Investigation of the magnetic dipole field at the atomic scale in quasi-one-dimensional paramagnetic conductor Li$_{0.9}$Mo$_{6}$O$_{17}$

TL;DR

This study uses $^{7}$Li-NMR spectroscopy and a paramagnetic electron model to analyze the atomic-scale magnetic dipole fields in Li$_{0.9}$Mo$_{6}$O$_{17}$, revealing the magnetic environment and ruling out charge effects in the metal-insulator crossover.

Contribution

It provides detailed measurements of magnetic dipole fields at the Li site in Li$_{0.9}$Mo$_{6}$O$_{17}$ and clarifies their role in the material's properties, especially concerning the metal-insulator transition.

Findings

Magnetic dipole field component has no lattice axial symmetry.

Maximum dipole field at the Li site is 0.35 G under 9 T field.

Mo ions have a small effective magnetic dipole moment of 0.015 μ_B.

Abstract

We report magnetic dipole field investigation at the atomic scale in a single crystal of quasi-one-dimensional (Q1D) paramagnetic conductor Li$_{0.9}$Mo$_{6}$O$_{17}$, using a paramagnetic electron model and $^{7}$Li-NMR spectroscopy measurements with an externally applied magnetic field $B_{0}$ = 9 T. We find that the magnetic dipole field component ($B_{||}^{\text{dip}}$) parallel to $B_{0}$ at the Li site from the Mo electrons has no lattice axial symmetry; it is small around the middle between the lattice $c$ and $a$ axes in the $ac$-plane with the minimum at the field orientation angle $\theta$ = +52.5$^{\circ}$, while the $B_{||}^{\text{dip}}$ maximum is at $\theta$ = +142.5$^{\circ}$ when $B_{0}$ is applied perpendicular to $b$ ($B_{0}$ $\perp$ $b$), where $\theta$ = 0$^{\circ}$ represents the direction of $B_{0}$ $\parallel$ $c$. Further estimate indicates that $B_{||}^{\text{dip}}$ has a maximum value of 0.35 G at $B_{0}$ = 9 T, and the Mo ions have a possible effective magnetic dipole moment 0.015 $\mu_{\text{B}}$ per ion, which is significantly smaller than that of a spin 1/2 free electron. By minimizing potential magnetic contributions to the NMR spectrum satellites with the NMR spectroscopy measurements at the direction where the value of the magnetic dipole field is the smallest, the behavior of the independent charge contributions is observed. This work demonstrates that the magnetic dipole field from the Mo electrons is the dominant source of the local magnetic fields at the Li site, and it suggests that the mysterious "metal-insulator" crossover at low temperatures is not a charge effect. The work also reveals valuable local field information for further NMR investigation which is suggested recently [Phys. Rev. B $\bf{85}$, 235128 (2012)] to be key important to the understanding of many mysterious properties of this Q1D material of particular interest.