Néel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures
Summary (3 min read)
Introduction
- Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing physical properties and integration with other 2D family for unique heterostructures.
- Here, the authors report the experimental observation of Néel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet Fe3GeTe2 (FGT).
- Using first principle calculations supported by experiments, the authors unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states.
- The authors then examine the stability of SkX against thermal fluctuation and magnetic fields, which eventually constitutes an experimental phase diagram of the SkX state.
A. Crystal structure and domain configuration
- Figure 1(a) schematically shows the crystal structures of monolayered FGT viewed from xy and yz planes and bilayered FGT exhibiting vdW bonding between monolayers.
- Each FGT monolayer consists of a Fe3Ge covalently bonded slab and two Te layers placed above and underneath the Fe3Ge, and each layer is separated by a 2.95-Å vdW gap in multilayered stack [28].
- It is noteworthy that Rxy measurements yield two distinct slopes (sharp and slanted slopes) in the temperature range 100 K T 180 K, and the slanted area becomes more prominent as temperature increases.
- Using this slanted area, the authors can drive the magnetization into the multidomain state at low temperatures and near zero magnetic fields, as shown in Fig. 1(d).
- The magnetization state of the FGT device was imaged by probing the intensity of transmitted circularly polarized x-ray at the Fe edge (L3 absorption edge), where x-ray magnetic circular dichroism (XMCD) provides contrasts corresponding to the out-of-plane magnetization.
B. Dynamic generation and stabilization of SkX
- Having established that multidomain states can be readily stabilized and observed in FGT, the authors then examined the currentinduced generation of magnetic skyrmions, as summarized in Fig.
- Thus, the spontaneous transition from the labyrinth random domain state to the skyrmionic state is triggered by both the external magnetic fields and strong current pulses.
- The authors performed the same procedure at slightly lower temperature, 100 K, and the consistent transformation into multiple skyrmions is observed and the generated skyrmions remain stable at zero magnetic field, Bz = 0 mT [highlighted in a blue-boxed area in Fig. 3(a)].
- After stabilizing the SkX state, the authors then plotted the experimental phase diagram of magnetic configurations in FGT, based on the real-space STXM measurements as summarized in Fig. 3(c).
- The authors observed three magnetic configuration phases: (i) SkX, (ii) the coexistence of SkX and multidomains, and (iii) saturated ferromagnetic states, where the representative STXM images of each state are included in the right panel of Fig. 3(c).
C. Lorentz transmission electron microscopy (LTEM) study of SkX
- To deeply understand magnetic configurations observed by STXM measurements, the authors first performed the LTEM measurement as summarized in Fig. 4 (see Methods for details [30], and Refs. [39–41] therein).
- Note here that Fresnel-LTEM is useful to detect the in-plane components of Bloch-type spin spirals at defocused modes, whereas it cannot directly observe Néel-type magnetic configurations with zone-axis beam 104410-4 irradiation, due to the cancellation of magnetic inductions between electrons and symmetric in-plane magnetic moments with opposite directions projected by Néel-type spin textures [39,42,43].
- Figure 4(a) first shows in-focus and defocused LTEM images of the FGT sample tilted about −20◦ along the x axis at zero field and 160 K, where dark/bright contrasts are only visible in defocused images.
- To generate SkX, the authors then performed the field cooling (FC) of FGT with an oblique magnetic field of B = −40 mT (the oblique angle is 20◦ to the zone axis).
D. Current-driven motion of isolated skyrmions
- To further highlight the potential of FGT-based 2D vdW heterostructures for skyrmion devices, the authors next demonstrate the current-driven motion of skyrmions in this material, as summarized in Fig.
- Figure 5(a) shows a schematic image of the FGT track and electric contacts fabricated on the Si3N4 membrane for STXM measurements.
- The current was applied 104410-6 along the +x direction, opposite to the electron flow along the −x direction as schematically indicated in each image.
- It is first noteworthy that skyrmions move upon the application of current pulses, and the propagation direction is along the electron-flow direction (against current flow), where this directionality indicates that the skyrmion is driven by spin-transfer torques (STTs) arising within the FGT.
E. First principle calculation on Dzyaloshinskii-Moriya interaction (DMI) from O-FGT
- With these experimental demonstrations of chiral skyrmions, their SkX state, and their current-driven motion in the FGT-based heterostructures, let us now discuss the physical origins of DMI in vdW FGT crystals.
- The authors first examined the possible DMI sources from the FGT crystal symmetry.
- For FGT crystal monolayer the calculated DMI, arising at both Fe/Te interfaces is of almost equal magnitude with opposite sign yielding negligible DMI as expected from the aforementioned crystal symmetry analysis.
- In particular, the concentration of Te atoms at both interfaces rapidly decreases and vanishes upon oxidation, while Fe and Ge concentrations only fluctuate and recover their original values near the largest oxidation areas (oxygen peaks).
- Nevertheless, the authors believe further systematic experimental studies probing the dependence of spin textures on the total FGT thickness, and/or the internal magnetization profile of skyrmions from top to bottom layers in FGT considering the role of van der Waals interactions could shed light into more precise tailoring of DMI and resulting magnetic textures in FGT crystal and heterostructures.
III. CONCLUSIONS
- In summary, the authors observed Néel-type chiral magnetic skyrmions and their lattice phase stabilization in a vdW ferromagnet FGT using high resolution magnetic microscopy.
- The authors examined the stability of SkX in FGT over a wide range of temperatures and magnetic fields, including its zero-field manifestation.
- The authors also demonstrated current-driven motion of individual skyrmions in FGT, highlighting its potential for device applications.
- The authors performed symmetry analysis and first principles calculations to unveil the origins of the emergent Neél-type spin textures, namely DMI at the oxidized interfaces of FGT, which also demonstrates the controllability of chiral states in vdW heterostructures by process and/or interfacial material engineering.
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Frequently Asked Questions (9)
Q2. What future works have the authors mentioned in the paper "Néel-type skyrmions and their current-induced motion in van der waals ferromagnet-based heterostructures" ?
The possibility to achieve and electrically manipulate magnetic skyrmions in vdW magnets marks a significant advance in vdW magnet-based spintronics. Along with the large potential of skyrmions for future spintronic devices to store, process, and transmit data with extremely low power cost, this work will pave a route towards vdW magnet-based topological magnetism and skyrmion electronics. Their results can support a further understanding of the fielddriven and current-driven dynamics of skyrmions in 2D vdW materials and provide guidelines for the design of magnetic devices based on 2D materials.
Q3. Why is the net DMI in the whole FGT structure cancelled?
due to the reflection symmetry of the system, the DMI contributions induced at the top and bottom FeIII sublayers are cancelledwith each other and the net DMI in the whole FGT structure vanishes.
Q4. What is the ESOC distribution of the single crystal monolayer DMI?
For the O-substitution case, the authors find that the single crystal monolayer DMI is nonmonotonic as a function of oxygen concentration, being weakly anticlockwise (respectively strongly clockwise) for low (respectively high) concentrations104410-7[Figs.
Q5. What is the potential of skyrmions in FGT?
The current-driven motion of skyrmion shows the potential of using skyrmions in FGT for functional device applications, such as the racetrack-type memory [20], where skyrmions act as moveable information carriers.
Q6. What is the significance of the DMI in the bulk FGT structures?
Regarding the DMI in the bulk O-substituted FGT structures, very importantly, the authors found additional DMI contributions arising from the proximity of the pure FGT cell with the oxidized layer O-FGT.
Q7. How many skyrmions were observed in Fig. 5(c)?
As shown in Fig. 5(c), the average skyrmion velocity was measured to be ∼1 m/s at a current density Je = 1.4 × 1011 A/m2, below which no skyrmion motion is observed.
Q8. Why is the net clockwise DMI present in the FGT sample?
In fact, even in the case of only the substitution scenario present, the overall net clockwise DMI will be present due to oxidation region asymmetry.
Q9. What is the DPCM measurement and analysis of the FGT flake?
The DPCM measurement and analysis present that the observed magnetic textures are Bloch type, in good agreement with previous LTEM results observed in a FGT flake without oxidized interfaces [23].