Magnetic field induced ab -plane rotation of the Eu magnetic moments in trigonal EuMg2Bi2 and EuMg2Sb2 single crystals below their Néel temperatures

Abstract

The thermodynamic and electronic-transport properties of trigonal ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$ in $ab$-plane magnetic fields ${\mathbf{H}}_{x}$ and the A-type antiferromagnetic structure have recently been reported. The Eu magnetic moments with spin $S=7/2$ remain locked in the $ab$ plane up to and above the $ab$-plane critical field ${\mathbf{H}}_{x}^{\mathrm{c}}=27.5$ kOe at which the Eu moments become parallel to ${\mathbf{H}}_{x}$. Here additional measurements at low fields are reported that reveal a new spin-reorientation transition at a field ${H}_{c1}\ensuremath{\approx}465$ Oe where the Eu moments remain in the $ab$ plane but become perpendicular to ${H}_{x}$. At higher fields, the moments cant toward the field resulting in $M\ensuremath{\propto}{H}_{x}$ up to ${\mathbf{H}}_{x}^{\mathrm{c}}$. Similar results are reported from measurements of the magnetic properties of ${\mathrm{EuMg}}_{2}{\mathrm{Sb}}_{2}$ single crystals, where ${H}_{\mathrm{c}1}\ensuremath{\approx}220$ 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 ${\mathrm{EuMg}}_{2}{\mathrm{Sb}}_{2}$ is found to be significantly smaller than in ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$.