Patent Publication Number: US-9407202-B2

Title: Spin-transfer torque type injection locking oscillator and design method thereof

Description:
TECHNICAL FIELD 
     The present invention relates to a spin-transfer torque type injection locking oscillator, and more specifically, to a spin-transfer torque type injection locking oscillator that improves both output power and noise performance by using a pair of spin-transfer torque devices which are optimized for high output power and low noise, respectively. 
     BACKGROUND ART 
     A spin-transfer torque oscillator is an oscillator which generates specific frequency signal resulted from spin precession inside magnetic-nonmagnetic multilayer structure by applying current to it. The spin-transfer torque oscillator has several advantages compared to prior art oscillators. These advantages are the nanometer scale structure fabrication using CMOS manufacturing process and the generation of wide range signals from a few MHz to GHz by varying input current. However, disadvantages of the spin-transfer torque oscillator are very low output levels in the range of nV and high phase noise characteristics. 
     The prior art inventions describe injection locking and mutual locking techniques for phase locking of spin oscillator as disclosed in the non-patent reference 1, 2, and 3. In this regard, the non-patent reference 1 reported that two nano-contact spin torque devices in the mutual phase locking are synchronized with a sudden narrowing of signal line-width with an increase in output power due to the coherence of the individual oscillators. However the output power of the oscillator is still in the range of a few pW, which is not suitable for telecommunication circuits. Injection locking uses locking or pulling phenomenon of periodic signals inside an oscillator as disclosed in the non-patent reference 2 and 3 experimentally and theoretically. The phase noise is reduced by locking the oscillating frequency into injection frequency. This is called as injection locking which makes the signal attenuation when input signal and oscillator signal having phase differences are combined, and at that time the phase has to vary to synchronize the phase of oscillator to the phase of injection signal for the maintaining of oscillation. However, the locking range in the silicon CMOS manufacturing process is very narrow and the phase noise reduction effect of the injection locking lacks of reliability due to the lack of suitable models and manufacturing process change. Moreover, the nanoscale integration advantage may disappear because additional oscillator is needed for injection locking. 
     THE NON-PATENT REFERENCES 
     
         
         (The non-patent reference 1) S. Kaka et al., “Mutual phase-locking of microwave spin torque nano-oscillators”, Nature, Vol. 437, pp. 389-392, 15 Sep. 2005. 
         (The non-patent reference 2) B. Razavi, “A study of injection locking and pulling in oscillators”, IEEE Journal of Solid-State Circuits, Vol. 39, pp. 1415-1424, September 2004. 
         (The non-patent reference 3) W. H. Rippard et al., “Injection locking and phase control of spin transfer nano-oscillators”, Physical Review Letters, Vol. 95, 067203, 5 Aug. 2005. 
       
    
     DISCLOSURE 
     Technical Problem 
     In order to solve the above problems, the present invention provides a spin-transfer torque type injection locking oscillator having both high output power and low phase noise characteristics by employing injection locking method using two oscillators. One is optimized for high output power characteristics and the other is optimized for low noise characteristics in the device manufacturing processing. 
     Technical Solution 
     According to the present invention in order to solve the above problems, the design method of the spin-transfer torque type injection locking oscillator comprises, the steps of: designing the structure of a low noise spin-transfer torque device; designing a high output power spin-transfer torque device; designing an injection oscillator using the low noise spin-transfer torque device; designing a free running oscillator using the high output power spin-transfer torque device; and designing an injection locking oscillator using the injection oscillator and the free running oscillator. The spin-transfer torque type injection locking oscillator has low noise and high output power characteristics. 
     In a preferable embodiment of the present invention, a spin-transfer torque type injection locking oscillator comprises an injection oscillator and a free running oscillator, which are a low noise spin-transfer torque device as the injection oscillator operated by current control and a high output power spin-transfer torque device as the free running oscillator operated by current control. The high output power spin-transfer torque device generates output signals by receiving output signals of the low noise spin-transfer torque device. 
     Advantageous Effects 
     The present invention can overcome disadvantages of a spin-transfer torque oscillator such as low output power and high phase noise characteristics by using injection locking method which combines two spin-transfer torque devices. One of two spin-transfer torque devices is fabricated as the optimized output power characteristics device while the other is fabricated as the optimized noise characteristics device in the device manufacturing process. 
     The present invention can reduce production cost of high quality spin-transfer torque device by removing alternative power supply and alternative generation circuits of additional injection oscillators and by integrating spin-transfer torque devices in nanoscale using CMOS process. The spin-transfer torque type injection locking oscillator can be used for FSK, ASK, and PSK modulator of wireless telecommunication systems. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a cross sectional view of a spin-transfer torque device according to the present invention. 
         FIG. 2  illustrates a circuit diagram of a spin-transfer torque oscillator which operated in basic mode according to a prior art. 
         FIG. 3  illustrates a circuit diagram of a spin-transfer torque oscillator comprising high frequency oscillating injection circuit for the operation of general injection locking according to a prior art. 
         FIG. 4  illustrates a block diagram of the spin-transfer torque type injection locking oscillator according to the present invention. 
         FIG. 5  shows an embodiment of the spin-transfer torque type injection locking oscillator described in  FIG. 4 . 
     
    
    
     MODE OF THE PRESENT INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described with drawings. In each drawing of the present invention, a size may be enlarged or reduced than an actual size to clarify the invention, and well known elements are omitted to emphasize a structural feature of the present invention. 
     Generally a spin-transfer torque device consists of multilayer structure of fixed magnetic layer/nonmagnetic layer/free magnetic layer as illustrated in  FIG. 1 , but may not be limited to that structure. Electrons in the spin-transfer torque device by applying current experience spin polarization when they are passing through nonmagnetic layer and magnetic layer. Since spin angular momentum of each electron has to be conserved, force corresponding to the change of spin angular momentum of each electron transfers to the magnetization of magnetic layer and consequently generates torque against the change of the magnetization direction of magnetic layer. 
     When certain amount of magnetic field or spin-transfer torque is simultaneously applied to a spin-transfer torque device to move opposite of the magnetization direction each other, the magnetization does not move to the direction of magnetic field or spin-transfer torque but rotate continually with constant frequency. The constant rotating frequency changes with changing the magnitude of applied magnetic field or current and can be controlled in the wide range of frequency from a few GHz to dozens of GHz. As the magnetization of free magnetic layer rotates with respect to the fixed direction of the magnetization of fixed magnetic layer, the magnetoresistance of the spin-transfer torque device changes continuously. That means it is possible to obtain alternative current signal from direct current signal and thus the spin-transfer torque device can be used as an oscillator. 
     A spin-transfer torque device can be embodied in a nano-contact or a nano-pillar structure using silicon CMOS manufacturing process. The nano-contact structure is fabricated on the multilayer structure of fixed magnetic layer/nonmagnetic layer/free magnetic layer by forming a contact window on the top layer to form electrodes. The nano-pillar structure is fabricated on the multilayer structure of fixed magnetic layer/nonmagnetic layer/free magnetic layer by etching outside of the multilayer structure to form a pillar. A free magnetic layer consists of soft magnetic materials which changes the magnetization direction by the applied magnetic field or current. A nonmagnetic layer consists of nonmagnetic metals or insulators. The nonmagnetic layer of an insulator should be thin for the tunneling of electrons or holes. A fixed magnetic layer consists of ferromagnetic materials which does not change the magnetization direction by the applied magnetic field or current. A spin-transfer torque device may become tunneling magneto-resistance (TMR) device by using an insulator for the nonmagnetic layer or a giant magneto-resistance (GMR) device by using a conducting layer for the nonmagnetic layer. A free magnetic layer may be a layer which includes magnetic vortex. 
       FIG. 2  illustrates a basic circuit of a spin-transfer torque oscillator, which includes a spin-transfer torque device  101 , a direct current source  102 , an inductor  103 , and a capacitor  104 . High frequency signal is generated by applying direct current on the spin-transfer torque device  101  using the direct current source  102 . The direct current component in the signals is blocked by the capacitor  104  and then high frequency signal only comes out in the output. 
     A spin-transfer torque device shows various oscillation frequency characteristics according to the amount of applied currents. One spin-transfer torque device increases oscillation frequency as increasing applied current according to materials used in magnetic layers, on the contrary the other spin-transfer torque device decreases oscillation frequency as increasing applied current. Or certain spin-transfer torque device decreases and after that increases as increasing applied current. Affecting factors on oscillation frequency in a spin-transfer torque device are degree of magnetic saturation freedom, magnetic attenuation constant, and spin polarization parameter, which are determined by material property parameters and form factors such as volume of a free magnetic layer. Thus spin-transfer torque oscillators with different oscillation frequencies or noise characteristics can be made by controlling volume, surface area, or thickness of the free magnetic layer. 
     Phase noise which well known as flicker noise (1/f noise) is known to be caused by material property and thus a spin-transfer torque device with low phase noise can be made by selecting certain magnetic materials. A spin-transfer torque device can be optimized by adjusting certain conditions as changing frequency as well as output signals and noise characteristics with increasing applied current. The magnetic layer is at least one or more, or their alloys selected from the group consisting of cobalt, nickel, chrome, platinum, boron, and iron. For example, Ni, NiFe, Co, CoFe, CoFeB, CoPt, or CoCrPt can be used, but the use may not be limited to them. Crystal structure of materials may be body centered cubic (BCC). Magnetization direction of magnetic layer can be selected by combining parallel or perpendicular directions of the magnetization like fixed magnetic layer. 
     Since a spin-transfer torque device has low output power and high phase noise, the array of spin-transfer torque device can be used for the improvement of output power and phase noise using mutual locking and synchronization method. The spin-transfer torque type injection locking oscillator according to the present invention uses the combining method of two different spin-transfer torque devices optimized using different materials and shapes in the CMOS manufacturing process. One is optimized for output power characteristics and the other is optimized for noise characteristics. 
     Low phase noise spin-transfer torque device can be made of magnetic layer having magnetic vortex. Magnetic vortex which spin direction&#39;s shape resembled vortex of tornado is randomly formed in the free magnetic layer. A spin-transfer torque oscillator with magnetic vortex structure involves more magnetic fields in an oscillation due to difference of shape. Thus the oscillation frequency becomes less dependent on the change of magnetic field and consequently has very narrow line width since the swing range of the oscillation frequency by magnetic field is relatively low. A spin-transfer torque oscillator with magnetic vortex layer has low output power and low phase noise. 
     A high output power spin-transfer torque oscillator can be made of TMR structure spin-transfer torque device. Since a spin-transfer torque oscillator which made of TMR structure normally has very high resistance, a high output power oscillator is obtained. A spin-transfer torque oscillator which made of TMR structure has high output power and high noise. 
       FIG. 3  illustrates an injection locking oscillator according to prior art invention. A spin-transfer torque oscillator  201  is used as a free running oscillator and an external oscillator  205  such as voltage controlled oscillator (VCO) is used as an injection oscillator. The locking range Δf lock  is described in mathematical formula 1. 
     
       
         
           
             
               
                 
                   
                     
                       
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                         o 
                       
                     
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                           φ 
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                   Mathematical 
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                   1 
                 
               
             
           
         
       
     
     Here, f drive  is the frequency of an injection oscillator, f o  is the frequency of a free running oscillator, V inj  is the alternative voltage of an injection oscillator, V osc  is the output voltage of a free running oscillator, φ is the phase difference between a free running oscillator and an injection oscillator. An injection oscillator according to prior art invention has difficulty of integration in nanoscale due to the use of VCO. 
       FIG. 4  illustrates a block diagram of a spin-transfer torque type injection locking oscillator according to the present invention, which includes a current control injection oscillator  309  and a current control free running oscillator  310 . The current control injection oscillator  309  consists of a low noise spin-transfer torque device  302  and the first power supply  304  and the current control free running oscillator  310  consists of a high output power spin-transfer torque oscillator and the second power supply  303 . The low noise spin-transfer torque device  302  operates as an oscillator and generates low noise oscillation output signals by controlling the first direct current (Idc 1 ) of the first power supply  304 . The high output power spin-transfer torque oscillator  301  operates as an oscillator by controlling the second direct current (Idc 2 ) of the second power supply  303 . The low noise signal generated by the low noise spin-transfer torque device  302  is injected into the high output power spin-transfer torque oscillator  301  and then the high output power spin-transfer torque oscillator  301  generates oscillation output signals which are synchronized in phase with low noise oscillation signals. 
     Since a spin-transfer torque oscillator according to the present invention uses spin-transfer torque devices, it can be operated by direct current as well as direct voltage. The low noise spin-transfer torque device  302  is a spin-transfer torque oscillator which is optimized for phase noise characteristics and the high output power spin-transfer torque oscillator  301  is a spin-transfer torque oscillator which is optimized for output power characteristics. They can be exchangeable. 
     The output signal of a free running oscillator is equal or increased when the phase signal of a free running oscillator is locked by injection, and the phase noise of output signals is equal or reduced to the phase of an injection oscillator. Thus, it is possible to embody an oscillator having both high output power and low phase noise characteristics. The oscillator can be fabricated using CMOS manufacturing process with the nanoscale integration due to simple circuits by using direct current usage only. 
       FIG. 5  illustrates a circuit diagram of a spin-transfer torque oscillator according to the present invention, which includes the first power supply  304 , the low noise spin-transfer torque device  302 , a coupler  311 , the second power supply  303 , and the high output power spin-transfer torque oscillator  301 . The low noise spin-transfer torque device  302  generates low noise oscillation output signals by controlling the first direct current (Idc 1 ) of the first power supply  304 , and the high output power spin-transfer torque oscillator  301  is controlled by the second direct current (Idc 2 ) of the second power supply  303 . The low noise signal generated by the low noise spin-transfer torque device  302  is injected into the high output power spin-transfer torque oscillator  301  and then the high output power spin-transfer torque oscillator  301  operates as a free running oscillator and generates low noise oscillation output signals. 
     The coupler  311  is a part to guide the direction of output signals to avoid interference between high frequency signals. A directional coupler, a combiner, or a circulator can be used. The coupler  311  inputs the output of the current control injection oscillator  309  to the current control free running oscillator  310 , but prevents from inputting the output of the current control free running oscillator  310  to current control the injection oscillator  309 . 
     Since a spin-transfer torque oscillator is easy for the integration, an integrated chip can be made by integrating a pair of the low noise spin-transfer torque device and the high output power spin-transfer torque oscillator using CMOS manufacturing process. As a spin-transfer torque type injection locking oscillator according to the present invention improves low output power and high phase noise, it can be used as FSK, ASK, or PSK modulator in wireless telecommunication system. 
     INDUSTRIAL APPLICABILITY 
     The present invention can apply to an oscillator, a modulator, a transmitter and receiver, and telecommunications.