Sustained phase separation and spin glass in Co-doped K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals

TL;DR

This study investigates how cobalt doping affects phase separation, magnetism, and superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals, revealing persistent spin glass behavior and transitions to ferromagnetic metallic states.

Contribution

It provides new insights into the effects of Co substitution on phase separation, magnetic states, and superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$, highlighting the coexistence of spin glass and superconducting phases.

Findings

Superconductivity is suppressed at 3.5% Co doping.

Semiconducting spin glass persists at high Co concentrations.

High Co induces ferromagnetic metallic state.

Abstract

We present Co substitution effects in K$_{x}$Fe$_{2-y-z}$Co$_{z}$Se$_{2}$ (0.06 $\leq$ $z$ $\leq$ 1.73) single crystal alloys. By 3.5\% of Co doping superconductivity is suppressed whereas phase separation of semiconducting K$_{2}$Fe$_{4}$Se$_{5}$ and superconducting/metallic K$_{x}$Fe$_{2}$Se$_{2}$ is still present. We show that the arrangement and distribution of superconducting phase (stripe phase) is connected with the arrangement of K, Fe and Co atoms. Semiconducting spin glass is found in proximity to superconducting state, persisting for large Co concentrations. At high Co concentrations ferromagnetic metallic state emerges above the spin glass. This is coincident with changes of the unit cell, arrangement and connectivity of stripe conducting phase.