  The trigonal compound EuMg2Bi2 has recently been discussed in terms of its
topological band properties. These are intertwined with its magnetic
properties. Here detailed studies of the magnetic, thermal, and electronic
transport properties of EuMg2Bi2 single crystals are presented. The Eu{+2}
spins-7/2 in EuMg2Bi2 exhibit an antiferromagnetic (AFM) transition at a
temperature TN = 6.7 K, as previously reported. By analyzing the anisotropic
magnetic susceptibility chi data below TN in terms of molecular-field theory
(MFT), the AFM structure is inferred to be a c-axis helix, where the ordered
moments in the hexagonal ab-plane layers are aligned ferromagnetically in the
ab plane with a turn angle between the moments in adjacent moment planes along
the c axis of about 120 deg. The magnetic heat capacity exhibits a lambda
anomaly at TN with evidence of dynamic short-range magnetic fluctuations both
above and below TN. The high-T limit of the magnetic entropy is close to the
theoretical value for spins-7/2. The in-plane electrical resistivity rho(T)
data indicate metallic character with a mild and disorder-sensitive upturn
below Tmin = 23 K. An anomalous rapid drop in rho(T) on cooling below TN as
found in zero field is replaced by a two-step decrease in magnetic fields. The
rho(T) measurements also reveal an additional transition below TN in applied
fields of unknown origin that is not observed in the other measurements and may
be associated with an incommensurate to commensurate AFM transition. The
dependence of TN on the c-axis magnetic field Hperp was derived from the
field-dependent chi(T), Cp(T), and rho(T) measurements. This TN(Hperp) was
found to be consistent with the prediction of MFT for a c-axis helix with S =
7/2 and was used to generate a phase diagram in the Hperp-T plane.
