structure calculations for x = 0 and 0.15 indicate that a flat band with a peak in the density of states just above the Fermi energy is responsible for this initial magnetic-ordering behavior on Ni doping. The AFM ordering is observed in the range 0.013 ≤ x ≤ 0.25 with the ordered moments aligned in the ab plane and with a maximum ordering temperature T N = 26.5 K at x = 0.10. The Curie-Weiss-like T dependence of χ in the paramagnetic (PM) state indicates dominant FM interactions. The behavior of the anisotropic susceptibilities below T N suggests a planar helical magnetic ground state with a composition-dependent pitch based on a local-moment molecular-field-theory model, with FM interactions in the ab plane and weaker AFM interactions along the helix c axis. However, the small ordered (saturation) moments ~0.1 μ B per transition metal atom, where μ B is the Bohr magneton, and the values of the Rhodes-Wohlfarth ratio indicate that the magnetism is itinerant. The high-field M (H) isotherms and the low-field χ-1 ( T > T N ) data were successfully analyzed within the framework of Takahashi's theory of FM spin fluctuations. The C p (T) at low T exhibits Fermi-liquid behavior for 0 ≤ x ≤ 0.15 , whereas an evolution to a logarithmic non-Fermi-liquid (NFL) behavior is found for x = 0.2 to 0.3. The logarithmic dependence is suppressed in an applied magnetic field. The low- T ρ ( H = 0 , T ) data show a T 2 dependence for 0 ≤ x ≤ 0.20 and a power-law dependence ρ (H = 0, T) = ρ 0 + AT n with n < 2 for x = 0.20 and 0.30. The exponent n shows a notable field dependence, suggesting both doping- and magnetic-field-tuned quantum critical phenomena. These low- T NFL types of behavior observed in the C p and ρ measurements are most evident near the quantum critical concentration x ≈ 0.3 at which a T = 0 composition-induced transition from the AFM phase to the PM phase occurs.