Low-field magnetic anomalies in single crystals of the A-type square-lattice antiferromagnet EuGa4

Abstract

The body-centered-tetragonal antiferromagnet ${\mathrm{EuGa}}_{4}$ was recently identified as a Weyl nodal-line semimetal that exhibits the topological Hall effect below its reported antiferromagnetic (AFM) ordering temperature ${T}_{\mathrm{N}}=15--16.5$ K which we find to be ${T}_{\mathrm{N}}=16.4(2)$ K. The ${\mathrm{Eu}}^{+2}$ ions are located at the corners and body centers of the unit cells. ${\mathrm{EuGa}}_{4}$ exhibits A-type antiferromagnetic order below ${T}_{\mathrm{N}}$, where the ${\mathrm{Eu}}^{2+}$ spin-7/2 moments are ferromagnetically aligned in the $ab$ plane with the Eu moments in adjacent Eu planes along the $c$ axis aligned antiferromagnetically. Low-field magnetization versus field $M({H}_{ab})$ data at $T=2$ K with the field aligned in the $ab$ plane are reported that exhibit anomalous positive curvature up to a critical field ${H}_{\mathrm{c}1}$ at which a second-order transition occurs with ${H}_{c1}\ensuremath{\approx}0.85$ kOe for $\mathbf{H}\ensuremath{\parallel}[1,1,0]$ and $\ensuremath{\approx}4.8$ kOe for $\mathbf{H}\ensuremath{\parallel}[1,0,0]$. For larger fields, the linear behavior ${M}_{ab}=\ensuremath{\chi}({T}_{\mathrm{N}}){H}_{ab}$ is followed until the critical field ${H}_{\mathrm{c}}$ is reached at which all moments become aligned with the applied field. A theory is formulated for $T=0$ K that fits the observed $M({H}_{ab})$ behavior at $T=2$ K well, where domains of $\mathrm{A}$-type AFM order with fourfold rotational symmetry occur in the AFM state in zero field. The moments in the four domains reorient to become almost perpendicular to ${\mathbf{H}}_{ab}$ at ${H}_{c1}$, followed by increasing canting of all moments toward the field with increasing field up to ${H}_{\mathrm{c}}$ which is reported to be 71 kOe. A first-order transition in $M({H}_{ab})$ at ${H}_{ab}={H}_{\mathrm{c}1}$ is predicted by the theory for $T=0$ K when ${\mathbf{H}}_{ab}$ is at a small angle from the [1,0,0] direction.