Patent Application: US-41895806-A

Abstract:
spin - torque devices are based on a combination of giant magnetoresistance and tunneling magnetoresistance effects . the basic structure has various applications , including amplifiers , oscillators , and diodes . for example , if the low - magnetoresistance contact is biased below a critical value , the device may function as a microwave - frequency selective amplifier . if the low - magnetoresistance contact is biased above the critical value , the device may function as a microwave oscillator . a plurality of low - and high - magnetoresistance contact pairs may be induced to oscillate in a phase - locked regime , thereby multiplying output power . the frequency of operation of these devices will be tunable by the external magnetic field , as well as by the direct bias current , in the frequency range between 10 and 100 ghz . the devices do not use semiconductor materials and are expected to be exceptionally radiation - hard , thereby finding application in military nanoelectronics .

Description:
this invention broadly resides in nano - scale , spin - torque devices that utilize current - driven spin waves for microwave signal processing . as an introduction , the device of fig2 comprises a metallic ferromagnetic nanowire with nominal dimensions of ˜ 5 nm thick , ˜ 50 nm wide , and ˜ 5 mm long . a number of ferromagnetic leads are attached via non - magnetic metallic spacers ( gmr leads ). these leads make contact to the wire in several places along its length forming a series of magnetic nanocontacts that serve as spin - polarized current injectors . when a large enough voltage is applied to a particular lead , spin current injected into the wire excites magnetization oscillations under the injector . these excitations of magnetization propagate along the wire as spin waves and induce microwave voltage signals at the neighboring dc - biased nanocontacts . thus , the nanowire acts as a one - dimensional ( 1d ) waveguide for the current - driven spin waves that provide coupling among the nanocontacts . an important advantage of the device geometry just described is the much stronger influence of the propagating spin waves excited by a particular nanocontact on the neighboring nanocontacts compared to the existing geometries . for example , the spin transfer device shown in fig1 ( b ) is a two - dimensional system , in which the intensity of the propagating spin waves decays with the distance r as ( 1 / r ) even in a conservative medium . in an actual experimental situation the factor ( 1 / r ) gives about a 10 - fold decrease of the strength of coupling between the neighboring nanocontacts . in contrast with the two - dimensional geometry , where the free layer of a nanocontact is an infinite film , the proposed one - dimensional geometry allows the spin wave propagation between the neighboring nanocontacts , but does not impose any geometric decay . therefore the interaction between neighboring nanocontacts is expected to be stronger than in other cases . recently observed diode effects in spin - transfer magnetic elements [ 10 ] further enables the development of a new class of phase - sensitive microwave detectors . being intrinsically resonant , these detectors will be much less sensitive to thermal noise than the conventional broadband semiconductor diode detectors . the new microwave diode effect results from spin — wave generation by spin - polarized current and the gmr effect . the microwave current i ( t )= i o e − iwt + c . c . passing through the nanocontact generates the magnetization precession at the same frequency , m = m o e − iωt + iφ + c . c , which modifies the nanocontact resistance via gmr . as a result , a constant voltage v =& lt ; r ( t ) i ( t )& gt ;˜ i 0 m 0 cos ( φ ) is induced at the nanocontact . the precession amplitude m o is significant only for the external ac current frequency that is close to the ferromagnetic resonance ( fmr ) frequency of the free layer . thus , the bandwidth of the diode effect is proportional to the fmr linewidth of the free layer ( typically ˜ 100 mhz ). the device geometry of fig2 , where the free layer has a shape of a one - dimensional waveguide , allows one to further decrease the bandwidth of the diode effect . if the ac current is applied to all the nanocontacts , the spin waves generated by each of the contacts will propagate inside the waveguide , and the amplitude of the magnetization precession at each contact will be a sum ( with the account of the phase ) of contributions of spin waves generated by all nanocontacts in the array . the individual spin waves will interfere either constructively or destructively , depending on the phase shift acquired during the spin wave propagation . as such , the one - dimensional array of magnetic nanocontacts works as a diffraction grating for spin waves . in this case , the bandwidth of generation will be determined by the spacing between the nanocontacts and by the number of the contacts in the array , and could be much smaller than the fmr linewidth in a continuous 2d free layer . as a result , the proposed device will operate as ultra - narrow - bandwidth microwave detector . spin - torque devices based on gmr and tmr nanocontacts connected by a spin - wave waveguide a new type of device according to the invention comprises one gmr contact ( thin spacer made of a nonmagnetic metal ) and one tmr contact ( the spacer made of an insulator ) formed on the same spin wave guide . as shown in fig3 , the geometry of this “ spin - torque transistor ” is based on a combination of gmr and tmr effects in two neighboring magnetic nanocontacts coupled to a spin - wave waveguide . the basic structure has various applications , including amplifiers , oscillators , and diodes ( as discussed above , wherein the tmr contact may not be necessary ). if the bias direct current through the gmr contact is below a critical value , the device will amplify the input microwave signal . by changing the magnitude of this sub - critical bias direct current we can control the amplification gain , analogous to a gate current . if the bias direct current through the gmr contact is above the critical value , the gmr contact will generate its own microwave signal , which , again , will be amplified due to the difference of gmr and tmr resistances , and the device will work as an independent microwave oscillator . the frequency of this oscillator may be varied by changing the bias magnetic field and by changing the bias direct current through the gmr contact . of course , the bias current through the gmr contact will also change the output amplitude . continuing the reference to fig3 , a small ac voltage v in applied to the left low resistance ( gmr ) nanocontact creates a spin - polarized current that excites a spin wave propagating in the nanowire waveguide towards the right high - resistance ( tmr ) nanocontact . the precession of magnetization under the tmr nanocontact , caused by the propagating spin wave , creates large microwave variations in the resistance of the tmr nanocontact biased by a direct current i tmr and , therefore , a large - amplitude microwave output voltage v out . the device combines the advantages of the gmr and tmr nanocontacts . the spin waves that propagate towards the tmr contact modifies its resistance , r → r 0 + δr ( t ). if the tmr contact is biased by the constant sub - critical current i 0 , the voltage at the tmr contact will have a significant ac component , δv ( t )≈ i 0 δr ( t ), which can be much larger than the ac voltage applied to the gmr contact , thereby functioning as a nanoscale - tunable frequency - selective amplifier . to increase the sensitivity of the gmr contact to small external ac signals , it is possible to bias the device by substantial , but sub - critical dc current that has been experimentally proven to reduce the effective magnetic dissipation in the gmr nanocontact [ 5 ]. in this case the device will work as a regenerative amplifier . in principle , this same device can work as a diode detector , as the direct - current voltage on the tmr contact will depend on the amplitude of the input microwave signal applied to gmr contact ( in the amplification regime ), but this is a minor effect . overall , the structure acts a magnetic analog of a semiconductor transistor , one that can operate in both amplification and generation regimes . the practical use of current - induced microwave generation in magnetic nanocontacts is hindered by the low microwave power (˜ 0 . 1 nw ) generated by a single nanocontact . according to this invention , this problem is addressed through mutual phase - locking in an array of weakly coupled spin - transfer nano - oscillators [ 6 - 9 ]. in the phase - locked regime all the coupled nanocontact oscillators generate coherently , which significantly increases the generated power (˜ n2 times , where n is the number of phase - locked oscillators ). a linear array of gmr / tmr pairs oscillating in a phase - locked regime may generate a significant power output of tens of microwatts or even milliwatts in the size 10 - 100 times smaller that that of microwave transistors based on semiconductor p - n junctions . an array of gmr contacts can also work as a frequency selective diode due to the signal interference . a preliminary theoretical analysis shows that the main effect leading to the self - phase - locking in an array of generating nanocontacts is the interaction through the spin waves propagating in the common free layer , rather than direct magneto - dipole interaction of nanocontacts or the coupling through the generated ac voltage . recent experiments [ 6 , 8 , 9 ] performed in the two - dimensional nanocontact geometry similar to that shown in fig1 ( b ) have shown that although the phase locking of two neighboring nanocontacts is possible , the frequency band of the phase locking is relatively narrow ( 100 mhz ) [ 8 ] and the effect is not well reproduced on all devices ( fig3 in [ 9 ]). thus , a stronger coupling between the neighboring nanocontacts is required for practical devices [ 7 ]. there are three important figures of merit for microwave semiconductor amplifiers , namely , gain ; gain - bandwidth product ( the frequency at which gain drops to 0 db ); and efficiency . as quantified below , these three parameters of the spin - torque amplifier are either similar or better than those of the conventional semiconductor transistors . the spin - torque amplifier using gmr and tmr nanocontacts connected by a spin wave waveguide ( length a = 50 nm , width w = 100 nm , thickness 4 - 5 nm ). due to this novel geometry it is possible to supply different ac and dc voltages on these two nanocontacts . a typical tmr nanocontact has 100 × 100 nm 2 in - plane size and is made with 1 . 15 - nm thick mgo dielectric between “ fixed ” and “ free ” layers made of ferromagnetic metals . it was recently demonstrated that the resistance - area product for the 1 . 15 - nm thick mgo is 3 ohm μm 2 , so the resistance of a 100 × 100 nm 2 tmr nanocontact is approximately r tmr = 300 ohm . the tmr coefficient ( maximum variation of resistance with variation of relative orientations of magnetizations in the “ free ” and “ fixed ” layers ) of such a nanocontact with cofe electrodes is k tmr = 115 % [ 14 ]. a typical gmr nanocontact has 60 × 120 nm 2 in - plane size and is made with 8 - nm thick cu spacer between the “ fixed ” and “ free ” layers made of ferromagnetic metals . a typical resistance of such a gmr nanocontact is r gmr = 5 ohm [ 15 ]. the gmr coefficient ( maximum variation of resistance with variation of relative orientations of magnetizations in the “ free ” and “ fixed ” layers ) of such a nanocontact with permalloy electrodes is k gmr ˜ 10 % [ 15 ]. the ac voltage v gmr at the gmr contact creates magnetization precession with the angle θ = ν gmr r gmr ⁢ σ γ ⁡ ( i gmr ) ( 1 ) where r gmr is the resistance of the gmr contact , σ is the spin - polarization parameter which is constant for a particular geometry of the contact , and γ ( i gmr ) is the dissipative parameter in the “ free ” ferromagnetic layer of the contact that can be varied from the magnitude proportional to the ferromagnetic resonance ( fmr ) linewidth in the “ free ” layer for zero dc bias current i gmr through the gmr contact ( γ = γ fmr for i gmr = 0 ) to almost zero for the bias current close to the critical current i c = γ fmr / σ ˜ 2 ma at which the microwave generation in the contact starts ( γ ≈ 0 for i gmr = i c = γ fmr / σ ) eq . ( 1 ) is valid for relatively small precession angles θ ≦ 30 °. note , that in has been proven experimentally in [ 2 ] that the dissipation parameter γ of the “ free ” layer of the gmr nanocontact can be reduced by at least 5 - 8 times if a moderate dc bias current i gmr ˜ 1 ma is applied to the gmr contact ( see fig3 d in [ 15 ] for details ). below we shall give a very conservative estimate of the amplifier gain coefficient in the case of zero dc bias current through the gmr contact , that will give us the lowest limit for the gain coefficient . the microwave precession excited by the input microwave signal in the gmr contact will excite propagating spin wave in the connecting waveguide , and that wave will create changes of the resistance of the tmr contact δr = δr max θ , where δr max = k tmr r tmr is the magnetoresistance of the tmr contact . if the tmr contact is biased by the dc current i tmr ( typical value is i tmr = 1 - 2 ma [ 1 ]) then the ac voltage at the tmr contact will be : ν tmr = i tmr ⁢ δ ⁢ ⁢ r = i tmr ⁢ k tmr ⁢ r tmr r gmr ⁢ σ γ ⁡ ( i gmr ) ⁢ ν gmr ( 2 ) and the gain coefficient g = v tmr / v gmr of the gmr / tmr amplifier is given by : g = ⁢ ν tmr ν gmr = k tmr ⁢ r tmr r gmr ⁢ i tmr [ γ ⁡ ( i gmr = 0 ) / σ ] = ⁢ 1 . 15 ⁢ ⁢ 300 ⁢ ⁢ ohm 5 ⁢ ⁢ ohm ⁢ 1 . 5 ⁢ ⁢ ma 2 ⁢ ⁢ ma ≈ 50 ( 3 ) if a moderate sub - critical dc current of i gmr ˜ 1 ma is applied to the gmr contact , the dissipation coefficient f can be reduced and the gain coefficient g can be increased by at least 5 times compared to the estimation given by eq . ( 3 ), resulting in 50 db power amplification . in semiconductor transistor amplifiers , gain depends on the frequency of operation but the maximum gain at a few ghz is typically between 10 db and 20 db . this means that spin - torque amplifier will exhibit substantially higher power gain , than a typical semiconductor transistor amplifier . a significant problem with conventional semiconductor high - frequency amplifiers is the roll - off of gain at high frequencies due to intrinsically large capacitance of the forward - biased junctions . the most important figure of merit for a high frequency transistor is the gain - bandwidth product , f t , which characterizes the frequency at which gain drops to 0 db . for silicon transistors the typical value of f t is 10 ghz while for special sige , gaas and gan transistors the typical value of f t is 20 ghz . this high - frequency gain roll - off presents a significant challenge for designing microwave amplifier with operational frequencies above 20 ghz ( k - band ). spin - torque amplifier according to this invention will have values of f t which are more than an order of magnitude higher than those of semiconductor transistors while essentially eliminating the gain roll - off problem of microwave signal amplification . indeed , the resistance and capacitance of the amplifier is dominated by the resistance and capacitance of the tunnel junction r tmr and c tmr . for an mgo tunnel junction described above r tmr = 300 ohm and c tmr = ɛɛ 0 ⁢ a d ≈ 0 . 0008 ⁢ ⁢ pf where a is the area of the tunnel junction a = 10 − 14 m 2 , d = 1 . 15 10 − 9 m is the mgo thickness and ε ≈ 10 is the dielectric constant of mgo . using a simple model for the calculation of the gain - bandwidth product [ 16 ], we obtain f t = 1 2 ⁢ π ⁢ ⁢ r tmr ⁢ c tmr ≈ 660 ⁢ ⁢ ghz . this high value of f t means that the gain of the spin - torque amplifier designed to operate at 50 ghz will be the same as that designed to operate at 2 ghz . this feature is truly unique to the spin - torque amplifier disclosed herein . there is a trade - off between the amplifier fidelity and its efficiency . the high values of magneto - resistance achievable with mgo ( up to 340 % [ 17 ]) allow efficiency parameters as high as 40 % for a 90 - degree - amplitude precession of magnetization in the spin - torque amplifier . however , this high efficiency comes at a cost of non - linear signal distortions . in the linear regime , for precession angles not exceeding 30 degrees the efficiency drops to about 10 %. these numbers are rather typical for microwave transistors . structural simplicity : currently - used frequency - selective amplifiers use a few transistors in combination with passive components including inductors which make the circuit very bulky ( a typical on - chip inductor takes about 0 . 01 mm 2 of chip space ). our design replaces all these active and passive components with one simple nano - scale device . this gives at least 2 orders of magnitude improvement of the chip real estate usage . as it was mentioned above , a typical tmr nanocontact has a resistance of r tmr = 300 ohm . for such a nanocontact , the power delivered to a 50 - ohm load will be reduced ( in comparison to the power delivered to a matched load ) by the power transmission coefficient ⁢ ⁢ 1 - γ 2 = 1 - ( 50 - r 50 + r ) 2 = 0 . 49 . therefore , the power delivered to a standard 50 - ohm load will be only a half of that delivered to a matched load . this 50 % reduction is not very significant considering the high gain expected for the spin - torque gmr / tmr amplifier . a few figures of merit characterize voltage - controlled oscillators ( vcos ), namely , quality factor ; modulation sensitivity ; output power ; tuning range ; and bias current and voltage . since our spin - torque oscillators are compatible with relatively inexpensive si cmos technology , we will compare the spin torque oscillator to the typical si - based on - chip oscillator used in wireless communications for microwave signal modulation / demodulation . quality factors of up to 20 , 000 were demonstrated for single generating spin - torque nanocontacts [ 14 ]. this quantity can only increase in a phase - locked array of generating nanocontacts ( roughly by n 1 / 2 times , where n is the number of phase - locked nanocontacts ), that we propose to use as a spin - torque oscillators . this high quality factor can be compared to the si - based vco which employ varactors and on - chip inductors for frequency tuning . typical quality factors achieved by these oscillators with active feedback are ˜ 100 , 000 . we expect that such quality factors for an array of phase - locked spin - torque oscillators without feedback will be achieved for n ˜ 100 phase - locked oscillators . it is feasible that with feedback circuitry in place the spin - torque oscillators will exceed the quality factors of the conventional on - chip vcos . spin - torque oscillators have much higher modulation sensitivity than conventional on - chip vcos . a typical modulation sensitivity of a spin torque oscillator is ˜ 100 , 000 mhz / v which is three orders of magnitude larger than the typical modulation sensitivity of a conventional vco ˜ 100 mhz / v . the output power generated by the spin - torque vco that is based on an array of 100 phase - locked nanocontacts is expected to be ˜ 0 . 01 mw . this number can be significantly improved if an array of phase - locked hybrid gmr / tmr devices is used as a tunable oscillator . for such a device , the output power in excess of 1 mw can be achieved , which is similar to that generated by conventional on - chip vcos . since the spin - torque oscillators can be produced in parallel by advanced photo - lithography and etching techniques , and they take a few orders of magnitude less area than the conventional vcos , the arrays of hybrid gmr / tmr spin - torque oscillators can generate microwave power per chip area significantly exceeding that achievable with any conventional vco . typical tuning range for a conventional vco is ˜ 10 %. spin - torque excitation frequency can be tuned in a range exceeding 50 % [ 15 , 18 ]. therefore , the tuning range of the spin - torque oscillators is at least several times higher than that of conventional vcos . note , also that spin - torque vcos can be tuned by both external magnetic bias field and by the bias dc current passing through the generating nanocontacts . typical central frequency can be chosen in the interval of 10 - 40 ghz [ 18 ] ( see e . g fig4 in [ 18 ]). it is important to note , that the presence of the bias magnetic field is not necessary neither for the operation of the spin - torque vco at a fixed frequency , nor for its tuning in the 50 % frequency range . the static bias field necessary for the saturation of the “ free ” ferromagnetic layer of the device ( typically made of permalloy ) can be provided by the shape anisotropy of the “ free ” layer made in the form of a rectangular nano - sized waveguide and will be sufficient to work in the 5 - 10 ghz frequency range . spin - torque vco requires much lower bias voltage ( v bias ˜ 20 mv , i bias ˜ 400 ma for an array for 100 contacts ) than the conventional vco which typically needs v bias ˜ 5 v , i bias ˜ 20 ma ), thus resulting in one order of magnitude lower power consumption than conventional vcos . 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