Thermal cycling induced alteration of the stacking order and spin-flip in the room temperature van der Waals magnet Fe5GeTe2

The magnetic properties of the quasi-two-dimensional van der Waals magnet Fe5-delta GeTe2 (F5GT), which has a high ferromagnetic ordering temperature T-C similar to 315 K, remain to be better understood. It has been demonstrated that the magnetization of F5GT is sensitive to both the Fe deficiency delta and the thermal-cycling history. Here, we investigate the structural and magnetic properties of F5GT single crystals with a minimal Fe deficiency (|delta| <= 0.1), utilizing combined x-ray and neutron scattering techniques. Our study reveals that the quenched F5GT single crystals experience an irreversible, first-order transition at T-S similar to 110 K upon first cooling, where the stacking order partly or entirely converts from ABC-stacking order to AA-stacking order. Importantly, the magnetic properties, including the magnetic moment direction and the enhanced T-C after the thermal cycling, are intimately related to the alteration of the stacking order. Our work highlights the significant influence of the lattice symmetry to the magnetism in F5GT.