In Situ Electric Field Skyrmion Creation in Magnetoelectric Cu2OSeO3

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In situ Electric Field Skyrmion Creation in Magnetoelectric CuOSeO Ping Huang, Marco Cantoni, Alex Kruchkov, Rajeswari Jayaraman, Arnaud Magrez, Fabrizio Carbone, and Henrik M. Rønnow Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02097 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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In situ Electric Field Skyrmion Creation in Magnetoelectric Cu2OSeO3 ∗,†,‡

Ping Huang,

Marco Cantoni,

§

Magrez,

†Laboratory

¶

Alex Kruchkov,

∗,‡

Fabrizio Carbone,

†

Rajeswari Jayaraman,

‡

Arnaud

∗,†

and Henrik M. Rønnow

for Quantum Magnetism (LQM), Institute of Physics, École Polytechnique

Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

‡Laboratory

for Ultrafast Microscopy and Electron Scattering (LUMES), Institute of

Physics, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

¶Centre

Interdisciplinaire de Microscopie Électronique (CIME), École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

§Crystal

Growth Facility, Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

E-mail: ping.huang@ep.ch; fabrizio.carbone@ep.ch; henrik.ronnow@ep.ch

Abstract Exploiting additional degrees of freedom in solid state materials may be the most promising solution when approaching the quantum limit of the Moore's law for the conventional electronic industry. Recently discovered topologically non-trivial spin textures, skyrmions, are outstanding among such possibilities. However, controlled creation of skyrmions, especially by electric means, remains a pivotal challenge towards technological applications. Here, we report that skyrmions can be created locally by

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electric eld in the magnetoelectric helimagnet Cu2 OSeO3 . Using Lorentz transmission electron microscopy, we successfully write skyrmions in situ from a helical spin background. Our discovery is highly coveted since it implies that skyrmionics can be integrated into modern eld eect transistor based electronic technology, where very low energy dissipation can be achieved, and hence realizes a large step forward to its practical applications.

Keywords skyrmions, Lorentz transmission electron microscopy, multiferroics, skyrmion dynamics, image processing

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Magnetic skyrmions are localized nanometric spin textures with quantized winding numbers as the topological invariant. 1 Rapidly increasing attention has been paid to the investigations of skyrmions since their experimental discovery in 2009, 2 due both to the fundamental properties and the promising potential in spintronics based applications. 3,4 To date, several categories of materials have been discovered to host skyrmions, including heavy metal/ferromagnet interfacial systems; 3 metallic chiral crystals such as MnSi, 2,5 FeGe, 6 Fex Co1−x Si 7 and β -Mn-type alloy Co-Zn-Mn; 8 and insulators GaV4 S8 9 and Cu2 OSeO3 . 10,11 As potential candidates of the building block for novel spintronic applications, eorts have been made to achieve creation and/or manipulation of skyrmions in these systems. Combination of electric currents and temperature gradients can rotate the skyrmion lattice (SkL) in MnSi. 5 Meta-stable skyrmions have been observed to exist in much larger regions in the phase diagram than the equilibrium ones via controlled temperature-magnetic eld paths in MnSi 12 and Co8 Zn8 Mn4 . 13 Individual laser pulses have also been demonstrated recently to induce the formation of metastable skyrmions in FeGe. 14 In the presence of a ferromagnetic background, creation of skyrmions at the heavy metal/ferromagnet interfaces have been realized by in-plane inhomogeneous electric currents, 15 and out-of-plane spin polarized currents 16 or electric elds 17 (E -elds) applied through a STM tip. However, most of the aforementioned attempts of creating and manipulating skyrmions used electric current as the controlling parameter. This is not ideal because, rstly it is energy dissipating, and more importantly it is not straightforward to integrate into modern electronic technology based on eld eect transistors. The insulating skyrmion host Cu2 OSeO3 , on the other hand, provides an alternative due to magnetoelectric coupling arising from spin-orbital eects. 10,11,18,19 As a result, emergent electric polarizations appear in the magnetically ordered phases of Cu2 OSeO3 . The coupling between the external E -eld and the emergent electric polarization (the −E · P coupling) in Cu2 OSeO3 provides a direct handle for tuning the relative energies of the competing magnetic textures, which enables the manipulation of skyrmions. For example, it has been demonstrated that the SkL in 3


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Figure 1: (a) Schematic illustration of the H-bar LTEM sample conguration with Pt electrodes deposited by FIB directly on the top surface of the sample. The 150 nm thick LTEM lamella is perpendicular to the [1¯ 10] direction, along which the out-of-plane magnetic eld is applied. The electric eld is along [11¯ 1] direction on the sample plane. (b) Schematic magnetic eld-temperature phase diagram for a nano-slab Cu2 OSeO3 sample. Below the magnetic ordering temperature of 60K, the system has a helical spin ground state existing at low magnetic eld. Phase transition to the skyrmion phase can be achieved by increasing the magnetic eld. The initial state of the sample for our experiment is set to be in the helical phase and close to the helical-skyrmion phase boundary, as indicated by the solid dot. (c) Schematic illustration of the transition of the spin textures from the helical phase to the skyrmion phase upon the application of the E-eld, and vice versa.

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Cu2 OSeO3 can be rotated by an E -eld. 20 An extremely interesting consequence of the −E · P coupling is the possibility of creating skyrmions. Controlling of the size of the skyrmion phase pocket in Cu2 OSeO3 by E -eld has recently been observed by magnetic susceptibility and microwave spectroscopy 21 and conrmed by neutron scattering, 22 yet local and real space/time demonstration has been lacking. Meanwhile, theoretical work of creating skyrmions in chiral crystal systems by E eld remains scarce and is limited to creating skyrmions from the ferromagnetic state, 23,24 rather than from the low-energy spin helical phase. Here, we report an experimental realization of locally creating skyrmions by E -eld from the helical phase observed in situ by Lorentz transmission electron microscopy (LTEM). LTEM is a very suitable tool for studying spin textures with in-plane long wavelength modulations. What makes it particularly exciting is the capability of the in situ tuning of conditions such as the E -eld, the magnetic eld and the temperature, while observing the system in real space and real time. Analysis of LTEM data can further give local and temporal information. 25 For the experiments reported here, a Cu2 OSeO3 single crystal was aligned by X-ray Laue diraction and cut into a rectangular plate so that the main surfaces are perpendicular to

[1¯10] direction and the two edges are along [11¯1] and [112] respectively. The plate was polished mechanically down to 20µm, and was attached onto a semi-circle Mo washer. Platinum electrodes for applying an in-plane E -eld were deposited directly on the top surface of the sample by focused ion beam (FIB) with a spacing of 56 µm. A 20 µm wide section in between the electrodes was further milled down by FIB to a thickness of 150 nm to allow LTEM observation. Previous susceptibility and neutron studies were performed in the geometry B k E k [111]. However, that geometry is not suitable for our LTEM experiments since the electrodes will have to be deposited on the path of the incident electron beam on both the top and the bottom surfaces of the sample. Instead, we chose a geometry of B k [1¯ 10] and E k [11¯ 1]. In this geometry, the spontaneous electric polarization of the skyrmions is 5


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thus along [00¯ 1], 11 which is on the sample plane and couples to the applied E -eld. The sample conguration is illustrated in Figure 1a. More details about the sample preparation and the experiments can be found in the Supporting Information. In the phase diagram of nano-slab Cu2 OSeO3 samples, as sketched in Figure 1b, the helical phase can be found at low magnetic elds, followed by the SkL phase until the ferrimagnetic phase is reached at high elds. Our measurements were performed at T = 24.7 K with the application of an out-of-plane magnetic eld 254 Oe, placing the sample in the helical phase but close to the SkL phase, as schematically indicated by the solid dot in the phase diagram in Figure 1b. The application of an E -eld along [11¯ 1] direction was observed to convert the helical magnetic texture into skyrmions, as schematically illustrated in Figure 1c.

Figure 2: (a) LTEM image with no applied E -eld. (b) LTEM image at E =+3.6 V/µm. (c) LTEM image at E =−3.6 V/µm. These real space observations clearly show that only the positive E -eld creates skyrmions. All gures are treated by a LoG lter. (d) Directions of the E -elds, the magnetic eld and the spontaneous electric polarization of the skyrmion phase are shown with correspondence to the LTEM images. A typical LTEM image without the application of the E -eld is shown in Figure 2a. The 6


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system is in the helical phase with multiple helical domains having propagation vectors close to [11¯ 1] and [00¯1] directions respectively. A few skyrmions can be found near the domain boundaries, demonstrating the proximity to the phase transition (More LTEM images with larger eld of view can be found in the Supporting Information). The existence of local skyrmions in the helical phase was recently addressed theoretically and experimentally. 26 After switching on a positive E -eld, a large domain of skyrmions appears, as can be seen in Figure 2b, demonstrating the creation of skyrmions by the E -eld. The application of a negative E -eld (Figure 2c), on the other hand, does not show any sign of creating skyrmions, and only aected are some of the helical domains, for example, the bottom right part of Figure 2c if compared with Figure 2a at the same region. This asymmetric response, where only one E -eld direction creates skyrmions, immediately rules out that the phase boundary is crossed by Joule heating due to the leakage current, which would have a symmetric E -eld response. 12 Therefore, it can be concluded that it is indeed the E -eld eect which creates the skyrmions. Furthermore, a sequence of E -elds, 0 V, +200 V (+3.6 V/µm), 0 V and −200 V (−3.6 V/µm), were applied with each eld lasting for 20 s, as shown in Figure 3b, and during such E -eld sequence, a LTEM movie was recorded with a frame interval of 150 ms. The applied in-plane E -elds caused horizontal shifts of the images, which allows for the direct determination of the directions, timings and relative strengthes of the E -elds. For |E| = 3.6 V/µm, the shift is about 1,000 nm. The blue curve in Figure 3b presents the shift as a function of time. It can be seen that the electric eld reaches the image area immediately. The slight relaxation after each shift reects weak screening due to the charging eect. In order to quantify the number of skyrmions as a function of time throughout the acquired LTEM movies, we developed an algorithm capable of counting skyrmions coexisting with the helical phase. First, the LTEM images are ltered by a Laplacian of Gaussian (LoG) lter with the size corresponding to that of a skyrmion in the image. Then local minima are identied as candidate skyrmion positions. To distinguish actual skyrmions 7


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from local minima along a helical line, the orientational distribution maps of local intensity are calculated. 2729 A local minima along a helical line will have the orientation parallel to its neighbours. Such candidates are therefore excluded. Manual inspection of selected frames conrms that the algorithm detects most of the skyrmions with a few false hits at helical domain walls and at bend contours in the images. Since imaging conditions stay constant during the movie, the false hits by the algorithm remain at a nearly constant background level of around 200 for all the images. More details on this algorithm, which was inspired by the algorithms used to identify ngerprints, can be found in the Supporting Information. a

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Figure 3: (a) The time dependence of the number of skyrmions in each image of the LTEM movie counted by the algorithm described in the main text. The solid line is a smoothing of the data for clarity. (b) The applied bipolar E -eld sequence and the corresponding shifts of the LTEM images as a function of time, respectively. The step-like shape of the data in (a) together with its perfect correspondence to the E -eld prole in (b) fully demonstrate the mono-polar feature of creating skyrmions by E -eld. (c) The average of the statistics into a single period of the applied E -eld. The solid lines are the tting to the exponential growth/decay as described in the main text. Employing this algorithm makes it possible to extract the number of skyrmions for each of the 1,800 images in the LTEM movie. The number of skyrmions as a function of time is shown in Figure 3a in the same time scale as in Figure 3b where the applied E -eld sequence is shown. The time evolution of the number of skyrmions shows repeated and 8


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reproducible writing of skyrmions by the positive E -elds. The process starts from E =0 V/µm, where the numbers of skyrmions are at the background level. An abrupt increase of the number of skyrmions is observed right at the time when switching on the E -eld to E =+3.6 V/µm. Switching o the E -eld results in a gradual monotonic decrease of the number of skyrmions and by the end of this E -eld value, the number returns to the background level. No obvious change in the number of skyrmions can be observed after switching on an E -eld in the opposite direction, E = −3.6 V/µm, consistent with the observation from the real space images that only the positive E -eld creates skyrmions. We averaged the counting results obtained from multiple E -eld cycles into a single period, as shown in Figure 3c, so that the dynamics of the E -eld whole process can be further analyzed. The number of skyrmions after switching on and o the positive E -eld can −

be described respectively by an exponential growth or decay: Ni (t) = Ai (1 − e

t−ti τi

) + N0 ,

where ti is the time points when the E -eld is switched. The response time for creating skyrmions is shorter, τwrite = 3.5 s for switching on E =+3.6 V/µm. The decay of skyrmions after switching o the positive creating E -eld is slower, τpersist = 6.4 s (The full tting results can be found in the Supporting Information). This dierence in time scales of the writing and relaxation processes implies dierent dynamics. Qualitative insight to this phenomena can be achieved by considering the free energy prole as sketched in Figure 4. Tuning the magnetic eld inside the helical phase close to the skyrmion phase, the two states are almost degenerate in energy, with a barrier between them, as illustrated in Figure 4a. The sample conguration we chose ensures that both the positive and the negative E -elds couple with the spontaneous polarizations of the skyrmions. Positive E -elds lower the energy of the skyrmion phase, which leads to the creation of skyrmions. Note that in Cu2 OSeO3 , the exchange interaction J and the DzyaloshinskiiMoriya interaction D are not aected by the application of E -elds. This is evidenced by the spacing among skyrmion, which is determined by the J/D ratio, 30 and it is the same as the spatial periodicity of the helical phase. Higher order terms of the anisotropy energy may 9


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be aected by E -eld 20 so that the SkL can be rotated, yet this is observed in our results. a

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Figure 4: Schematic illustration of the free energy proles (a) at no applied E -eld, (b) when applying a creating E -eld, and (c) after switching o the creating E -eld are illustrated respectively. The equilibrium states are indicated by the green dots while the meta-stable state by a faded one. The arrows indicate the change of the actual state of the system. The dotted lines in gray and green schematically indicate the energy levels of the helical phase and the SkL phase respectively in each situation. Furthermore, the separation of the time scales in the writing and relaxation processes suggests the possibility of potential barrier engineering so as to increase τpersist , e.g. through pinning, while reducing τwrite through larger elds or thinner devices. This is because the eective barrier in the presence of a large positive electric eld can be dierent and smaller (Figure 4b) than the eective barrier in the absence of E -eld (Figure 4a and c). In summary, we report the experimental realization of writing skyrmions in the magnetoelectric compound Cu2 OSeO3 conducted in situ and observed in real space/time by LTEM. Rather than starting from the spin polarized state mostly considered in theoretical proposals, we start from the helical phase. In our B k [1¯ 10] and E k [11¯1] sample geometry, the transverse E -eld is found to write skyrmions in only one polarity and to induce no obvious eects in the other one, implying strongly the possibility of converting voltage signals into topological bits. More interestingly, using an algorithm to count the number of skyrmions in each of the 1,800 frames of a LTEM movie, we extract respectively the reaction and the holding times of the E -eld created skyrmions, which sheds lights on the engineering of the eective energy barriers to achieve ecient writing of skyrmions by E -eld while keeping information lossless even when powered o. 10


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While in this demonstration experiment we prioritized a transverse E -eld conguration for the LTEM investigation, the concept can readily be generalized to a grid pattern where arrays of electrodes arranged orthogonally on either side of a 50-100 nm thick slab would allow to apply longitudinal E -elds locally, hence forming an addressable array of skyrmion bits. We therefore believe that our observation opens the possibility for an alternative route towards skyrmionic memory than the so-called skyrmion race-track concept.

Acknowledgement The authors thank Danièle Laub and Barbora Bártová for their help in sample fabrication. The work was supported by the Swiss National Science Foundation (SNSF) through project 166298, Ambizione Fellowship 168035, the Sinergia network 171003 for Nanoskyrmionics, and the National Center for Competence in Research 157956 on Molecular Ultrafast Science and Technology (NCCR MUST), as well as the ERC project CONQUEST. The authors declare no competing interests.

Supporting Information Available Supporting Information available: experimental details, image processing, skyrmion identication from image data, quantitative analysis and a LTEM movie of the raw data as well as the skyrmion identication results.

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Nano Letters

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In Situ Electric Field Skyrmion Creation in Magnetoelectric Cu2OSeO3

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