Skyrmions are typically manipulated by driving them with an electrical current in a racetrack and have been proven to be advantageous, due to their high mobility, energy efficient drivability and potential in overcoming edge roughness-related pinning occurring in domain wall (DW)- based racetrack devices [1]-[3]. However, to have high density skyrmion based memory devices, it is essential to create and manipulate skyrmions in magnetic tunnel junctions (MTJs). Previously, it has been shown with micromagnetic simulations that voltage-controlled magnetic anisotropy (VCMA) can be used in perpendicular MTJs to create an intermediate skyrmion starting from a ferromagnetic up/down state and subsequently annihilating it to form a ferromagnetic down/up state [4] and thus energy efficient electrical field switching robust to defects and thermal noise can be achieved. Towards the experimental realization of this switching strategy, skyrmions have to be created and manipulated with VCMA in a confined geometry where the skyrmion lateral dimension equals the lateral dimension of a nanodot. However, so far creation and annihilation of fixed magnetic skyrmions using VCMA has only been experimentally observed through magnetic force microscopy (MFM) imaging in thin ferromagnetic films where the skyrmions are much smaller in lateral dimension compared to the thin films and the imaging was performed after withdrawal of the applied electric field (Fig.1) [5]. Here, we will study experimental creation and annihilation of skyrmions using VCMA in a confined geometry of &lt;1μm lateral dimension with in-situ application of voltage while imaging. This will enable visualization of the skyrmion/ferromagnet state as a function of the applied electric field and the mechanism through which skyrmions can be manipulated with VCMA induced change in PMA in the presence of interactions with the boundary of the patterned nanodot. Fig. 2 shows an example of a nanostructure in which such experiments will be carried out.<br/><br/>Acknowledgement: NSF grant CCF #1909030

![](https://assets.underline.io/uploads/markdown_image/1/image/8ddae612ae2d0e4ae2fc5e04ca640dfb.jpg)

Fig. 1. Magnetization state before application of any electric field (a) and after application of negative (b), positive (c) voltage pulse [5]

![](https://assets.underline.io/uploads/markdown_image/1/image/a01a9085cbf4c95176982da23375450e.jpg)

Fig. 2. Skyrmions to be studied in confined geometries



Creation and Annihilation of Magnetic Skyrmions in Confined Geometry using Voltage Control of Magnetic Anisotropy

Young Professionals

Although current induced motion of magnetic skyrmions have been extensively studied in recent work [1-3], manipulation of magnetic skyrmions that are fixed in space can also be utilized to implement such devices. We have recently shown that, skyrmion core reversal and switching between ferromagnetic states via an intermediate skyrmion state can be achieved in the free layer of an MTJ using Voltage Control of Magnetic Anisotropy (VCMA) which could result in energy efficient memory devices with smaller footprint [4-5]. In this work, we demonstrate experimental evidence of voltage controlled switching of fixed skyrmions in both continuous film [6] and confined geometry. The continuous heterostructure film stack consists of IrMn/CoFeB/MgO layers. The exchange biased structure allows stabilization of skyrmions without any external magnetic field. Upon application of a voltage pulse, the perpendicular magnetic anisotropy (PMA) changes at the ferromagnet/oxide interface. When the PMA is increased by applying a negative voltage pulse, skyrmions are annihilated. On the other hand, skyrmions can be recreated using a positive voltage pulse. We will also present micromagnetic simulations which reveal the detailed magnetization dynamics of this switching. Next, we will show voltage control of skyrmions in patterned nanodots composed of Ta/CoFeB/MgO multilayers. The geometric confinement is expected to influence the switching dynamics as shown in Fig. 1 [4,5]. To systematically modulate the confinement strength, we fabricated nanodots of varying lateral dimensions. The magnetization configurations are imaged using Magnetic Force Microscopy (MFM). Preliminary result of MFM imaging of skyrmion states under 1300 mV applied voltage in a ~5 µm dot is shown in Fig. 2. We will present detailed analysis of the confinement effect on skyrmion switching using further in-situ MFM imaging and rigorous micromagnetic simulations and compare these observations with the switching in thin films. Acknowledgement: NSF CCF collaborative grants: 1909030 and 1909416.References: [1] A. Fert et al, Skyrmions on the track, Nature Nanotechnology volume 8, pages152–156(2013) [2] K. Everschor-Sitte et al, Perspective: Magnetic skyrmions—Overview of recent progress in an active research field, Journal of Applied Physics 124, 240901 (2018) [3] G. Yu et al, Room-Temperature Skyrmion Shift Device for Memory Application, Nano Lett. 17, 1, 261–268 (2017) [4] D. Bhattacharya et al, Voltage controlled core reversal of fixed magnetic skyrmions without a magnetic field, Sci. Rep. 6, 31272 (2016) [5] D. Bhattacharya et al, Skyrmion-mediated voltage-controlled switching of ferromagnets for reliable and energy-efficient two-terminal memory, Applied Mater. & Interfaces 10 (20), 17455-17462 (2018). [6] D. Bhattacharya et al, Creation and annihilation of non-volatile fixed magnetic skyrmions using voltage control of magnetic anisotropy, Nature Electronics (2020)

Switching fixed magnetic skyrmions in continuous film and patterned nanodots using Voltage Control of Magnetic Anisotropy

GOD - Skyrmion Stabilization, Voltage Control, and Neuromorphic Applications

technical paper

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Dhritiman Bhattacharya

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Creation and Annihilation of Magnetic Skyrmions in Confined Geometry using Voltage Control of Magnetic Anisotropy

Young Professionals

Switching fixed magnetic skyrmions in continuous film and patterned nanodots using Voltage Control of Magnetic Anisotropy

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