The present invention relates to a spin texture storage device, a method for moving spin textures in such a spin texture storage device and a racetrack memory device comprising such a spin texture storage device.

Background

A new paradigm came into electronics with the advent of spin-transfer electronics, or 'spintronics', where the spin degree of freedom, previously unutilized in conventional electronics, serves as the information carrier. The seminal discoveries of the tunnelling magnetoresistance (TMR) effect and the giant magnetoresistance (GMR) effect led to deployment of spintronic concepts in commercial storage devices, including hard disk drives (HDD) and random access memory (RAM). Magnetic states can be switched by spin- polarized currents, rather than by magnetic fields, an effect commonly referred to as spin- transfer torque (STT).

Of special interest for next-generation spintronics are the topological spin solitons, like magnetic domain walls, which can encode data in the form of localized discontinuities in the magnetization and where STT can be leveraged for data transfer. E.g. in so called 'racetrack memory' domain walls or other solitons are utilized for encoding of data as disclosed in US 2004/0252539 A1.

EP 3 166 138 A1 discloses a skyrmion memory circuit capable of circularly transferring a magnetic element skyrmion, comprising one or more current paths in a magnet having a closed-path pattern that are provided surrounding an end region including an end portion of the magnet in a plane of the magnet with the closed-path pattern, and applying current between an outer terminal connected to an outer circumferential portion of the closed-path pattern and an inner circumference electrode connected to an inner circumferential portion of the closed-path pattern, transferring the skyrmion in a direction substantially perpendicular to the direction of the applied current, and circulating the skyrmion in the magnet with the closed-path pattern. WO 2018/092611 A1 discloses a similar setup.

Disclosure of the Invention

According to aspects of the present invention a spin texture storage device, a method for moving spin textures in such a spin texture storage device, a racetrack memory device comprising such a spin texture storage device and a use of such a racetrack memory device having the features of the independent claims are provided. Advantageous further developments form the subject matter of the dependent claims and of the subsequent description.

Advantageously, the inventions provide for devices and methods that can surpass current RAM technologies in terms of data access times and reduced power consumption. With no obvious failure mechanism, racetrack memory signifies a promising transfer to spintronic devices in memory devices. The inventions provide for special advantages for topologically protected spin textures, such as skyrmions and skyrmionic bubbles, referred to as “skyrmions” throughout the application for simplicity.

Skyrmions represent particle-like spin-configurations with a non-trivial real-space topology. Skyrmions have been observed to couple strongly to spin-polarized currents, showing compatibility with low-power applications. As information carriers, skyrmions have the intriguing properties that they can be written, deleted, and moved on demand and their dimensions have been found to approach atomic length scales, facilitating ultrahigh storage density.

According to one aspect, the invention relates to a spin texture storage device with a closed- loop storage track comprising electric conductive and ferromagnetic material and having a track length, a track width and a track height, the track width being at most 2 pm and the track height being at most 2 pm, and with an accelerator segment having at least one, or at least two electrode(s) connected to the storage track so that electric current can be caused to flow along the storage track through an accelerator section having an accelerator section length, the accelerator section length being less than half of the track length. It has to be stressed that the optimal dimensions particularly depend on the material and its magnetic properties. Dimension ranges that have shown particular advantages, e.g. for Fe3Sn2, are characterized by the track width being between 200 nm and 2 pm and/or the track height being between 100 nm and 1 pm. Contrary to EP 3 166 138 A1, the current that drives the skyrmions is applied along the storage track, i.e. in the direction of the track length.

In that the accelerator section length is less than half of the track length, or at most 40%, 30%, 25%, 20%, 10% or 5% of the track length, the power demand for moving the spin textures along the track can be significantly reduced. This especially allows for upscaling the device to bigger track lengths without significantly increasing the losses. Skyrmions have been observed in non-centrosymmetric systems and as well in centrosymmetric systems. Material which has been proven to be well suitable for storing information using skyrmions is centrosymmetric material having competing magnetocrystalline anisotropy (magnetocrystalline anisotropy constant K _u ) and dipolar energies (dipolar anisotropy constant K _d ), in particular with 0.1 < K _u /K _d < 1, such as Fe3Sn2.

In this class of materials, the skyrmions can exhibit different helicities, which can be utilized to encode information. Other materials that, in principle, provide the right K _u /K _d ratio include Co and Fe3GeTe2. Fe3Sn2 is ferromagnetic at room temperature and has competing magnetic exchange interactions, correlated electron behaviour, weak magnetocrystalline anisotropy, and lattice (spatial) anisotropy.

In order to reliably and easily create and store spin textures such as skyrmions in the storage track, the track width is in an embodiment at most 800 nm, 600 nm, 500 nm, 400 nm, 300 nm or smaller. Alternatively, the track width is in an embodiment at most 1 ,750 nm,

1 ,500 nm, 1 ,250 nm, or 1 ,000 nm. These dimensions simultaneously allow for good electric and magnetic properties and a high storage density.

According to an embodiment, the storage track has a corner-free geometry, especially a ring shape or an oval shape or an elliptical shape. However, also other corner-free geometries as e.g. serpentines or meanders are advantageous for implementing the invention. Having a corner-free geometry avoids spin textures being pinned on a corner of the track. In case of a ring shape, oval shape or elliptical shape the outer diameter or outer minor axis, respectively, of the track is in an embodiment at least 2 pm and/or at most 10 pm.

In order to reduce interactions between neighbouring sections of the track, especially when creating spin textures in the track, and to avoid short circuits, a shortest distance between two neighbouring sections is in an embodiment larger than zero, in another embodiment at least one or in the order of the track width. The distance may be determined perpendicular to a tangent on the track.

According to an embodiment, the spin texture storage device has stored therein spin textures, especially skyrmions. In an embodiment, each of the spin textures has a helicity for representing a bit. In relation to the dimensions of the spin texture parallel to the respective dimension of the track, the track height is in an embodiment in the order of the height of the spin texture. Further, in an embodiment, the track width is in the order of the width of the spin texture. Additionally or alternatively, the track width is in an embodiment at most the one that allows only a 1 D chain of skyrmions to form, and not a 2D hexagonal packing of skyrmions. For Fe3Sn2, this is about 800 nm. These dimensions simultaneously allow for good electric and magnetic properties and a high storage density. In an embodiment, the accelerator section length is in the order of the length of the spin texture and/or in the order of the distance between two neighbouring spin textures. Thus, the accelerator section length section can be as small as possible.

According to a further aspect of the invention, there is provided a method for moving spin textures such as skyrmions in a spin texture storage device according to the invention by causing a current to flow through the accelerator section of the storage track, i.e. in the direction of the track length. In an embodiment, the current density of the current is in the range of 10 ¹² Am ^-2 or lower. This provides for an easy possibility to reliable move spin textures in a storage device with low power demands.

In an embodiment, spin-polarized currents are used to move the spin textures. Metallic electrodes may be used and currents get spin-polarized in the ferromagnet out of which the storage ring is made. Alternatively, to inject currents that are already spin-polarized, half metals can be used as contact material.

According to a further aspect of the invention, there is provided a magnetic racetrack memory device comprising a spin texture storage device according to the invention, a write- in element for manipulating a feature of a spin texture, especially for reversing the helicity of a skyrmion, and a read-out element for determining the feature of the spin texture, especially the helicity of a skyrmion. This provides for a memory device with high density and low losses.

An intriguing property of the skyrmions in centrosym metric materials is the degeneracy of the skyrmion helicity, associated with the winding direction of the skyrmion, as the handedness of the rotation is equivalent in the absence of chiral interactions. This provides the solitons with additional flexibility, which can be utilized for encoding of data. Helicity indicates the in plane magnetic-moment swirling direction of a skyrmionic configuration and can be classified as the phase offset of the polar angle of the spins. It can be utilized to represent bits "0" and

The helicity of a skyrmion can be reversed (i.e. a bit can be written) via a spin-polarized current as shown by Zhipeng Hou et al. , Current-Induced Helicity Reversal of a Single Skyrmionic Bubble Chain in a Nanostructured Frustrated Magnet, Adv. Mater. 2020, 32, 1904815. The helicity of a skyrmion can be determined (i.e. a bit can be read) by techniques utilizing the Topological Hall Effect (THE, cf. Bruno, P. et al. Topological Hall Effect and Berry Phase in Magnetic Nanostructures. Phys. Ref. Lett. 93, 096806; Neubauer. A. et al. Topological Hall Effect in the A Phase of MnSi. Phys. Rev. Lett. 102, 186602; Kanazawa, N. er al. Large Topological Hall Effect in a Short-Period Helimagnet MnGe. Phys. Rev. Lett.

106, 156603) and techniques based on magnetoresistance (Gobel. B. et al. Topological Hall Signatures of Magnetic Hoptions. Phys. Rev. Research 2, 013315; Hanneken, C. et al. Electrical Detection of Magnetic Skyrmions by Tunnelling Non-Collinear Magnetoresistance. Nature Nanotechnology 10, 1039-1042; Du, H. et al. Electrical Probing of Field-Driven Cascading Quantized Transitions of Skyrmion Cluster States in MnSi Nanowires. Nature Communications 6, 7637). Assuming the conduction electrons are able to follow the local spin direction (adiabatically), they pick up a phase as they traverse the skyrmion.

Instead of writing and erasing skyrmions, during use of a magnetic racetrack memory device a number of skyrmions in the track is kept constant and the information is stored in the helicity (e.g., positive = 1, negative = 0).

Further advantages and embodiments of the invention will become apparent from the description and the appended figures.

It should be noted that the previously mentioned features and the features to be further described in the following are usable not only in the respectively indicated combination, but also in further combinations or taken alone, without departing from the scope of the present invention.

In the drawings:

Figure 1 shows, in a perspective schematic view, an embodiment of a magnetic racetrack memory device comprising a number of skyrmions in a spin texture storage device.

Figure 2a shows, in a schematic top view, two skyrmions having different helicity with Y = +TT/2 and g = -tt/2.

Figure 2b shows, in a perspective schematic view, a row of skyrmions having different helicity usable to represent bits "0" and "1" in a memory. Figure 3 shows a theoretical phase diagram for the creation of skyrmions in Fe3Sn2 rings.

Figure 4 shows simulations of Fe3Sn2 nanorings having different diameters with a constant width of 500 nm and a thickness of 200 nm in zero field and with an applied field of 300 mT.

Figure 1 shows, in a schematic perspective view, an embodiment of a magnetic racetrack memory device 200 according to the invention. The racetrack memory device 200 comprises an embodiment of a spin texture storage device 100 according to the disclosure, a write-in element 210 and a read-out element 220.

The spin texture storage device 100 comprises a closed-loop storage track 110 having here an oval shape consisting of two half circles having an outer diameter D connected via straight lines having a length . The storage track 110 has a width w, a height h, and a length of 2 + (D-W)TT. In the shown geometry, the shortest distance between two neighbouring sections of the storage track is D-2w. These sizes lie on a nanoscale and microscale. In an embodiment, for the case of Fe3Sn2, the width is between 400 nm and 1pm, especially about 500 nm, and the height is between 100 nm and 500 nm, especially about 200 nm. The outer diameter D is in an embodiment 2 pm or smaller. For other materials, the dimensions will vary according to the size of the spin textures.

The storage track 110 comprises (or especially consists of) electric conductive and ferromagnetic material, here Fe3Sn2. The storage track is shown to store numerous spin textures in the form of magnetic skyrmions 300. Skyrmions are particle-like textures of nanometre size and can be created by applying a magnetic field along the height of the storage track 110 (i.e. perpendicular to the track plane). The strength of the field may lie in the range of 200 - 500 mT (material and size dependent). Fig. 3 and 4 show further details of creating skyrmions in a ring structure.

As illustrated in Fig. 2a and 2b, skyrmions represent particle-like spin-configurations with a non-trivial real-space topology. Fig. 2a schematically shows two skyrmions in a centrosymmetric material where it can have different helicities, which indicates the in-plane magnetic-moment swirling direction of a skyrmionic configuration. The helicity can be utilized for encoding of data by bits "0" and "1", as shown in Fig. 2b. The spin texture storage device 100 further comprises an accelerator segment 120 having two electrodes 121, 122 connected to the storage track 110 allowing current I to flow through an accelerator section 123 along the storage track, i.e. along the length of the track. The two electrodes 121, 122 are connected to a front end and a back end, respectively, of the accelerator section 123 of the storage track, and are placed close enough together that the majority of the current (i.e. more than 50%, in another embodiment more than 75%, 80%, 90%) flows along the accelerator section 123. Some current will naturally flow in the opposite direction, but this fraction can be minimized by optimizing the spacing of the electrodes and hence lower the round-trip impedance. For the sake of clarity, it is noted that the current flows through the material (here Fe3Sn2) of the storage track.

The accelerator section 123 has an accelerator section length L _A , which is significantly shorter than the track length. Here, the accelerator section length is about 15% of the track length.

In an embodiment, spin-polarized currents are used to move the spin textures. To inject currents that are already spin-polarized, half metals can be used as contact material. Alternatively, metallic electrodes may be used and currents get spin-polarized in the ferromagnetic material out of which the storage ring is made.

In order to shift the data, current having a current density of about 10 ¹² Am ^-2 (material dependent), e.g. about 1.5-10 ¹² Am ^-2 , is sourced through the two electrodes 121, 122 and flows along the accelerator section 123 of the track. Skyrmions can be moved based on non local effects, wherein the accelerated skyrmions transmit a torque to the rest of the skyrmions that creates a domino effect along the track (indicated by the two big arrows in Fig. 1). The circulation of the skyrmions can be visualized as the rolling-action of the ball elements in a mechanical ball bearing.

In order to read and write the data, the write-in element 210 is utilized for reversing the helicity of a skyrmion, and the read-out element 220 is utilized for determining the helicity of a skyrmion. In the shown example, the write-in element 210 comprises two electrodes connected to the track so that electric current can flow perpendicular to the track, i.e. along the width of the track. The helicity of a skyrmion can e.g. be reversed via a spin-polarized current pulse. Suitable current densities for triggering the helicity reversal usually lie in the range of 10 ⁹ — 10 ¹⁰ Am ^-2 , with a corresponding pulse-width varying from 1 ps to 100 ns (material dependent, shorter pulse-width is preferable for faster operation). As explained above, the helicity of a skyrmion can be determined by techniques utilizing the Topological Hall Effect (THE) and techniques based on magnetoresistance.

Fig. 3 shows a theoretical phase diagram for the creation of skyrmions in Fe3Sn2 rings. Micromagnetic simulations were carried out with varying track width and applied fields, while the thickness was kept constant at 200 nm. In order to obtain representative simulations of the ring geometry without simulating an entire ring, periodic boundary conditions were applied in the horizontal direction (parallel to the x-axis). The simulations are hence representative of a small segment of the ring with a length of about 2 pm, where it is assumed that the ring diameter is large enough that the segment can be approximated as a straight track. In the phase diagram the horizontal axis corresponds to the field Bz applied perpendicular to the plane of the track, and the vertical axis corresponds to the track width w.

Five phases of particular interest to the construction of nanorings are distinguished in the phase diagram. In narrow tracks with w « 400nm, symmetric stripes are found to occur in the ground states, characterized by a strong dependence on the track geometry, aligning predominantly parallel or perpendicular to the track. In applied magnetic fields, these structures do not form isolated skyrmions, but rather half-skyrmionic bubbles localized at the edges. At greater thicknesses with w ³ 400nm, random stripe patterns are found to emerge, characterized by a stripe direction that is relatively unconnected with the track geometry. In applied magnetic fields of around 200 - 450mT, the random stripe patterns are found to contract into isolated skyrmions. Hence, the region referenced with "Bubbles" in the phase diagram represents the stability region where skyrmions can be created in the circular track. The saturation field is also found to be strongly dependent on the track width and the values will vary depending on the actual material that is used to build the ring.

Based on the phase diagram presented in Fig. 3, complete nanorings were simulated with varying outer diameters and track widths. Fig. 4 shows simulations with a fixed track width of 500nm and an outer diameter varying from 1 pm to 4 pm, both in zero field and with an applied magnetic field of 300 mT. According to the phase diagram presented in Fig. 3, these values lie in the stability region of isolated skyrmions, however the curvature of the ring was not taken into account in the construction of the phase diagram. Indeed, the simulations reveal that for rings with an outer diameter below 2 pm, skyrmions with a well-defined topological charge do not form, even though the track width and applied field lie in the stability region. In the ring with an outer diameter of 4 pm, stable skyrmions with a mixed helicity can be observed. According to embodiments of the invention, skyrmions (skyrmionic bubbles) with two energetically equivalent helicity states are densely packed in a corner-free geometry to form a skyrmion chain, where the helicity property of individual skyrmions encodes the data. For this, a racetrack is utilized built from a centrosymmetric magnet, where the absence of chiral interactions facilitates the two equivalent helicity states, stabilized by anisotropy, dipolar interactions and external magnetic field. Control of the helicity is facilitated by applying a spin-polarized current in the direction perpendicular to the racetrack. Shifting of the data is accomplished by applying a spin-polarized current in the direction parallel to the track, over a limited distance, termed the accelerator track. By utilizing long-ranged interactions between the skyrmions, the accelerator track drives the data array without the need to apply a current over the entire track. The latter represents one of the main advantages of the invention.

The memory device according to the invention allows for low-energy memory and storage applications. The device is fully solid state and data is stored in a non-volatile format. The current is applied only over a limited volume due to the inclusion of the accelerator track, limiting Joule heating and facilitating efficient heat sinking.