Strong spin frustration and magnetism in kagomé antiferromagnets LnCu ₃(OH) ₆Br ₃ ( Ln = Nd, Sm, and Eu)

To study the effects of lanthanide ions on the geometrically frustrated antiferromagnets and their magnetic properties, we grew high-quality single crystals of LnCu ₃(OH) ₆Br ₃ ( Ln = Nd, Sm, and Eu) by hydrothermal method and studied their crystal structures and magnetic properties. The refinements of the crystal structure referred to the powder x-ray diffraction data show that LnCu ₃(OH) ₆Br ₃ adopt a Kapellasite-type layer structure, which is isostructural to their chlorine analogue. Magnetic susceptibilities demonstrate that LnCu ₃(OH) ₆Br ₃ have strong antiferromagnetic coupling and a pronounced magnetic frustration effect. Magnetization measurements indicate canted antiferromagnetic ordering of Cu ²⁺ ions around 16 K within the kagomé plane and weak ferromagnetic coupling. Moreover, shoulder-like anomalies in specific heat around 16 K could be a signature of emergent of magnetic ordering. The low-temperature negative magnetization and specific heat of LnCu ₃(OH) ₆Br ₃ ( Ln = Nd, Sm, and Eu) indicate that Ln ³⁺ ions induce more exotic magnetic ground state properties.