Patent Publication Number: US-9418648-B1

Title: Effective generation of ultra-high frequency sound in conductive ferromagnetic material

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is the continuation-in-part of the U.S. patent application Ser. No. 14/517,801, filed on Oct. 18, 2014 and entitled “USING TUNNEL JUNCTION AND BIAS FOR EFFECTIVE CURRENT INJECTION INTO MAGNETIC PHONON-GAIN MEDIUM”, which is the continuation-in-part application for the U.S. patent application Ser. No. 13/661,053, filed on Oct. 26, 2012, and entitled “GENERATION OF ULTRA-HIGH FREQUENCY SOUND”, now U.S. Pat. No. 8,891,335. 
    
    
     TECHNICAL FIELD 
     The technology relates to the generation of ultra-high frequency sound in the GHz region. 
     BACKGROUND 
     In the parent U.S. patent application Ser. No. 13/661,053, filed on Oct. 26, 2012, and entitled “GENERATION OF ULTRA-HIGH FREQUENCY SOUND”, (now U.S. Pat. No. 8,891,335) the generation of ultra-high frequency (1-10) GHz sound was disclosed. In the U.S. patent application Ser. No. 14/517,801, filed on Oct. 18, 2014 and entitled “USING TUNNEL JUNCTION AND BIAS FOR EFFECTIVE CURRENT INJECTION INTO MAGNETIC PHONON-GAIN MEDIUM” an efficient technique for injection of electrical current into sub-band having spin opposite to the direction of magnetization of the conductive ferromagnetic material was disclosed. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts that are further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     An apparatus for generation of an ultra-high frequency sound waves with frequencies between (1 GHz-10 GHz) is proposed. 
     The apparatus of the present technology comprises a spin injector coupled to a tunnel junction. The tunnel junction is coupled to a conductive ferromagnetic material including magnon gain medium. The spin injector is configured to inject minority non-equilibrium electrons into the conductive ferromagnetic material via the tunnel junction. The non-equilibrium magnons are generated in the magnon gain medium of the conductive ferromagnetic material. 
     The apparatus of the present technology further comprises a ferromagnetic dielectric material coupled to the conductive ferromagnetic material. The ferromagnetic dielectric material includes the magnetic phonon-gain medium. The non-equilibrium magnons propagated into the ferromagnetic dielectric material and having the magnon velocity exceeding the sound velocity in the phonon-gain medium of the ferromagnetic dielectric material cause generation of ultra-high frequency non-equilibrium phonons in the ferromagnetic dielectric material. 
     The apparatus of the present technology further comprises an ultra-high frequency sound waveguide coupled to the ferromagnetic dielectric material. The ultra-high frequency sound waveguide is configured to output the ultra-high frequency sound generated in the ferromagnetic dielectric material. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the technology and, together with the description, serve to explain the principles below: 
         FIG. 1  depicts a block diagram of the apparatus of the present technology comprising a spin injector, a tunnel junction, a conductive ferromagnetic material, a ferromagnetic dielectric material, and an ultra-high Frequency Sound Waveguide configured to output the non-equilibrium high frequency phonons having frequency in the range of (1-10) GHz. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now is made in detail to the embodiments of the technology, examples of which are illustrated in the accompanying drawings. While the present technology will be described in conjunction with the various embodiments, it will be understood that they are not intended to limit the present technology to these embodiments. On the contrary, the present technology is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the various embodiments as defined by the appended claims. 
     Furthermore, in the following detailed description, numerous specific-details are set forth in order to provide a thorough understanding of the presented embodiments. However, it will be obvious to one of ordinary skill in the art that the presented embodiments may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the presented embodiments. 
     In an embodiment of the present technology,  FIG. 1  depicts a block diagram  10  of the apparatus comprising a spin injector  18 , a tunnel junction  16 , a conductive ferromagnetic material  12 , and a ferromagnetic dielectric material  14  further including a magnetic phonon-gain medium (not shown). A bias voltage  20  is applied to the spin injector  18 . 
     The Ultra-High Frequency Sound Waveguide  24  is configured to output the non-equilibrium high frequency phonons having frequency in the range of (1-10) GHz. 
     The Ultra-High Frequency Sound Waveguide  24  can be implemented by using an ultrasonic horn. Ultrasonic horn (also known as acoustic horn, sonotrode, acoustic waveguide, ultrasonic probe) is necessary because the amplitudes provided by the transducers themselves are insufficient for most practical applications of power ultrasound. Another function of the ultrasonic horn is to efficiently transfer the acoustic energy from the ultrasonic transducer into the treated media, which may be solid (for example, in ultrasonic welding, ultrasonic cutting or ultrasonic soldering) or liquid (for example, in ultrasonic homogenization, sonochemistry, milling, emulsification, spraying or cell disruption). 
     Ultrasonic processing of liquids relies of intense shear forces and extreme local conditions (temperatures up to 5000 K and pressures up to 1000 atm) generated by acoustic cavitation. The ultrasonic horn is commonly a solid metal rod with a round transverse cross-section and a variable-shape longitudinal cross-section—the rod horn. Another group includes the block horn, which has a large rectangular transverse cross-section and a variable-shape longitudinal cross-section, and more complex composite horns. The devices from this group are used with solid treated media. The length of the device must be such that there is mechanical resonance at the desired ultrasonic frequency of operation—one or multiple half wavelengths of ultrasound in the horn material, with sound speed dependence on the horn&#39;s cross-section taken into account. In a common assembly, the ultrasonic horn is rigidly connected to the ultrasonic transducer using a threaded stud. 
     In an embodiment of the present technology, as was shown in the parent case U.S. Pat. No. 8,891,335 (that is incorporated herein in its entirety), the conductive ferromagnetic material  12  comprise a conduction band (not shown) that is split into two sub-bands separated by an exchange energy gap, a first sub-band having spin up, and a second sub-band having spin down. 
     In an embodiment of the present technology, as was shown in the U.S. patent application Ser. No. 14/517,801, filed on Oct. 18, 2014 and entitled “USING TUNNEL JUNCTION AND BIAS FOR EFFECTIVE CURRENT INJECTION INTO MAGNETIC PHONON-GAIN MEDIUM” (the U.S. patent application Ser. No. 14/517,801 is incorporated herein in its entirety), the application of the bias voltage  20  is used to shift the Fermi level of the spin injector  18  with respect to the Fermi level of the conductive ferromagnetic material  12  so that the injected electrons tunneling via the tunnel junction  16  into the second sub-band of the conductive ferromagnetic material  12  having spin down, flip their spin, pass into the first sub-band having spin up, and generate non-equilibrium magnons during this process. 
     In an embodiment of the present technology, the conductive ferromagnetic material ( 12  of  FIG. 1 ) is selected from the group consisting of: a ferromagnetic semiconductor; a dilute magnetic semiconductor (DMS); a half-metallic ferromagnet (HMF); and a ferromagnetic conductor, with a gap in the density of states of the minority electrons around the Fermi energy. 
     Recently some dilute magnetic semiconductors (DMS), with Tc above room temperature, have been studied intensively. These are oxides doped with magnetic cations. The examples are: GaN, doping Mn-9%, Tc=940 K; AlN, doping Cr-7%, Tc&gt;600 K; TiO2 (anatase), doping Co-7%, Tc=650 K; SnO2, doping Co-5%, Tc=650 K. These magnets can be used as a magnon gain medium (MGM) to generate nonequilibrium magnons and photons at room temperatures. 
     In an embodiment of the present technology, the half-metallic ferromagnet (HMF) is selected from the group consisting of a spin-polarized Heusler alloy; a spin-polarized Colossal magnetoresistance material; and CrO2. 
     Half-metallic ferromagnets (HMF) are ferromagnetic conductors, with a gap in the density of states of the minority electrons around the Fermi energy, Ef. Thus, the electrons in these materials are supposed to be 100% spin polarized at Ef. Thermal effects and spin-orbital interactions reduce the electron polarization. However, the electron polarization is close to 100% in half-metallic ferromagnets with spin-orbital interaction smaller than the minority electron gap and at temperatures much lower than the Curie temperature Tc. 
     Half-metallic ferromagnets (HMF) form a quite diverse collection of materials with very different chemical and physical properties. 
     Chromium Dioxide, CrO 2 . 
     Tc=390 K. Magnetic moment per Cr=2 μB. The polarization measured at low temperatures is close to 100%. There are some other known half-metallic ferromagnetic oxides, e.g. Sr 2 FeMoO 6 . 
     Heusler Alloys. 
     Most of the predicted HMF is Heusler alloys. In general, these are ternary X2YZ-compounds, X and Y are usually transition metals and Z is a main group element. The most studied of them is NiMnSb: Tc=728 K, with magnetic moment close to 4 μB. Experiments show that NiMnSb is a half-metallic ferromagnet at low temperatures. But there is evidence that at T≈90 K a phase transition into a usual ferromagnetic state takes place, and it seems unlikely that NiMnSb is a half-metallic ferromagnet near room temperature. 
     There are many other Heusler alloys with half-metallic ferromagnet properties, like: (1) Co 2 MnSi (CMS) having Tc of 1034 K and magnetic moment of 5 μB; (2) Co 2 MnGe having Tc of 905 K and magnetic moment close to 5 μB; and (3) Co 2 MnSn having Tc of 826 K and magnetic moment of 5.4 μB; etc. 
     Colossal Magnetoresistance Materials: 
     La 1-x Sr x MnO 3  (for intermediate values of x) is presumably a half-metallic ferromagnet having Tc close to room temperature. Photoelectron emission experiments confirm the half-metallicity of La 0.7 Sr 0.3 MnO 3 , with Tc=350 K. The polarization degree at T=40K is 100±5%, the gap for the minority spins is 1.2 eV. 
     In an embodiment of the present technology, the spin-polarized Heusler alloy is selected from the group consisting of Co 2 FeAl 0.5 Si 0.5 ; NiMnSb; Co 2 MnSi (CMS); Co 2 MnGe; Co 2 MnSn; Co 2 FeAl and Co 2 FeS (CFS). 
     It has been shown recently (S. Wurmehl et al.,  PRB  72, 184434 (2005)), that the alloy with the highest magnetic moment and Tc is Co 2 FeSi having Tc of 1100 K (higher than for Fe), and having magnetic moment per unit cell of 6 μB. The orbital contribution to the moments is small, while the exchange gap is large, of order 2 eV. Therefore, the effect of thermal fluctuations and spin-orbit interaction on the electron polarization is negligible. One should expect, therefore, that the electrons in Co 2 FeSi (CFS) are polarized at high temperatures, sufficiently close to Tc. Indeed, according to the experiment the magnetic moment at 300 K is the same as at 5 K. 
     Note that HMF, as well as ferromagnetic semiconductors, differ from “normal” metallic ferromagnets by the absence of one-magnon scattering processes. Therefore, spin waves in HMF, as well as in magnetic insulators, are well defined in the entire Brillouin zone. This was confirmed by neutron scattering experiments performed on some Heusler alloys. For references, please see: (1) Y. Noda and Y. Ishikawa ( J. Phys. Soc. Japan v.  40, 690, 699 (1976)) have investigated the following Heusler alloys: Pd 2 MnSn and Ni 2 MnSn; (2) K. Tajima et al. ( J. Phys. Soc. Jap. v.  43, 483 (1977)), have investigated Heusler alloy Cu 2 MnAl. 
     However, in the present application, the above disclosed magnets are used as a magnon gain medium to generate the non-equilibrium magnons. Please, see the discussion below. 
     In an embodiment of the present technology, referring still to  FIG. 1 , the spin injector  18  comprises a half-metal (please, see discussion above) having magnetization oriented antiparallel to the magnetization of the conductive ferromagnetic material  12 . The antiparallel orientation of the magnetization of the spin injector relatively to the orientation of the magnetization of the conductive ferromagnetic material can be achieved by using antiferromagnetic pinning layers (not shown) having an exchange bias effect on the spin injector and “pinning” its soft magnetization in one selected direction. 
     Exchange bias (or exchange anisotropy) occurs in bilayers (or multilayers) of magnetic materials where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the soft magnetization curve of a ferromagnetic film. The exchange bias phenomenon is of tremendous utility in magnetic recording, where it is used to pin the state of the read back heads of hard disk drives at exactly their point of maximum sensitivity; hence the term “bias.” 
     Currently exchange bias is used to pin the harder reference layer in spin valve read back heads and MRAM memory circuits that utilize the giant magnetoresistance or magnetic tunneling effect. Desirable properties for an exchange bias material include a high Néel temperature, a large magnetocrystalline anisotropy and good chemical and structural compatibility with NiFe and Co, the most important ferromagnetic films. The most technologically significant exchange bias materials have been the rock salt-structure antiferromagnetic oxides like NiO, CoO and their alloys and the rock salt-structure intermetallics like FeMn, NiMn, IrMn and their alloys. 
     In an embodiment of the present technology, referring still to  FIG. 1 , the tunnel junction  16  is selected from the group consisting of a thin insulating layer between the spin injector  18  and the conductive ferromagnetic material  12 . The tunnel junction is selected from the group consisting of: AlO; Al 2 O 3  and MgO. 
     The current densities of 10 7  A/cm 2  (well above the critical pumping currents of order of (10 5 -10 6 ) A/cm 2  that we need) were achieved by using very thin MgO tunnel junctions. For reference, please see: “ Spin - transfer switching in full - Heusler Co   2   FeAl - based magnetic tunnel junctions ;” by Hiroaki Sukegawa, Zhenchao Wen, Kouta Kondou, Shinya Kasai, Seiji Mitani, and Koichiro Inomata,  Applied Physics Letters,  100, 182403 (2012). Thus, applying the threshold current density for achieving the magnon lasing threshold is feasible in the proposed apparatus  10  of  FIG. 1 . 
     In an embodiment of the present technology, because the tunnel junction  16  separates electronic systems of the conductive ferromagnetic material  12  and of the electronic system of spin injector  18 , the external bias voltage  20  can be applied to the spin injector  18  to shift its Fermi level with respect to the Fermi level of the conductive ferromagnetic material  12 . 
     In an embodiment of the present technology, as was shown in the parent cases (the U.S. Pat. No. 8,891,335, and the U.S. patent application Ser. No. 14/517,801), the electrons injected into the conductive ferromagnetic material  12  via tunnel junction  16  are tunneling into the upper sub-band with spin down, flip their spin and emit magnons by entering the sub-band with spin up, thus generating non-equilibrium magnons inside the conductive ferromagnetic material  12 . 
     In an embodiment of the present technology, however, the process of generation of ultra-high frequency sound waves with frequencies between (1 GHz-10 GHz) in the conductive ferromagnetic material  12  by using the non-equilibrium magnons having magnon velocity higher than speed of sound u, is suppressed by selecting the dimensions of the conductive ferromagnetic material L 1x    26  and L 1y    28  both below critical:
 
 L   1x   ≦Lc ≈(10 −2 −10 −3 ) cm,  (Eq. 1)
 
 L   1y   ≦Lc ≈(10 −2 −10 −3 ) cm,  (Eq. 2)
 
so that phonon instability relation is not satisfied within the geometrical region (L 1x , L 1y ) of the conductive ferromagnetic material  12 .
 
     In an embodiment of the present technology, however the instability relationship is satisfied in the ferromagnetic dielectric material  14  by selecting its dimension L 2y    30  above critical:
 
 L   2y   ≧Lc ≈(10 −2 −10 −3 ) cm,  (Eq. 3)
 
but L 2x    31  below critical
 
 L   2x   ≦Lc ≈(10 −2 −10 −3 ) cm,  (Eq. 4)
 
so that the ultra-high frequency sound will be generated only along the axis y  11  and will be outputted by the ultra-high frequency waveguide  24  also in the y direction.
 
     Indeed, the main source of phonon damping in half-metals is phonon-electron scattering. That is why to achieve the effective generation of ultra-high frequency sound we should provide the instability region inside the ferromagnetic dielectric material  14  (that has no free electrons). Thus, the effective generation of ultra-high frequency sound inside the ferromagnetic dielectric material  14  means that that the generated ultra-frequency sound will experience low damping inside the ferromagnetic dielectric material  14 . 
     A non-equilibrium magnon generated in the conductive ferromagnetic material  12  including the magnon-gain medium has an exchange energy that is far greater than the relativistic energy with which the non-equilibrium magnon interacts at the border area  32  between the conductive ferromagnetic material  12  and the ferromagnetic dielectric material  14 . That is why the non-equilibrium magnon interacts at the border area  32  can either propagate into the ferromagnetic dielectric material  14  with the probability P or reflect back into the in the conductive ferromagnetic material  12  with the probability R. This is the classic description. 
     However, the non-equilibrium magnon having an exchange energy that is far greater than the relativistic energy is substantially a quantum object. That is why the non-equilibrium magnon having an exchange energy that is far greater than the relativistic energy can at the same time do both: 
     (i) propagate into the ferromagnetic dielectric material  14  with the probability P; 
     and 
     (ii) reflect back into the conductive ferromagnetic material  12  with the probability R. 
     But, the sum of P and R should be 1:
 
 P+R= 1.  (Eq. 5)
 
     The coefficients P and R depend on both the magnon stiffness D 1    34  inside the conductive ferromagnetic material  12  and the magnon stiffness D 2    36  inside the ferromagnetic dielectric material  14 . Typically, the value of magnon stiffness correlates with the temperature Curie of the material: the greater the temperature Curie the greater the stiffness D. 
     For a typical ferromagnetic dielectric material  14  let us take Yttrium iron garnet (YIG). Indeed, YIG is a kind of synthetic garnet, with chemical composition  Y   3   Fe   2 (Fe O   4 ) 3 , or Y 3 Fe 5 O 12 . It is a ferrimagnetic material with a temperature Curie of 560 K. YIG may also be known as Yttrium ferrite garnet, or as Iron yttrium oxide or Yttrium iron oxide, the latter two names usually associated with powdered forms. 
     For a typical conductive ferromagnetic material let us take a half-metal CMS having a temperature Curie temperature Curie of 1034 K. 
     Thus, we assume that the magnon stiffness D 1  inside the conductive ferromagnetic material  12  is greater than the magnon stiffness D 2  inside the ferromagnetic dielectric material. If this is the case, we have the following:
 
 P≈D   2   /D   1 ;  (Eq. 6)
 
and:
 
 R≈ 1− D   2   /D   1 ,  (Eq. 7)
 
so that P+R=1.
 
     If D 2  is zero, than the ferromagnetic dielectric material  14  is a non-magnetic material with temperature Curie equal to zero as well, and P is also zero as magnons cannot propagate into the non-magnetic material. 
     Thus, only a part P≈D 2 /D 1  of non-equilibrium magnons takes part in the process of Cherenkov-type generation of non-equilibrium phonons having ultra-high frequency in the ferromagnetic dielectric material  14 . But the non-equilibrium phonons having ultra-high frequency generated in the ferromagnetic dielectric material  14  do not damp on free electrons (which is the main source of damping of non-equilibrium phonons having ultra-high frequency) because free electrons do not exist inside the ferromagnetic dielectric material  14 . That is why the generation of the ultra-high frequency sound is much more effective in the apparatus of the present technology as opposed to the apparatus disclosed in the parent U.S. Pat. No. 8,891,335. 
     The above discussion has set forth the operation of various exemplary systems and devices, as well as various embodiments pertaining to exemplary methods of operating such systems and devices. In various embodiments, one or more steps of a method of implementation are carried out by a processor under the control of computer-readable and computer-executable instructions. Thus, in some embodiments, these methods are implemented via a computer. 
     In an embodiment, the computer-readable and computer-executable instructions may reside on computer useable/readable media. 
     Therefore, one or more operations of various embodiments may be controlled or implemented using computer-executable instructions, such as program modules, being executed by a computer. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. In addition, the present technology may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote computer-storage media including memory-storage devices. The present technology may also be implemented in a real time, or in a post-processed, or a time-shifted implementation where sufficient data is recorded to permit calculation of final results at a later time. 
     Although specific steps of exemplary methods of implementation are disclosed herein, these steps are examples of steps that may be performed in accordance with various exemplary embodiments. That is, embodiments disclosed herein are well suited to performing various other steps or variations of the steps recited. Moreover, the steps disclosed herein may be performed in an order different than presented, and not all of the steps are necessarily performed in a particular embodiment. 
     Although various electronic and software based systems are discussed herein, these systems are merely examples of environments that might be utilized, and are not intended to suggest any limitation as to the scope of use or functionality of the present technology. Neither should such systems be interpreted as having any dependency or relation to any one or combination of components or functions illustrated in the disclosed examples. 
     Although the subject matter has been described in a language specific to structural features and/or methodological acts, the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as exemplary forms of implementing the claims.