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Conventional electromagnetic (EM) antennas cannot be aggressively miniaturized since their gain and radiation efficiency plummet when their sizes become much smaller than the radiated wavelength. Recently, we demonstrated a new genre of unconventional extreme subwavelength nano-antennas that are several orders of magnitude smaller than the wavelength they radiate, and yet they radiate efficiently, beating the conventional Harrington limits on the gain and radiation efficiency by many orders of magnitude. This is made possible by their unique unconventional mechanism of activation. These nano-antennas are implemented with 2-D periodic arrays of ∼100-nm-sized nanomagnets deposited on piezoelectric substrates. A surface acoustic wave (SAW) launched in the substrate excites resonant spin waves in the nanomagnets at discrete (GHz) frequencies via phonon–magnon coupling, which radiates EM waves very efficiently at those frequencies via magnon–photon coupling. Normally, one would expect such ultrasmall antennas to behave as point sources that radiate isotropically. Surprisingly, they do not because of the intrinsic anisotropy in the nanomagnet array. The radiation patterns in the plane of the nanomagnets and the two transverse planes are anisotropic. By changing the direction of SAW propagation in the plane of the nanomagnets, one can change the radiation patterns in all three planes, which heralds a new method of beam steering or active electronic scanning. 

Abstract Magnetic straintronics made its debut more than a decade ago as an extremely energy-efficient paradigm for implementing a digital switch for digital information processing. The switch consists of a slightly elliptical nano-sized magnetostrictive disk in elastic contact with a poled ultrathin piezoelectric layer (forming a two-phase multiferroic system). Because of the elliptical shape, the nanomagnet’s magnetization has two stable (mutually antiparallel) orientations along the major axis, which can encode the binary bits 0 and 1. A voltage pulse of sub-ns duration and amplitude few to few tens of mV applied across the piezoelectric generates enough strain in the nanomagnet to switch its magnetization from one stable state to the other by virtue of the inverse magnetostriction (or Villari) effect, with an energy expenditure that is roughly an order of magnitude smaller than what it takes to switch a modern-day electronic transistor. That possibility, along with the fact that such a switch is non-volatile unlike the conventional transistor, generated significant excitement. However, it was later tempered by the realization that straintronic switching is also extremelyerror-prone, which may preclude many digital applications, particularly in Boolean logic. In this perspective, we offer the view that there is plenty of room for magnetic straintronics in theanalogdomain, which is much more forgiving of switching errors, and where the excellent energy-efficiency and non-volatility are a boon. Analog straintronics can have intriguing applications in many areas, such as a new genre of aggressively miniaturized electromagnetic antennas that defy the Harrington limits on the gain and radiation efficiency of conventional antennas, analog arithmetic multipliers (and ultimately vector matrix multipliers) for non-volatile deep learning networks with very small footprint and excellent energy-efficiency, and relatively high-power microwave oscillators with output frequency in the X-band. When combined with spintronics, analog straintronics can also implement a new type of spin field effect transistor employing quantum materials such as topological insulators, and they have unusual transfer characteristics which can be exploited for analog tasks such as frequency multiplication using just a single transistor. All this hints at a world of new possibilities in the analog domain that deserves serious attention. 

Mechanical strain provides a knob for controlling the magnetization of the magnetostrictive-free layer of magnetic tunnel junctions (MTJs), with many applications for energy-efficient memory and computing. This requires integrating materials with high magnetostriction coefficient into MTJs, while still preserving the CoFeB-MgO tunnel barrier for high tunnel magnetoresistance (TMR). One way to accomplish this is to replace the CoFeB free layer of the MTJ with an exchange-coupled bilayer of CoFeB and a highly magnetostrictive ferromagnet like Galfenol (FeGa). Here, FeGa, a thermally stable magnetostrictive material, is integrated into CoFeB-based MTJs. We show that engineering a thin layer of CoFeB and FeGa provides a means of controlling the magnetic properties and switching field in FeGa-based MTJs, and that the exchange-coupled FeGa-CoFeB layer can be used as both a free layer and a fixed layer in the MTJ stack with TMR as high as 100%. 

Abstract We observed strong tripartite magnon-phonon-magnon coupling in a two-dimensional periodic array of magnetostrictive nanomagnets deposited on a piezoelectric substrate, forming a 2D magnetoelastic “crystal”; the coupling occurred between two Kittel-type spin wave (magnon) modes and a (non-Kittel) magnetoelastic spin wave mode caused by a surface acoustic wave (SAW) (phonons). The strongest coupling occurred when the frequencies and wavevectors of the three modes matched, leading to perfect phase matching. We achieved this condition by carefully engineering the frequency of the SAW, the nanomagnet dimensions and the bias magnetic field that determined the frequencies of the two Kittel-type modes. The strong coupling (cooperativity factor exceeding unity) led to the formation of a new quasi-particle, called a binary magnon-polaron, accompanied by nearly complete (~100%) transfer of energy from the magnetoelastic mode to the two Kittel-type modes. This coupling phenomenon exhibited significant anisotropy since the array did not have rotational symmetry in space. The experimental observations were in good agreement with the theoretical simulations. 

Abstract In Part I of this topical review, we discussed dynamical phenomena in nanomagnets, focusing primarily on magnetization reversal with an eye to digital applications. In this part, we address mostly wave-like phenomena in nanomagnets, with emphasis on spin waves in myriad nanomagnetic systems and methods of controlling magnetization dynamics in nanomagnet arrays which may have analog applications. We conclude with a discussion of some interesting spintronic phenomena that undergird the rich physics exhibited by nanomagnet assemblies. 

Abstract When magnets are fashioned into nanoscale elements, they exhibit a wide variety of phenomena replete with rich physics and the lure of tantalizing applications. In this topical review, we discuss some of these phenomena, especially those that have come to light recently, and highlight their potential applications. We emphasize what drives a phenomenon, what undergirds the dynamics of the system that exhibits the phenomenon, how the dynamics can be manipulated, and what specific features can be harnessed for technological advances. For the sake of balance, we point out both advantages and shortcomings of nanomagnet based devices and systems predicated on the phenomena we discuss. Where possible, we chart out paths for future investigations that can shed new light on an intriguing phenomenon and/or facilitate both traditional and non-traditional applications. 

The Landau-Lifshitz-Gilbert (LLG) equation, used to model magneto-dynamics in ferromagnets, tacitly assumes that the angular momentum associated with spin precession can relax instantaneously when the real or effective magnetic field causing the precession is turned off. This neglect of “spin inertia” is unphysical and would violate energy conservation. Recently, the LLG equation was modified to account for inertia effects. The consensus, however, seems to be that such effects would be unimportant in slow magneto-dynamics that take place over time scales much longer that the relaxation time of the angular momentum, which is typically few fs to perhaps ~100 ps in ferromagnets. Here, we show that there is at least one very serious and observable effect of spin inertia even in slow magneto-dynamics. It involves the switching error probability associated with flipping the magnetization of a nanoscale ferromagnet with an external agent, such as a magnetic field. The switching may take ~ns to complete when the field strength is close to the threshold value for switching, which is much longer than the angular momentum relaxation time, and yet the effect of spin inertia is felt in the switching error probability. This is because the ultimate fate of a switching trajectory, i.e. whether it results in success or failure, is influenced by what happens in the first few ps of the switching action when nutational dynamics due to spin inertia holds sway. Spin inertia increases the error probability, which makes the switching more error-prone. This has vital technological significance because it relates to the reliability of magnetic logic and memory. 

Binary switches, which are the primitive units of all digital computing and information processing hardware, are usually benchmarked on the basis of their ‘energy–delay product’, which is the product of the energy dissipated in completing the switching action and the time it takes to complete that action. The lower the energy–delay product, the better the switch (supposedly). This approach ignores the fact that lower energy dissipation and faster switching usually come at the cost of poorer reliability (i.e., a higher switching error rate) and hence the energy–delay product alone cannot be a good metric for benchmarking switches. Here, we show the trade-off between energy dissipation, energy–delay product and error–probability for an electronic switch (a metal oxide semiconductor field effect transistor), a magnetic switch (a magnetic tunnel junction switched with spin transfer torque) and an optical switch (bistable non-linear mirror). As expected, reducing energy dissipation and/or energy–delay product generally results in increased switching error probability and reduced reliability.