No abstract

Achieving large-scale growth of two-dimensional (2D) ferromagnetic materials with high Curie temperature T C and perpendicular magnetic anisotropy (PMA) is highly desirable for the development of ultracompact magnetic sensors and magnetic memories. In this context, van der Waals (vdW) Cr 2 Te 3 appears to be a promising candidate. Bulk Cr 2 Te 3 exhibits strong PMA and a T C of 180 K. Moreover, both PMA and T C might be adjusted in ultrathin films by engineering composition or strain or applying an electric field. In this work, we demonstrate the molecular beam epitaxy (MBE) growth of vdW heterostructures of five-monolayer quasifreestanding Cr 2 Te 3 on three classes of 2D materials: graphene (semimetal), WSe 2 (semiconductor), and Bi 2 Te 3 (topological insulator). By combining structural and chemical analysis down to the atomic level with ab initio calculations, we confirm the single-crystalline character of Cr 2 Te 3 films on the 2D materials with sharp vdW interfaces. They all exhibit PMA and T C close to the bulk Cr 2 Te 3 value of 180 K. Ab initio calculations confirm this PMA and show how its strength depends on strain. Finally, Hall measurements reveal a strong anomalous Hall effect, which changes sign at a given temperature. We theoretically explain this effect by a sign change of the Berry phase close to the Fermi level. This transition temperature depends on the 2D material in proximity, notably as a consequence of charge transfer. MBE-grown Cr 2 Te 3 /2D material bilayers constitute model systems for the further development of spintronic devices combining PMA, large spin-orbit coupling, and sharp vdW interface.

We present a microscopic magnetic domain imaging study of single-shot all-optical magnetic toggle switching of a ferrimagnetic Gd26Fe74 film with out-of-plane easy axis of magnetization by X-ray magnetic circular dichroism photoelectron emission microscopy. Individual linearly polarized laser pulses of 800 nm wavelength and 100 fs duration above a certain threshold fluence reverse the sample magnetization, independent of the magnetization direction, the so-called toggle switching. Local deviations from this deterministic behavior close to magnetic domain walls are studied in detail. Reasons for nondeterministic toggle switching are related to extrinsic effects, caused by pulse-to-pulse variations of the exciting laser system, and to intrinsic effects related to the magnetic domain structure of the sample. The latter are, on the one hand, caused by magnetic domain wall elasticity, which leads to a reduction of the domain-wall length at features with sharp tips. These features appear after the optical switching at positions where the line of constant threshold fluence in the Gaussian footprint of the laser pulse comes close to an already existing domain wall. On the other hand, we identify the presence of laser-induced domain-wall motion in the toggle-switching event as a further cause for local deviations from purely deterministic toggle switching.

We present a microscopic magnetic domain imaging study of single-shot all-optical magnetic toggle switching of a ferrimagnetic Gd 26 Fe 74 film with out-of-plane easy axis of magnetization by x-ray magnetic circular dichroism photoelectron emission microscopy. Individual linearly polarized laser pulses of 800 nm wavelength and 100 fs duration above a certain threshold fluence reverse the sample magnetization, independent of the magnetization direction, the so-called toggle switching. Local deviations from this deterministic behavior close to magnetic domain walls are studied in detail. Reasons for nondeterministic toggle switching are related to extrinsic effects, caused by pulse-to-pulse variations of the exciting laser system, and to intrinsic effects related to the magnetic domain structure of the sample. The latter are, on the one hand, caused by magnetic domain wall elasticity, which leads to a reduction of the domain-wall length at sharp tipped features. These features appear after the optical switching at positions where the line of constant threshold fluence in the Gaussian footprint of the laser pulse comes close to an already-existing domain wall. On the other hand, we identify the presence of laser-induced domain-wall motion in the toggleswitching event as a further cause for local deviations from purely deterministic toggle switching.