Observation of a 1/3 magnetization plateau in Pb₂Cu₁₀O₄(SeO₃)₄Cl₇ arising from (Cu²⁺)₇ clusters of corner-sharing (Cu²⁺)4 tetrahedra

A mixed-valence compound Pb₂Cu₁₀O₄(SeO₃)₄Cl has a complex structure consisting of one nonmagnetic Cu⁺ (S = 0) ion and four nonequivalent magnetic Cu²⁺ (S = 1/2) ions. It exhibits antiferromagnetic ordering at T_N = 10.2 K. At a temperature below T_N, a sequence of spin-flop transition at B_spin-flop = 1.3 T and 1/3 plateau formation at B_spin-flip = 4.4 K is observed in the magnetization curve M(B). The 1/3 magnetization plateau persists at least up to 53.5 T. The spin exchanges of Pb₂Cu₁₀O₄(SeO₃)₄Cl₇ evaluated by performing energy-mapping analysis based on DFT+U calculations show that the magnetic properties of Pb₂Cu₁₀O₄(SeO₃)₄Cl₇ are described by the (Cu²⁺)₇ cluster of corner-sharing (Cu²⁺)₄ tetrahedra, and that each (Cu²⁺)7 cluster has a S = 3/2 spin arrangement in the ground state. The 1/3 magnetization plateau observed for Pb₂Cu₁₀O₄(SeO₃)₄Cl7 is explained by the field-induced flip of every second (Cu²⁺)₇ cluster within a unit cell.