Patent ID: 12217890

DETAILED DESCRIPTION

FIG.1shows an antenna system100. The system100comprises an array101of nanomagnets over a substrate102. One or more additional films or materials103,104, etc. may lie between the substrate102and the array101. Alternatively, the array101(and the nanomagnets thereof) may be in direct contact with the substrate102. In this disclosure, “nanomagnets” means a ferromagnetic material of a few tens to a few hundreds of nanometer dimensions (e.g., between 20 and 500 nm in the largest dimension). The nanomagnets are two-phase multiferroic, i.e. they are magnetostrictive and are in elastic contact with a piezoelectric material. Alternatively, they could be magnetostrictive and in electrical contact with a heavy metal. One or more additional films may be interposed between the nanomagnet and heavy metal. “Heavy metals” means metals and other materials that can sustain a current flow and exhibit strong spin-orbit coupling. Passage of current through a heavy metal exerts a spin-orbit torque on the nanomagnet that they are in contact with. The magnetizations of the multiferroic nanomagnets (that is to say, the orientations or directions of their polarizations) may be changed by an electric field applied on the piezoelectric component of the multiferroic nanomagnet that exerts a strain on the nanomagnets. The magnetizations of nanomagnets in electrical contact with a heavy metal can be changed by passing electric current through the heavy metal that results in the exertion of a spin-orbit torque on the nanomagnets. If the electric field or the current oscillates in time, then so will the magnetizations. These oscillations radiate electromagnetic waves, thereby implementing an electromagnetic antenna that transmits electromagnetic waves.

Because the radiation is caused by oscillating magnetizations and not oscillating charges, the antenna's radiation efficiency can beat the (l/λ)2limit. At the same time, these magnetization oscillations make the nanomagnets expand and contract periodically, which causes a periodic strain on the substrate underneath. This generates a surface acoustic wave in the substrate, making the system also act as an acoustic antenna that transmits acoustic waves.

The system can also act as a receiving antenna. If an electromagnetic wave is incident on the nanomagnets, it will make their magnetizations oscillate, which will generate periodic spin pumping into the heavy metal and hence an alternating voltage across it because of the inverse spin Hall effect. In this case, the array of nanomagnets acts as a receiving antenna. Similarly, if an acoustic wave is incident on the nanomagnets, the nanomagnets will respond to the wave by periodically expanding and contracting, which will make their magnetizations oscillate due to the inverse magnetostriction effect and the resulting spin pumping into the heavy metal will cause an oscillating voltage owing to the inverse spin Hall effect. Thus, the array can also act as a receiving acoustic antenna.

The array101of nanomagnets functions as the antenna element of system100. Depending on the specific configuration, the array101may transmit a signal, receive a signal, or both transmit and receive. The term “array” as used herein may refer to a single array or multiple arrays. Thus “an array” may comprise a plurality of arrays. Generally the smallest transmitting or receiving element of an array is a nanomagnet.

Antenna system100further comprises a module105. Module105may be a transmitter, a receiver, or a transceiver. Module105may be configured to deliver power to or near the array101. Module105may be configured to control the intensity, waveform, frequency, or amplitude of signals sent to the array101and emitted from the array101. Module105is configured or configurable to periodically change magnetizations of the nanomagnets such that the array101emits one or more waves (one or more forms of radiation). In addition or in the alternative, module105is configured or configurable to filter, convert, and/or process a signal detected by array101.

The antenna system100may be configured to transmit (emit) electromagnetic and/or acoustic waves. The antenna system100may be configured to receive electromagnetic and/or acoustic waves, converting such incoming signal to an electrical signal such as a current or voltage that varies based on a predetermined signal or message content.

The individual nanomagnets as illustrated inFIG.1are elliptical in shape. Different embodiments may have differently shaped nanomagnets. That is, an array101may have nanomagnets which are circular, elliptical, square, rectangular, polygonal, or any of variety of other shapes. The shape may affect the direction of the poles and may be selected to control the propensity of the nanomagnet to flip or rotate under predetermined environmental conditions, e.g., the strength of an applied electric or magnetic field.

FIG.2Ashows another array200of nanomagnets. The array comprises a plurality of rows (which may be called columns depending on the reference frame). As illustratedFIG.2Ashows ten rows201athrough201j, but embodiments may have significantly more than ten rows, e.g., hundreds or thousands of rows. Each row comprises a plurality of nanomagnets. As illustratedFIG.2Ashows four nanomagnets per row, e.g., row201jhas nanomagnets202a,202b,202c, and202d. To avoid overcrowdingFIG.2A, only the nanomagnets of row201jare labeled. Embodiments may however have tens, hundreds, thousands, or more nanomagnets per row (or column). The nanomagnets of any one row are collectively in contact with a heavy metal strip203. To avoid overcrowdingFIG.2A, only the heavy metal strip203of row201ais labeled, but each row has a separate heavy metal strip. All of the heavy metal strips203are electrically connected with conductive contacts204and205. The radiation from the antenna may be increased or decreased by respectively increasing or decreasing the number of nanomagnets in the array.

FIG.2Bshows a zoomed out portrayal of the array200. At this zoom only four rows are distinguishable.FIG.2Bshows that conductive contacts204and205are respectively conductively connected with electrodes206and207. The connection to successively larger conductive elements may be used to provide for the connection of the antenna array with other circuitry elements. As a reminder of scale of the illustrated elements, each nanomagnet may be, for example, only 100-500 nm in length and/or 100-500 nm in width.

FIG.3is a schematic that incorporates the exemplary array200into a larger system300that includes both transmitting and receiving elements for respectively emitting and detecting transmission signals which are surface acoustic waves (SAWs). The substrate302is a piezoelectric substrate and the nanomagnets are magnetostrictive. The nanomagnets and substrate302are arranged with respect to one another such that physical forces in one transfer or are transferable to the other. Generally this is most readily achieved by the nanomagnets and substrate302being in direct physical contact with one another. However, some embodiments may have one or more materials (e.g., layers, films) partially or completely between the piezoelectric substrate and the nanomagnets.

InFIG.3the array200is only illustrated with three rows for simplicity of illustration. As discussed above, the actual number of rows may be tens, hundreds, or thousands. The number of rows may be configured according to the signal strength desired or required of the antenna as determined prior to manufacture. Generally, more nanomagnets, and thus more rows, corresponds with greater emitted signal strength (or sensitivity to received signals). The electrodes206and207are connected to a driver305which may comprise or else is connected to an AC source. The electrodes206and207are conductive traces and may be, for example, gold or aluminum. The driver305is configured to periodically change magnetizations of the nanomagnets of the array200such that the array emits one or more waves, in this case acoustic waves.

The receiving elements in system300are interdigitated transducers303and304. Two receiving elements303and304are shown, but systems like system300may have only one or, alternatively, a plurality (e.g., 3, 4, 5, tens, or more). A transducer303is connected to electrodes306and307which facilitate electrical connection with downstream circuitry.FIG.3shows the antenna-transmitter system (comprising the nanomagnet array and driver) and the antenna-receiver system (comprising the interdigitated transducers) physically situated near one another. This proximity may be representative for some embodiments but particularly functions inFIG.3the illustrative objective of fitting both systems in the same drawing. In practice, the distance over which the signal is transmitted may vary substantially, depending on the particular use scenario. The distances over which a surface acoustic wave (SAW) can travel depends on the attenuation constant which, in turn, depends on the material in which the SAW propagates and also the crystallographic orientation along the direction of propagation. Typically, the attenuation constant in suitable materials is very small. Hence the SAW can easily propagate over several tens of cm (e.g., at least 50, 60, 70, 80, or 90 cm) without losing much signal strength. The direction of the SAW propagated by the nanomagnet array200is generally in all directions on the surface plane of the piezoelectric substrate302. That is to say the exemplary antenna is omnidirectional in the plane of the piezoelectric substrate.

FIGS.4A and4Billustrate the mechanism by which the magnetizations of the nanomagnets are periodically changed, resulting in the emission of one or more waves. Each ofFIGS.4A and4Bshow the same three nanomagnets401a,401b,401cbelonging to the same row of an array such as array200(seeFIGS.2A,2B, and3).

FIG.4Cshows a perspective view of a single nanomagnet401, representative of nanomagnets401a,401b, and401c. Each nanomagnet401comprises a ledge431and a remaining portion432. The remaining portion432may be square, for example, with the ledge431being substantially smaller in surface area and volume than the remaining portion432. Portions431and432may be a single body produced in a single step or by the same manufacturing steps (e.g., of a photolithographic process).

Returning toFIGS.4A and4B, a single heavy metal strip402is arranged across the ledge of each of the nanomagnets401a,401b, and401c. That is to say, each ledge is underneath the heavy metal nanostrip. An exemplary heavy metal is platinum. The bulk of each nanomagnet (the portion432) is outside the strip402and hence its expansion/contraction is not clamped by the nanostrip. Only the ledge is clamped.

FIG.4Ashows the application of a charge current to the heavy metal nanostrip in a first direction of current, whereasFIG.4Bshows the application of a charge current to the heavy metal nanostrip in a second direction of current, where the first and second directions are opposite. Both directions may be result of an applied alternating current. Thus in practice, the driver would periodically alternate the current between these two states. When a charge current is injected into the nanostrip, the top and bottom surfaces of the nanostrip become spin-polarized because of the giant spin Hall effect in the heavy metal (here, Pt). The two surfaces have antiparallel polarizations. The polarizations of spins in either surface depends on the direction of the current and changes sign when the current reverses direction. The accumulated spins in the bottom surface of the nanostrip402diffuse into the ledges431that they are in contact with, and from there into the remaining portion of the nanomagnets401a,401b, and401c. This transfer exerts a spin-orbit torque on the nanomagnets401a,401b, and401cand rotates their magnetizations.FIGS.4A and4Bare labeled with a number of arrows to show the spin on top surfaces of the nanostrip402, the bottom surfaces of the nanostrip402, and in the nanomagnets401a,401b, an401c.

When the driver reverses the direction of the injected charge current, the change in current reverses the spin polarization of the bottom surface of the nanostrip and hence rotates the magnetizations of the nanomagnets in the opposite direction because the spin-orbit torque will reverse direction. Generally, this will happen only as long as the period of the current is longer than the time of magnetization rotation. Thus, if the driver passes an alternating current through the nanostrip, it will rotate or flip the magnetizations of the nanomagnets periodically, as long as the frequency of the current is considerably smaller than the inverse of the spin rotation times of the nanomagnets. One or more of the shape, size, and material of the nanomagnets, the nanostrip, and the physical arrangement of the nanomagnets and nanostrips may be configured at the time of manufacture to set the threshold to a predetermined value.

The alternating rotation of the nanomagnets in the array causes the array to emit an electromagnetic wave, hence the system300inFIG.3will act as an electromagnetic antenna. At the same time, the nanomagnets will periodically expand and contract because they are magnetostrictive, thus executing a “breathing mode” oscillation. This will generate periodic strain in the piezoelectric substrate302underneath the nanomagnets and set up a SAW that is detectable with interdigitated transducers (IDTs). Thus, the system300acts as a dual electromagnetic and acoustic antenna. The driver305may be configured to modulate a frequency of the emitted wave solely by controlling a frequency of the alternating charge current.

The wavelength of the acoustic wave may be determined solely by the frequency of the alternating charge current (which is the dominant frequency of the generated acoustic wave) and the velocity of acoustic wave propagation in the piezoelectric substrate. Therefore, it has no relation to the size of the nanomagnets (antenna elements). The antenna elements may be much smaller than the size of the acoustic wavelength. The result is a subwavelength acoustic antenna with a radiation efficiency that exceeds the theoretical limit for an acoustic antenna excited at acoustic resonance.

From the physics perspective, electrical signals (photons) are used to generate acoustic waves (phonons). The antenna converts photons in the input (low-frequency electromagnetic) alternating signal applied to the Pt strip to magnons via the spin Hall effect (or spin-orbit torque) and then to phonons in the surface acoustic wave via magnetoelastic coupling, which are finally detected at the IDT. The overall efficiency of this three-step process is at least ≈1%.

Example 1 below show an extreme subwavelength acoustic antenna that emits acoustic waves (in addition to electromagnetic waves) with an efficiency of ≈1%. The antenna dimension was 67 times smaller than the acoustic wavelength. The antenna is not driven at acoustic resonance, thereby avoiding the expected efficiency limit. For instance, for an antenna 67 times smaller than the acoustic wavelength, the efficiency of such antenna if driven at acoustic resonance would be limited to ( 1/67)2=0.02%. The present embodiment overcomes that limit and achieves an efficiency of ≈1%, which is 50 times larger than the limit. This is achieved using a principle of actuation different from acoustic resonance. The antenna dimension can be made substantially smaller than 1/67 times the wavelength with still reasonable radiation efficiency.

The excitation frequency (frequency of the electrical signal applied to the Pt nanostrip to produce the giant spin Hall effect) in Example 1 was around 3.6 MHz. The maximum allowable frequency (or alternately the minimum signal period) is determined by the time it takes for the nanomagnets' magnetizations to rotate over a significantly large angle. The signal period must exceed the latter time by a factor of e.g. ≈10 to ensure that nearly all the nanomagnets have ample time to rotate their magnetizations through a large enough angle and produce a strain in the piezoelectric substrate underneath. The time taken by the magnetization to rotate through a large enough angle depends on the size, shape, nanomagnet material, and also the strength of the spin-orbit torque, which, in turn, is determined by the spin-orbit interaction strength in the heavy metal (in this case platinum) and the magnitude of the current injected into it. For reasonable values of these parameters, the switching time is estimated to be no less than ≈1 ns. (Past simulations by the inventors based on the Landau-Lifshitz-Gilbert equation suggests that the switching time is on the order of 1 ns.) Of course, there will be distribution of the switching time because of defects in the nanomagnets, pinning sites, variations in shape and size, etc. As a result, some embodiments may use a safe estimate threshold for switching of 10 ns, which would then limit the maximum frequency to 100 MHz. This is high enough for many on-chip acoustic applications.

The system300ofFIG.3was described above according to a configuration in which antenna array200transmitted a signal and IDTs303and304received the signal. In some embodiments, the nanomagnet array200may instead by configured as the receiving element. The transmitting element in such an arrangement may be another nanomagnet array like array200, or else it may be IDTs. To recap, an antenna according to the present disclosure may be configured as a transmitting antenna, receiving antenna, or both. Antennas according to the present disclosure may furthermore be configured as an electromagnetic antenna, acoustic antenna, or both.

InFIG.3, the IDTs303and304may be used as input ports, and the two terminals or electrodes206and207of the heavy metal strips may be used as output ports. An oscillating electrical signal applied at the input port produces an oscillating electrical signal at the output port because of reciprocity. The input signal at the IDT304or303launches a SAW in the substrate302. The SAW in the substrate302periodically rotates the magnetizations of the magnetostrictive nanomagnets owing to the Villari effect. This results in periodic spin pumping into the heavy metal (e.g., Pt) lines, resulting in an alternating spin current which would have been converted into a charge current by the inverse spin Hall effect, resulting in an oscillating electrical signal across the heavy metal lines. However, to observe the reciprocal effect, the design should be such that sufficient spin pumping can occur. Here, the details of system300may differ from what is disclosed above inFIGS.4A,4B, and4C. When configuring system300for optimal performance as an acoustic antenna that transmits a SAW, the exemplary nanomagnets401included ledges to avoid clamping. Such configuration may preclude sufficient spin pumping because a small section of the nanomagnets are in contact with the Pt strip. Therefore, the reciprocal effect is small in the arrangement of heavy metal strip and nanomagnets represented byFIGS.4A,4B, and4C. Accordingly, for embodiments in which an array200of a system300is configured for receiving an acoustic wave and converting it to an electrical signal, the nanomagnets should have a larger overlap with the adjacent heavy metal strip for more efficient spin pumping.

The actuation of the nanomagnets as described in the preceding paragraph is similar for a configuration in which the array200is used for emitting an electromagnetic wave as a result of actuation by an acoustic wave. The physics underlying configuration of the array200as electromagnetic antenna actuated by an acoustic wave is a converse of the physics underlying the configuration of the array200as an acoustic antenna actuated by an alternating current that causes alternating spin-orbit torque. Configured as an electromagnetic antenna actuated by an acoustic wave, array200of system300converts phonons to magnons to photons in order to realize a subwavelength electromagnetic antenna implemented with magnetostrictive nanomagnets that were periodically strained with a surface acoustic wave.

Exciting the antenna at acoustic resonance instead of electromagnetic resonance allows for considerable miniaturization of the antenna. Since the acoustic wave velocity in many piezoelectric solids is roughly five orders of magnitude smaller than the speed of light in vacuum, the acoustic wavelength is five orders of magnitude smaller than the electromagnetic wavelength at the same frequency. Consequently A/λac2˜1010×A/λEM2, where A is the radiating area of the antenna, λacis the acoustic wavelength and λEMis the electromagnetic wavelength.

The surface acoustic waves were also found to amplify the magnetization response of nanomagnets resonating in GHz frequencies, which may have different applications of its own. These extreme sub-wavelength antennas allow dramatic downscaling of communication systems and may open up new high frequency applications. They may also allow the miniaturization of two-dimensional phased array antennas for electronic beam steering with the entire array occupying an area much smaller than the square of the emission wavelength.

FIGS.5A and5Billustrate yet a further embodiment. Here a sub-wavelength electromagnetic antenna500is implemented with anti-ferromagnets that undergo a metamagnetic transition to a ferromagnetic state at a critical temperature. For example, FeRh is an antiferromagnet below 350 K temperature, but becomes a ferromagnet above that temperature. The transition temperature is suppressed to below room temperature using strain. Thus, by applying strain, antiferromagnetic FeRh is converted to ferromagnetic FeRh at room temperature.

InFIG.5A, islands506a,506b,506c, and506dof anti-ferromagnetic material are on a piezoelectric substrate504. Islands (of lateral dimensions such as 100 nm and thickness 5-10 nm) may be sputter deposited on a patterned piezoelectric substrate. The islands may be made of, for example, FeRh. The piezoelectric substrate may be LiNbO3for example. The antiferromagnetic state has no net magnetization.

InFIG.5Ba bias electric field is induced by an applied voltage from power source505applied between electrodes502and503on opposite sides of the substrate504. Application of a voltage across the piezoelectric substrate strains the islands such that they undergo a metamagnetic transition to the ferromagnetic phase. The strain in the substrate505resulting from the bias field causes the islands506a,506b,506c, and506dto convert to a ferromagnetic state with orientations fixed in a common direction.

Application of an oscillating voltage to the piezoelectric periodically generates a magnetic field in the direction of the bias field, and this oscillating field will produce an electromagnetic wave. The islands506a,506b,506c, and506dthus become an extreme subwavelength electromagnetic antenna.

The highest frequency that is producible is limited by the piezoelectric response time of 10-100 ps and hence is between 10 and 100 GHz (a higher frequency than embodiments described above since we are relying on the metamagnetic transition and not the Giant Spin Hall Effect, spin diffusion and magnetic reversal of the nanomagnets). The electromagnetic wavelength at these frequencies is in the range 0.3-3 mm, while the antenna dimension is ≈100 nm, making the wavelength to antenna dimension ratio at least 3000:1.

FIG.5Cshows an acoustic-to-electromagnetic converter500where the stress is generated with a surface acoustic wave. To implement an antenna for lower frequencies (≈1 GHz), then the strain in the piezoelectric can also be produced by launching a surface acoustic wave of frequency ≈1 GHz in the substrate.

FIG.6shows as yet a further embodiment an extreme sub-wavelength electromagnetic antenna600. The antenna comprises an array601of nanomagnets arranged on an insulating film602over a topological insulator film603deposited on a substrate604. An alternating current is passed through the topological insulator film via contacts605and606. The contacts may be placed directly on top of the topological insulator film. The topological insulator film alternately injects spins of opposite polarization into the nanomagnets. This makes their magnetizations oscillate and emit electromagnetic radiation. The spin Hall angle associated with spin injection from a topological insulator is larger than that associated with spin injection from a heavy metal. Hence, a topological insulator may deliver a stronger spin-orbit torque and increase the radiation efficiency as well as the output power. The driver in this embodiment may be a microwave source or any alternating current source that includes a microwave source.

EXAMPLES

Example 1. Demonstration of an Extreme Subwavelength Nanomagnetic Acoustic Antenna Actuated by Spin-Orbit Torque from a Heavy Metal Nanostrip

The nanomagnets and Pt strip were fabricated on a 128° Y-cut LiNbO3substrate. The substrate was spin-coated with bilayer polymethyl methacrylate (PMMA) e-beam resists of different molecular weights to obtain good undercut: PMMA 495 diluted 4 vol % in Anisole, followed by PMMA 950 also diluted 4 vol % in Anisole. The spin coating was carried out at a spin rate of 2500 rpm. The resists were subsequently baked at 110° C. for 5 min. Next, electron-beam lithography was performed using a Hitachi SU-70 scanning electron microscope (at an accelerating voltage of 30 kV and 60 pA beam current) with a Nabity NPGS lithography attachment. Finally, the resists were developed in methyl isobutyl ketone and isopropyl alcohol (1:3) for 270 s followed by a cold IPA rinse. For nanomagnet delineation, a 5 nm thick Ti adhesion layer was first deposited on the patterned substrate using e-beam evaporation at a base pressure of ≈2×10−7Torr, followed by the deposition of Co. Pt was deposited similarly. The lift-off was carried out using Remover PG solution.

FIG.7Ashows the pattern of the acoustic antenna (with the nanomagnets, Pt lines, contact pads to the Pt lines, and IDTs).FIG.7Bshows a scanning electron micrograph of the fabricated nanomagnets. The nanomagnets are rectangular with long dimension≈250 nm, short dimension≈200 nm, and the ledge length is ≈100 nm. The ledge has a tapered shape and the full width at half maximum is ≈70 nm.

FIGS.8A and8Bshow scanning electron micrographs of the Pt line and nanomagnet assembly. There are 40 Pt lines and hence 40 rows of nanomagnets that are contacted.

FIGS.9A and9Bshow both the oscilloscope traces of the sinusoidal voltage applied across the Pt lines to actuate the acoustic antenna via spin-orbit torque and the voltage detected at the IDT. These traces are shown for two frequencies, 3.63 MHz which is the resonant frequency of the IDT, and 6.87 MHz.

The resistance of the 40 parallel Pt lines varied between 98 and 108 ohms from sample to sample. Therefore, the resistance of each Pt nanostrip or line is on the order of 4 kΩ For the case inFIG.9A, the input voltage of 11.25 V peak-to-zero (measured from the oscilloscope trace) will produce a current of 2.8 mA peak-to-zero in a Pt line. The line has a length of 15 μm, width 1 μm, and thickness 200 nm. Therefore, the peak current density in each Pt line is 1.4×1010A m−2, which should be well above the critical current density needed to produce sufficient spin-orbit torque. In the case ofFIG.9B, the input voltage of 12.85 V peak-to-zero produced a peak-to-zero current of 3.21 mA in a Pt line, resulting in a peak-to-zero current density of 1.6×1010Am-2in each line.

The input power to the acoustic antenna is calculated as Vin2/2RPtwhere Vinis the peak-to-zero input voltage and RPtis the resistance of the 40 Pt lines in parallel. For the case inFIG.9A, this quantity is 633 mW, while for the case inFIG.9B, it is 825 mW.

In order to calculate the radiation efficiency, first the power in the acoustic wave that has been produced is determined. The power carried by an acoustic wave of amplitude φ is given by

P=12⁢γ0⁢Wλ⁢φ2(1)
where γ0is the characteristic admittance of the SAW line and has a value of 2.1×10−4S for LiNbO3, W is the width of the IDT and λ is the wavelength of the SAW. The IDTs were designed and fabricated for W/λ=40.

Neglecting capacitive and inductive effects, the voltage Voutdetected at the IDT is related to the SAW amplitude φ as
φ≈μVout(2)
where μ is the response function of an IDT operating in the transmitting mode. For the present system, this quantity was calculated as ≈2 (see V. Sampath, N. D'Souza, D. Bhattacharya, G. M. Atkinson, S. Bandyopadhyay, J. Atulasimha,Nano Lett.2016, 16, 5681.) Hence, for the case shown inFIG.9A(where Vout=0.45 V peak-to-zero as seen in the oscilloscope trace) φ=0.9 V peak-to-zero and for the case shown inFIG.9B, φ=1.6 V peak-to-zero. Therefore, from Equation (2), the SAW power produced in the two cases are 3.4 and 10.7 mW, respectively. The corresponding efficiencies of SAW production are 0.54% and 1.3%, respectively. These numbers are approximate because of some simplifying assumptions such as neglecting inductive and capacitive effects. It is also assumed that the IDT detection efficiency is 100%, which is an overestimate, especially at frequencies that are not the IDT resonant frequency. Hence, the estimates of the antenna efficiencies presented here are conservative.

In the LiNbO3substrate, the acoustic wave velocity is ≈3300 m s−1. Therefore, for a frequency of 3.63 MHz, the acoustic wavelength is ≈1 mm. The nanomagnet assembly acting as the antenna has a dimension of ≈15 μm in the direction of SAW propagation. Hence the ratio of acoustic wavelength to antenna dimension is ≈67, making it an extreme subwavelength antenna (an antenna whose linear dimension is smaller than one-tenth of the wavelength). If the antenna was excited by an acoustic wave and driven at the acoustic resonance, the radiation efficiency would have been limited to ≈( 1/67)2=0.02%. The measured efficiency here is about 50 times larger. The present embodiment was able to overcome the limit because the antenna was actuated via spin-orbit torque as opposed to being driven with an acoustic wave.

It was considered whether the output voltage detected at the IDT (shown inFIGS.9A/9B) could have been due to direct electromagnetic pick up through the air from the input, instead of being from an acoustic wave in the substrate. Electromagnetic pickup, however, could never have produced the phase shifts (time delay) seen between the input and output signals inFIGS.9A/9B. The measured average distance between the input and output ports is ≈6 mm and the time Δt that it would take an electromagnetic wave to traverse this distance is 20 ps. At a frequency f of 3.63 MHz (FIG.9A), this would produce a phase lag of 2πfΔt=4.56×10−4radians between the output and input, which is much smaller than what is observed. For an acoustic wave traveling in the substrate, with a velocity 5 orders of magnitude smaller than that of an electromagnetic wave propagating through the air, Δt will be five orders of magnitude larger and the phase shift at this frequency will be 45.6 radians=(14π+1.62) radians. When the modulo 2π value of this phase shift is taken, this is 1.62 radians. The observed phase shift is about 2.2 radians which is much closer to the acoustic phase shift than the electromagnetic phase shift. This gives confidence that the detected signal is not due to electromagnetic pick up.

Repeating this exercise for the 6.87 MHz frequency will yield an electromagnetic phase shift of 8.63×10−4radians and an acoustic phase shift of 86.3 radians=(26π+4.6) radians. The modulo 2π value of this phase shift is 4.6 radians which is close to the observed value of 3.3 radians. This again gives confidence that the observed output voltage at the IDT is indeed due to the generated surface acoustic wave.

In conclusion, this Example demonstrates an acoustic antenna actuated by the spin-orbit torque from a heavy metal nanostrip. The use of this novel actuation mechanism allows for an extreme subwavelength acoustic antenna with a radiation efficiency over 50 times larger than the limit for an acoustic antenna actuated by an acoustic wave and operated at acoustic resonance.

Example 2. Extreme Sub-Wavelength Magneto-Elastic Electromagnetic Antenna Implemented with Multiferroic Nanomagnets

This Example demonstrates an extreme sub-wavelength EM antenna whose radiation efficiency exceeds the A/λEM2limit by a factor exceeding 105.

An extreme-sub-wavelength EM antenna was constructed as an array of magnetostrictive (Co) nanomagnets of dimension ˜300 nm fabricated on a piezoelectric 128° Y-cut LiNbO3substrate. The substrate was spin-coated with bilayer polymethyl methacrylate (PMMA) e-beam resists of different molecular weights to obtain good undercut: PMMA 495 diluted 4% by volume in Anisole, followed by PMMA 950 also diluted 4% by volume in Anisole. The spin coating was carried out at a spinning rate of 2500 rpm. The resists were subsequently baked at 110° C. for 5 min. Next, electron-beam lithography was performed using a Hitachi SU-70 scanning electron microscope (at an accelerating voltage of 30 kV and 60 pA beam current) with a Nabity NPGS lithography attachment. Finally, the resists were developed in methyl isobutyl ketone and isopropyl alcohol or MIBK-IPA (1:3) for 270 s followed by a cold IPA rinse.

For nanomagnet delineation, a 5-nm-thick Ti adhesion layer was first deposited on the patterned substrate using e-beam evaporation at a base pressure of ˜2×10−7Torr, followed by the deposition of Co. The lift-off was carried out using Remover PG solution.

A surface acoustic wave (SAW) was launched in the substrate with electrodes and the SAW periodically strained the nanomagnets, causing their magnetizations to rotate owing to the inverse magnetostriction (Villari) effect. The rotating magnetizations emit EM waves (at the frequency of the SAW), which were detected in the far field by a dipole antenna coupled to a spectrum analyzer. The SAW (excitation) frequency was 144 MHz. The inventors were able to detect EM emissions at the same frequency that was 8 dBm above ambient emissions, at a distance>2 m from the antenna. A control sample (that contained no nanomagnets, but was otherwise identical to the actual sample) was used for background subtraction. The inventors were thus able to measure the EM emission from the nanomagnets at the exclusion of all other emitters (e.g. surface currents in the electrodes that are used to launch the SAW and any other spurious source radiating at or near 144 MHz).

FIGS.10A,10B, and10Cshow the scanning electron micrographs of the nanomagnet arrays that were fabricated. Two sets of samples were made: Sample A and Sample B. The former contained 55,000 nanomagnets and the latter 275,000 nanomagnets.FIGS.10A and10Bshow low magnification images of several rectangular arrays in the two samples (each white speck is an array).FIG.10Cshows a zoomed image of one such array (where the magnification is not enough to resolve individual nanomagnets).FIGS.11A and11Bshow the nanomagnets at higher magnifications that allow one to resolve individual nanomagnets.

The magnetic behavior of the nanomagnets was characterized with static magneto-optical Kerr effect (S-MOKE) at room temperature.FIG.12shows the Kerr rotation versus magnetic field characteristics (hysteresis loops) under two situations: when the magnetic field is directed along the horizontal axis of the arrays and when directed along the vertical axis. The hysteresis loops confirm that the fabricated nanostructures are ferromagnetic at room temperature. The coercivity is 100 Oe higher when the magnetic field is directed along the horizontal axis which is parallel to the minor axes of the elliptical nanomagnets because this is the hard axis while the major axis is the easy axis.

In order to demonstrate the antenna function and also measure the antenna characteristics, SAW signal was launched in the substrate at two different frequencies (fSAW=144 MHz and 900 MHz). Any detectable EM emission (above the noise floor determined by ambient emissions) was measured at a distance>2 m from the samples. The detector was a dipole antenna calibrated to specific frequencies and these two frequencies belonged to that set, which is why they were chosen.

EM emissions were detected at 144 MHz, but not at 900 MHz which was too high a frequency for the magnetization of the nanomagnets to rotate. At 144 MHz, the EM wavelength is 2 m. Since the separation between the detector and the antenna was greater than the EM wavelength, we were measuring the far-field emission.

The SAW velocity in the LiNbO3substrate is about 4100 m/sec, and hence the SAW wavelength is 28.4 μm at 144 MHz, while the EM wavelength is 2 m at that frequency. The ratio of the SAW to EM wavelength is thus 1.42×10−5.

Measurements were made for both samples A and B containing nanomagnets, as well as control samples that were otherwise identical to the real samples but had no nanomagnets.FIG.13shows the detection results at fSAW=900 MHz for Sample A (screenshots of the spectrum analyzer are shown in the Supplementary Material). There is ˜1 dB difference between the 900 MHz emissions from the real sample (with nanomagnets) and the control sample (without nanomagnets), indicating that the nanomagnets are radiating very weakly at 900 MHz, if at all. Most likely, the nanomagnets do not radiate sufficiently because they are not able to rotate their magnetizations through sufficiently large angles at this high rate (900 MHz). Surface currents induced in the electrodes used to launch the SAW in both samples also radiate electromagnetic waves at 900 MHz and the detected emissions are primarily due to that. Similar (negative) result was obtained with Sample B.

Past simulations by the inventors have shown that strain induced large angle magnetization rotation in single domain elliptical nanomagnets (of lateral dimension ˜100 nm) typically takes place in about 1 ns. The nanomagnets used here were larger (>300 nm lateral dimension) and multidomain. Hence, it is possible that they rotate slower and therefore, the period of the 900 MHz signal (1.1 ns) does not allow them enough time to rotate through a large angle and radiate electromagnetic waves. Subsequently, the excitation frequency was reduced to 144 MHz. The detection results are shown inFIG.14.

FIG.14clearly shows a measurable difference between the real samples and the control samples. The detected radiation power from Sample B is 8 dB higher than that from the control sample, while that from Sample A is 3 dB higher than that from the control sample. These differences indicate that the nanomagnets are able to rotate their magnetizations at this lower frequency and emit EM waves.

Time-resolved magneto-optical Kerr effect (TR-MOKE) measurements were also carried out on the nanomagnets at room temperature at various amplitudes of SAW excitation to verify that the launched SAW indeed has an effect on the magnetization rotation. The oscillations in time-resolved Kerr rotations were measured with a micro-focused optical pump-probe set up as shown inFIG.15A. Details of the set-up (e.g. beam spot size, pulse width, repetition rate, etc.) can be found elsewhere and hence not repeated here. See, for example, S. Mondal, M. A. Abeed, K. Dutta, A. De, S. Sahoo, A. Barman and S. Bandyopadhyay, Hybrid magneto-dynamical modes in a single magnetostrictive nanomagnet on a piezoelectric substrate arising from magnetoelastic modulation of precessional dynamics,ACS Appl. Mater. Interfaces,10, 43970-43977 (2018). See also A. Barman and J. Sinha inSpin Dynamics and Damping in Ferromagnetic Thin Films and Nanostructures, Springer International Publishing AG, (2018).

The measurements were done in the absence of any bias magnetic field. The ultrashort laser pulses used in the TR-MOKE measurements set up very high frequency (˜4 GHz) oscillations of the nanomagnets' magnetizations and surprisingly, it was found that the amplitudes of the Kerr oscillations resulting from these high frequency oscillations are significantly increased by the launched SAW with fSAW=144 MHz. The amplitudes are markedly different in the absence of SAW versus in the presence of SAW. The amplitudes also show a rather weak dependence on the launched SAW power (P) for P>−15 dBm. These results are shown inFIGS.15B and15C.

It is noted that the SAW frequency (fSAW=144 MHz) is more than an order of magnitude lower than the Kerr oscillation frequencies which are in the neighborhood of 4 GHz. The Kerr oscillations are not caused by the launched SAW. Instead, they are caused by the ultrashort laser pulses in the TR-MOKE set-up. The excitation by the femtosecond laser causes an ultrafast demagnetization of the nanomagnets followed by two-step relaxation (not shown) which also launches an ultrafast internal field to trigger magnetization precession of the nanomagnets. The absence of any bias magnetic field ensures that the magnetization precesses around an effective magnetic field due to the dipolar coupling fields between the nanomagnets, which leads to a dominant natural resonance frequency at around 4 GHz. Clearly the launched SAW strongly affects the amplitude of this resonant oscillation of magnetization despite being highly off-resonant and having a very weak SAW power.FIG.15Dshows the fast Fourier transforms (power spectral densities) of the Kerr oscillations for various SAW power. The ensuing power spectral densities are also affected by the SAW due to the variation in the magnetization oscillation amplitudes.

In the TR-MOKE experiments, it was not possible to detect any Kerr oscillation having a frequency component at the launched SAW frequency of 144 MHz because the time delay between the pump and probe laser (Δt) was only up to 3 ns and hence the lowest frequency component that could be resolved was about ⅓ ns, i.e. 333 MHz. Therefore, a SAW was launched of frequency fSAW=350 MHz. The time-resolved Kerr rotations and their fast Fourier transforms are shown inFIGS.16A and16B. InFIG.16Bit is still not possible to detect any clear and consistent peak at 350 MHz. This indicates that either the SAW, by itself, cannot induce sufficient magnetization rotation at this high frequency of 350 MHz, or any signature of that rotation is being drowned by the much stronger Kerr oscillations caused not by the SAW, but by the ultrashort laser pulses. However, the clear observation that the SAW power affects Kerr oscillations significantly indicates unambiguously that the SAW affects magnetization oscillations and thus could be the source of the electromagnetic emission which is observed.

InFIG.17, we show the reflection coefficient S11(measured at the electrodes that launch the SAW) at the input power of 5 dbm (3.16 mW) as a function of frequency. The measurements are carried out with a network analyzer for Sample A, as well as the control samples. At the frequency of 144 MHz, ˜85% of the input power is reflected back to the source because of impedance mismatch and hence only about 15% of the input power is coupled into the SAW. Therefore, the actual power fed to the antenna is 3.16×0.15=0.474 mW. This is the input power to Sample A, and the input power to Sample B is about the same.

The EM power from Sample B that is detected by the receiving dipole antenna is −73 dBm (seeFIG.14) which is about 50 pW. The power radiated by the control sample that is detected by the antenna is −81 dBm, which is 8 pW. Hence the power detected from the nanomagnets is 42 pW. The actual radiated power from Sample B is radiated over 4π solid angle and the fraction that is incident on the receiving dipole antenna is

lw4⁢π⁢r2
where l is the length of the receiving antenna, w is its width and r is the separation between the source and the detector. In our case l=1 m, w=0.5 cm and r=2 m. Hence the ratio

l⁢w4⁢π⁢r2
is 10−4and consequently, the power actually radiated by Sample B is 42×104pW=0.42 μW. Consequently, the radiation efficiency, which is the ratio of the radiated power to the input power, is 0.42 μW/0.474 mW=0.088% in the case of Sample B. In the case of Sample A, the detected power was −78 dBm, which is about 15 pW. Therefore, the power radiated by the nanomagnets in sample A was 15 pW−8 pW=7 pW. In this case, the efficiency is 0.07 μW/0.474 mW=0.014%. Since Sample A had 55,000 nanomagnets and Sample B had 275,000 nanomagnets, it is expected that the radiation from Sample A be weaker than the radiation from Sample B.

Now the A/λEM2limit is calculated for both samples. The area of a nanomagnet is

π4⁢(a×b)
where a is the major axis dimension (360 nm) and b is the minor axis dimension (330 nm). Since there are 55,000 nanomagnets in Sample A, the radiating area is

A=π4⁢(3⁢6⁢0×3⁢3⁢0)×1⁢0-1⁢8×55,TagBox[",", NumberComma, Rule[SyntaxForm, "0"]]000=5×10-9⁢m2.
Hence, in the case of Sample A, A/λEM2=1.25×10−9, which means that the measured efficiency of 0.014% was able to beat the A/λEM2limit by 112,000 times. In the case of Sample B, the radiating area is

A=π4⁢(3⁢6⁢0×3⁢3⁢0)×1⁢0-1⁢8×275,TagBox[",", NumberComma, Rule[SyntaxForm, "0"]]000=2.5×10-8⁢m2.
Hence for Sample B, A/λEM2=6.25×10−9, which means that the measured efficiency of 0.088% was able to beat the A/λEM2limit by 140,800 times in Sample B.

This Example demonstrates extreme sub-wavelength electromagnetic antennas whose radiation efficiencies greatly exceed the theoretical limit of A/λEM2(A<λEM) [where A is the emitting area and λEMis the wavelength of the emitted electromagnetic wave] by a factor exceeding 105. This allows new extents to the miniaturization of electromagnetic antennas. In this Example, the emitting areas of the antennas are about 2×108times smaller than the square of the emission wavelength. This drastic miniaturization was made possible by exciting the antennas at acoustic resonance instead of electromagnetic resonance. The surface acoustic waves were also found to amplify the magnetization response of these nanomagnets resonating in GHz frequencies, which is yet another application of the technology.

In this disclosure, where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, representative illustrative methods and materials are described.

While exemplary embodiments of the present invention have been disclosed herein, one skilled in the art will recognize that various changes and modifications may be made without departing from the scope of the invention as defined by the following claims.