Microscopic structures and magnetic properties are investigated for Fe5−xGeTe2 single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet. An Fe atom (Fe(1)) located in the outermost Fe5Ge sublayer has two possible split‐sites which are either above or below the Ge atom. Scanning tunneling microscopy shows √3 × √3 superstructures which are attributed to the ordering of Fe(1) layer. The √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the observed √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisymmetric exchange interaction. The temperature dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe5−xGeTe2 due to its non‐centrosymmetricity. Analytical study also supports that the observed ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties and the origin of the new vdW ferromagnet, Fe5−xGeTe2 for future topology‐based spin devices.

The recent discovery of magnetic van der Waals (vdW) materials provides a platform to answer fundamental questions on the two-dimensional (2D) limit of magnetic phenomena and applications. An important question in magnetism is the ultimate limit of the antiferromagnetic layer thickness in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures to observe the exchange bias (EB) effect, of which origin has been subject to a long-standing debate. Here, we report that the EB effect is maintained down to the atomic bilayer of AFM in the FM (Fe3GeTe2)/AFM (CrPS4) vdW heterostructure, but it vanishes at the single-layer limit. Given that CrPS4 is of A-type AFM and, thus, the bilayer is the smallest unit to form an AFM, this result clearly demonstrates the 2D limit of EB; only one unit of AFM ordering is sufficient for a finite EB effect. Moreover, the semiconducting property of AFM CrPS4 allows us to electrically control the exchange bias, providing an energy-efficient knob for spintronic devices.

Ultrathin layered crystals of coordinated chromium­(III) are promising not only as two-dimensional (2D) magnets but also as 2D near-infrared (NIR) emitters due to long-range spin correlation and efficient transition between high- and low-spin excited states of Cr3+ ions. In this study, we report on the dual-band NIR photoluminescence (PL) of CrPS4 and show that its excitonic emission bifurcates into fluorescence and phosphorescence depending on thickness, temperature, and defect density. In addition to the spectral branching, the biexponential decay of PL transients, also affected by the three factors, could be well described within a three-level kinetic model for Cr­(III). In essence, the PL bifurcations are governed by activated reverse intersystem crossing from the low- to high-spin states, and the transition barrier becomes lower for thinner 2D samples because of surface-localized defects. Our findings can be generalized to 2D solids of coordinated metals and will be valuable in realizing groundbreaking magneto-optic functions and devices.

Two-dimensional magnets with high Curie temperatures have recently been produced by incorporating the prototypical ferromagnetic element, iron, into 2D van der Waals layers using the iron-based layered compounds known as Fe x GeTe2 (x = 3, 4, 5). In these materials, each layer consists of an Fe x Ge slab sandwiched by chalcogen tellurium (Te) atoms and the layers are stacked by van der Waals interactions. Their intrinsically high Curie temperatures, as high as room temperature, and large magnetization have drawn significant attention. Moreover, their ability to be tuned using their extrinsic parameters, such as gating and doping, makes their application in spintronics promising. Since they are primarily metallic materials with high electrical conductivity, it is easy to characterize their transport properties and thus directly use them for spintronic devices. In this review, we summarize the magnetic properties of layered iron-based magnetic materials and the spin transport phenomena in devices made of these materials, and provide perspective and insight into the characteristics of these unique materials.

FexGeTe2 (x = 3, 4, 5) systems, two-dimensional (2D) van der Waals (vdW) ferromagnet (FM) metals with high Curie temperature (TC), have been intensively studied to realize all-2D spintronic devices....