Physical properties and thermal stability of Fe5-x GeTe2 single crystals

The magnetic and transport properties of Fe-deficient Fe5GeTe2 single crystals (Fe5-xGeTe2 with x≈0.3) were studied and the impact of thermal processing was explored. Quenching crystals from the growth temperature has been previously shown to produce a metastable state that undergoes a strongly hysteretic first-order transition upon cooling below ≈100 K. The first-order transition impacts the magnetic properties, yielding an enhancement in the Curie temperature TC from 270 to 310 K. In the present work, THT≈550 K has been identified as the temperature above which metastable crystals are obtained via quenching. Diffraction experiments reveal a structural change at this temperature, and significant stacking disorder occurs when samples are slowly cooled through this T range. The transport properties are demonstrated to be similar regardless of the crystal's thermal history. The scattering of charge carriers appears to be dominated by moments fluctuating on the Fe(1) sublattice, which remain dynamic down to ≈100-120 K. Maxima in the magnetoresistance and anomalous Hall resistance are observed near 120 K. The Hall and Seebeck coefficients are also impacted by magnetic ordering on the Fe(1) sublattice. The data suggest that both electrons and holes contribute to conduction above 120 K, but that electrons dominate at lower T when all of the Fe sublattices are magnetically ordered. This study demonstrates a strong coupling of the magnetism and transport properties in Fe5-xGeTe2 and complements the previous results that demonstrated strong magnetoelastic coupling as the Fe(1) moments order.