NSRRC Activity Report 2023

086 NSRRC ACTIVITY REPORT 2023 T he 2D-THA systems are of particular interest owing to their geometric frustration and potential for novel spin states. The material investigated, Lu y Y 1−y MnO 3 , belongs to the family of hexagonal R MnO 3 compounds, where R represents a rare-earth ion. It has been studied for its multiferroic properties and intriguing magnetic behavior. Lu y Y 1−y MnO 3 adopts P 6 3 cm symmetry; Shinichiro Yano (NSRRC) and his collaborators focused on the six irreducible representations as candidates of magnetic structures of Lu y Y 1−y MnO 3 and its high-resolution spin dynamics. Weak Trimerization in 2D Heisenberg Antiferromagnet Detailed neutron scattering studies revealed fragile trimerization in an almost perfect 2D Heisenberg Antiferromagnet. Fig. 1 : (a,b) Exchange interactions in the plane. Mn position ( x Mn , 0, 0) x ~ 1/3 is a critical parameter because there are four J 1 and two J 2 when x Mn > 1/3, while there are two J 1 and four J 2 when x Mn < 1/3. These are the next nearest neighbour J 1 and the subsequent nearest neighbour J 2 . (c) There are six candidates of magnetic structure under P63cm symmetry, Γ 1 to Γ 4 are one-dimensional irreducible representations and Γ 5 and Γ 6 are two-dimensional irreducible representations. [Reproduced from Ref. 1] Fig. 2 : Neutron power diffraction data revealed the detailed crystal and magnetic structures. (a) Magnetic phase transition temperatures are determined for y = 0.15, 0.30, and 0.45. (b) Trimerization at manganese x position was not observed, and x Mn remains at a constant temperature below 100 K. (c) It was found that two irreducible representations Γ 3 and Γ 4 are needed to describe the magnetic structures. (d) Temperature dependence is observed for the ratio of the linear combination of the two irreducible representations Γ 3 and Γ 4 . [Reproduced from Ref. 1] Yano utilized neutron powder diffraction at WOMBAT in ANSTO to determine the magnetic structure of Lu y Y 1−y MnO 3 , which was described as a mixture of two magnetic space groups at different compositions. He especially focused on y = 0.3, where x Mn is close to the 1/3; therefore, an almost perfect triangular lattice is realized by the system. This approach, combined with temperature variation, gave insight into the system’s trimerization and exchange interactions.