TECHNICAL FIELD

5 The present invention is direct to a spin oscillator device, and use of such a device.

BACKGROUND

Spin oscillator devices, such as spin-torque oscillators (STO:s) that utilize spin angular momentum are known. 0 A spin polarized electric current can transfer its angular momentum to the magnetization of a thin film in a process known as spin transfer torque. At large enough current densities, which can be obtained by passing the current through a nanoscale contact, a so-called nanocontact (NC), the intrinsic damping of the magnetic thin film can be fully compensated and auto- oscillations of the magnetization can be sustained. Under suitable applied fields these auto-5 oscillations manifest themselves as propagating spin waves which radiate away from the NC.

Such intrinsically nanoscale devices are attractive candidates for applications where a highly tunable broad-band oscillator is needed. However, the low output power and high phase noise of the oscillator has stalled their progress. A generally accepted technique to improve oscillator performance has been to synchronize many of them together. 0 Beyond oscillator technology, such devices have other potential uses. For example, while complementary metal-oxide semiconductor (CMOS) integrated circuits have been the prior art technology in data processing, one such "beyond CMOS" technology relies on the functionalization of spin waves, a sub-field within spintronic generally called magnonics. The typical components of a magnonics device include a mechanism to generate, manipulate, and5 detect spin waves as well as a magnetic medium for the spin waves to travel in. An extension of these technologies is to utilize massively parallel networks of spintronic oscillators for neuromorphic functions that aim to mimic the functionality found in the human brain. It is therefore crucial to develop new techniques to manipulate and reliably control spin waves on the nanoscale. Additionally, due to damping and losses within the ferromagnetic medium there is a finite propagation length of the spin waves. It is also technologically advantageous to increase the range of spm wave propagation such that information can be transported over greater distances.

The synchronization and phase locking of coupled non-linear oscillators is a common natural phenomenon. The coupling is typically described as mutual since each oscillator plays an active role in the resulting synchronized state. Mutual synchronization is considered the primary vehicle to achieve sufficient signal quality required for applications. However, since the early experiments demonstrating spin wave (SW) based mutual synchronization of two nano contact STOs (NC-STOs), progress has been relatively slow in terms of synchronizing more oscillators. In 2009 four sub-GHz vortex-based oscillators were synchronized through direct exchange, and it was not until 2013 that SW based mutual synchronization of three high-frequency NC-STOs was demonstrated.

SUMMARY OF THE INVENTION

According to an aspect of the present disclosure, propagating spin waves are the dominant coupling mechanism giving rise to synchronization. Furthermore, a unique combination of the NC geometry, Oersted field, and an externally applied magnetic field provide that directional spin wave beams can be generated. The directionality of the SWs allows for a variety of new functionalities in regards to oscillator synchronization and SW propagation.

According to a first embodiment of the first aspect, there is provided a spin oscillator device comprising a first spin torque oscillator (STO) having an extended multilayered magnetic thin- film stack, wherein a nano-contact (NC) is provided on the magnetic thin-film stack providing an NC-STO comprising a magnetic free-layer and having a nanoscopic region. The NC is configured to focus electric current to the nanoscopic region, configured to generate the necessary current densities needed to excite propagating spin waves in the magnetic free layer, wherein a circumferential magnetic field surrounds the NC. An externally applied field is configured to control the propagation of the spin waves forming a spin wave beam to a second spin oscillator device arranged in spin wave communication and synchronized to the first NC. According to an aspect, by daisy-chaining multiple NCs in vertical array geometry, one can propagate spin wave information (that is spin wave phase, frequency, amplitude, and their modulation) from one NC to another NC over much larger distances than would be allowed by a single NC. This is technologically advantageous, since it increases the range of spin-wave propagation such that information can be transported over greater distances.

By means of the embodiment disclosed above and various other embodiments, driven synchronization, addressable synchronization, SW repeater, tuning the width of the SW beam and synchronization of 2D arrays can be provided. All these will be explained as follows in more detail with reference to drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become further apparent from the following detailed description and the accompanying drawing, of which:

Fig. 1 shows a spin oscillator device according to an embodiment of the invention in a sectional view from the side, illustrating the stacked layers.

Fig la and Fig. lb show a similar spin oscillator device according to the embodiment shown in Fig. 1 of the invention in a perspective view from, illustrating typical stacked layers.

Fig. 2 shows a spin oscillator device according to an embodiment of the invention in a principal view showing driven synchronization.

Fig. 3 shows a spin oscillator device according to an embodiment of the invention in a principal view showing addressable synchronization.

Fig. 4 shows a spin oscillator device according to an alternative embodiment of the invention in a principal view showing addressable synchronization.

Fig. 5 shows a spin oscillator device according to an alternative embodiment of the invention in a principal view showing a SW repeater.

Fig. 6 shows a spin oscillator device according to an alternative embodiment of the invention in a principal view showing tuning the width of the SW beam. Fig. 7 shows a spin oscillator device according to an alternative embodiment of the invention in a principal view showing synchronization of 2D arrays of NCs.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described as follows.

Fig. 1 shows a spin oscillator device according to an embodiment of the invention.

The spin oscillator device 1 comprises a spin oscillator 2 having a magnetic layer 3. The spin oscillator device 1, is configured to generate spin waves (SWs) and SW beams. The spin oscillator 2 is configured to be controlled by means of injecting current Id ₀ , and/or applying magnetic fields. In this embodiment, the spin oscillator 2 is a spin torque oscillator, STO, wherein a nano-contact (NC) 6 is provided on the magnetic film 3 providing a so-called "NC- STO" 2, 6.

Typically, in operation, spin polarized current Id ₀ is injected through the NC 6 and excites oscillations of magnetization of an active magnetic layer, also referred to as a free layer 3, herein positioned under a cap layer 9, directly under the NC 6. These oscillations generate spin waves in the free layer 3 that propagate away from the NC 6. In this NC-STO device 1, also in short- referred to as a NC device, current flows perpendicular to a stack of layers 9, 3, 7, 8, wherein a polarizing fixed layer 8 serves as a current spin polarizer.

Alternatively the spin oscillator 2 can have its active magnetic layer 3 underneath the spacer layer and its fixed polarizing layer 8 above the spacer layer, i.e. layer 3 and 8 switch places.

Alternatively, either the free layer 3, or the fixed layer 8, or both, can be composite layers, and/or multilayers, where different magnetic and non-magnetic materials are combined to tailor their magnetic properties M. Alternatively, the polarizing fixed layer 8 can be omitted. The spin oscillator 2 generates an output signal Vrf through a magneto resistive effect (spacer layer 7 is metal such as Cu) such as giant magnetoresistance, tunneling magnetoresistance (spacer layer 7 is a tunneling barrier), or anisotropic magnetoresistance, or a combination thereof.

Now is referred to Fig. la and lb. Fig la and Fig. lb show a similar spin oscillator device 1 according to the embodiment shown in Fig. 1 of the invention in a perspective view from a side and above, illustrating typical stacked layers 2 as non-limiting examples.

Fig. la and lb show a nanocontact spin torque oscillator (NC-STO) device 1 architecture as shown in Fig. 1. A lithographically defined nanocontact 6, typically circular in cross-section and from 5 to 500 nm in diameter, on top of an extended spin valve stack 2 acts to focus electric current Id ₀ to a nanoscopic region. Such focusing of the electric current Id ₀ generates the necessary current densities needed to excite spin waves in the "free layer" 3, which is shown specifically for NiFe in Fig. la. Along with the electrical current Id ₀ which passes through the nanocontact NC 6, there is a circumferential magnetic field, called an Oersted field (H _0e ) that surrounds the nanocontact NC 6. This field alters the propagation of the spin waves, promoting the formation of highly directional spin wave beams, denoted SW beam in Fig. la, which travel "upwards" towards positive Z-values in the particular coordinate system shown in Figs, la- b. When the nanocontacts are situated horizontally these spin wave beams do not overlap significantly and the spin wave communication between neighboring nanocontacts is minimal. However, when placed vertically, see NCI and NC2 Fig. 1(a) with respect to each other the communication channel is optimal and the spin waves will synchronize their frequencies. More importantly, the frequency of the bottom-most nanocontact, herein the first NC, NC 1, sets the frequency of the pair, as it drives the synchronization of the other NC.

As explained, according to an aspect of the present invention, a unique combination of the NC geometry, the Oersted field, and the externally applied magnetic field provide that directional spin wave beams can be generated. The directionality of the SWs allows for a variety of new functionalities in regards to oscillator synchronization and SW propagation, of which three different concepts of directional synchronization via SW beams will be explained as follows in Figures 2, 3 and 4.

Now is referred to Fig. 2, which shows a spin oscillator device according to an embodiment of the invention in a principal view showing driven synchronization.

An externally applied field (H _ext ) is configured to control the control the propagation of the spin waves (SWs) forming a spin wave beam (SW beam) to a second spin oscillator device (NCn), which is arranged in spin wave communication and synchronized to the first NC (NCI).

In another embodiment of the first aspect, at least a second NC having a second frequency f2 is arranged vertically stacked onto the first NC in spin wave communication and synchronized to the first NC having a first frequency, wherein the second frequency f2 = fl, and the externally applied field has an in-plane component pointing in the direction shown. In this way, "driven synchronization" can be provided, as illustrated in Fig. 2.

The directionality of the SW beams is used to promote driven synchronization, where the frequency, herein the first frequency, fl of the bottom-most oscillator NCI sets the frequency, herein the second frequency, £2 of a synchronized array of NCs.

The term "driven synchronization" is further explained below.

The impact of the NC array geometry dramatically enhances synchronization of stacked NC- STOs, if they are aligned essentially perpendicular to the in-plane component of the applied magnetic field H _ext . Synchronization is provided by a highly collimated and directional SW beam arising from a combination of the Oersted field and applied field induced asymmetric field landscape. As a consequence of this directionality, synchronization can no longer be considered mutual, but driven, as the final SW mode is enforced by the NC-STO from which the SW beam originates. The demonstrated driven synchronization has the potential to greatly increase the number of synchronized NC-STOs as one can daisy-chain an arbitrary number of NC-STOs and extend synchronization over distances much greater than the SW propagation length. This will be further explained with reference to Figure 5. Addressable synchronization via iWsNow is referred to Figure 3 and 4, which respectively shows a spin oscillator device according to an embodiment of the invention in a principal view showing addressable synchronization. By tuning the angle of the in-plane component of the externally applied field the SW beam can point in an arbitrary direction within the free layer as shown in Figure 3, and 4 respectively. The SW beam will propagate in a direction perpendicular to the in-plane component of the field, Η. _χί; ¾. Thus synchronization along any in-plane direction is achieved. SW repeater

As opposed to light waves, which can travel through a vacuum, spin waves much travel through a magnetic medium. Due to damping and losses within this medium there is a finite propagation length of the spin waves. It is technologically advantageous to increase the range of spin wave propagation such that information can be transported over greater distances. In analogy to radio, cellular, optical, and repeaters, the embodiment shown in Fig. 5, which can be regarded as a "spin wave repeater" is able to greatly extend the range of the propagating spin waves.

Now is referred to Figure 5, which shows a spin oscillator device according to an alternative embodiment of the invention in a principal view showing a SW repeater.

Due to the driven and directional nature of the synchronization, by simply daisy-chaining oscillators, herein the first NC, NCI, the second NC, NC2, a third NC, NC3,. , . , and a n:th NC, NCn together, the synchronized chain of oscillators will act as a SW repeater. Therefore, the frequency, herein fl, of the bottom-most oscillator, herein the first NC, NCI, can be propagated over distances much larger than would be allowed by the intrinsic SW propagation length in the free layer. This chain of oscillators can be extended indefinitely. Thus, in principle an infinite number of oscillators can be daisy-chained and not limited to 5 only.

Tuning the width of the SWheam

Now is referred to Figure 6, which shows a spin oscillator device according to an alternative embodiment of the invention in a principal view showing tuning of the SW beam.

By tuning the angle of the out-of- plane field component of the external applied field H _ex -L the width of the SW ^T beam is tuned, forming an opening angle Θ. Synchronization of 2D arrays ofNCs

Now is referred to Figure 7, which shows a spin oscillator device according to an alternative embodiment of the invention in a principal view showing synchronization of 2D arrays of NCs. A broad SW beam with opening angle Θ of a first NC, herein denoted NCI , will be able to synchronize at least two neighboring oscillators, herein denoted NC2 and NC3, which in turn will synchronize their nearest neighbors, herein NC4 and NC5, and NC5 and NC 6, respectively. Therefore synchronization can be extended to a 2D array 11 of NCs.

According to an aspect, the NC device can be used in one or more of: spintronic, magnonics, hard disk drives (reading head), micro wave signal generators and detectors or domain-wall devices.

According to an aspect, the device can alternatively be based on magnetic tunnel junctions.

The foregoing detailed description is intended to illustrate and provide easier understanding of the invention, and should not be construed as limitations. Alternative embodiments will become apparent to those skilled in the art without departing from the spirit and scope of the present invention.