Muon spin relaxation studies of the frustrated quasi-two-dimensional square-lattice spin system Cu(Cl,Br)La(Nb,Ta)2O7: Evolution from spin-gap to antiferromagnetic state

We report muon spin relaxation (μSR) and magnetic-susceptibility measurements on Cu(Cl,Br)La(Nb,Ta)2O7, which demonstrate: (a) the absence of static magnetism in (CuCl)LaNb2O7 down to 15 mK confirming a spin-gapped ground state; (b) phase separation between partial volumes with a spin-gap and static magnetism in (CuCl)La(Nb,Ta)2O7; (c) history-dependent magnetization in the (Nb,Ta) and (Cl,Br) substitution systems; (d) a uniform long-range collinear antiferromagnetic state in (CuBr)LaNb2O7; and (e) a decrease in Néel temperature with decreasing Br concentration x in Cu(Cl1−xBrx)LaNb2O7 with no change in the ordered Cu moment size for 0.33≤x≤1. Together with several other μSR studies of quantum phase transitions in geometrically frustrated spin systems, the present results reveal that the evolution from a spin-gap to a magnetically ordered state is often associated with phase separation and/or a first-order phase transition.