Local microscopic properties and annealing effect of Rb_0.85Fe_1.9Se₂ single crystals.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid State Chemistry

Background:

• Rb$_{0.85}$Fe$_{1.9}$Se$_{2}$ exhibits mesoscopic phase separation, influencing its physical properties.
• Understanding the interplay between magnetic and nonmagnetic iron sites is crucial for characterizing this material.

Purpose of the Study:

• To investigate the effects of annealing on the phase-separated structure of Rb$_{0.85}$Fe$_{1.9}$Se$_{2}$ single crystals.
• To analyze the dynamics of nonmagnetic and magnetic iron sites and their response to thermal treatment.

Main Methods:

• Mössbauer spectroscopy was employed to identify and characterize distinct iron sites.
• Single crystals were subjected to annealing at the phase separation temperature (T$_{p}$).

Main Results:

• Two distinct iron sites (nonmagnetic and magnetic) were identified via Mössbauer spectroscopy.
• Annealing at T$_{p}$ led to smaller, more homogeneously distributed domains and softened dynamics of nonmagnetic iron sites.
• Magnetic iron sites exhibited a strong magnetic texture with moments oriented perpendicular to the ab-plane.
• A systematic decrease in the isomer shift of the doublet by 0.02(1) mm s$^{-1}$ was observed after annealing.

Conclusions:

• Annealing Rb$_{0.85}$Fe$_{1.9}$Se$_{2}$ effectively controls mesoscopic phase separation and domain morphology.
• The study provides insights into the spin dynamics and magnetic ordering of iron sites in this material.
• Thermal treatment offers a pathway to tune the physical properties of Rb$_{0.85}$Fe$_{1.9}$Se$_{2}$.