The magnetic structure of the $zigzag$ chain family Na$_{x}$Ca$_{1-x}$V$_2$O$_4$ determined by muon-spin rotation

TL;DR

This study uses muon-spin rotation to investigate the magnetic structures of Na$_x$Ca$_{1-x}$V$_2$O$_4$ compounds, revealing a transition from incommensurate spin-density-wave order to complex sub-phases with a modulated helical structure as Na content varies.

Contribution

First muon-spin rotation analysis of the entire Na$_x$Ca$_{1-x}$V$_2$O$_4$ family, identifying magnetic phase transitions and detailed spin structures across different compositions.

Findings

Identified a critical Na concentration at x_c~0.833 separating different magnetic ground states.

Observed incommensurate spin-density-wave order in low Na compounds.

Revealed a modulated helical incommensurate spin structure in the metallic phase.

Abstract

We present muon-spin rotation measurements on polycrystalline samples of the complete family of the antiferromagnetic (AF) $zigzag$ chain compounds, Na$_x$Ca$_{1-x}$V$_2$O$_4$. In this family, we explore the magnetic properties from the metallic NaV$_2$O$_4$ to the insulating CaV$_2$O$_4$. We find a critical $x_c(\sim0.833)$ which separates the low and high Na-concentration dependent transition temperature and its magnetic ground state. In the $x<x_c$ compounds, the magnetic ordered phase is characterized by a single homogenous phase and the formation of incommensurate spin-density-wave order. Whereas in the $x>x_c$ compounds, multiple sub-phases appear with temperature and $x$. Based on the muon data obtained in zero external magnetic field, a careful dipolar field simulation was able to reproduce the muon behavior and indicates a modulated helical incommensurate spin structure of the metallic AF phase. The incommensurate modulation period obtained by the simulation agrees with that determined by neutron diffraction.