Suppression of antiferromagnetic order and strong ferromagnetic spin fluctuations in Ca(Co1-xNix)2-y As2 single crystals
CaCo2–yAs2 is a unique itinerant system having strong magnetic frustration. Here, we report the effect of electron doping on the physical properties resulting from Ni substitutions for Co. The single crystals of Ca(Co1–xNix)2–y As2 were characterized by single-crystal x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetization M versus temperature T, magnetic field H, time t, and heat capacity Cp (H, T) measurements. The A-type antiferromagnetic (AFM) transition temperature TN = 52 K for x = 0 decreases to 22 K with only 3% Ni substitution and is completely suppressed for x > 0.16. For 0.11 ≤ x ≤ 0.52 strong ferromagnetic (FM) fluctuations develop as revealed by magnetic susceptibility χ(T) = M(T) / H measurements. For x = 0.11 and 0.16 competing AFM and FM interactions result in a reentrant spin-glass behavior below TN, as evidenced by the observations of thermomagnetic hysteresis and magnetic relaxation. Enhanced FM fluctuations are also found for the x = 0.21 and 0.31 crystals, where χc increases significantly at low T. A large χ anisotropy in these compositions where χc is up to a factor of two larger than χab suggests that the FM spin fluctuations are quasi-1D in nature. Weak ferromagnetic contributions to the magnetization are found at T = 2 K for x = 0.11 –0.31. Heat-capacity Cp(T) measurements reveal the presence of FM quantum spin fluctuations for 0.11 ≤ x ≤ 0.52, where a logarithmic T dependence of Cp(T) / T is observed at low T. Here, the suppression of AFM order by the development of strong FM fluctuations in Ca(Co1–xNix)2–yAs2 crystals suggests the presence of a FM quantum-critical point at x ≈ 0.20. Our density-functional theory (DFT) calculations confirm that FM fluctuations are enhanced by Ni substitutions for Co in CaCo2–yAs2. The Sommerfeld electronic heat-capacity coefficient is enhanced for x = 0, 0.21, and 0.42 by about a factor of two compared to DFT calculations of the density of states (DOS) at the Fermi energy, suggesting an enhancement of the DOS from electron-phonon and/or electron-electron interactions. The crystals with x > 0.52 do not exhibit FM spin fluctuations or magnetic order at T ≥ 1.8 K, which was found from the DFT calculations to arise from a Stoner transition. Superconductivity is not observed above 1.8 K for any of the compositions. Neutron-diffraction studies of crystals with x = 0.11 and 0.16 in the crossover regime (0.1 ≲ x ≲ 0.2) show no evidence of A-type ordering as observed in the parent compound with x = 0. Furthermore, no other common magnetic structures, such as ferromagnetic (FM), helical stacking of in-plane FM layers, or in-plane AFM structure, are found with an ordered moment greater than the uncertainty of 0.05 μB per transition-metal atom.
- Research Organization:
- Ames Laboratory (AMES), Ames, IA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- AC02-07CH11358
- OSTI ID:
- 1820990
- Report Number(s):
- IS-J-10,594; TRN: US2214307
- Journal Information:
- Physical Review B, Vol. 104, Issue 9; ISSN 2469-9950
- Publisher:
- American Physical Society (APS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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