$\mu$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

TL;DR

This study uses muon spin relaxation and magnetic susceptibility to explore the magnetic phases of Cu(Cl,Br)La(Nb,Ta)$_{2}$O$_{7}$, revealing a transition from spin-gap to antiferromagnetic states involving phase separation and first-order transitions.

Contribution

It provides detailed experimental insights into the evolution of magnetic states in a frustrated quasi-2D square-lattice system, highlighting phase separation and transition characteristics.

Findings

No static magnetism in CuClLaNb$_{2}$O$_{7}$ down to 15 mK

Phase separation between spin-gap and static magnetism in certain compositions

Long-range antiferromagnetic order in CuBrLaNb$_{2}$O$_{7}$

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 N\'eel 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 $0.33 \leq x \leq 1$. 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.