The existence of long-range magnetic order in low-dimensional magnetic systems, such as the quasi-two-dimensional van der Waals (vdW) magnets, has attracted intensive studies of new physical phenomena. The vdW Fe_{N}GeTe_{2} (N=3, 4, 5; FGT) family is exceptional, owing to its vast tunability of magnetic properties. In particular, a ferromagnetic ordering temperature (T_{C}) above room temperature at N=5 (F5GT) is observed. Here, our study shows that, by nickel (Ni) substitution of iron in F5GT, a record high T_{C}=478(6) K is achieved. Importantly, pervasive, beyond room-temperature ferromagnetism exists in almost the entire doping range of the phase diagram of Ni-F5GT. We argue that this striking observation in Ni-F5GT can be possibly due to several contributing factors, including increased 3D magnetic couplings due to the structural alterations. 

/pdf/pervasive-beyond-room-temperature-ferromagnetism-in-a-doped-3hejj9ip.pdf

Pervasive beyond Room-Temperature Ferromagnetism in a Doped van der Waals Magnet

With the advent of the Internet of Things and big data, massive data must be rapidly processed and stored within a short timeframe. This imposes stringent requirements on memory hardware implementation in terms of operation speed, energy consumption, and integration density. To fulfill these demands, 2D materials, which are excellent electronic building blocks, provide numerous possibilities for developing advanced memory device arrays with high performance, smart computing architectures, and desirable downscaling. Over the past few years, 2D‐material‐based memory‐device arrays with different working mechanisms, including defects, filaments, charges, ferroelectricity, and spins, have been increasingly developed. These arrays can be used to implement brain‐inspired computing or sensing with extraordinary performance, architectures, and functionalities. Here, recent research into integrated, state‐of‐the‐art memory devices made from 2D materials, as well as their implications for brain‐inspired computing are surveyed. The existing challenges at the array level are discussed, and the scope for future research is presented.

Integrated Memory Devices Based on 2D Materials

The role of the crystal lattice, temperature and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 is a key open question. Using Lorentz transmission electron microscopy, we experimentally observe topological spin structures up to room temperature in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temperature and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of a centrosymmetric crystal structure. In the pre-cooling phase, these bubbles are observable even without the application of an external field, while in the post-cooling phase, a transformation from bubble domains to stripe domains is seen. To understand the magnetic order in Fe5GeTe2 we compare macroscopic magnetometry characterization results with microscopic density functional theory calculation. Our results show that even up to room temperature, topological spin structures can be stabilized in centrosymmetric van der Waals magnets. 

/pdf/skyrmionic-spin-structures-in-layered-fe5gete2-up-to-room-2rstepwk.pdf

Skyrmionic spin structures in layered Fe5GeTe2 up to room temperature

Recent experiments on Fe5GeTe2 suggested the presence of a symmetry breaking of its conventional crystal structure. Here, using density functional theory calculations, we elucidate that the stabilization of the (√3 × √3)R30° supercell structure is caused by the swapping of Fe atoms occurring in the monolayer limit. The swapping to the vicinity of Te atoms is facilitated by the spontaneous occurrence of Fe vacancy and its low diffusion barrier. Our calculated magnetic exchange parameters show the simultaneous presence of ferromagnetic and antiferromagnetic exchange among a particular type of Fe atom. The Fe sublattice projected magnetization obtained from Monte Carlo simulations clearly demonstrates an exotic temperature-dependent behavior of this Fe type along with a large canting angle at T = 0 K, indicating the presence of a complex noncollinear magnetic order. We propose that the low-temperature crystal structure results from the swapping between two sublattices of Fe, giving rise to peculiar magnetization obtained in experiments.

https://pubs.acs.org/doi/pdf/10.1021/acs.jpclett.2c00692

Unusual Magnetic Features in Two-Dimensional Fe5GeTe2 Induced by Structural Reconstructions

The discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room‐temperature operation of lateral spin‐valve devices using the vdW itinerant ferromagnet Fe5GeTe2 in heterostructures with graphene is demonstrated. The room‐temperature spintronic properties of Fe5GeTe2 are measured at the interface with graphene with a negative spin polarization. Lateral spin‐valve and spin‐precession measurements provide unique insights by probing the Fe5GeTe2/graphene interface spintronic properties via spin‐dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe5GeTe2 along with the presence of negative spin polarization at the Fe5GeTe2/graphene interface. These findings open opportunities for vdW interface design and applications of vdW‐magnet‐based spintronic devices at ambient temperatures.

A Room‐Temperature Spin‐Valve with van der Waals Ferromagnet Fe5GeTe2/Graphene Heterostructure

Novel magnetic ground states have been stabilized in two-dimensional (2D) magnets such as skyrmions, with the potential next-generation information technology. Here, we report the experimental observation of a Néel-type skyrmion lattice at room temperature in a single-phase, layered 2D magnet, specifically a 50% Co-doped Fe5GeTe2 (FCGT) system. The thickness-dependent magnetic domain size follows Kittel's law. The static spin textures and spin dynamics in FCGT nanoflakes were studied by Lorentz electron microscopy, variable-temperature magnetic force microscopy, micromagnetic simulations, and magnetotransport measurements. Current-induced skyrmion lattice motion was observed at room temperature, with a threshold current density, jth = 1 × 106 A/cm2. This discovery of a skyrmion lattice at room temperature in a noncentrosymmetric material opens the way for layered device applications and provides an ideal platform for studies of topological and quantum effects in 2D. 

ARINDAM DAS

Papers

Room-temperature skyrmion lattice in a layered magnet (Fe <sub>0.5</sub> Co <sub>0.5</sub> ) <sub>5</sub> GeTe <sub>2</sub>