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

Abstract

Resonant x-ray Bragg diffraction data for gadolinium tetraboride (GdB{sub 4}), gathered at the Gd L{sub 3} edge above and below a transition temperature T{sub 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 P2{sub 1}2{sub 1}2, which is a continuous structural phase transition in Landau and renormalization-group theories. Below T{sub N}, a configuration of Gd moments is established, which is consistent with the magnetic Shubnikov group P2{sub 1}2{sub 1}2, 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.