Phase transitions, noncollinear magnetism, and magnetoelectric symmetry in gadolinium tetraboride

Resonant x-ray Bragg diffraction data for gadolinium tetraboride (GdB₄), gathered at the Gd L₃ edge above and below a transition temperature T_N=42 K, is shown to imply that a structural phase transition occurs at temperatures above the onset of long-range magnetic order in the Gd moments. Specifically, the room-temperature tetragonal structure P4∕mbm would appear to change to orthorhombic P2₁2₁2, which is a continuous structural phase transition in Landau and renormalization-group theories. Below T_N, a configuration of Gd moments is established, which is consistent with the magnetic Shubnikov group P2₁2₁2, where moments are constrained to lie in the plane normal to the c axis. The crystal structure and magnetic point symmetry (222) inferred, is one of the class of 58 magnetic groups that can show the magnetoelectric effect.