Unusual Van der Waals Magnetoresistance in Stacked Ferromagnetic FeGeTe: The Role of Atomically Sharp Interfaces.

Interfaces can significantly influence the physical properties of systems, especially in 2D van der Waals (vdW) magnets, where atomically sharp interfaces (ASI) are intrinsic. However, the role of the ASI fields on magnetoresistance (MR) in vdW magnetic layers has largely been overlooked. Here, we investigate the angular dependence of MR in stacked ferromagnetic FeGeTe (FGT). Remarkably, the MR exhibits similar universal behaviors associated with unusual anisotropic magnetoresistance (UAMR), which has been widely observed in almost all magnet/non-magnet bilayers at the nanometer scale, featuring distinct characteristics. Unlike the exponential decay of UAMR with thickness in nanometer-thick bilayers, the UAMR of stacked FGT layers remains insensitive to thickness. The MR in the film plane displays dominant two-fold oscillation, while high-order oscillations of MR exceeding 1.4% are observed in the planes perpendicular to the film, nearly an order of magnitude larger than in-plane anisotropic MR. The UAMR of the FGT films cannot be explained by the well-known spin Hall MR theory based on spin/orbital current and charge current interconversion. Instead, it aligns with the predictions of the two-vector MR theory, particularly its sum-rule constraints. The results provide direct experimental evidence that the ASI field, rather than spin/charge current interconversion, governs the UAMR in vdW ferromagnets.