Magneto-optical Kerr effect microscopy is used to show that monolayer chromium triiodide is an Ising ferromagnet with out-of-plane spin orientation. The question of what happens to the properties of a material when it is thinned down to atomic-scale thickness has for a long time been a largely hypothetical one. In the past decade, new experimental methods have made it possible to isolate and measure a range of two-dimensional structures, enabling many theoretical predictions to be tested. But it has been a particular challenge to observe intrinsic magnetic effects, which could shed light on the longstanding fundamental question of whether intrinsic long-range magnetic order can robustly exist in two dimensions. In this issue of Nature, two groups address this challenge and report ferromagnetism in atomically thin crystals. Xiang Zhang and colleagues measured atomic layers of Cr2Ge2Te6 and observed ferromagnetic ordering with a transition temperature that, unusually, can be controlled using small magnetic fields. Xiaodong Xu and colleagues measured atomic layers of CrI3 and observed ferromagnetic ordering that, remarkably, was suppressed in double layers of CrI3, but restored in triple layers. The two studies demonstrate a platform with which to test fundamental properties of purely two-dimensional magnets. Since the discovery of graphene1, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin–valley coupling2, Ising superconductors3,4,5 that can be tuned into a quantum metal6, possible Mott insulators with tunable charge-density waves7, and topological semimetals with edge transport8,9. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered10,11,12,13,14; such a crystal would be useful in many technologies from sensing to data storage15. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin–Wagner theorem16. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals17,18,19. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect20, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics12, and van der Waals engineering to produce interface phenomena15.

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Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

Graphene plasmons are rapidly emerging as a viable tool for fast electrical manipulation of light. The prospects for applications to electro-optical modulation, optical sensing, quantum plasmonics, light harvesting, spectral photometry, and tunable lighting at the nanoscale are further stimulated by the relatively low level of losses and high degree of spatial confinement that characterize these excitations compared with conventional plasmonic materials. We start with a general description of the plasmons in extended graphene, followed by analytical methods that lead to reasonably accurate estimates of both the plasmon energies and the strengths of coupling to external light in graphene nanostructures, including graphene ribbons. We discuss several possible strategies to extend these plasmons towards the visible and near infrared, including a reduction in the size of the graphene structures and an increase in the level of doping. Specifically, we discuss plasmons in narrow ribbons and molecular-size graphene structures. We further formulate prescriptions based on geometry to increase the level of electrostatic doping without causing electrical breakdown. Results are also presented for plasmons in highly-doped single-wall carbon nanotubes, which exhibit similar characteristics as narrow ribbons and show a relatively small dependence on the chirality of the tubes. We further discuss perfect light absorption by a single-atom carbon layer, which we illustrate by investigating arrays of ribbons using fully analytical expressions. Finally, we explore the possibility of exploiting optically pumped transient plasmons in graphene, whereby the optically heated graphene valence band can sustain collective plasmon oscillations similar to those of highly doped graphene, and well-defined during the picosecond time window over which the electron is at an elevated temperature.

Graphene Plasmonics: Challenges and Opportunities

Graphene plasmons are rapidly emerging as a viable tool for fast electrical manipulation of light. The prospects for applications to electro-optical modulation, optical sensing, quantum plasmonics, light harvesting, spectral photometry, and tunable lighting at the nanoscale are further stimulated by the relatively low level of losses and high degree of spatial confinement that characterize these excitations compared with conventional plasmonic materials, alongside the large nonlinear response of graphene. We start with a general description of the plasmonic behavior of extended graphene, followed by analytical methods that lead to reasonably accurate estimates of both the plasmon energies and the strengths of coupling to external light in graphene nanostructures, including graphene ribbons. Although graphene plasmons have so far been observed at mid-infrared and longer wavelengths, there are several possible strategies to extend them toward the visible and near-infrared, including a reduction in the size ...

TOPICAL REVIEW: Nanomagnetics

The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.

https://iopscience.iop.org/article/10.1088/0034-4885/61/7/001/pdf

Ferromagnetic resonance of ultrathin metallic layers

The magnetic anisotropy of nanometer thin films and of nanosize structures is discussed. Experimental methods for the quantitative determination of magnetic anisotropy are described. Magnetocrystalline, shape, and magnetoelastic anisotropy contributions are reviewed, and recent examples for the non-bulk-like magnetic anisotropy and of the temperature dependence of both the magnetization and magnetic anisotropy of nanoscale materials are presented. It is shown that film strain and its relaxation give rise to film thickness dependent anisotropy, which can be misinterpreted as a surface anisotropy. The decisive role of the surface anisotropy for adsorbate-induced spin-reorientation transitions (SRT) is elucidated. The application of x-ray magnetic circular dichroism (XMCD) for the determination of magnetic anisotropy of nanosize islands down to the single atom size is presented.

http://iopscience.iop.org/article/10.1088/0953-8984/16/20/R01/pdf

The magnetic anisotropy and spin reorientation of nanostructures and nanoscale films

The heteroepitaxy of Fe on W(110) is governed mainly by the large lattice mismatch f › 9.4% derived from the elemental lattice constants aW › 3.165 A and aFe › 2.866 A [1]. As a consequence of this mismatch only the first monolayer (ML) [2] iron grows pseudomorphically on W(110) at 300 K [3]. Misfit dislocations caused by the reduction of the strain energy of the Fe film are already created in islands of the second monolayer at a Fe coverage of about 1.5 pseudomorphic monolayers, as shown in a recent scanning tunneling microscopy (STM) study [4]. A central-force model [5] involving bulk iron elastic constants yields an elastic energy per surface atom of order 0.3 eV, which is a formidable contribution to any energy consideration regarding growth, structure, and magnetism of the iron film. Thus, stress induced effects are likely to affect the behavior of the film and will be discussed in this Letter. Not only the morphology of the film undergoes a transition from 1 to 2 ML thickness [4], but also four different magnetic regimes are of interest: (i) a submonolayer region, paramagnetic due to the absence of magnetic percolation [6], (ii) a ferromagnetic one monolayer region, characterized by a pronounced twofold in-plane anisotropy [7], (iii) an intermediate, “ sesquilayer” region, consisting of second layer islands on top of a one monolayer sea exhibiting, reportedly [8], antiferromagnetic order, and (iv) a 2 ML region without striking magnetic properties. The subject of this work is the investigation of the sesquilayer region at a coverage of 1.5 ML. We present for the first time stress measurements with submonolayer sensitivity taken during the growth of ultrathin iron films on W(110). The magnetism of the sesquilayer films is investigated by in situ Kerr effect measurements; the observed coercivity is explained by a novel domain wall pinning mechanism which is estimated to be 10 times stronger than the stress induced coercivity increase. The iron films were grown under ultrahigh vacuum (UHV) conditions on clean W(110) substrates at 300 K. Film and sample cleanliness were checked by Auger electron spectroscopy, the contamination level due to the only contaminants oxygen and carbon was found to be less than

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Film Stress and Domain Wall Pinning in Sesquilayer Iron Films on W(110).

Quantum interference is a coherent quantum phenomenon that takes place in confined geometries. Using spin-polarized scanning tunneling microscopy, we found that quantum interference of electrons causes spatial modulation of spin polarization within a single magnetic nanostructure. We observed changes in both the sign and magnitude of the spin polarization on a subnanometer scale. A comparison of our experimental results with ab initio calculations shows that at a given energy, the modulation of the spin polarization can be ascribed to the difference between the spatially modulated local density of states of the majority spin and the nonmodulated minority spin contribution.

https://science.sciencemag.org/content/sci/327/5967/843.full.pdf

Spin-dependent Quantum Interference within a Single Magnetic Nanostructure

Experimental investigations of spin-polarized electron confinement in nanostructures by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are reviewed. To appreciate the experimental results on the electronic level, the physical basis of STM is elucidated with special emphasis on the correlation between differential conductance, as measured by STS, and the electron density of states, which is accessible in ab initio theory. Experimental procedures which allow one to extract the electron dispersion relation from energy-dependent and spatially resolved STM and STS studies of electron confinement are reviewed. The role of spin polarization in electron confinement is highlighted by both experimental and theoretical insights, which indicate variation of the spin polarization in sign and magnitude on the nanometer scale. This review provides compelling evidence for the necessity to include spatial-dependent spin-resolved electronic properties for an in-depth understanding and quantitative assessment of electron confinement in magnetic nanostructures and interaction between magnetic adatoms.

Dirk Sander

Papers

The magnetic anisotropy and spin reorientation of nanostructures and nanoscale films

Film Stress and Domain Wall Pinning in Sesquilayer Iron Films on W(110).

Spin-dependent Quantum Interference within a Single Magnetic Nanostructure

Spin-polarized quantum confinement in nanostructures: Scanning tunneling microscopy

Stress and magnetic properties of surfaces and ultrathin films