A-type antiferromagnetic order and magnetic phase diagram of the trigonal Eu spin-7/2 triangular-lattice compound EuSn2As2

The trigonal compound EuSn2As2 was recently discovered to host Dirac surface states within the bulk band gap and orders antiferromagnetically below the Neel temperature TN = 23.5(2) K from our neutron-diffraction measurements. Here the magnetic ground state of single-crystal EuSn2As2 and the evolution of its properties versus temperature T and applied magnetic field H are reported. Included are the zero-field single-crystal neutron diffraction measurements versus T, magnetization M(H, T ), magnetic susceptibility chi(H, T ) = M(H, T )/H, heat capacity Cp(H, T ), and electrical resistivity rho(H, T ) measurements. The neutron-diffraction and chi(T ) measurements both indicate a collinear A-type antiferromagnetic (AFM) structure below TN, where the Eu2+ spins S = 7/2 in a triangular ab-plane layer (hexagonal unit cell) are aligned ferromagnetically in the ab plane, whereas the spins in adjacent Eu planes along the c axis are aligned antiferromagnetically. The chi(Hab, T ) and chi(Hc, T ) data together indicate a smooth crossover between the collinear AFM alignment and an unknown magnetic structure at H approximate to 0.12 T. Dynamic spin fluctuations up to 60 K are evident in the chi(T ), Cp(T ) and rho(H, T ) measurements, a temperature that is more than twice TN. The rho(H, T ) is consistent with a low-carrierdensity metal with strong magnetic scattering and does not reflect a contribution of the topological state of the material as reported earlier by ARPES measurements. This observation is consistent with previous ones for other topological insulators where the chemical potential is above the Dirac point so that ARPES readily detects the surface states, whereas resistivity measurements do not. The magnetic phase diagrams for both H II c and H II ab in the H-T plane are constructed from the TN(H ), chi (H, T ), Cp(H, T ), and rho(H, T ) data.