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

The body-centered-tetragonal antiferromagnet EuGa4 was recently identified as a Weyl nodal-line semimetal that exhibits the topological Hall effect below its reported antiferromagnetic (AFM) ordering temperature T-N = 15-16.5 K which we find to be T-N = 16.4(2) K. The Eu+2 ions are located at the corners and body centers of the unit cells. EuGa4 exhibits A-type antiferromagnetic order below T-N, where the Eu2+ 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 Hc1 at which a second-order transition occurs with Hc1 approximate to 0.85 kOe for H parallel to [1, 1, 0] and approximate to 4.8 kOe for H parallel to [1, 0, 0]. For larger fields, the linear behavior M-ab = chi(T-N)H-ab is followed until the critical field Hc 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 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 H-ab at H-c1, followed by increasing canting of all moments toward the field with increasing field up to H-c which is reported to be 71 kOe. A first-order transition in M(H-ab) at H-ab = H-c1 is predicted by the theory for T = 0 K when Hab is at a small angle from the [1,0,0] direction.

Automated summary

The body-centered-tetragonal antiferromagnet EuGa4 is found to be a Weyl nodal-line semimetal exhibiting topological Hall effect below its reported antiferromagnetic ordering temperature T-N = 15-16.5 K, with an actual T-N = 16.4(2) K. EuGa4 shows A-type antiferromagnetic order below T-N, where the Eu2+ 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 data at T = 2 K reveals anomalous positive curvature up to a critical field Hc1, followed by a second-order transition. The theory explains the observed behavior at T = 2 K, where domains of A-type antiferromagnetic order with fourfold rotational symmetry occur in zero field.