$\mu$SR studies of the frustrated quasi-2d 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 ($\mu$SR) and magnetic susceptibility measurements on Cu(Cl,Br)La(Nb,Ta)${}_2$O${}_7$, which demonstrate: (a) the absence of static magnetism in (CuCl)LaNb${}_2$O${}_7$ 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)${}_2$O${}_7$; (c) history-dependent magnetization in the (Nb,Ta) and (Cl,Br) substitution systems; (d) a uniform long-range collinear antiferromagnetic state in (CuBr)LaNb${}_2$O${}_7$; and (e) a decrease of Néel temperature with decreasing Br concentration $x$ in Cu(Cl${}_{1-x}$Br${}_x$)LaNb${}_2$O${}_7$ with no change in the ordered Cu moment size for $0.33\le x\le 1$. Together with several other $\mu$SR 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.

10 pages 10 color figures; revised from letter paper style to full paper style; Figs. 2, 4, 5, 7c, 10 newly added

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• Energy-scale phenomenology and pairing via resonant spin-charge motion in FeAs, CuO, heavy-fermion and other exotic superconductors
• Ferromagnetic frustrated spin systems on the square lattice: a Schwinger boson study
• Ferromagnetically coupled dimers on the distorted Shastry-Sutherland lattice: Application to (CuCl)LaNb2O7
• (CuCl)LaTa2O7 and quantum phase transition in the (CuX)LaM2O7 family (X = Cl, Br; M = Nb, Ta)
• Magnetic glassy phase in FeSeTe single crystals
• Short-range order of Br and three-dimensional magnetism in (CuBr)LaNb2O7
• Synthesis, Structural and magnetic properties of the solid solution (CuCl1-xBrx)LaNb2O7 (0<x<1)