Helical antiferromagnetic ordering in EuNi_1.95As₂ single crystals

The Eu^{+ 2} spins-7/2 in EuNi₂ As₂ with the body-centered tetragonal ThCr₂ Si₂ structure order antiferromagnetically below the Néel temperature T_N = 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 χ, isothermal magnetization M, heat capacity Cp, and electrical resistivity ρ measurements versus applied magnetic field H and temperature T. Vacancies are found on the Ni sites corresponding to the composition EuNi_{1.95 (1)} As 2. A good fit of the ρ (T) data by the Bloch-Grüneisen theory for metals was obtained. The _χab (T) data below T_N 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 T_N is also fitted by MFT. The isothermal in-plane magnetization M_ab exhibits two metamagnetic transitions versus H, whereas M_c (T = 2 K) is nearly linear up to H = 14 T, both behaviors being consistent with MFT. The M_c (H, T) , ρ(H_c, T) , and Cp (H _c , T) data yielded a H_c- T phase diagram separating the AFM and paramagnetic phases in good agreement with MFT. Anisotropic χ(T) literature data for the ThCr ₂ Si₂ -type helical antiferromagnet EuRh₂ As₂ were also fitted well by MFT. A comparison is made between the crystallographic and magnetic properties of ThCr₂ Si₂-type Eu M₂ Pn₂ 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.