  The thermodynamic and electronic-transport properties of trigonal EuMg2Bi2 in
ab-plane magnetic fields Hx and the A-type antiferromagnetic structure have
recently been reported. At a temperature of 1.8 K < TN, the Eu magnetic moments
with spin S = 7/2 remain locked in the ab plane up to and above the ab-plane
critical field Hxc = 27.5 kOe at which the Eu moments become parallel to Hx.
Here additional measurements at low fields are reported that reveal a new
spin-reorientation transition at a field Hc1 = 465 Oe where the Eu moments
remain in the ab plane but become perpendicular to Hx. At higher fields, the
moments cant towards the field resulting in M proportional to Hx up to Hxc.
Similar results are reported from measurements of the magnetic properties of
EuMg2Sb2 single crystals, where Hc1 = 220 Oe is found. Theory is formulated
that models the low-field magnetic behavior of both materials, and the
associated anisotropies are calculated. The ab-plane trigonal anisotropy in
EuMg2Sb2 is found to be significantly smaller than in EuMg2Bi2.
