France



Magnetic skyrmions are long-lived topological excitations in a small subset of magnetic materials, including some heavy-metal / ferromagnet multilayers with strong interfacial spin-orbit interactions. Skyrmions are a rare example of a compact, unbound, and switchable topological state of matter. Here, “compact” means that each skyrmion is confined to a very small area of the sample. Thanks to this property, even micrometer-sized samples can host a very large number of such topological objects and thereby acquire a very large topological charge. “Unbound” means that skyrmions, unlike vortices or hedgehogs, can exist without their topological counterpart, the antiskyrmion. In fact, skyrmions are often found in pure form, such as ordered (crystalline) or disordered (amorphous) arrays. These dense skyrmion states can be considered a topological phase, with the order parameter being the topological charge. Finally, “switchable” means that, due to the discrete nature of the lattice, skyrmions can be nucleated and annihilated, leading to a change of the global topological charge.&lt;br/&gt;&lt;br/&gt;Switching from a topological trivial state to a skyrmion state is a topological phase transition. Such a phase transition requires the overcoming of strong energy barriers—the same energy barriers that also allow skyrmions to exist at room temperature [1]. Topological phase transitions are therefore expected to be of first order, with a transition dynamic characterized by slow and heterogeneous nucleation, as known, e.g., from the freezing of water. In fact, heterogeneous nucleation is the underlying principle of electrical skyrmion nucleation, which crucially relies on material defects to break the translational symmetry [2].&lt;br/&gt;&lt;br/&gt;Surprisingly, however, we find picosecond homogeneous nucleation of an extended topological phase, comprising a dense array of nanometer-scale magnetic skyrmions, after a single femtosecond laser pulse in metallic ferromagnetic multilayers. Lorentz transmission electron microscopy and skyrmion Hall effect measurements confirm that only skyrmions, and not antiskyrmions, are nucleated by the light pulse, even in an achiral Pt/Co/Pt material.&lt;br/&gt;&lt;br/&gt;This presentation will focus on the nucleation dynamics of this all-optical topological phase transition, which we were able to follow in real time during the early user operation of beamline SCS at the European XFEL [3]. Using time-resolved small angle x-ray scattering, we discovered that rapid, homogeneous nucleation of the skyrmion phase is mediated by a previously undisclosed transient fluctuation state. This state, which is characterized by high spatial frequency magnetic fluctuations, persists for approximately 100 ps after exciting our magnetic multilayer with a femtosecond, infrared laser pulse. The topological phase emerges from these fluctuations by nucleation and coalescence, a mechanism that goes beyond existing theories of topological phase transitions such as the Kibble–Zurek mechanism and the Berezinskii–Kosterlitz–Thouless transition. The process is completed on a time scale of 300 ps. Using atomistic spin dynamics simulations, we confirm that the fluctuation state is key to the ultrafast increase of the global topological charge, enabled by an almost complete elimination of the topological energy barrier in this transient state of matter.

**References:**

[1] F. Büttner, I. Lemesh, and G. S. D. Beach, Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications. Scientific Reports 8, 4464 (2018).&lt;br/&gt;&lt;br/&gt;[2] F. Büttner, I. Lemesh, M. Schneider, B. Pfau, C. M. Günther, P. Hessing, J. Geilhufe, L. Caretta, D. Engel, B. Krüger, J. Viefhaus, S. Eisebitt, and G. S. D. Beach, Field-free deterministic ultrafast creation of magnetic skyrmions by spin–orbit torques. Nature Nanotechnology 12, 1040 (2017).&lt;br/&gt;&lt;br/&gt;[3] F. Büttner, B. Pfau, M. Böttcher, M. Schneider, G. Mercurio, C. M. Günther, P. Hessing, C. Klose, A. Wittmann, K. Gerlinger, L.-M. Kern, C. Strüber., C. von Korff Schmising, J. Fuchs , D. Engel, A. Churikova, S. Huang, D. Suzuki, I. Lemesh, M. Huang, L. Caretta, D. Weder, J. H. Gaida, M. Möller, T. R. Harvey, S. Zayko, K. Bagschik, R. Carley, L. Mercadier, J. Schlappa, A. Yaroslavtsev, L. Le Guyarder, N. Gerasimova , A. Scherz, C. Deiter, R. Gort, D. Hickin, J. Zhu, M. Turcato, D. Lomidze, F. Erdinger, A. Castoldi, S. Maffessanti, M. Porro, A. Samartsev, J. Sinova, C. Ropers, J. H. Mentink, B. Dupé, G. S. D. Beach, and S. Eisebitt, Observation of fluctuation-mediated picosecond nucleation of a topological phase. Nature Materials 20, 30 (2021).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/32457bd794a7357a1245dc6e188b51db.jpg)
Magnetic state of the sample before (left) and after (right) exposure with a single fs laser pulse. The circle (1.5 µm diameter) indicates our field of view. Black (white) indicates magnetization pointing up (down).

![](https://assets.underline.io/uploads/markdown_image/1/image/97b56242ca4e46e74620e4de652ae946.jpg)
Schematic of the process of all-optical topological switching. Starting with a uniform state (1), the sample is heated to a transient fluctuations state (2) by the laser. The fluctuations remove the topological energy barrier and allow skyrmion seeds to nucleate. The final state (3) is a skyrmion state obtained after coalescence and growth of the skyrmion nuclei.

INTERMAG 2021

 Colossal Topological Hall Effect at the Transition Between Isolated and Lattice-Phase Interfacial Skyrmions

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Magnetic skyrmions are long-lived topological excitations in a small subset of magnetic materials, including some heavy-metal / ferromagnet multilayers with strong interfacial spin-orbit interactions. Skyrmions are a rare example of a compact, unbound, and switchable topological state of matter. Here, “compact” means that each skyrmion is confined to a very small area of the sample. Thanks to this property, even micrometer-sized samples can host a very large number of such topological objects and thereby acquire a very large topological charge. “Unbound” means that skyrmions, unlike vortices or hedgehogs, can exist without their topological counterpart, the antiskyrmion. In fact, skyrmions are often found in pure form, such as ordered (crystalline) or disordered (amorphous) arrays. These dense skyrmion states can be considered a topological phase, with the order parameter being the topological charge. Finally, “switchable” means that, due to the discrete nature of the lattice, skyrmions can be nucleated and annihilated, leading to a change of the global topological charge. Switching from a topological trivial state to a skyrmion state is a topological phase transition. Such a phase transition requires the overcoming of strong energy barriers—the same energy barriers that also allow skyrmions to exist at room temperature [1]. Topological phase transitions are therefore expected to be of first order, with a transition dynamic characterized by slow and heterogeneous nucleation, as known, e.g., from the freezing of water. In fact, heterogeneous nucleation is the underlying principle of electrical skyrmion nucleation, which crucially relies on material defects to break the translational symmetry [2]. Surprisingly, however, we find picosecond homogeneous nucleation of an extended topological phase, comprising a dense array of nanometer-scale magnetic skyrmions, after a single femtosecond laser pulse in metallic ferromagnetic multilayers. Lorentz transmission electron microscopy and skyrmion Hall effect measurements confirm that only skyrmions, and not antiskyrmions, are nucleated by the light pulse, even in an achiral Pt/Co/Pt material. This presentation will focus on the nucleation dynamics of this all-optical topological phase transition, which we were able to follow in real time during the early user operation of beamline SCS at the European XFEL [3]. Using time-resolved small angle x-ray scattering, we discovered that rapid, homogeneous nucleation of the skyrmion phase is mediated by a previously undisclosed transient fluctuation state. This state, which is characterized by high spatial frequency magnetic fluctuations, persists for approximately 100 ps after exciting our magnetic multilayer with a femtosecond, infrared laser pulse. The topological phase emerges from these fluctuations by nucleation and coalescence, a mechanism that goes beyond existing theories of topological phase transitions such as the Kibble–Zurek mechanism and the Berezinskii–Kosterlitz–Thouless transition. The process is completed on a time scale of 300 ps. Using atomistic spin dynamics simulations, we confirm that the fluctuation state is key to the ultrafast increase of the global topological charge, enabled by an almost complete elimination of the topological energy barrier in this transient state of matter.

**References:**

[1] F. Büttner, I. Lemesh, and G. S. D. Beach, Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications. Scientific Reports 8, 4464 (2018). [2] F. Büttner, I. Lemesh, M. Schneider, B. Pfau, C. M. Günther, P. Hessing, J. Geilhufe, L. Caretta, D. Engel, B. Krüger, J. Viefhaus, S. Eisebitt, and G. S. D. Beach, Field-free deterministic ultrafast creation of magnetic skyrmions by spin–orbit torques. Nature Nanotechnology 12, 1040 (2017). [3] F. Büttner, B. Pfau, M. Böttcher, M. Schneider, G. Mercurio, C. M. Günther, P. Hessing, C. Klose, A. Wittmann, K. Gerlinger, L.-M. Kern, C. Strüber., C. von Korff Schmising, J. Fuchs , D. Engel, A. Churikova, S. Huang, D. Suzuki, I. Lemesh, M. Huang, L. Caretta, D. Weder, J. H. Gaida, M. Möller, T. R. Harvey, S. Zayko, K. Bagschik, R. Carley, L. Mercadier, J. Schlappa, A. Yaroslavtsev, L. Le Guyarder, N. Gerasimova , A. Scherz, C. Deiter, R. Gort, D. Hickin, J. Zhu, M. Turcato, D. Lomidze, F. Erdinger, A. Castoldi, S. Maffessanti, M. Porro, A. Samartsev, J. Sinova, C. Ropers, J. H. Mentink, B. Dupé, G. S. D. Beach, and S. Eisebitt, Observation of fluctuation-mediated picosecond nucleation of a topological phase. Nature Materials 20, 30 (2021).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/32457bd794a7357a1245dc6e188b51db.jpg)
Magnetic state of the sample before (left) and after (right) exposure with a single fs laser pulse. The circle (1.5 µm diameter) indicates our field of view. Black (white) indicates magnetization pointing up (down).

![](https://assets.underline.io/uploads/markdown_image/1/image/97b56242ca4e46e74620e4de652ae946.jpg)
Schematic of the process of all-optical topological switching. Starting with a uniform state (1), the sample is heated to a transient fluctuations state (2) by the laser. The fluctuations remove the topological energy barrier and allow skyrmion seeds to nucleate. The final state (3) is a skyrmion state obtained after coalescence and growth of the skyrmion nuclei.

technical paper

The Pt-Co-Ni multi-layer system has shown promising tunability of its interfacial magnetic properties (including Dzyaloshinskii-Moriya interaction (DMI) and magnetic anisotropy) for the development of future chiral spintronic devices. Previous work has shown that the system supports stabilization of Néel Skyrmions [1] and efficient movement of Dzyaloshinskii domain walls (DW) [2]. However, the vast majority of the work has focused on the case of a Pt/Co deposition order (from substrate side) leading to predominantly left-handed chiral Neel domain walls. Moreover, it is expected that there will be interesting properties that could result from combining left and right-handed domain walls in more complicated heterostructures [3]. Here, we leverage the Pt-Co/Ni multi-layer system to compare the role of the Pt/Co interface when on the bottom vs top of the asymmetric repeat unit. The film stacks under consideration are [Pt/(Co/Ni)M] N or [Pt/(Ni/Co)M]N and were deposited by DC magnetron sputtering in an argon plasma with pressure fixed at bar (2.5 mTorr). Film stacks were grown on oxidized Si substrates with a Ta/Pt seed layer to induce FCC (111) texture. Pt/(Co/Ni)M]N multi-layers show strong perpendicular magnetic anisotropy (figure 1a) with a pinched perpendicular MH loop characteristic for bubble materials. Lorentz Transmission Electron Microscopy (LTEM) shows contrast only in the tilted orientation confirming the presence of Néel-type Skyrmions and domain walls, which suggests a significant interfacial DMI strength. This is likely due to differences in the DMI associated with Pt/Co and Ni/Pt interfaces. For these samples, Brillouin Light Scattering (BLS) spectroscopy shows a shift in the spin wave spectra corresponding to |D|= 0.21 mJ/m2, which is sufficient to stabilize the Néel configuration. For the [Pt/(Ni/Co) M] N sample with all deposition conditions otherwise identical, there is a marked drop in perpendicular magnetic anisotropy with a less clearly defined pinch in the MH loop (Figure 2 a). Lorentz TEM shows only Bloch type domain walls with no preferred chirality (Figure 2 b). Furthermore, application of a perpendicular field stabilizes achiral Bloch bubbles along with a large number of topologically trivial bubbles (or type 2 bubbles) with two vertical Bloch lines inside. It is reasonable to expect that the reduction in magnetic anisotropy is related to the Pt/Co interface quality, which would in turn impact the DMI strength explaining the absence of Neel DWs. It has previously been proposed that the origin of the deposition order effect is due to intermixing between the Pt and Co [4]. It is possible the that higher adatom energy of Pt results in a higher degree of intermixing at the interface when deposited onto Co, which will be discussed along with possible steps to mitigate the problem experimentally. As a secondary note, although it has a relatively small DMI, this system can serve as a playground for evaluating the role of topology on the annihilation field of the magnetic bubbles. In-situ Lorentz TEM images show an appreciably larger annihilation field for the non-trivial bubbles (i.e. Bloch Skyrmions) than for the trivial bubbles. Statistics on the annihilation field will be presented in detail in this paper.

**References:**

1. D.Lau et al., Physical Review B, vol. 9. P. 184410 (2018) 2. S. Parkin, et al., Science, vol. 320, no. 5873, p. 190–194. (2008) 3. N. Pandey, et al., AIP Advances, vol. 10, no. 1, p. 015233. (2020) 4. R. Lavrijsen et al, Physical review B, vol 91.10, p. 104414. (2015)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/843ae3e7665d6c08eae328e403cd669f.jpg)
Fig 1: a) M-H loop of [Pt(5Å)/Co(4Å)/Ni(6Å)]x50 multilayer structure. Fresnel mode LTEM with: b) 0° tilt and 0 T OOP field showing no DW contrast. c) 20° tilt and 0 T OOP field showing Néel wall contrast. d) 20° tilt and 0.42 T OOP field showing formation of Néel Skyrmions.

![](https://assets.underline.io/uploads/markdown_image/1/image/fb745423377868d2dcda1b3f2ed19a99.jpg)
Figure 2: a) M-H loop of [Pt(5Å)/Ni(6Å)/Co(4Å)]x50 multilayer structure. Fresnel mode LTEM with: b) 0 T OOP field showing labryinth-like DW contrast. c) 0.39 T OOP field showing growth of of bubbles from bloch domain walls. d) 0.49 T OOP field showing a mixed bubble system; red and green circles – achiral bloch bubbles, blue circle – topologically trivial bubbles (type II).

 Consequences of Pt/Co vs Co/Pt deposition order on interfacial magnetic properties in Pt-Co-Ni based asymmetric superlattices

Ultrafast helicity-independent all-optical switching (HI-AOS)1 as well as ps current-pulse driven magnetization reversal2 of ferrimagnetic GdFeCo suggest the pathway of realizing ultrafast magnetization reversal of a MTJ based memory element. Previously, MTJ devices were reported using GdFeCo as a free layer3. However, due to the low spin polarization of GdFeCo and lack of thermal treatment, the TMR was too small (0.6 %) for a practical memory device. Therefore, it is of much interest to switch a ferromagnet, which is known to have higher spin-polarization, with ultrafast excitation. More recently, two routes of HI-AOS switching of a ferromagnet have been demonstrated. Firstly by exploiting ballistic spin polarized current injection, which is shown in a GdFeCo/Cu/[Co/Pt] spin-valve structure4. In this structure, the spin current, generated due to the switching of GdFeCo under optical excitation, is responsible for the switching of Co/Pt multilayers. A second route is RKKY exchange coupling of HI-AOS of GdFeCo to a ferromagnet5,6. On this particular platform, 7 ps switching speed of Co/Pt multilayers has been demonstrated when ferromagnetically coupled with a GdFeCo ferrimagnet. However, GdFeCo does not hold PMA for a patterned dot with a diameter less than about one mm7. Hence, GdFeCo is not desirable for building a coupled ferromagnet-ferrimagnet heterostructure as a storage layer of a nano-patterned memory cell. On the other hand, CoGd ferromagnetic alloy has been shown to maintain its PMA, as well as exhibit HI-AOS when it is patterned down to at least 200 nm diameter7. Therefore, in this work, we investigated the static magnetic properties, all-optical switching and time-resolved magnetization dynamics of a CoGd/Pt spacer (1 to 4 nm)/ [Co/Pt] multilayer heterostructure. We demonstrate that the strength and type of indirect RKKY exchange coupling between the CoGd-ferrimagnet and Co/Pt MLs-ferromagnet depends on the Pt spacer thickness. Irrespective of the type of RKKY coupling, optical toggle switching of both the Co/Pt multilayers and CoGd alloy has been observed. Only for decoupled case (Pt spacer 4 nm), the magnetization of Co/Pt multilayer is not reversed. Figure 1(a) shows that, for 4 nm spacer, although the magnetization of CoGd is reversed after 1.5 ps, the Co/Pt multilayer is only partially demagnetized followed by a slow recovery to its initial magnetization direction. In contrast to the decoupled sample, a ferromagnetically coupled sample (Pt spacer 1nm) exhibits switching of both the CoGd alloy and Co/Pt multilayer, as shown in Fig. 1(b). The CoGd reverses its magnetization after ~ 1.5 ps. The Co/Pt multilayer on the other hand, although has slightly higher demagnetization rate, stays demagnetized then switching at 3 ps. To the best of our knowledge, this is the fastest switching of a ferromagnet ever reported. In order to qualitatively understand the microscopic mechanism of magnetization reversal of Co/Pt MLs and both sublattices of the CoGd alloy, we performed theoretical calculations extending the microscopic three temperature model proposed by Beans et al8. Considering inter-sublattice, intra-sublattice and indirect RKKY exchange scattering, we explain the magnetization reversal dynamics of different magnetic components of ferromagnetically and antiferromagnetically coupled as well as decoupled ferromagnet-ferrimagnet heterostructures. The calculated values of switching times are in agreement with the experimental observations. These results represent a significant step towards ultrafast, nanoscale spintronic memory technologies.

**References:**

1 I. Radu, K. Vahaplar, C. Stamm et al Nature 472, 205 (2011). 2 Y. Yang, R.B. Wilson, J. Gorchon, C.-H. Lambert et al. Sci. Adv. 3, e1603117 (2017). 3 J.Y. Chen, L. He, J.P. Wang, and M. Li, Phys. Rev. Appl. 7, 021001 (2017). 4 S. Iihama, Y. Xu, M. Deb, et al. Adv. Mater. 30, 1 (2018). 5 J. Gorchon, C.H. Lambert, Y. Yang, et al., Appl. Phys. Lett. 111, 042401 (2017). 6 L. Avilés-Félix, L. Álvaro-Gómez, G. Li, et al. AIP Adv. 9,125328, (2019). 7 A. El-Ghazaly, B. Tran, A. Ceballos, et al. Appl. Phys. Lett. 114, 232407 (2019). 8 M. Beens, M.L.M. Lalieu, A.J.M. Deenen, et al., Phys. Rev. B 100, 220409R (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fcdfd06d01c59df2587ebc9398cd4b41.jpg)
Fig. 1. Depth resolved time resolved magnetization dynamics of (a) decoupled ([Co/Pt]3/Co/Pt 4 nm/CoGd/cap) and (b) ferromagnetically ([Co/Pt]3/Co/Pt 1 nm/CoGd/cap) coupled stacks.

 RKKY exchange coupling mediated ultrafast all-optical switching of a ferromagnet

Fast domain wall (DW) motion in magnetic nanostructures is crucial for future spintronic devices, such as racetrack memory [1], spin logic [2] and neuromorphic computing [3]. The discovery of fast DW transport governed by spin-orbit torque (SOT) and the Dzyaloshinskii-Moriya interaction (DMI) in ultrathin magnetic layers on heavy metals was a breakthrough in the development of low energy devices based on DW motion. However, a lack of all-electrical control of DWs in nanoscale devices impedes to bring these advanced materials to practical applications. Therefore, finding energy-efficient ways to electrically write, read and transport DWs is a necessity. As demonstrated in STT-MRAM technology, the magnetic tunnel junction (MTJ) based on a CoFeB/MgO free layer (FL) [4], which offers efficient spin-transfer torque (STT) write and a high tunneling magnetoresistance (TMR) readout signal, could be a potential solution to electrically operate a full functional DW device. However, such an MTJ stack poses significant challenges to realize a DW device, which particularly relate to low DW speed and poor manufacturability of a CoFeB/MgO based DW conduit using industrial integration platforms [5]. Here, we propose and develop a new type of MTJ stack that incorporates typical high DW velocity materials (i.e., Pt/Co) as the second FL into a conventional MTJ with a CoFeB/MgO based FL. We firstly demonstrate that the fundamental properties of a standard MTJ device are not compromised by the integration of DW conduit materials. Complete functional DW devices, consisting of multiple MTJ pillars connected by a common FL, have been fully integrated using such novel MTJ stack, Figure 1 (a). We then demonstrate that all-electrical control of a DW in a nanoscale device, i.e., STT write, TMR read and SOT driven DW motion, can be achieved. As an example, figure 1 (b) presents field-driven domain expansion from P2 through the whole track, after the domain was nucleated by STT in P2. Figure 1 (c) demonstrates the current-driven DW transport from P2 to P1, with small field assistance. No change in resistance was observed in P3, as expected from directional current-driven DW motion. Finally, we show that these DW devices can be used to investigate the SOT driven DW dynamics in nanoscale devices where conventional magnetic imaging techniques become ineffective. Our DW devices show good TMR read-out and efficient STT writing, comparable to current STT-MRAM devices. The devices are fabricated in imec’s 300 mm CMOS fab on full wafers which clears the path for large scale integration. This proof-of-concept thus offers potential solutions for high performance and low-power DW-based devices for logic and neuromorphic applications [5,6].

**References:**

1. Parkin et al., Science 320, 190 (2008) 2. Zhaochu., et al., Nature 579, 214 (2020) 3. Siddiqui., et al., Nano Lett. 20, 1033 (2019) 4. Ikeda, et al., Nat. Mater. 9, 721 (2010) 5. Raymenants, et al., 2018 IEDM, 36.4 6. Raymenants, et al., 2020 IEDM, 21.5

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/84febf49913847a9fbe7f5b8a24e7a0e.jpg)
Figure 1 | (a) TEM image of a DW device. The figure highlights how current can flow through the FL track, transporting DWs from pillar to pillar. (b) Domain was nucleated in P2 by STT, after which external field expanded it to the outer pillars, P1 and P3. (c) A domain was nucleated in P2 by STT, after which directional DW transport was achieved from P2 to P1 when driven by SOT and a small assist field. No DW propagation was observed to P3 as expected [6].

 All-electrical control of nanoscale domain wall devices using magnetic tunnel junction read and write INVITED

Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) has become one of the leading candidates for next generation memory applications, for which a key challenge is the simultaneous achievement of low switching current, high thermal stability and large TMR. Low switching current can be achieved with small device size; however, the thermal stability of perpendicular magnetic tunnel junction is extremely reduced at ultrafine scales. A new approach has been recently proposed by revisiting the shape anisotropy [1, 2]. The concept of Perpendicular Shape Anisotropy (PSA) MTJ is developed with a thicker free layer to keep its thermal stability in single-digit nanometers. Due to the PSA emerges as the main source of perpendicular anisotropy, high thermal stability can be achieved even down to sub-10-nm. The device shows possible application in the nanometer scale and is compatible with the developed CMOS technology. In this manner, the SPICE model of the device is highly required since its operation principle is different from the incumbent STT-MTJ device. In this paper, a dynamic model of the PSA-MTJ is developed, including its micromagnetic simulation using LLG equation. In the nanometer scale, the stochastic term is also included. By analyzing the instantaneous magnetization vector, the switching time is extracted. The impacts of write current scaling on the switching time, the energy consumption and the write failure rate of PSA-MTJ are studied. Based on the model, both the static and dynamic behaviors can be captured. The scale effect of the PMA region with different diameters is studied based on the developed model. Finally, hybrid MTJ/CMOS simulation is performed to validate the developed model for circuit designs and simulations in the single-digit nanometer scale. Figure 1 (a) shows the thermal stability factor Δ as a function of free layer thickness t and diameter D. Figure 1 (b) shows the region achieving both high thermal stability and low critical voltage VC0. Figure 2 shows the dynamic properties of z-component of normalized magnetization with different device sizes, applied current is 70 uA with 15 ns width. The switching time can be extracted from the 90% magnetization reversal point.

**References:**

[1] K. Watanabe, B. Jinnai, S. Fukami, H. Sato, and H. Ohno, Nat Commun., Vol. 9, no. 1, p. 663 (2018). [2] N. Perrissin et al., Nanoscale., Vol. 10, no. 25, pp. 12187-12195 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/637a58d53e5665843272ca818863a115.jpg)
Fig. 1 (a) as function of t and D. (b) Regions satisfying Δ>80 and VC0<0.5 V.

![](https://assets.underline.io/uploads/markdown_image/1/image/f8097a7b412557a5264c57489468377d.jpg)
Fig.2 The z-component of magnetization with different D (t = 15 nm).

 Modeling of Single-Digit Nanometer Perpendicular Shape Anisotropy Magnetic Tunnel Junction Driven by Spin-Transfer-Torque

Spin-orbit torque (SOT) offers a fast and efficient way to control magnetization, making this technique bear exciting prospect both in magnetic random access memories (SOT-MRAM) and data processing, especially in the fields of neural computing and spin logics. Conventionally, only spin-up and spin-down states of a magnetic system are utilized to encode binary data. Meanwhile, spin torques including spin-orbit torque and spin-transfer torque (STT) can switch magnetization between the two states and thus write digits 0 or 1 into the magnetic system. In this work, a bi-SOT strategy (two orthogonal SOTs provided by two vertical in-plane currents) can switch a perpendicular system (Pt/Co/MgO) from the spin-up/spin-down states to a random state with zero averaged mz, which makes three-state storage or ternary computing physically potential. Furthermore, we can use the newly-introduced random state by electric method to reverse chirality (clockwise or counterclockwise direction) of the SOT switching which used to be realized by a direction-reversible magnetic field. The multistate nature and the flexible chirality offered by this bi-SOT technique may find matched applications in ternary computing, neural computing and spin logics.

**References:**

[1] X. Zhang C. H. Wan, Z. H. Yuan, C. Fang, W. J. Kong, H. Wu, Q. T. Zhang, B. S. Tao, X. F. Han, J. Mag. Mag. Mater. 428, 2017, 401-405 [2] C. Wan, X. Zhang, Z. H. Yuan, C. Fang, W. J. Kong, Q. T. Zhang, H. Wu, U. Khan, X. F. Han, Adv. Electron. Mater. 3, 2017, 1600282 [3] X. Wang, C. H. Wan, W. J. Kong, X. Zhang, Y. W. Xing, C. Fang, B. S. Tao, W. L Yang, L. Huang, H. Wu, M. Irfan and X. F. Han et al. Adv. Mater. 30, 2018, 1801318. [4] W. J. Kong, C. H. Wan, X. Wang, B. S. Tao, L. Huang, C. Fang, C. Y. Guo, Y. Guang, M. Irfan, and X. F. Han, Nature Commun. 10, 2019, 233.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2295103c04b09a81d2d0b2aa48b4347e.jpg)

 Chirality-reversible multistate switching via Bi-SOT in a perpendicularly magnetized system

Introduction Spin torque oscillators (STOs) and MRAM devices can exhibit non-linear behaviour which can be exploited to carry out tasks which would otherwise take a long time or require a huge amount of processing power when using conventional, von Neumann computing architecture. Arrays of phase-locked STOs can be used to measure the degree of matching between a test value and a reference value, and this can be used for applications such as image recognition. STOs which are sufficiently close together can be synchronised via the magnetostatic interactions between the devices. The strength of the magnetostatic interactions depends upon the relative location of the STOs, their shape and their magnetic properties. This work examines the requirements for STO synchronisation and ways to extend the correlation time (the time during which the STO magnetisation is phase-locked). The model Arrays of STOs were modelled using micromagnetic simulations. Each STO had a cylindrical shape with a diameter, D, of 60 nm and consisted of a 5 nm thick spin injection layer (SIL) and a 10 nm thick free layer (FL), separated by a 2.5 nm non-magnetic interlayer. The saturation magnetisation, Ms, of the SIL and FL was 637 and 1591 emu/cm3, respectively, with uniaxial anisotropy, Ku, of 8×106 erg/cm3 and zero, respectively. The SIL easy axis was along the z, or vertical axis. An electric current flowing through the STOs exerted a spin torque on the magnetisation, causing the free layer magnetisation to rotate in the x-y plane. In this work the STO diameter was sufficiently small that the free layer magnetisation was largely uniform and no vortices were formed. Results The average free layer magnetisation correlation, 〈M1.M2〉, was calculated for two STOs as a function of their separation and the temperature; the results are shown in fig. 1. When the STOs were very close together (dx &lt; 140 nm) the magnetostatic field was so strong that the electric current could not induce oscillation. As the STO separation increased, the magnetostatic field from one STO acting on the other decayed rapidly, limiting the range of separations over which the magnetisation was synchronised. Increasing the temperature decreased the degree of correlation between the magnetisation. Even when the value of 〈M1.M2〉was high, there were occasions when the magnetisation of the STOs slipped out of phase, i.e. the phase difference exceeded 180°. Such events are referred to as phase slips and the time interval between phase slips is denoted here as the correlation time. For practical applications the magnetisation should remain phase-locked for 1000 - 10000 oscillations, equivalent to correlation times of 200 - 2000 ns in the case of the STOs described here, which had an oscillation frequency of about 5 GHz. The correlation time decreased as the number of STOs in the array increased. Fig. 2 shows the average correlation time for an array of nine STOs in which the angle between the STOs, θ, was varied from zero (STOs arranged in a straight line along the x axis) to 60°. The correlation time was calculated using the magnetisation of the STOs at either end of the array. When the STOs were in a straight line (θ = 0°) the maximum correlation time was about 30 ns for STOs with a separation of 180 nm. Introducing an angle between the STOs increased the correlation time. When θ = 60° the correlation time was increased by more than an order of magnitude compared to θ = 0°. When dx = 180 nm and θ = 60° the correlation time exceeded the maximum simulation time of 10 μs, and for dx = 220 nm the correlation time was about 300 ns. The reason for the increase in correlation time with θ can be understood by plotting the magnetostatic field in the fifth (central STO) as a function of θ for the case when the FL magnetisation was perpendicular to the array of STOs, i.e. along the y axis, as shown in the inset to fig. 2. When θ = 0° the component of Hd along the y axis was negative and the magnetostatic energy favoured an anti-parallel alignment of the magnetisation in adjacent STOs, increasing the probability of a phase slip. As the angle between STOs was increased the magnetostatic field approached zero and became positive for 44° &lt; θ &lt; 61°, i.e. the magnetostatic energy favoured parallel alignment of the FL magnetisation over this range of θ and the correlation time was increased. The total output power of arrays of STOs also increased when an angle was introduced between the STOs. The effect of STO size and position distributions will also be discussed.

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8212c931a853017bc62c0e888bc7f90a.jpg)
Fig. 1 Correlation of free layer magnetisation of two STOs as a function of separation and temperature.

![](https://assets.underline.io/uploads/markdown_image/1/image/2667bb73da57e86310396a6f9eb251e6.jpg)
Fig. 2 Correlation time of STO magnetisation for arrays of nine STOs. T = 300 K. Inset: Hd in centre STO vs. angle between STOs for dx = 200 nm.

 Increasing the Correlation Time of Spin Torque Oscillators Synchronised by Magnetostatic Interactions

Human brain-inspired neuromorphic computing (NC), which emulates neural activities, is considered for the low-power implementation of artificial intelligence. It gives the benefit of parallel processing, ultralow power consumption, self-learning, and fault tolerance. The realization of hardware elements, such as artificial synaptic devices, which are the basic building blocks of neuromorphic computing systems, is an essential path towards this goal. Two terminal artificial synaptic devices have been studied using resistive random-access memory and memristors and have emulated the synaptic functions. However, in these two-terminal devices, both learning and signal transmission cannot occur concurrently, which leads to an incomplete emulation of a synapse. On the other hand, in a three-terminal synaptic device, signal transmission and learning occur independently through the channel and external gate respectively, leading to complete emulation of a synapse. Much progress has been made in three-terminal devices based on oxides [1], organic materials [2,3], and 2D materials [4]. Electrolyte gating has been widely used for conductance modulation by massive carrier injections at a very low gate voltage. However, studies based on metallic ferromagnetic systems remain highly unexplored. Additionally, a synaptic device based on magnetic metal has a spin degree of freedom that can be tuned along with. In this work, we have demonstrated a three-terminal coplanar synaptic transistor based on permalloy by electrolyte gating. The channel consists of a stack of Ta (3 nm)/NiFe (3 nm)/Ta (1 nm) on the silicon substrate. EMIM-TFSI ionic liquid was used as the gate dielectric. Fig. 1(a) shows the transfer curve measured at different sweeping rates. A giant anticlockwise loop opening in the transfer curve shows a non-volatile and reversible change of channel conductance suitable for emulating synaptic functions. We have obtained the highest room temperature conductance modulation of 11 % for a metal at the slowest sweeping rate, as shown in Fig. 1(b). The transistor's working principle is based on the formation of an electric double layer (EDL) at the interface of ionic liquid and channel and by intercalation/ extraction of oxygen ions. Multilevel non-volatile states are a prerequisite for realizing analog computing. We have obtained gating controlled multilevel, non-volatile, reversible conducting states by applying positive gate pulses of different amplitude shown in Fig. 1(c). The channel conductance remained constant at zero gating showing the non-volatile nature of the state. Furthermore, the channel conductance can be brought back to the initial state by applying negative gate pulses. The retention of conducting states is checked separately, which shows retention of up to a few hours or more. In this permalloy based artificial synapse, the gate pulses act as the external stimuli, and the channel conductance act as the synaptic weight. We have emulated the essential synaptic functions at room temperature. A basic form of short-term plasticity like excitatory postsynaptic conductance (EPSC) and paired-pulse facilitation (PPF) has been emulated, as shown in Fig. 2(a) and Fig. 2(b), respectively. Moreover, we have shown a transition from short-term memory (STM) to long-term memory (LTM) by varying the gate pulse amplitude, duration, and number shown in Fig. 2(c). The change in channel conductance is more significant at higher amplitude, duration, and number due to the intercalation/extraction of more oxygen ions into the channel. Long-term plasticity behavior like long-term potentiation (LTP) and multiple potentiation and depression cycles have been demonstrated in Fig. 2(d)-(f). The writing and reading energy per synaptic event are calculated at different gate amplitude and duration showing a linear escalation. The lowest writing energy consumed in our device is 0.6 nJ, and can be further reduced by reducing the channel dimension and the pulse width. The variation of magnetic transport properties of the device with gating is also studied. Along with the emulation of synaptic functions, the device showed dynamic filtering behavior and can act as a high-pass filter. These results provide an insight into the potential application of ferromagnetic metal-based synaptic transistor for large-scale, low power consuming, silicon compatible neuromorphic computing network.

**References:**

1 J.T. Yang, C. Ge, J.Y. Du, H.Y. Huang, M. He, C. Wang, H. Bin Lu, G.Z. Yang, and K.J. Jin, Adv. Mater. 30, 1 (2018). 2 W. Xu, S.Y. Min, H. Hwang, and T.W. Lee, Sci. Adv. 2, 1 (2016). 3 Y. Van De Burgt, E. Lubberman, E.J. Fuller, S.T. Keene, G.C. Faria, S. Agarwal, M.J. Marinella, A. Alec Talin, and A. Salleo, Nat. Mater. 16, 414 (2017). 4 C.J. Wan, Y.H. Liu, P. Feng, W. Wang, L.Q. Zhu, Z.P. Liu, Y. Shi, and Q. Wan, Adv. Mater. 28, 5878 (2016).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/4dab6706c8eb7a634a11f93f16285802.jpg)
Fig. 1(a) Transfer curve measured at a different sweeping rate, ISD= 10 µA, channel conductance shown as a function of gate voltage. (b) The conductance modulation with the sweeping rate. (c) Multilevel, nonvolatile, reversible conducting states realized by applying 30 s gate pulses of different amplitude and brought back to the initial state by applying a negative pulse.

![](https://assets.underline.io/uploads/markdown_image/1/image/869a416cb7a3e2e10b9634698752e378.jpg)
Fig. 2 (a) Excitatory postsynaptic conductance (EPSC) triggered by positive presynaptic spike, (2 V, 20 s), ISD= 10 µA (b) Paired pulse facilitation (PPF) by applying pair of pulses (2 V, 5 s), 3s apart (c) EPSC triggered by presynaptic spikes of different amplitude (Δt= 30 s) showing the transition from STM to LTM (d) Long term potentiation (LTP) triggered by 50 consecutive presynaptic spikes (2 V, 3 s) (d) Biological potentiation and depression in series with 20 positive and negative pulses, VG = ± 1 V (e) Cycles of potentiation and depression showing repeatability of emulation of synaptic function.

 Based Synaptic Transistor for Neuromorphic Computing

Over the last decade, magnetization manipulation based on spin-orbit torque has attracted wide research attention as it shows tremendous potential for the next generation ultrafast and power efficient nonvolatile memories. Very recently, centrosymmetric L10 FePt is reported to achieve the room temperature SOT switching in single layer and a new type of inversion asymmetry called composition gradient is responsible for the observed SOT. Due to its inherent superior perpendicular magnetic anisotropy (PMA), the FePt in L10 phase enables high thermal stability when device size reduced to nanoscale, which may provide a simple structure towards industry application. In this work, current-induced perpendicular magnetization switching is obtained in 6 nm thick L10 FePt single layer and in Ta/6nm FePt bilayer, respectively. The switching ratio of L10 FePt single-layer (Figure 1 (e)) in the optimum applied magnetic field (500Oe) is relatively small, about 3.3%. However, the Ta/FePt bilayers have a slightly larger switching ratio (about 4.3%) than the L10 FePt single-layer. In addition, the stability of the switching loop of the Ta/FePt bilayers film is relatively better and the critical current density is reduced in the case of a Ta/FePt bilayer. Moreover, we found that the SOT effective fields become smaller in FePt/Ta bilayer compared to that in single FePt layer by measuring the harmonic Hall voltages, which may attribute to the combined effect of the opposite spin current generated by Ta HM layer and the smaller perpendicular anisotropy. The SOT efficiency βDL of the Ta/FePt bilayers is 3.2×10-10 Oe m2A-1, which is several times of the traditional Ta/CoFeB/MgO structure reported before. The results indicate a new scheme of current-induced magnetization switching for high PMA and high coercive materials, which is expected to be applied to spintronics devices with high thermal stability and ultra-high storage density in the future.

**References:**

Fukami S, Zhang C, DuttaGupta S, et al, Nature materials, 2016, 15(5): 535-541. Liu L, Yu J, Gonzalez-Hernandez R, et al, Physical Review B, 2020, 101(22): 220402. Manchon A, Zelezny J, Miron I M, et al., Reviews of Modern Physics, 2019, 91(3): 035004. Tang M, Shen K, Xu S, et al , Advanced Materials, 2020, 32(31): 2002607. R. Ramaswamy, J. M. Lee, K. Cai, et al, Appl. Phys. Rev. 5, 031107(2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c8bb1f4b27f941b96b8e5a272593f346.jpg)
Fig.1 (a) A schematic of Ta/FePt bilayer structure.(b) Schematic of Hall bar structure (c)Schematic illustration of Hall device together with the measurement circuit.(d) R-H hysteresis characteristic measured by AHE. Positive RAHE corresponds to“upward” magnetization, whereas negative RAHE corresponds to “downward” magnetization.(e)and (f) SOT switching loops obtained under varying Hx for (e) 6-nm FePt single layer and (f) 5-nm Ta/ 6-nm FePt bilayer. (g)-(j) Field dependence of first and second-harmonic signals for 6-nm FePt film (Iac = 6mA).

 Current induced magnetization switching in L10 FePt and Ta/FePt films with large perpendicular magnetic anisotropy through spin-orbit torque

Magnetic skyrmions are topologically-nontrivial spin textures with particle-like properties [1]. Their size, topological stability, and mobility suggest their use in future generations of spintronic devices, the prototype of which is the skyrmion racetrack [2]. To realise a racetrack requires three basic operations: the nucleation (writing), propagation (manipulation), and detection (reading) of a skyrmion, all by electrical means. Here we show that all three are experimental feasible at room temperature in Pt/Co/Ir or Pt/CoB/Ir multilayers in which the different heavy metals above and below the magnetic layer break inversion symmetry and induce chirality by means of the Dzyaloshinskii-Moriya interaction, defining the structure of Néel skyrmion spin textures [3]. We show deterministic nucleation on nanosecond timescales using an electrical point contact on top of the multilayer [4] (Figure 1), current-driven propagation along a wire in which the skyrmions are channelled by defects in the multilayer [5], and their detection by means of the Hall effect (Figure 2) that reveals an unexpectedly large contribution to the Hall signal that correlates with the topological winding number [6].

**References:**

[1] N. Nagaosa & Y. Tokura, Nat. Nanotech. 8, 899 (2013). [2] A. Fert et al. Nature Nanotech. 8, 152 (2013). [3] K. Zeissler et al. Sci. Rep. 7, 15125 (2017). [4] S. Finizio et al., Nano Lett. 19, 7246 (2019). [5] K. Zeissler et al., Nature Comm. 11, 428 (2020) [6] K. Zeissler et al. Nature Nanotech. 13, 1161 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e2d829c356fc36105eeb0bb2e9c2db72.jpg)
Figure 1. STXM images (a) before and (b) after electrical nucleation of a skyrmion at a 500 nm wide injector contact to a Pt/CoB/Ir multilayer.

![](https://assets.underline.io/uploads/markdown_image/1/image/031a069098aadac7fb343bb1f3bd715d.jpg)
Figure 2. STXM image of two skyrmions in an electrically-connected 1 µm diameter Pt/Co/Ir multilayer dot. 500 nm scale bar.

 Skyrmions in chiral magnetic multilayers

Systems in which magnetic skyrmions occur at room temperature could enable new computing architectures as well as compact and energetically-efficient storage such as racetrack memories. There are two fundamental problems that prevent the successful implementation of skyrmions in a racetrack device. A first problem is not being able to restrict the skyrmion motion along straight paths and one solution is the use of antiferromagnetically-coupled skyrmions [1]. The second problem is not having stable inter-skyrmion distances, which translates into fluctuating distances among bits. A potential solution was revealed with the first experimental observation of coexisting skyrmions and chiral bobbers [2], opening the possibility that a chain of binary data bits could be encoded by two different solitons. However, this has only been observed in B20-type single crystalline materials and at low temperatures. With our work, we present a ferro-/ferri-/ferromagnetic multilayer system that can host two distinct skyrmion phases at room temperature. The two phases represent a tubular (strong constrast) and a partial (weak contrast) skyrmion (see Figure 1), as revealed from quantitative magnetic force microscopy data complemented by micromagnetic simulations. Furthermore, the tubular skyrmion can be converted into a partial skyrmion. Given its high tunability, this hybrid system serves as a promising platform for future skyrmionic devices.

**References:**

[1] W. Legrand et al., Nat. Mater. 19, 34–42 (2020) [2] F. Zheng et al., Nat. Nanotechnol. 13, 1–7 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8658ad08d7d440f2f349350bb470ce35.jpg)
Fig. 1 Upper: Schematics of the trilayer sample consisting of two ferromagnetic skyrmion (SK) layers separated by a 3.8-nm thick ferrimagnetic (Fi) layer; SK = [Ir(1nm)/Fe(0.3nm)/Co(0.6nm)/Pt(1nm)]×5 and Fi = [TbGd(0.2m)/Co(0.4nm)]×6/TbGd(0.2nm). The weak skyrmions are depicted by the vertical pale blue rectangles penetrating through the SK layers only. The strong skyrmions are indicated by wider dark blue rectangles penetrating through all layers. Lower: 2 × 2 µm2 magnetic force microscopy image taken in a magnetic field of about 133 mT, highlighting a weak and a strong skyrmion in the lower two panels.

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Consequences of Pt/Co vs Co/Pt deposition order on interfacial magnetic properties in Pt-Co-Ni based asymmetric superlattices

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