Helical Antiferromagnetic Ordering in EuNi{1.95}As2

The Eu{+2} spins-7/2 in EuNi2As2 with the body-centered tetragonal ThCr2Si2 structure order antiferromagnetically below the Neel temperature TN =15 K into a helical antiferromagnetic (AFM) structure with the helix axis aligned along the tetragonal c axis and the Eu ordered moments aligned ferromagnetically within the ab plane as previously reported from neutron diffraction measurements [T. Jin, et al., Phys. Rev. B 99, 014425 (2019)]. Here we study the crystallographic, magnetic, thermal, and electronic transport properties of Bi-flux-grown single crystals using single-crystal x-ray diffraction, anisotropic magnetic susceptibility chi, isothermal magnetization M, heat capacity Cp, and electrical resistivity rho measurements versus applied magnetic field H and temperature T. Vacancies are found on the Ni sites corresponding to the composition EuNi{1.95}As2. The chi{ab}(T) data below TN are fitted well by molecular field theory (MFT), and the helix turn angle kd and the Eu-Eu Heisenberg exchange constants are extracted from the fit parameters. The kd value is in good agreement with the neutron-diffraction result. The magnetic contribution to the zero-field heat capacity below TN is also fitted by MFT. The isothermal in-plane magnetization Mab exhibits two metamagnetic transitions versus H, whereas Mc(T = 2 K) is nearly linear up to H =14 T, both behaviors being consistent with MFT. The Mc(H,T), rho(Hc,T), and Cp(Hc,T) data yielded a Hc-T phase diagram separating the AFM and paramagnetic phases in good agreement with MFT. Anisotropic chi(T) literature data for the ThCr2Si2-type helical antiferromagnet EuRh2As2 are also fitted well by MFT. A comparison is made between the crystallographic and magnetic properties of ThCr2Si2- type EuM2Pn2 compounds with M = Fe, Co, Ni, Cu, or Rh, and Pn = P or As, where only ferromagnetic and c-axis helical AFM structures are found.