Collinear Spin-density-wave Order and Anisotropic Spin Fluctuations in the Frustrated $J_1$--$J_2$ Chain Magnet NaCuMoO$_4$(OH)

TL;DR

This study investigates the magnetic phases and spin fluctuations in the quasi-one-dimensional frustrated $J_1$-$J_2$ chain magnet NaCuMoO$_4$(OH) using NMR and heat capacity measurements, revealing complex orderings and anisotropic excitations.

Contribution

It provides detailed experimental characterization of magnetic order and spin dynamics in NaCuMoO$_4$(OH), supporting a theoretical model of frustrated $J_1$-$J_2$ chains with high-quality crystals.

Findings

Identification of spiral and SDW magnetic orders below and above critical field

Observation of anisotropic spin fluctuations with gapped transverse and gapless longitudinal excitations

Validation of the $J_1$-$J_2$ chain model for this compound

Abstract

The phase diagram of the quasi-one-dimensional magnet NaCuMoO$_4$(OH) is established through single-crystal NMR and heat-capacity measurements. The $^{23}$Na and $^1$H NMR experiments indicate a spiral and a collinear spin-density-wave (SDW) order below and above $B_c$ = 1.5-1.8 T, respectively. Moreover, in the paramagnetic state above the SDW transition temperature, the nuclear spin-lattice relaxation rate $1/T_1$ indicates anisotropic spin fluctuations that have gapped excitations in the transverse spectrum but gapless ones in the longitudinal spectrum. These static and dynamic properties are well described by a theoretical model assuming quasi-one-dimensional chains with competing ferromagnetic nearest-neighbor interactions $J_1$ and antiferromagnetic next-nearest-neighbor interactions $J_2$ ($J_1$-$J_2$ chains). Because of the excellent crystal quality and good one dimensionality, NaCuMoO$_4$(OH) is a promising compound to elucidate the unique physics of the frustrated $J_1$-$J_2$ chain.