Experimental observation of decoupled spin subsystems in decorated square kagomé lattice magnets of the nabokoite family

The square kagomé lattice (SKL) offers a model platform for investigating geometric frustration in 2D systems. Nabokoite-family compounds \nabok{A}{X} (A=Na, K, Cs, Rb and X=Cl, Br) extend this physics to a 3D network, where 2D SKL layers are decorated by interlayer spins. Using electron paramagnetic resonance (EPR), we demonstrate a dramatic splitting of this complex exchange network into two virtually decoupled spin subsystems: absolute calibration of the electron paramagnetic resonance (EPR) absorption reveals that only a fraction of all copper spins in nabokoites is EPR-active and this fraction of the spins orders at the Néel point. Comparison of the EPR absorption and static susceptibility indicates that contribution of the EPR-silent spin subsystem to total magnetic susceptibility decreases on cooling. This direct observation of coexisting magnetic order and possible spin-liquid dynamics within a single compound challenges conventional models of unified exchange networks in decorated frustrated lattices.