Unusual antiferromagnetic order and fluctuations in RbMn$_{6}$Bi$_{5}$

TL;DR

This study uncovers complex antiferromagnetic order and fluctuations in RbMn$_{6}$Bi$_{5}$, revealing magnetic structures and dynamics crucial for understanding its superconductivity, using advanced NMR techniques.

Contribution

It provides the first detailed magnetic structure and fluctuation analysis of RbMn$_{6}$Bi$_{5}$ using NMR, challenging previous models and highlighting magnetic excitations' role in superconductivity.

Findings

Strong antiferromagnetic fluctuations above T_N

First-order transition to complex magnetic order below T_N

Magnetic structure with localized moments and charge rearrangement

Abstract

Quasi-one-dimensional RbMn$_{6}$Bi$_{5}$, the first pressure-induced ternary Mn-based superconductor, exhibits a phase diagram analogous to those of cuprate and iron-based superconductors, with superconductivity neighboring antiferromagnetic order. Here, we use $^{55}$Mn and $^{87}$Rb nuclear magnetic resonance (NMR) to unravel its magnetic structure and fluctuations. Above the N\'eel temperature ($T_{\rm N}$), strong antiferromagnetic fluctuations dominate, characteristic of a paramagnetic state with pronounced spin-lattice relaxation rate enhancement. Below $T_{\rm N}$, a first-order phase transition establishes a commensurate antiferromagnetic order, where Mn atoms at the pentagon corners exhibit distinct magnetic moments with different orientations, while the central Mn atom carries no magnetic moment. The complex magnetic architecture, revealed by zero-field and high-magnetic-field NMR spectra, contrasts with earlier neutron diffraction models proposing uniform spin density waves, instead supporting localized moments ordering with charge rearrangement. The proximity of robust antiferromagnetic fluctuations to the high-pressure superconducting phase suggests a potential role for magnetic excitations in mediating unconventional Cooper pairing, akin to paradigmatic high-$T_c$ systems. These findings provide critical insights into the interplay between geometric frustration, magnetic order, and superconductivity in manganese-based materials.