Magnetism of kagome metals $\left(\text{Fe}_{1-x} \text{Co}_{x}\right) \text{Sn}$ studied by $\mu$SR

TL;DR

This study investigates the magnetic properties of kagome metal $ ext{Fe}_{1-x} ext{Co}_x ext{Sn}$ using $ ext{mu}$SR, susceptibility, and STM, revealing a new phase in FeSn and spin glass behavior in Co-rich samples, highlighting the role of doping-induced randomness.

Contribution

It provides the first detailed $ ext{mu}$SR analysis of $ ext{Fe}_{1-x} ext{Co}_x ext{Sn}$, identifying a new magnetic phase and clarifying the origin of spin glass behavior due to doping randomness.

Findings

Identified a new magnetic phase in FeSn below 50 K.

Observed spin glass behavior in Co-rich $ ext{Fe}_{0.2} ext{Co}_{0.8} ext{Sn}$.

Contrasted spin dynamics with those in geometrically frustrated insulators.

Abstract

We study the magnetic properties of the metallic kagome system $\left(\mathrm{Fe}_{1-x} \mathrm{Co}_{x}\right) \mathrm{Sn}$ by a combination of Muon Spin Relaxation ($\mu \mathrm{SR}$), magnetic susceptibility and Scanning Tunneling Microscopy (STM) measurements, in single crystal specimens with Co concentrations $\mathrm{x}=0,0.11,0.8$. In the undoped antiferromagnetic compound FeSn, we find possible signatures for a previously unidentified phase that sets in at $T^*\sim 50$ K, well beneath the Neel temperature $T_N \sim 376$ K, as indicated by a peak in the relaxation rate $1/T_1$ observed in zero field (ZF) and longitudinal field (LF) $\mu \mathrm{SR}$ measurements, with a corresponding anomaly in the ac and dc-susceptibility, and an increase in the static width $1/T_2$ in ZF measurements. No signatures of spatial symmetry breaking are found in STM down to $7$ K. In $\mathrm{Fe}_{0.2} \mathrm{Co}_{0.8} \mathrm{Sn}$, we find canonical spin glass behavior with freezing temperature $T_{g} \sim 3.5 \mathrm{~K}$; the ZF and LF time spectra exhibit results similar to those observed in dilute alloy spin glasses CuMn and AuFe, with a critical behavior of $1 / T_{1}$ at $T_{g}$ and $1 / \mathrm{T}_{1}\rightarrow 0$ as $T \rightarrow 0$. The absence of spin dynamics at low temperatures makes a clear contrast to the spin dynamics observed by $\mu \mathrm{SR}$ in many geometrically frustrated spin systems on insulating kagome, pyrochlore, and triangular lattices. The spin glass behavior of CoSn doped with dilute Fe moments is shown to originate primarily from the randomness of doped Fe moments rather than due to geometrical frustration of the underlying lattice.