Magnetic skyrmions are particle-like nanometre-sized spin textures of topological origin found in several magnetic materials, and are characterized by a long lifetime. Skyrmions have been observed both by means of neutron scattering in momentum space and microscopy techniques in real space, and their properties include novel Hall effects, current-driven motion with ultralow current density and multiferroic behaviour. These properties can be understood from a unified viewpoint, namely the emergent electromagnetism associated with the non-coplanar spin structure of skyrmions. From this description, potential applications of skyrmions as information carriers in magnetic information storage and processing devices are envisaged.

Topological properties and dynamics of magnetic skyrmions

Skyrmions are topologically protected field configurations with particle-like properties that play an important role in various fields of science. Recently, skyrmions have been observed to be stabilized by an external magnetic field in bulk magnets. Here, we describe a two-dimensional square lattice of skyrmions on the atomic length scale as the magnetic ground state of a hexagonal Fe film of one-atomic-layer thickness on the Ir(111) surface. Using spin-polarized scanning tunnelling microscopy we can directly image this non-collinear spin texture in real space on the atomic scale and demonstrate that it is incommensurate to the underlying atomic lattice. With the aid of first-principles calculations, we develop a spin model on a discrete lattice that identifies the interplay of Heisenberg exchange, the four-spin and the Dzyaloshinskii-Moriya interaction as the microscopic origin of this magnetic state.

Spontaneous atomic-scale magnetic skyrmion lattice in two dimensions

In the field of molecular spintronics, the use of magnetic molecules for information technology is a main target and the observation of magnetic hysteresis on individual molecules organized on surfaces is a necessary step to develop molecular memory arrays. Although simple paramagnetic molecules can show surface-induced magnetic ordering and hysteresis when deposited on ferromagnetic surfaces, information storage at the molecular level requires molecules exhibiting an intrinsic remnant magnetization, like the so-called single-molecule magnets (SMMs). These have been intensively investigated for their rich quantum behaviour but no magnetic hysteresis has been so far reported for monolayers of SMMs on various non-magnetic substrates, most probably owing to the chemical instability of clusters on surfaces. Using X-ray absorption spectroscopy and X-ray magnetic circular dichroism synchrotron-based techniques, pushed to the limits in sensitivity and operated at sub-kelvin temperatures, we have now found that robust, tailor-made Fe(4) complexes retain magnetic hysteresis at gold surfaces. Our results demonstrate that isolated SMMs can be used for storing information. The road is now open to address individual molecules wired to a conducting surface in their blocked magnetization state, thereby enabling investigation of the elementary interactions between electron transport and magnetism degrees of freedom at the molecular scale.

http://www.molspinqip.org/wp-content/uploads/file/magnetic%20memory_1.pdf

Magnetic memory of a single-molecule quantum magnet wired to a gold surface.

Discoveries of intrinsic two-dimensional (2D) ferromagnetism in van der Waals (vdW) crystals provide an interesting arena for studying fundamental 2D magnetism and devices that employ localized spins1–4. However, an exfoliable vdW material that exhibits intrinsic 2D itinerant magnetism remains elusive. Here we demonstrate that Fe3GeTe2 (FGT), an exfoliable vdW magnet, exhibits robust 2D ferromagnetism with strong perpendicular anisotropy when thinned down to a monolayer. Layer-number-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism for thicknesses less than 4 nm (five layers), accompanied by a fast drop of the Curie temperature (TC) from 207 K to 130 K in the monolayer. For FGT flakes thicker than ~15 nm, a distinct magnetic behaviour emerges in an intermediate temperature range, which we show is due to the formation of labyrinthine domain patterns. Our work introduces an atomically thin ferromagnetic metal that could be useful for the study of controllable 2D itinerant ferromagnetism and for engineering spintronic vdW heterostructures5. Metallic ferromagnetism is reported in an exfoliated monolayer of the van der Waals material Fe3GeTe2.

Two-dimensional itinerant ferromagnetism in atomically thin Fe3GeTe2.

Thin film nanoscale elements with a curling magnetic structure (vortex) are a promising candidate for future nonvolatile data storage devices. Their properties are strongly influenced by the spin structure in the vortex core. We have used spin-polarized scanning tunneling microscopy on nanoscale iron islands to probe for the first time the internal spin structure of magnetic vortex cores. Using tips coated with a layer of antiferromagnetic chromium, we obtained images of the curling in-plane magnetization around and of the out-of-plane magnetization inside the core region. The experimental data are compared with micromagnetic simulations. The results confirm theoretical predictions that the size and the shape of the vortex core as well as its magnetic field dependence are governed by only two material parameters, the exchange stiffness and the saturation magnetization that determines the stray field energy.

https://science.sciencemag.org/content/sci/298/5593/577.full.pdf

Direct Observation of Internal Spin Structure of Magnetic Vortex Cores

A two-dimensional antiferromagnetic structure within a pseudomorphic monolayer film of chemically identical manganese atoms on tungsten(110) was observed with atomic resolution by spin-polarized scanning tunneling microscopy at 16 kelvin. A magnetic superstructure changes the translational symmetry of the surface lattice with respect to the chemical unit cell. It is shown, with the aid of first-principles calculations, that as a result of this, spin-polarized tunneling electrons give rise to an image corresponding to the magnetic superstructure and not to the chemical unit cell. These investigations demonstrate a powerful technique for the understanding of complicated magnetic configurations of nanomagnets and thin films engineered from ferromagnetic and antiferromagnetic materials used for magnetoelectronics.

https://science.sciencemag.org/content/sci/288/5472/1805.full.pdf

Real-space imaging of two-dimensional antiferromagnetism on the atomic scale

Scanning tunneling spectroscopy (STS) of thin Fe films on W(110) shows that the electronic structure of domains and domain walls is different. This experimental result is explained on the basis of first-principles calculations. A detailed analysis reveals that the spin-orbit induced mixing between minority ${d}_{xy+xz}$ and minority ${d}_{{z}^{2}}$ spin states depends on the magnetization direction and changes the local density of states in the vacuum detectable by STS. As a consequence nanometer-scale magnetic structure information is obtained even by using nonmagnetic probe tips.

/pdf/magnetization-direction-dependent-local-electronic-structure-5ff8n9lbbk.pdf

Magnetization-direction-dependent local electronic structure probed by scanning tunneling spectroscopy.

In this paper we establish a monolayer of Mn on W~110! as a model system for two-dimensional itinerant antiferromagnetism. Combining scanning tunneling microscopy ~STM!, low-energy electron diffraction, and ab initio calculations performed with the full-potential linearized augmented plane wave method we have studied the structural, electronic, and magnetic properties of a Mn monolayer on W~110!. Our experimental results indicate that in spite of the huge tensile strain Mn grows pseudomorphically on W~110! up to a thickness of three monolayers. Intermixing between the Mn overlayer and the W substrate can be excluded. Using these structural data as a starting point for the ab initio calculations of one monolayer Mn on W~110! we conclude that ~i! Mn is magnetic and exhibits a large magnetic moment of 3.32m B , ~ii! the magnetic moments are arranged in a c(232) antiferromagnetic order, ~iii! the easy axis of the magnetization is in plane and points along the @11 ¯ 0# direction, i.e., the direction along the long side of the ~110! surface unit cell with a magnetocrystalline anisotropy energy of 1.3‐1.5 meV, and ~iv! the Mn-W interlayer distance is 2.14 A. The calculated electronic structure of a Mn monolayer on W~110! is compared with experimental scanning tunneling spectroscopy results. Several aspects are in nice agreement, but one cannot unambiguously deduce the magnetic structure from such a comparison. The proposed two-dimensional antiferromagnetic ground state of a Mn monolayer on W~110! is directly verified by the use of spin-polarized STM ~SP-STM! in the constant-current mode, and an in-plane easy magnetization axis could be confirmed using tips with different magnetization directions. We compare the measurements with theoretically determined SP-STM images calculated combining the Tersoff-Hamann model extended to SP-STM with the ab initio calculation, resulting in good agreement.

/pdf/structural-electronic-and-magnetic-properties-of-a-mn-2ah6wku1c2.pdf

Structural, electronic, and magnetic properties of a Mn monolayer on W(110)

Electric-field-induced changes in scanning tunneling microscopy images of metal surfaces

Scanning tunnelling spectroscopy (STS) of thin Fe films on W(110) shows that the electronic structures of magnetic domains and domain walls are different. This experimental result is explained on the basis of first-principles calculations. A detailed analysis reveals that the spin–orbit induced mixing between minority dxy+xz and minority dz2 spin states depends on the magnetization direction and changes the local density of states in the vacuum detectable by STS. The effect scales in second or fourth order with the magnetization angle relative to the easy axis. Our finding implies that nanometre-scale magnetic structure information can be obtained even by using non-magnetic probe tips. Magnetization dependent measurements show that the canting of adjacent spins has no major influence on the electronic structure of the sample.

https://iopscience.iop.org/article/10.1088/0953-8984/15/5/320/pdf

X. Nie

Papers

Real-space imaging of two-dimensional antiferromagnetism on the atomic scale

Magnetization-direction-dependent local electronic structure probed by scanning tunneling spectroscopy.

Structural, electronic, and magnetic properties of a Mn monolayer on W(110)

Electric-field-induced changes in scanning tunneling microscopy images of metal surfaces

Spin-orbit induced local band structure variations revealed by scanning tunnelling spectroscopy