Electromagnetic detection apparatus and methods

Systems and methods for detecting electromagnetic waves are disclosed. A system for detecting such waves includes a device having a first magnetic layer having a fixed magnetization direction, a second magnetic layer having an unfixed magnetization direction responsive to the electromagnetic wave, and a barrier layer positioned between the first and second magnetic layers. The device may have an impedance dependent on a relative angle between the fixed magnetization direction and the unfixed magnetization direction. The system further has a detector configured to detect a change in the impedance indicative of the electromagnetic wave. The electromagnetic wave may be detected by positioning the device in order to detect the electromagnetic wave, determining a change in the impedance of the device, and detecting the electromagnetic wave based on the change in the impedance of the device. Characteristics of the wave such as frequency, power, and phase may also be detected.

FIELD OF THE INVENTION

The present invention relates to the field of spintronics and, more particularly, to electromagnetic wave detection systems and methods.

BACKGROUND OF THE INVENTION

Electromagnetic wave detectors are used to detect electromagnetic waves. Conventional detectors make direct use of the electric field portion of the electromagnetic wave for detection. Conventional detectors, however, may have difficulty detecting high power electromagnetic waves and may be bulky.

SUMMARY OF THE INVENTION

Aspects of the present invention are embodied in systems and methods for detecting electromagnetic waves. A system for use in detecting an electromagnetic wave includes a device having a first magnetic layer having a fixed magnetization direction, a second magnetic layer having an unfixed magnetization direction responsive to the electromagnetic wave, and a barrier layer positioned between the first and second magnetic layers. The device has an impedance dependent on a relative angle between the fixed magnetization direction and the unfixed magnetization direction. The relative angle is initially configured to be approximately 90 degrees. The system further has a detector configured to measure a change in the impedance indicative of the electromagnetic wave.

Another aspect of the invention is embodied in a system for use in detecting an electromagnetic wave. The system includes a device having a first magnetic layer having a fixed magnetization direction, a second magnetic layer having an unfixed magnetization direction responsive to the electromagnetic wave, and a barrier layer positioned between the first and second magnetic layers. The device has an impedance dependent on a relative angle between the fixed magnetization direction and the unfixed magnetization direction. The system also includes an external magnetic source for applying a magnetic field to the device. The system further has a detector configured to measure a change in the impedance indicative of the electromagnetic wave.

Yet another aspect of the invention is embodied in a system for use in detecting an electromagnetic wave. The system includes a plurality of devices, each device having a first magnetic layer having a fixed magnetization direction, a second magnetic layer having an unfixed magnetization direction responsive to the electromagnetic wave, and a barrier layer positioned between the first and second magnetic layers. Each device has an impedance dependent on a relative angle between the fixed magnetization direction and the unfixed magnetization direction. The system further includes a receiver for receiving the electromagnetic wave and transmitting the electromagnetic wave to the plurality of devices. The receiver has a plurality of sections corresponding to the plurality of devices. The plurality of sections are configured such that the electromagnetic wave will have a different power density in each section of the receiver. The system further includes a detector configured to detect a change in the impedance of one of the plurality of devices indicative of the electromagnetic wave.

Another aspect of the invention is embodied in a system for use in detecting a phase of a received electromagnetic wave. The system includes a reference electromagnetic wave generator and a device having a first magnetic layer having a fixed magnetization direction, a second magnetic layer having an unfixed magnetization direction at least partially dependent on the received electromagnetic wave and the reference electromagnetic wave, and a barrier layer positioned between the first and second magnetic layers. The device further has an impedance dependent on a relative angle between the fixed magnetization direction and the unfixed magnetization direction. The system also includes a detector configured to detect a change in the impedance of the device indicative of the relative phase of the received electromagnetic wave.

Still another aspect of the invention is embodied in a method for detecting an electromagnetic wave. An electromagnetic wave may be detected by positioning a spintronic device in order to detect the electromagnetic wave, the device having a first magnetic layer having a fixed magnetization direction, a second magnetic layer having an unfixed magnetization direction responsive to the electromagnetic wave, and a barrier layer positioned between the first and second magnetic layers. The device has an impedance dependent on a relative angle between the fixed magnetization direction and the unfixed magnetization direction. The relative angle is initially configured to be approximately 90 degrees. A change in the impedance of the device is then determined. An electromagnetic wave is detected based on the change in the spin property of the device.

Another aspect of the invention is embodied in a method for detecting an electromagnetic wave. An electromagnetic wave may be detected by positioning a spintronic device in order to detect the electromagnetic wave, the device having a first magnetic layer having a fixed magnetization direction, a second magnetic layer having an unfixed magnetization direction responsive to the electromagnetic wave, and a barrier layer positioned between the first and second magnetic layers. The device has an impedance dependent on a relative angle between the fixed magnetization direction and the unfixed magnetization direction. The relative angle is initially configured to be one of approximately 0 degrees and 180 degrees. An external DC magnetic field is applied to the device. The external DC magnetic field is swept to cause the relative angle to switch from the one of approximately 0 degrees and approximately 180 degrees to the other one of approximately 0 degrees and approximately 180 degrees. A change in the impedance of the device adjacent to a switching region of the relative angle is then determined. An electromagnetic wave is detected based on the change in the spin property of the device.

DETAILED DESCRIPTION OF THE INVENTION

Electrons have both charge and spin properties. The field of electronics is based on the charge property of electrons. The field of spintronics is based on the spin property of electrons. Spintronics generally concerns the detection and/or manipulation of electron spin within a device, which can influence the charge properties of the device. Electron spin is a vector quantity with its direction defined as the direction of magnetization of the electron. There are generally two categories of spin, spin-up and spin-down. Consequently, electrons may be grouped into spin-up and spin-down electrons. Charges or currents having any arbitrary spin direction can be constructed from the combination of these two bases.

In magnetic materials, one type of electron spin may be more common than the other, in which case they are defined as majority and minority spins. In such materials, an electrical current through the material can be thought of as consisting of two parallel channels corresponding to a flow of majority spin and minority spin electrons. When the number of electrons in each channel is different, the overall current carries a net spin direction, termed as spin-polarized current. Additionally, the electrical impedance in the majority spin channel and the minority spin channel may be different. Similarly, these impedances combine to create a separate overall impedance, termed a spin-dependent impedance. As magnetic materials are configured in a multilayer system, the electrical transport properties of the system will depend on the magnetization direction of each magnetic layer. The electrical transport properties of a material or system may include, for example, the electrical current through the system, the impedance of the system, or the voltage across the system. These electrical transport properties may vary depending on the spin of the electrons passing through the magnetic layers, and can therefore also be understood as spin-polarized transport properties. It will be understood that any reference herein to the electrical properties of a device such as current, impedance, or voltage will be referencing the spin-polarized transport properties of the respective material or device, which are dependent on the magnetic properties of the material or device.

As used herein, the term “impedance” refers to the dominant affect, change in impedance and/or resistance, presented by the device. In an exemplary embodiment, where the dominant affect presented by the device is a change in impedance, impedance will be determined, and where the dominant affect presented by the device is resistance, resistance will be determined.

The invention will now be described with reference to the accompanying drawings.FIG. 1Adepicts a system100for use in describing exemplary systems and methods for detecting an electromagnetic wave in accordance with aspects of the present invention. The electromagnetic wave may optimally be in the microwave range; however, it is contemplated that system100may detect electromagnetic radiation outside of the microwave range. As a general overview, system100includes a device102and a detector112. The device102includes two magnetic layers104and106and a barrier layer108. Device102may also include a fixing layer110. Additional details of system100are provided below.

Magnetic layers104and106are layers of magnetic material. In an exemplary embodiment, magnetic layers104and106are formed from ferromagnetic material. However, it is contemplated that magnetic layers104and106may be formed from other magnetic materials including, for example, ferrimagnetic materials, antiferromagnetic materials, or a combination of magnetic materials. Suitable magnetic materials for magnetic layers104and106may include, for example, at least one of the elements Ni, Fe, Mn, Co, or their alloys, or half-metallic ferromagnets such as NiMnSb, PtMnSb, Fe3O4, or CrO2. Other suitable magnetic materials for magnetic layers104and106will be understood by one of ordinary skill in the art from the description herein.

Barrier layer108is positioned between magnetic layers104and106. In an exemplary embodiment, barrier layer108is formed from an insulating material such as, for example, an oxide or nitride of one or more of Al, Mg, Si, Hf, Sr, Zn, Zr, or Ti. In another exemplary embodiment, barrier layer108may be formed from conducting materials. Such conducting materials may allow electrons to easily pass from one magnetic layer to the other. Suitable conducting materials for barrier layer108will be understood by one of ordinary skill in the art from the description herein.

Fixing layer110may be positioned adjacent magnetic layer104. In an exemplary embodiment, fixing layer110fixes the magnetization direction of magnetic layer104. Fixing layer110may consist of a single layer of material or may consist of a stack of layers of one or more materials, as would be know to one of ordinary skill in the art. Fixing layer110may optimally be formed from antiferromagnetic or ferromagnetic materials such as, for example, FeMn, NiMn, FeNiMn, FeMnRh, RhMn, CoMn, CrMn, CrMnPt, CrMnRh, CrMnCu, CrMnPd, CrMnIr, CrMnNi, CrMnCo, CrMnTi, PtMn, PdMn, PdPtMn, IrMn, NiO, CoO, SmCo, NdFeB, FePt, or a combination of these materials, which fix the magnetization direction of magnetic layer104. Other suitable materials for fixing layer110will be understood by one of ordinary skill in the art from the description herein.

Device102has an associated impedance dependent on layers104-110of device102. In an exemplary embodiment, the impedance of device102is dependent on the magnetization directions of magnetic layers104and106. Magnetic layers104and106each have an associated magnetization direction (depicted by arrows inFIGS. 1B-1C). In an exemplary embodiment, the magnetization direction of magnetic layer104is fixed in a single direction and the magnetization direction of magnetic layer106is unfixed, or free.

The magnetization direction of magnetic layer104may be fixed by positioning fixing layer110adjacent magnetic layer104. The unfixed magnetization direction of magnetic layer106may be configured to initially have a given direction relative to the fixed magnetization direction of magnetic layer104. For example, the initial magnetization direction of magnetic layer106may be parallel to the magnetization direction of magnetic layer104, as depicted inFIG. 1B. Alternatively, the initial magnetization of magnetic layer106may be perpendicular to the magnetization direction of magnetic layer104, as depicted inFIG. 1C. In either configuration, however, the unfixed magnetization direction of magnetic layer106may be free to rotate away from the initially configured direction, e.g., through a full 360°. The initial magnetization direction of magnetic layer106may be selected by applying an external DC magnetic field to device102in the desired direction of the unfixed magnetization. The external DC magnetic field may be generated by an external electromagnet or by a DC current adjacent device102.

The impedance of device102is dependent on a relative angle between the magnetization directions of magnetic layers104and106.FIG. 1Ddepicts a graph of impedance of exemplary device102based on the relative angle between the magnetization directions of magnetic layers104and106. In an exemplary embodiment, the magnitude of impedance of device102is lowest when the relative angle between the fixed magnetization direction and the unfixed magnetization direction is 0°, i.e., when the directions are parallel. The magnitude of impedance of device102is highest when the relative angle between the fixed magnetization and the unfixed magnetization is 180°, i.e., when the directions are antiparallel, or opposite.

The magnetization direction of magnetic layer106is at least partially dependent on a magnetic field received by device102. Accordingly, as will be discussed in greater detail below, exposure of layer106to an electromagnetic wave, which will have electric and magnetic field portions, may cause the magnetization direction of magnetic layer106to change. A change in the unfixed magnetization direction of magnetic layer106causes a change in the relative angle, which in turn changes the impedance of device102. Accordingly, the impedance of device102may change when exposed to a magnetic field, and therefore is at least partially dependent on a received electromagnetic wave.

Although device102illustrates layers104-110having the same width, it is contemplated that any of the layers of device102could be wider or narrower as necessary to optimize the impedance and magnetization orientation of device102. In a preferred embodiment, device102is a spintronic device having a relatively large magnetoimpedance (e.g., greater than 5%), including, for example, a magnetic tunnel junction or a spin valve. However, device102may be any suitable spintronic device. Suitable devices102for use with the present invention will be understood by one of skill in the art from the description herein.

Detector112measures the voltage across device102. In an exemplary embodiment, detector112is a voltage detector such as, for example, a lock-in amplifier. However, detector112may be any suitable voltage detector. The voltage measured by detector112is dependent on the impedance of device102. As described above, exposure to an electromagnetic wave may change the impedance of device102. Accordingly, system100may detect an electromagnetic wave based on a change in the impedance of device102which is reflected in a change in the voltage measured by detector112. A suitable voltage detector will be known to one of ordinary skill in the art from the description herein.

The interaction between an electromagnetic wave and the unfixed magnetization direction will now be described. In an exemplary embodiment, the free magnetic layer is sensitive to ferromagnetic resonance. This means that, when exposed to an electromagnetic wave, the unfixed magnetization direction precesses in response to the magnetic field portion of the electromagnetic wave. The free magnetic layer has a specific ferromagnetic resonant frequency at which the unfixed magnetization direction experiences the largest angle of precession. This frequency may be located in the microwave range.

The angle of precession of the unfixed magnetization direction is dependent on the magnetic field portion and the frequency of the electromagnetic wave. For example, as the magnitude of the magnetic field portion of the electromagnetic wave increases, the angle of precession of the magnetization direction increases. For another example, as the frequency of the electromagnetic wave approaches the ferromagnetic resonant frequency of the magnetic layer, the angle of precession of the magnetization direction also increases. In a configuration where the fixed and unfixed magnetic layers are initially configured to a specific relative angle (e.g., parallel or perpendicular), exposure to an electromagnetic wave may cause the relative angle between the fixed and unfixed magnetization directions to precess around the pre-configured angle. Precession of the relative angle thereby causes a change in the impedance of the device, which can be measured by a suitable voltage detector. This allows an exemplary device of the present invention to convert a received electromagnetic wave into a voltage signal which can be measured with a detector.

FIG. 2is a flow chart200depicting exemplary steps for detecting an electromagnetic wave in accordance with an aspect of the present invention. To facilitate description, the steps ofFIG. 2are described with reference to the system components ofFIG. 1. It will be understood by one of skill in the art from the description that different components may be utilized without departing from the scope of the present invention.

In step202, a spintronic device is positioned to receive an electromagnetic wave. In an exemplary embodiment, device102is positioned to receive an electromagnetic wave. As described above, device102has an impedance that is at least partially dependent on the relative angle between the fixed and unfixed magnetization directions of layers104and106. The relative angle between the fixed and unfixed magnetization directions may be initially configured to be 90°, which may provide advantages that will be discussed herein. When device102receives an electromagnetic wave, the unfixed magnetization direction precesses, causing a change in the relative angle and a change in the impedance of device102.

In step204, a change in the impedance of the device is determined. In an exemplary embodiment, the impedance of device102combines with a current to generate a voltage across device102. Detector112measures the voltage across device102. Any change in the voltage measured across device102corresponds to a change in impedance of device102.

In step206, an electromagnetic wave is detected. In an exemplary embodiment, a change in the voltage measured by detector112corresponds to a change in the impedance of device102. The change in impedance is caused by exposure to an electromagnetic wave. Thus, a change in the voltage across device102indicates the detection of an electromagnetic wave. Additional characteristics of the wave, such as power, frequency, and/or phase, may further be determined from the change in voltage measured by detector112, as will be described in further detail below.

FIG. 3Adepicts an exemplary system300for detecting an electromagnetic wave in accordance with an aspect of the present invention. As a general overview, system300includes a device302, a current source311, a detector312, and an electromagnetic wave source314. System300may further include a receiver316. System300may be configured to detect electromagnetic waves having frequencies far below the ferromagnetic resonant frequency of device302. Additional details of system300are provided below.

Device302is a spintronic device substantially as described above with respect to device102. Device302includes a first magnetic layer having a fixed magnetization direction and a second magnetic layer having an unfixed magnetization direction. Device302includes a barrier layer positioned between the two magnetic layers. Device302may further include a fixing layer to fix the magnetization direction of one of the magnetic layers. Device302has an impedance dependent at least in part on a relative angle between the magnetization directions of the two magnetic layers.

The magnetization directions of device302are configured to be either parallel or antiparallel. Accordingly, in one exemplary embodiment of device302, the initial unfixed magnetization direction is oriented in the same direction as the fixed magnetization direction, and the relative angle between the fixed and unfixed magnetization directions is approximately 0°. In another exemplary embodiment of device302, the initial unfixed magnetization direction is oriented in the opposite direction of the fixed magnetization direction, and the relative angle between the fixed and unfixed magnetization directions is 180°.

Current source311is configured to provide a current through device302. In an exemplary embodiment, current source311is a constant direct-current (DC) source. A suitable current source for use with the present invention will be understood by one of skill in the art.

Detector312measures the voltage across device302. In an exemplary embodiment, detector312is a voltage detector such as, for example, a lock-in amplifier. However, detector312may be any suitable voltage detector. The voltage measured by detector312results from the impedance of device302combined with the current provided by current source311. As described above, exposing device302to an electromagnetic wave may change the impedance of device302. Accordingly, system300may detect an electromagnetic wave based on a change in the impedance of device302which is reflected in the voltage measured by detector312. A suitable voltage detector will be known to one of ordinary skill in the art from the description herein.

Electromagnetic wave source314emits electromagnetic waves. Electromagnetic wave source314may optimally emit electromagnetic radiation in the microwave range; however, it is contemplated that electromagnetic wave source314may emit other electromagnetic radiation. Detector302is exposed to the electromagnetic waves emitted by source314. In an exemplary embodiment, electromagnetic waves from source314cause the unfixed magnetization direction of the free magnetic layer to precess. Precession of the unfixed magnetization direction causes the relative angle of the fixed and unfixed magnetizations of device302to precess around either 0° or 180°. Correspondingly, precession of the relative angle causes a change in the impedance of device302responsive to receiving the electromagnetic waves from source314. Electromagnetic wave source314may be any source of electromagnetic radiation desired to be detected.

Receiver316may be used to receive electromagnetic waves from electromagnetic wave source314and to transmit them to device302. Receiver316may be coupled to device302such that receiver316focuses the magnetic field portion of the received electromagnetic waves on device302. In an exemplary embodiment, receiver316may be a waveguide such as a shorted coplanar waveguide. However, receiver may be any waveguide or antenna suitable for receiving electromagnetic waves from source314and transmitting them to device302. A suitable receiver316for use with the present invention will be understood by one of skill in the art from the description herein.

FIG. 3Bdepicts a graph350of the impedance of exemplary device302as a function of time. As described above, device302has a maximum impedance when the relative angle between the fixed and unfixed magnetization directions is 180°. Device302is depicted having a maximum impedance of 10,000Ω, as shown by line352of graph350. As disclosed above, when an electromagnetic wave is applied to device302, the unfixed magnetization direction precesses from the antiparallel configuration, causing the relative angle to precess back and forth around 180°. Because the maximum relative angle is 180°, precession periodically decreases the relative angle, which periodically lowers the impedance of device302.FIG. 3Bdepicts the periodic lowering of impedance of device302caused by the electromagnetic wave. In this configuration, the average impedance of device302becomes 9,900Ω, as shown by line354of graph350.

Referring back toFIG. 3A, the change in impedance combines with the current from current source311to generate a change in voltage. Detector112measures this change in voltage across device302. Accordingly, exposure to an electromagnetic wave may cause a change in voltage across device302. Measurement of such a change in voltage by detector312indicates the detection of an electromagnetic signal by system300.

System300may also be configured to determine characteristics of the electromagnetic wave based on the voltage measured by detector312. For example, system300may be configured to determine the frequency of the received electromagnetic wave. In an exemplary embodiment, the frequency of precession of the relative angle of device302, and correspondingly the frequency of the change in impedance of device302, is dependent on the frequency of the electromagnetic wave. Accordingly, the frequency of the electromagnetic wave may be proportional to the frequency of the voltage change measured by detector312. By measuring the frequency of the voltage, the frequency of the electromagnetic wave may thereby be determined.

In an alternative embodiment, the frequency of the magnetic wave may be determined by measuring the voltage measured by detector312as a function of an externally applied DC magnetic field. An external DC magnetic field may be applied from an electromagnetic or current adjacent device302(not shown). Applying a DC magnetic field to device302may change the ferromagnetic resonant frequency of the unfixed magnetic layer. The resonance frequency of the unfixed magnetic layer is equal to:
f=γ√{square root over ((Hdc+Han)(Hdc+Han+Ms))}{square root over ((Hdc+Han)(Hdc+Han+Ms))}
where γ is the gyromagnetic ratio, Hdcis the applied DC magnetic field, Hanis the anisotropy field, and Msis the saturation magnetization. The values of γ, Hanand Msall depend on the magnetic material used in the unfixed magnetic layer and may be predetermined. Therefore, the applied DC magnetic field, Hdc, may be swept to tune the ferromagnetic resonant frequency to match the frequency of the electromagnetic wave. As described above, when the electromagnetic wave frequency and the ferromagnetic resonant frequency match, the magnetic field portion of the electromagnetic wave will precess the unfixed magnetization with a maximum angle, thereby causing the largest change in impedance of device302. To determine the frequency of the electromagnetic wave, one may sweep the applied DC magnetic field and observe the field Hdcthat produces the largest voltage change measured by detector312, which corresponds to the largest impedance change of device302. The frequency may then be determined using value of Hdcat which the largest impedance change occurs.

Additionally, system300may be configured to determine the power of the received electromagnetic wave. In an exemplary embodiment, the magnitude of the precession of the relative angle of device302, and correspondingly the magnitude of the change in impedance of device302, is dependent on the power of electromagnetic wave. Specifically, the magnitude of the change in impedance may be proportional to the square root of the power of the electromagnetic wave. Accordingly, the power of the electromagnetic wave may be proportional to the magnitude of the voltage measured by detector312. By measuring the magnitude of the voltage, and thereby the magnitude of the change in impedance, the power of the electromagnetic wave may be determined.

System300may also be configured as an electromagnetic wave demodulator. In an exemplary embodiment, the electromagnetic wave received from source314may be an AC signal wave modulating a high frequency electromagnetic carrier wave, i.e. a microwave carrier. By applying a current from current source311, the change in impedance of device302will result in a change in voltage. This change in voltage may be proportional to the AC signal modulated by the electromagnetic wave. For amplitude modulation, detector312may employ a low-pass filter to filter out the electromagnetic carrier wave. In this case, the voltage measured by detector312may be proportional to the DC current from current source311, the AC signal from the wave, and harmonics. The AC signal may then be extracted. A similar technique may be used to extract an AC signal from a frequency modulated or phase modulated electromagnetic wave.

System300may further be specially configured to detect the power of electromagnetic waves having a frequency lower than the ferromagnetic resonant frequency of the unfixed magnetization direction.FIG. 3Cis a graph360of voltage measured across exemplary device302as a function of an applied external magnetic field strength. In an exemplary embodiment, an external DC magnetic field is applied to device302, as described above. The external DC magnetic field may be produced by an electromagnet or external current adjacent the device (not shown). When the external magnetic field is swept from negative to positive strength, the unfixed magnetization direction may switch between the antiparallel and the parallel configuration (as indicated by arrow sets362and364). In the parallel configuration, the measured voltage can be expressed as Vb+aχp2(Hp)hrf2, where Vbis the background signal, a is a coefficient, χp(Hp) is the susceptibility of the unfixed magnetic layer of the device in the parallel configuration, and hrfis the magnetic field portion of the electromagnetic wave. Similarly, the measured voltage in the antiparallel configuration can be expressed as Vb−aχap2(Hap)hrf2, where χap(Hap) is the susceptibility of the unfixed magnetic layer of the device in the antiparallel configuration. The voltage difference between the parallel and the antiparallel configuration (ΔV) can thus written as a[χp2(Hp)+χap2(Hap)]hrf2. Therefore, the value of ΔV can be used to determine the power of the electromagnetic wave. In a preferred embodiment, the value of ΔV may be obtained by measuring the voltage across device302directly adjacent to the location where the external magnetic field causes the device to switch from parallel to antiparallel, or vice versa (the switching region). This measurement of ΔV is illustrated between points366and368inFIG. 3C. This configuration may be particularly useful for detecting low frequency electromagnetic waves (e.g., from approximately 100 Hz to 500 MHz). However, for electromagnetic waves having frequencies closer to the ferromagnetic resonant frequency, detection may be preferable using the ferromagnetic resonance effect, as described above.

FIG. 4Adepicts another exemplary system400for detecting an electromagnetic wave in accordance with an aspect of the present invention. As a general overview, system400includes a device402, a detector412, and an electromagnetic wave source414. System400may further include a receiver416. Additional details of system400are provided below.

Device402is a spintronic device substantially as described above with respect to device102. Device402includes a first magnetic layer having a fixed magnetization direction and a second magnetic layer having an unfixed magnetization direction. Device402includes a barrier layer positioned between the two magnetic layers. Device402may further include a fixing layer to fix the magnetization direction of one of the magnetic layers. Device402has an impedance dependent at least in part on a relative angle between the magnetization directions of the two magnetic layers.

The magnetization directions of the first and second magnetic layers of device402are configured to be perpendicular to one another. Accordingly, in an exemplary embodiment of device402, the unfixed magnetization direction is initially configured to be at a right angle to the fixed magnetization direction, such that the relative angle between the fixed and the initial unfixed magnetization directions is approximately 90°.

As will be described in greater detail below, system400may operate without the need for an external current source, as provided in system300. Accordingly, system400may be a passive system. However, it is contemplated that system400may also include a current source similar to current source311described above with respect to system300. When coupled with a current source, it is contemplated that system400would function similar to system300, as described above.

Detector412measures the voltage across device402. In an exemplary embodiment, detector412is a voltage detector such as, for example, a lock-in amplifier. However, detector412may be any suitable voltage detector. The voltage measured by detector412is dependent on the impedance of device402. As described above, exposure to an electromagnetic wave may change the impedance of device402. Additionally, as will be described, exposure of device402to an electromagnetic may induce a current in device402. Accordingly, system400may detect an electromagnetic wave based on a change in the impedance of device402which is reflected in the voltage measured by detector412. A suitable voltage detector will be known to one of ordinary skill in the art from the description herein.

Electromagnetic wave source414emits electromagnetic waves. Electromagnetic wave source414may optimally emit electromagnetic radiation in the microwave range; however, it is contemplated that electromagnetic wave source414may emit other electromagnetic radiation. Detector402is exposed to the electromagnetic waves emitted by source414. In an exemplary embodiment, electromagnetic waves from source414cause the unfixed magnetization direction to precess. Precession of the unfixed magnetization direction causes the relative angle of the fixed and unfixed magnetizations of device402to precess around 90°. Precession of the relative angle causes a change in the impedance of device402responsive to the electromagnetic waves from source414. Electromagnetic wave source414may be any source of electromagnetic radiation desired to be detected.

Receiver416may be used to receive electromagnetic waves from electromagnetic wave source414and to transmit them to device402. Receiver416may be coupled to device402in order to focus the magnetic field portion of the received electromagnetic waves on device402. In an exemplary embodiment, receiver416may be a waveguide such as a loaded coplanar waveguide. However, receiver may be any waveguide or antenna suitable for receiving electromagnetic waves from source414and transmitting them to device402. A suitable receiver416for use with the present invention will be understood by one of skill in the art from the description herein.

FIG. 4Bdepicts a graph450of the impedance of exemplary device402as a function of time. Device402is depicted as having an impedance of 8,500Ω at a relative angle of 90°, as shown by line452of graph450. When an electromagnetic wave is applied to device402, the unfixed magnetization direction precesses from the perpendicular configuration, causing the relative angle to precess back and forth around 90°. As the relative angle periodically raises and lowers, the impedance of device402similarly raises and lowers.FIG. 4Bdepicts the periodic raising and lowering of the impedance of device402caused by the electromagnetic wave. In this configuration, the precession does not change the average impedance of device402.

The electromagnetic wave received by device402generates both electric and magnetic fields. As described above, the magnetic field portion of the electromagnetic wave may cause precession of the unfixed magnetization direction, thereby causing a change in the impedance of device402. Additionally, the electric field portion of the electromagnetic wave may induce a current in device402. The changing impedance and the induced current may then generate a voltage across device402. Accordingly, voltage across device402may be generated without the need for an external current source, as described in system300.

FIG. 4Cdepicts graphs460and470of the change in impedance, induced current, and induced voltage across device402. Specifically, graph460corresponds to the exposure of device402to an electromagnetic wave having a frequency substantially different from the ferromagnetic resonance of the unfixed magnetic layer (off-resonance), while graph470corresponds to the exposure of device402to an electromagnetic wave having a frequency the same as the ferromagnetic resonant frequency of the unfixed magnetic layer (on-resonance). In the off-resonance configuration, the magnitude of the change in impedance462caused by the magnetic field portion is comparatively low (e.g., 20Ω). Additionally, the change in impedance462is out of phase with the current464induced by the electric field portion. Accordingly, no voltage466is induced across device402. In the on-resonance configuration, the magnitude of the change in impedance472caused by the magnetic field portion is comparatively high. Additionally, the change in impedance472is in phase with the current474induced by the electric field portion. Accordingly, a non-zero voltage476is induced across device402. As detector412measures the voltage across device402, a measurement of non-zero voltage indicates the detection of an electromagnetic signal.

Referring back toFIG. 4A, system400may also be configured to determine characteristics of the electromagnetic wave based on the voltage measured by detector412. System400may be used to determine the frequency and/or power of the received electromagnetic wave using the steps described above with respect to system300. Similarly, system400may be configured as an electromagnetic wave demodulator using the steps described above with respect to system300.

System400may further be configured as an electromagnetic wave modulator or amplifier. In an exemplary embodiment, a current source similar to current source311may be added to system400. To function as a modulator, the current source may apply an alternating current (AC) through device402. This current combines with the impedance of device402to create an output voltage across device402. The voltage measured by detector412may be in the form of a modulated electromagnetic wave, with the AC signal from the current device modulating the received electromagnetic wave. To function as an amplifier, the current source may apply a direct current (DC) through device402. This current combines with the impedance of device402to create an output voltage across device402. The output voltage measured by detector412will have the same frequency of the received electromagnetic wave, amplified by the application of the DC current.

System400may also be configured to detect the relative phase between the electric field portion and the magnetic field portion of the electromagnetic wave. In an exemplary embodiment, a sweeping external DC magnetic field is applied to device402, as described above. Detector412may then measure the spectrum of voltage resulting from the magnetic field sweep. The relative phase between the electric field portion and magnetic field portions of the electromagnetic wave will result in a relative phase between the induced current and impedance of device402. The relative phase between the induced current and the impedance may be reflected in the voltage spectrum measured by detector412, i.e. a specific profile of the field-swept voltage spectrum may correspond to a relative phase of the electromagnetic wave. Accordingly, the relative phase of the received electromagnetic wave may be discerned from the voltage spectrum measured during the sweep of the external DC magnetic field. Determination of the relative phase from this voltage spectrum will be understood by one of skill in the art from the description herein.

FIG. 5Adepicts an exemplary system500for detecting an electromagnetic wave in accordance with another aspect of the present invention. System500is configured to detect electromagnetic waves over a wider range of power. As a general overview, system500includes devices502a-502c, a current source511, a detector512, an electromagnetic wave source514, and a receiver516. Additional details of system500are provided below.

Devices502a-502care spintronic devices substantially as described above with respect to device102. The fixed magnetization direction and unfixed magnetization direction of devices502a-502cmay be configured to either be parallel/antiparallel or perpendicular, as described above. The impedance of devices502a-502care changed by exposure to an electromagnetic wave. WhileFIG. 5Adepicts three devices, it is contemplated that any number of devices502may be used to achieve a wider range of electromagnetic wave detection.

Current source511is configured to provide a current through devices502a-502c. Detector512measures the voltage across devices502a-502c. System500may detect an electromagnetic wave based on a change in the impedance of devices502a-502cwhich is reflected in the voltage measured by detector512. Electromagnetic wave source514emits the electromagnetic waves to be detected, as described above. Electromagnetic wave source514may optimally emit microwave radiation, however, it is contemplated that source514may emit other electromagnetic radiation.

System500detects electromagnetic waves based on the voltage measured by detector512. As described above, system500may be configured to obtain information about the electromagnetic wave, such as frequency and/or power, where such properties of the wave are proportional to the voltage generated across devices502a-502cand measured by detector512. In an exemplary embodiment, each device502a-502chas a region in which the voltage across the device502a-502cis linearly proportional to the power of the received electromagnetic wave.FIG. 5Bis a graph550of the voltage response across an exemplary device502n(Voutput) as a function of the power of the electromagnetic wave, where n represents any of devices502a-502c. Graph550illustrates that exemplary device502nhas a region of linearly proportional voltage response552n. Linear response range552ncovers approximately 20-30 decibels of the electromagnetic wave's power spectrum. Above this range552n, device502ndoes not produce output voltage linearly proportional to electromagnetic wave power. The voltage across device502ndue to an electromagnetic wave in the non-linear range may be less useful for detecting or obtaining information about the electromagnetic wave. Accordingly, the useful range of electromagnetic wave detection for a single exemplary device502nmay be limited to approximately 20-30 decibels.

System500may increase this range of electromagnetic wave detection through the use of multiple devices502a-502ccoupled to receiver516. Receiver516receives electromagnetic waves from source514and transmits the waves to devices502a-502c. In an exemplary embodiment, receiver516is a cascading coplanar waveguide having three sections518a-518c. However, receiver516may be any suitable waveguide or antenna employing cascading circuits for receiving an electromagnetic wave from source514and transmitting it to devices502a-502c. Additionally, receiver516may have any number of sections518, e.g., corresponding to the number of devices502. Each section518a-518cis coupled to a corresponding device502a-502cin order to focus the magnetic field portion of the received electromagnetic wave on the corresponding device. Specifically, the magnetic field portion of the received electromagnetic wave may be inversely proportional to the width of a section518nof the receiver514. Accordingly. sections518a-518cof receiver516are configured such that the power density of a received electromagnetic wave increases as the electromagnetic wave cascades, or passes from a larger section to a smaller section.

For example, section518amay be configured to have a width of 100 μm and section518bmay be configured to have a width of 10 μm. In this configuration, an electromagnetic wave in section518amay have a power density one hundred times less than the same electromagnetic wave in section518b. Accordingly, if the electromagnetic wave has a power above the linear response range552bof device502b, the wave's power density will be 20 decibels less in section518a, and may fall within the linear response range552aof device502a.

Configuration of receiver516with cascading sections effectively increases the range of electromagnetic wave detection of system500by combining the linear response ranges of devices502a-502c. As noted above, the linear response range of system500may be optimized by using any number of devices502in conjunction with receiver516. Where the electromagnetic wave falls within the linear response range of only one device502n, a switch520may be employed to allow detector512to measure the voltage across only that device502n.

System500may also be configured to determine characteristics of the electromagnetic wave based on the voltage measured by detector512. System500may be used to determine the frequency and/or power of the received electromagnetic wave using the steps described above with respect to system300. Similarly, system500may be configured as an electromagnetic wave demodulator using the steps described above with respect to system300.

FIG. 6Adepicts an exemplary system600for detecting an electromagnetic wave in accordance with yet another aspect of the present invention. System600is further configured to detect a phase of received electromagnetic waves. As a general overview, system600includes a device602, a current source (not shown), a detector (not shown), an electromagnetic wave source614, a receiver616, a reference wave source622, a phase tuner624, and a reference wave receiver626. Additional details of system600are provided below.

Device602is a spintronic device substantially as described above. The fixed magnetization direction and initial unfixed magnetization direction of device602are configured to be parallel or antiparallel, as described above. The impedance of device602may be changed by exposure to an electromagnetic wave. System600includes a current source (not shown) which creates a current through device602, thereby creating a voltage across device602. System600further includes a detector (not shown) for measuring the voltage across device602. System600may detect an electromagnetic wave based on a change in the impedance of device602which is reflected in the voltage measured by the detector. Electromagnetic wave source614emits electromagnetic waves as described above. Receiver616receives electromagnetic waves from source614and transmits them to device602.

Reference wave source622emits a reference electromagnetic wave. In an exemplary embodiment, reference wave source622is any frequency-tunable electromagnetic wave source. Reference wave source622emits a reference electromagnetic wave tuned to the same frequency as the received electromagnetic wave from source614. In an alternative embodiment, the reference wave can be obtained by splitting the received electromagnetic waves from source614, e.g. as in conventional vector network analyzers. In this embodiment, reference wave source622may be omitted. A suitable reference wave source will be understood by one of skill in the art from the description herein.

Phase tuner624adjusts the phase of the reference electromagnetic wave from reference source622. In an exemplary embodiment, phase tuner624receives the reference electromagnetic wave from reference source622. Phase tuner adjusts the phase of the reference electromagnetic wave and transmits the wave to reference wave receiver626. A suitable phase tuner will be understood by one of skill in the art from the description herein.

Reference wave receiver626receives the reference electromagnetic waves from phase tuner624and transmits them to device602. In an exemplary embodiment, reference wave receiver is a waveguide or antenna of substantially the same form as receiver616.

System600may detect the frequency or power of a received electromagnetic wave as described above. Additionally, system600may detect a phase of the received wave. In an exemplary embodiment, system600detects the frequency of a received electromagnetic wave from source614. Reference electromagnetic wave source622is then tuned to emit a reference electromagnetic wave having the same frequency as the received wave from source614. Phase tuner624sweeps the phase of the reference wave from 0° to 360°. Reference wave receiver626receives the reference wave from phase tuner624and transmits the wave to device602. The detector of system600measures the voltage across device602as the phase of the reference wave is swept.

FIG. 6Bis a graph650of the voltage measured across exemplary device602in accordance with an aspect of the present invention. Graph650depicts the measured voltage corresponding to one sweep of the phase of the reference wave from 0° to 360°. Where the phase of the received wave and the reference wave are the same, the measured voltage reaches a maximum value. Accordingly, peak652in the measured voltage corresponds to an area where the detected wave from electromagnetic wave source614and the reference wave for the reference wave from reference electromagnetic wave source622have the same phase. Accordingly, system600may determine the phase of the received wave by noting the phase of the reference wave at the point during the phase sweep where a voltage peak is detected.

Referring back toFIG. 6A, system600may further be configured for use as an electromagnetic wave vector network analyzer. The power of the received electromagnetic wave may be determined by system600based on proportionality with the measured voltage, as described above. Additionally, the phase of a received electromagnetic wave may be determined by system600using the above steps. Configuration of system600as a vector network analyzer would be understood by one of ordinary skill in the art. Alternatively, system600could be configured as a spectrum analyzer. As described, the measured voltage across device602varies by phase only when the reference electromagnetic wave has the same frequency as the receive electromagnetic wave. Accordingly, the frequency of the received electromagnetic wave may be determined by tuning the frequency of reference electromagnetic wave source622until a phase-dependent voltage is measured, indicating that reference wave source622and received wave source614are emitting at the same frequency. Configuration of system600as a spectrum analyzer would thereby be understood to one of ordinary skill in the art.

FIG. 7Adepicts another exemplary system700for detecting an electromagnetic wave in accordance with an aspect of the present invention. System700is also configured to detect a phase of received electromagnetic waves. As a general overview, system700includes a device702, a detector (not shown), an electromagnetic wave source714, a receiver716, a reference wave source722, a phase tuner724, and a reference wave receiver726. Additional details of system700are provided below. It will be understood that phase tuner724may not be necessary for phase detection when a field swept ferromagnetic resonance profile is obtained, as would be understood by one of ordinary skill in the art from the description herein.

Device702is a spintronic device substantially as described above. The fixed magnetization direction and unfixed magnetization direction of device702are configured to be perpendicular, as described above. The impedance of device702may be changed by exposure to an electromagnetic wave. System700further includes a detector (not shown) for measuring the induced voltage across device702. System700may detect an electromagnetic wave based on a change in the impedance of device702which is reflected in the voltage measured by the detector. Electromagnetic wave source714emits electromagnetic waves as described above.

Receiver716receives electromagnetic waves from source714. In an exemplary embodiment, receiver716is a shorted coplanar waveguide. However, receiver716may be any suitable waveguide or antenna. Receiver716does not couple directly to device702. Instead, receiver716receives electromagnetic wave from source714and then irradiates a magnetic field from this wave. Device702is exposed to this irradiated magnetic field, causing a change in the impedance of device702, as described above.

In an exemplary embodiment, reference wave source722emits a reference electromagnetic wave tuned to the same frequency as the received electromagnetic wave from source714. Phase tuner724adjusts the phase of the reference electromagnetic wave and transmits the wave to reference wave receiver726.

Reference wave receiver726receives the reference electromagnetic wave from phase tuner724. In an exemplary embodiment, reference wave receiver is an open coplanar waveguide. However, reference wave receiver726may be any suitable waveguide or antenna. Reference wave receiver726focuses an electric field from the reference wave onto device702. Device702receives this electric field, creating a current through device702.

As described above with reference to system400, the combination of the change in impedance of device702and the induced current in device702creates a voltage across device702. The detector of system700then measures this voltage. System700may detect the frequency or power of the received magnetic wave as described above using the measured voltage. Additionally, system700may detect a phase of the received wave. In an exemplary embodiment, the voltage profile measured by the detector is at least partially dependent on the difference in phase of the magnetic field of the received wave irradiated by receiver716and the electric field of the reference wave transmitted by reference wave receiver726.

FIG. 7Bis a computer image750depicting four voltage profiles760-790measured across exemplary device702.FIG. 7Bincludes graphs760-790, which include exemplary profiles of measured voltage. Each graph760-790corresponds to a phase difference between the magnetic field from the received wave and the electric field from the reference wave. Phase tuner724may be used to determine the phase of the reference wave. Accordingly, system700may determine the phase of the received wave from source714based on the profile of the measured voltage and the phase of the reference wave.

Referring back toFIG. 7A, for the purpose of determining a phase of the received wave, phase tuner724may be omitted from system700where the phase of the reference wave is known. However, system700may further be configured for use as an electromagnetic wave vector network analyzer or a spectrum analyzer, as described above with respect to system500, using phase tuner724, for example. Configuration of system700as a wave vector network analyzer or a spectrum analyzer will be understood by one of ordinary skill in the art from the description herein.

Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention. For example, due to the miniaturized dimension of the device, this device may be used for electromagnetic wave imaging with micron/submicron size resolution. It is further contemplated that the device be used as an antenna to receive electromagnetic wave signals. Such a miniaturized antenna could find many applications in communication system such as cellular phones.