Muon spin relaxation studies of the frustrated quasi-two-dimensional square-lattice spin system Cu(Cl,Br)La(Nb,Ta)(2)O-7: Evolution from spin-gap to antiferromagnetic state

Abstract

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.