Magnetic properties of the frustrated magnet Cu5(PO4)3(OH)4 on a peculiar spin network composed of pentagons and triangles

Abstract

Orthorombic Cu5(PO4)3(OH)4 (pseudomalachite) is a layered compound. Each Cu-O layer in this compound is separated by PO4 tetrahedra. The spin network in the bc-plane of a pseudomlachite is composed of triangles and pentagons, which will induce the spin frustration effect. This network may be a new type of geometrically frustrated spin lattice. Magnetic susceptibility, specific heat, high field magnetization measurements are used to study the magnetic properties of Cu5(PO4)3(OH)4 in natural mineral samples. Magnetic ordering is observed at around 4.2 K. The spin entropy change at the transition temperature is about one-fifth times smaller than that expected for a usual long-range order. A 1/5 magnetization plateau is observed in the high-field magnetization curve. The plateau is explained based on a simple model in which the spin network is assumed to be composed of chain, dimer and monomer units.