It was recently reported that heavy-ion irradiation to layered ferromagnetic insulators $\mathrm{Cr}X{\mathrm{Te}}_3$ ($X=\mathrm{Si}$ and Ge) produces amorphous ferromagnetic metals with approximately three times higher transition temperatures ($T_C$) than those of single crystals. This high-$T_C$ metallic state is reminiscent of the correlated metallic phase realized by the Mott transition in single-crystalline $\mathrm{Cr}X{\mathrm{Te}}_3$; however, the detailed mechanism behind this unusual change in the electric and magnetic properties upon amorphization remains elusive. Here, we study the electronic and structural properties of amorphous $\mathrm{Cr}X{\mathrm{Te}}_3$ using transport and optical measurements, as well as x-ray absorption fine structure (XAFS) measurements. In line with the transport dominated by disorder scatterings, bad metallic behavior was observed in the optical conductivity spectra for irradiated samples. The analysis of the extended XAFS oscillations suggests that the basic Cr-Te octahedral structure is likely to remain after amorphization, although distortion and defects are present. As possible mechanisms for the high-$T_C$ amorphous ferromagnetic state, ferromagnetic superexchange interaction is more likely than double-exchange interaction, based on the shrinkage of the charge-transfer gap, local structure change, and change in carrier density.

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• Metal-insulator transition
• Amorphous materials
• Disordered systems
• Mott insulators
• X-ray absorption spectroscopy