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In quantum mechanics, spin is an intrinsic form of angular momentum carried by elementary particles. Spintronics is “A branch of physics concerned with the storage and transfer of information by means of electron spins in addition to electron charge as in conventional electronics.” Spin-based electronics focuses on devices whose functionality is based primarily on the spin degree of freedom of the carriers. This is in contrast to conventional electronics, which exploits only the charge of the carriers. Using either the spin in tandem with the charge or alone, spintronics has some advantages over conventional semiconductor electronics, including higher integration density, non-volatility, decreased power dissipation and faster processing speeds.
Over the last few decades, spintronic sensors working as solid-state magnetic sensors have gained continuous attention and research effort because of their traits such as superb sensitivity, low power consumption, compactness, wide bandwidth, room-temperature operation and CMOS compatibility.
Spintronic sensors can provide measurement data of magnetic field from which magnetic-field-related parameters of some real-world objects can be derived. Spintronic sensors, working as sensing devices, possess superb measuring abilities, and they are of small size and low cost. They can be used as pervasive magnetic field sensors in electrical current sensing, transmission and distribution lines monitoring, vehicle detection and biodetection.
Spintronic sensors with superb measuring ability and multiple unique advantages can be an important piece of cornerstone for IoT. The IoT paradigm envisions to connect billions of “Things” that surround us to the Internet and expects to use the information of these “Things” to enhance utilization effectiveness and efficiency of various public resources, thus accelerating the actualization of smart living. Spintronic sensors, can provide the information of magnetic field and magnetic-field-related parameters (e.g., current, vehicle speed, analyte concentration, etc.) in several applications, which can be a cornerstone for the IoT.
By virtue of the IoT technologies, spintronic sensors are seamlessly integrated into WSNs, which provide the pervasive WSSN monitoring systems for smart living. These WSSN based or related solutions enable the pervasive building energy management system, the wide-area transmission and distribution network monitoring system, the all-round traffic monitoring and management system, and the pervasive health diagnosis system.
The ultrafast detection and spectral analysis of enemy radio transmissions is vital for survivability applications to allow achieving the active interference with these signals on the time scale of the signal propagation time. This problem arises in antiradar defense (to detect incoming radar pulses and jam them or determine the radar position), counterterrorist activity (to detect and jam triggering microwave signals of radiotriggered explosive devices) and military intelligence (to intercept and/or jam radio messages sent using the frequencyhopping spread spectrum method). In all these tasks the detector should be able to determine the frequency of a microwave signal very fast ― on submicrosecond time scale ― to take appropriate counteractions during the time intervals comparable with the time of the pulse propagation.
US Army researchers have developed “Spintronic Radar Detectors for Multifunctional Armor systems,” the fast and very accurate spintronic sensor system for detection and analysis of radar threats for ground combat vehicles.
Basic principles of spintronic sensors
Typically, the resistance of spintronic sensors depends on both the magnitude and direction of the external magnetic field, known as the magnetoresistance (MR) effect. Based on distinct underlying mechanisms, spintronic sensors are normally categorized into anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR) sensors.
The AMR effect in ferromagnetic metals and alloys stems from the anisotropic scattering of conduction electrons induced by spin-orbit interaction. The spin-orbit interaction leads to a mixing of spin-up and spin-down electron states in the ferromagnetic materials.
Since the magnetization direction of the AMR material can be changed by the magnitude and direction of an external magnetic field, a link between the resistance of AMR sensors and the external magnetic field can be established. Based on this principle, the information of magnetic field (e.g., direction, magnitude) can be obtained by using AMR sensors. Furthermore, the directional sensing ability of the AMR sensor can be utilized to determine the rotational conditions (e.g., angle, rotational speed) of a magnetic object. As a result, nowadays AMR sensors can work as contactless magnetometers, angle or rotation sensors.
One spintronic device already in use is the giant magnetoresistive, or GMR, sandwich structure, which consists of alternating ferromagnetic (that is, permanently magnetized) and nonmagnetic metal layers. Depending on the relative orientation of the magnetizations in the magnetic layers, the electrical resistance through the layers’ changes from small (parallel magnetizations) to large (antiparallel magnetizations).
Unlike the AMR sensor where the electron spins are randomly oriented, the FM layer in GMR sensors serves as a spin filter to polarize the spin of electrons.
Investigators discovered that they could use this change in resistance (called magnetoresistance, and “giant” because of the large magnitude of the effect in this case) to construct exquisitely sensitive detectors of changing magnetic fields, such as those marking the data on a computer hard-disk platter. These disk drive read/write heads have been wildly successful, permitting the storage of tens of gigabytes of data on notebook computer hard drives, and have created a billions of dollar per year industry.
Compared to spin valves, a typical TMR sensor has a similar basic structure but with the metallic spacer replaced by a thin (0.5 ~ 2 nm) insulating barrier. As a result, instead of the spin-dependent scattering effect, the spin-dependent tunneling effect is involved in TMR sensors, which are therefore called magnetic tunnel junctions (MTJs).
For sensor applications, not only the sensitivity is required to be improved but also the noise level should be suppressed in order to boost up the signal-to-noise ratio (SNR). The MR ratio of TMR sensors is 1 ~ 2 orders of magnitude higher than that of GMR sensors, but the noise level of TMR sensors is much higher than that of GMR sensors
Sensor Applications
Spintronic sensors can provide measurement data of magnetic field from which magnetic-field-related parameters of some real-world objects can be derived. Spintronic sensors, working as sensing devices, possess superb measuring abilities, and they are of small size and low cost. They can be used as pervasive magnetic field sensors in electrical current sensing, transmission and distribution lines monitoring, vehicle detection and biodetection.
Electrical current sensing
Magnetic field is emanated from a current-carrying conductor and the magnitude of the magnetic field at a sensing point can be deduced from the Biot-Savart law. Based on the above-mentioned principle, spintronic sensors are increasingly gaining popularity in current sensing compared to current transformers (CT), shunt resistors, Rogoswski coils, fluxgates and Hall effect sensors.
Vehicle detection
Vehicle detectors based on spintronic sensors have been widely used for vehicle detection applications. The Earth provides a uniform and stable magnetic field over the planet surface. A ferrous or metal object, like a vehicle, can be considered as a model consisting of a number of bipolar magnets with N-S polarization direction. A vehicle can cause a local disturbance in the Earth’s field when it moves or stands still. The disturbance depends on the ferrous material, the size and the moving orientation of this object.
By analyzing the disturbance signal, the presence, moving speed, direction and classification of this vehicle can be determined. To obtain a smoother magnetic field signal, a digital filtering algorithm is usually used to eliminate noise, which may utilize fast Fourier transform, median filter, and Gaussian filter, and so on. Each category of vehicle signal has its own characteristics due to the different structures and sizes.
Biodetection
With the significant technological advancement of spintronic sensors, they have been becoming increasingly important not only in the industrial area but also in biomedical applications. Spintronic-based devices have become powerful tools for highly sensitive and rapid biological detections. The spintronic biodetection technology aims at sensing the concentration of target analyte molecules in solution, such as DNAs and proteins. In the spintronic biodetection process, magnetic labels are utilized to tag the target analyte molecules, and then spintronic sensors are used to detect the magnetic signals generated from the labels.
Firstly, the surface of the sensing area is functionalized with the probe molecules, which can specifically bind to the target analyte molecules. The target analytes in recent researches are dominantly DNAs or proteins, and the approaches to capturing them are rather different. In the DNA sequence detection, the complementary DNA chains are used as probes to capture the target DNA chain. While for the detection of protein molecules, antibodies are employed to bind to the target protein through the specific immune (antigen-antibody) recognition. Then the sample solution (e.g., plasma, sera, urine) is applied on the sensor surface. After the interrogation of the probe array, the target analyte molecules in the solution are specifically captured onto the sensor surface. The target analyte molecules are tagged by the magnetic labels before or after the capturing process depending on different mechanisms (direct labeling or indirect labeling). As a result, the magnetic labels are attached to the sensor surface. By exciting with the applied magnetic field, the magnetic labels produce a fringe field that is detectable by the underneath sensors. This detected magnetic signal quantitatively represents the concentration level of the target analytes in the sample solution.
Military Applications
Research from a team led by North Carolina State University is opening the door to smarter sensors by integrating the smart material vanadium dioxide (VO2) onto a silicon chip and using lasers to make the material magnetic. The advance paves the way for multifunctional spintronic smart sensors for use in military applications and next-generation spintronic devices.
VO2 is currently used to make infrared sensors. By integrating VO2 as a single crystal onto a silicon substrate, the researchers have made it possible to create infrared smart sensors, in which the sensor and computational function are embedded on a single chip. This makes the sensor faster and more energy efficient, since it doesn’t have to send data to another chip to be processed. Smart sensors are also lighter than conventional ones, since separate chips aren’t necessary.
“For military applications, sensor technology needs to be able to sense, manipulate, and respond to data quickly – and this work achieves that,” says Dr. Jay Narayan, John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and senior author of a paper describing the work.
Scientists Develop Novel “Spintronic” Sensors for the Army
U.S. Army Tank Automotive Research, Development and Engineering Center (TARDEC) researchers have made significant contributions to spintronic microwave detector development theory in collaboration with the research group led by Physics Department Professor Andrei Slavin at Oakland University, Rochester, MI.
Through crucial collaboration, we co-developed mathematical models and computer programs for optimization of parameters and geometrical dimensions of future spintronic detectors to achieve maximum sensitivity. Ten spintronic microwave detectors have been built based on theoretical calculations and computer modeling by the research group led by Professor Ilya Krivorotov at the University of California at Irvine.
The system is based on arrays of nano-scale radiation-hard frequency-selective spintronic microwave diodes (SMD). The spintronic radar detectors and planar microwave antennas could be embedded directly into the vehicle’s armor without compromising its structural integrity.
Spintronic Radar Detector receives a microwave signal from a planar antenna. This signal creates a resonance magnetization precession in the “free layer” of a nanoscale SMD. A precession signal is detected by giant magnetoresistance effect. Radar detector (array of SMDs with different inplane shapes and, thus, different resonance frequencies) works as a fast as ~500 nanoseconds (ns).
The fast and reliable detection of radar threats will provide sufficient time to undertake the relevant countermeasures (e.g., active jamming of the enemy radar, reposition of a sacrificial armor component, etc.) which will lead to greatly improved survivability of ground combat vehicles. The characteristic time of frequency determination will be substantially shorter than the return propagation time of a transmitted radar or control pulse that typically is in the order of a microsecond.
The next step in developing the novel spintronic radar detectors is the integration of SMD arrays into protective surfaces of ground vehicles. The authors have begun measuring the effects of various protective materials on the SMD detector with CPW antenna.
The objective is to develop prototype spintronic devices or systems and nanoengineered metamaterials for radar detection, signature management and active smart armor protection systems. Ultimately, these devices and materials will be integrated into ground combat vehicles.
References and Resources also include:
https://arxiv.org/ftp/arxiv/papers/1611/1611.00317.pdf
https://www.army.mil/article/144834/scientists_develop_novel_spintronic_sensors_for_the_army
