Spin disorder state in a site-depleted 3D coupled trillium spin-lattice system Pb1.5Fe2(PO4)3

Abstract

The study of spin liquids has emerged as a prominent subject of investigation in the field of magnetism owing to their notable characteristics encompassing long-range entanglement, fractional excitations, and topological order. The spin-liquid state is an exotic state of matter with no conventional magnetic long-range order and is rarely observed in S = $\frac{5}{2}$ systems. Here, we have studied the magnetic behavior of Pb_1.5Fe₂(PO₄)₃ (PbFPO) through magnetic and heat capacity measurements on polycrystalline samples. The PbFPO compound has two Pb sites with 25% site depletion in the lattice structure. The compound holds two trillium lattices of S = $\frac{5}{2}$ moments in the unit cell, which are connected with additional magnetic couplings similar to that of KSrFe₂(PO₄)₃. Despite the comparatively large Curie–Weiss temperature of -68 K (antiferromagnetic), the magnetic susceptibility data shows no sign of magnetic long-range order down to 2 K. The absence of field-dependent behavior in the heat capacity data up to 110 kOe field indicates that the PbFPO system possesses elevated spin correlations. The magnetic heat capacity data follows the power law behavior, probably stemming from the system’s gapless excitations from the spin-disordered state. Although the compound has a heavy site depletion, the spin disorder state is quite robust and is not even affected by the large magnetic fields.