By Dr. Elena N. Bankowski and Dr. Thomas J. Meitzler, U.S. Army TARDECJune 3, 2015
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.
Based on our landmark research, the authors received a 2013 Army Outstanding Technical Research and Development Achievement Award for the "Spintronic Radar Detectors for Multifunctional Armor." We are continuing our research, development and integration of the fast and very accurate spintronic sensor system for detection and analysis of radar threats for ground combat vehicles. 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 as depicted in Figure 1.
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 structure of a Spintronic Radar Detector is shown in Figure 2. The SMD receives a microwave signal from a planar antenna. This signal creates a resonance magnetization precession in the "free layer" of a nano-scale SMD. A precession signal is detected by giant magneto-resistance effect. Radar detector (array of SMDs with different in-plane shapes and, thus, different resonance frequencies) works as a fast ― ~500 nanoseconds (ns) ― passive spectrum analyzer.
The authors, in collaboration with university spintronic researchers, developed a new spintronic detector design and received United States Patent No. US 8,860,159 B2, for Spintronic Electronic Device and Circuits, on Oct. 14, 2014, for their invention. None of the prior spintronic sensors utilized Ta, Ru and CuN layers, which were used to produce a very smooth (small crystallographic grain size) conductive bottom layer. This was necessary for deposition of a pinhole-free, magnetic tunnel junction (MTJ) on top. None of the prior art spintronic sensors described the layers of Ta, Ru and Cu on the top, which were needed for making a reliable contact to the top lead of the tunnel junction.
The SMD schematic shown in Figure 3 depicts a horn antenna used to transmit the incoming microwave frequency signal. The coplanar waveguide (CPW) antenna (#1) is used to receive this signal and feed it to the spintronic detector (#2).
Turning the set screw (#5) changes the distance from the magnet (#6) to the detector and varies the magnetic field. This changes the spintronic sensor's resonance frequency, based on the MTJ and SMD sensitivity. The spintronic detectors are tuned for maximum sensitivity. The experimental results have demonstrated that SMDs coupled to CPW antennas were capable of receiving external microwave signals in a laboratory experiment performed in the anechoic chamber developed in TARDEC's Electrified Armor Laboratory. Figure 4 depicts the experimental setup inside the anechoic chamber.
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. Two types of materials, Alumina and silicon carbide (SiC), were placed between the transmitting horn antenna and the receiving CPW antenna and the signal strength was measured. Figure 5 shows the detector plots. More work towards system integration is planned. The objective is to develop prototype spintronic devices or systems and nano-engineered metamaterials for radar detection, signature management and active-smart armor protection systems. Ultimately, these devices and materials will be integrated into ground combat vehicles.
The fast and reliable detection of radar threats will provide sufficient time to undertake the relevant counter-measures (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 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 anti-radar defense (to detect incoming radar pulses and jam them or determine the radar position), counter-terrorist activity (to detect and jam triggering microwave signals of radio-triggered explosive devices) and military intelligence (to intercept and/or jam radio messages sent using the frequency-hopping spread spectrum method). In all these tasks the detector should be able to determine the frequency of a microwave signal very fast ― on sub-microsecond time scale ― to take appropriate counter-actions during the time intervals comparable with the time of the pulse propagation.
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