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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. 

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. 

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.