  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 ~100K. The first-order
transition impacts the magnetic properties, yielding an enhancement in the
Curie temperature T_C from 270 to 310K. In the present work, T_HT ~550K 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 temperature 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 100-120K. Maxima in the
magnetoresistance and anomalous Hall resistance are observed near 120K. 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 120K, but that electrons dominate at lower temperature 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. The published version of this manuscript
is DOI:10.1103/PhysRevMaterials.3.104401 (2019)
