Nanomagnetic implementations of Boolean logic have attracted attention because of their nonvolatility and the potential for unprecedented overall energy-efficiency. Unfortunately, the large dissipative losses that occur when nanomagnets are switched with a magnetic field or spin-transfer-torque severely compromise the energy-efficiency. Recently, there have been experimental reports of utilizing the Spin Hall effect for switching magnets, and theoretical proposals for strain induced switching of single-domain magnetostrictive nanomagnets, that might reduce the dissipative losses significantly. Here, we experimentally demonstrate, for the first time that strain-induced switching of single-domain magnetostrictive nanomagnets of lateral dimensions ∼200 nm fabricated on a piezoelectric substrate can implement a nanomagnetic Boolean NOT gate and steer bit information unidirectionally in dipole-coupled nanomagnet chains. On the basis of the experimental results with bulk PMN-PT substrates, we estimate that the energy dissipation for logic operations in a reasonably scaled system using thin films will be a mere ∼1 aJ/bit.

Strain-mediated voltage control of magnetization in piezoelectric/ferromagnetic systems is a promising mechanism to implement energy-efficient spintronic memory devices. Here, we demonstrate giant voltage manipulation of MgO magnetic tunnel junctions (MTJ) on a Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) piezoelectric substrate with (001) orientation. It is found that the magnetic easy axis, switching field, and the tunnel magnetoresistance (TMR) of the MTJ can be efficiently controlled by strain from the underlying piezoelectric layer upon the application of a gate voltage. Repeatable voltage controlled MTJ toggling between high/low-resistance states is demonstrated. More importantly, instead of relying on the intrinsic anisotropy of the piezoelectric substrate to generate the required strain, we utilize anisotropic strain produced using local gating scheme, which is scalable and amenable to practical memory applications. Additionally, the adoption of crystalline MgO-based MTJ on piezoelectric layer lends itself to high TMR in the strain-mediated MRAM devices. *Corresponding author. Tel: (612) 625-9509. E-mail: jpwang@umn.edu 2 Information storage technology is constantly challenged by an increasing demand for storage units that are small, retain information for the longest time, and dissipate miniscule amount of energy to store (write) and retrieve (read) information. Magnetic random access memory (MRAM) meets these requirements to a large extent and has been proposed as a universal storage device for computer memory. [1][2][3] In MRAM technology, magnetic tunneling junctions (MTJ) comprise the main storage cells. Low-energy writing of bits requires an electrically tunable mechanism to reorient the magnetization of the MTJ. However, the widely studied switching mechanisms based on utilizing current induced spin-transfer-torques (STT) 4,5 or spin-orbit-torques (SOT) 6-8 incur high energy dissipation because of the relatively large writing current density. 9,10In recent years, several mechanisms based on using voltage to control magnetization have emerged as promising routes for ultra-low power writing of data. 11-15 Among these approaches, the strain induced control of the magnetic anisotropy in multiferroic heterostructures (a magnetostrictive layer elastically coupled with an underlying piezoelectric layer) stands out as a remarkably energyefficient switching mechanism. 16-21It has been widely investigated in various piezoelectric/ferromagnetic bilayer thin films [22][23][24][25][26] or nano-structures. [27][28][29][30] There are also several theoretical predications 31-33 that such a method will dissipate only a few atto-Joules (aJ) of energy to write data. This establishes the promise of using strain to control the resistance of an MTJ for ultra-energy-efficient memory applications.The key for strain control of the in-plane magnetization is that the in-plane strain should be anisotropic. In most of the previous reports, [24][25][26][27]34 single crystalline piezoelectric substrates Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT...

We report manipulation of the magnetic states of elliptical cobalt magnetostrictive nanomagnets (of nominal dimensions ~ 340 nm × 270 nm × 12 nm) delineated on bulk 128° Y-cut lithium niobate with Surface Acoustic Waves (SAWs) launched from interdigitated electrodes. Isolated nanomagnets that are initially magnetized to a single domain state with magnetization pointing along the major axis of the ellipse are driven into a vortex state by surface acoustic waves that modulate the stress anisotropy of these nanomagnets. The nanomagnets remain in the vortex state until they are reset by a strong magnetic field to the initial single domain state, making the vortex state "non-volatile". This phenomenon is modeled and explained using a micromagnetic framework and could lead to the development of extremely energy efficient magnetization switching methodologies.

Determining the force required to detach a droplet from a fiber or from an assembly of fibers is of great importance to many applications. A novel technique is developed in this work to measure this force experimentally by using ferrofluid droplets in a magnetic field. Unlike previous methods reported in the literature, our technique does not require air flow or a mechanical object to detach the droplet from the fiber(s); therefore, it simplifies the experiment and also allows one to study the capillarity of the droplet-fiber system in a more isolated environment. In this article, we investigated the effects of the relative angle between intersecting fibers on the force required to detach a droplet from the fibers in the in-plane or out-of-plane direction. The in-plane and through-plane detachment forces were also predicted via numerical simulation and compared with the experimental results. Good agreement was observed between the numerical and experimental results. It was found that the relative angle between intersecting fibers has no significant effect on the detachment force in the out-of-plane direction. However, the detachment force in the in-plane direction depends strongly on the relative angle between the fibers, and it increases as this angle increases.

Self-polarized Gd5Si4-polyvinylidene fluoride (PVDF) nanocomposite films have been synthesized via a facile phase-inversion technique. For the 5 wt% Gd5Si4-PVDF films, the enhancement of the piezoelectric β-phase and crystallinity are confirmed using Fourier transform infrared (FTIR) spectroscopy (phase fraction, Fβ, of 81% as compared to 49% for pristine PVDF) and differential scanning calorimetry (crystallinity, ΔXc, of 58% as compared to 46% for pristine PVDF), respectively. The Gd5Si4 magnetic nanoparticles, prepared using high-energy ball milling were characterized using Dynamic Light Scattering and Vibrating Sample Magnetometry (VSM) to reveal a particle size of ∼470 nm with a high magnetization of 11 emu/g. The VSM analysis of free-standing Gd5Si4-PVDF films revealed that while the pristine PVDF membrane shows weak diamagnetic behavior, the Gd5Si4-PVDF films loaded at 2.5 wt% and 5 wt% Gd5Si4 show enhanced ferromagnetic behavior with paramagnetic contribution from Gd5Si3 phase. The interfacial interactions between Gd5Si4 and PVDF results in the preferential crystallization of the β-phase as confirmed via the shift in the CH2 asymmetric and symmetric stretching vibrations in the FTIR. These results confirm the magnetic Gd5Si4 nanoparticles embedded in the PVDF membrane lead to an increased β-phase fraction, which paves the way for future efficient energy harvesting applications using a combination of magnetic and piezoelectric effects.

The authors show how to implement a 4-state universal logic gate (NOR) using three straincoupled magnetostrictive-piezoelectric multiferroic nanomagnets (e.g. Ni/PZT) with biaxial magnetocrystalline anisotropy. Two of the nanomagnets encode the 2-state input bits in their magnetization orientations and the third nanomagnet produces the output bit via dipole interaction with the input nanomagnets. A voltage pulse alternating between -0.2 V and +0.2 V is applied to the PZT layer of the third nanomagnet and generates alternating tensile and compressive stress in its Ni layer to produce the output bit, while dissipating ~ 57,000 kT (0.24 fJ) of energy per gate operation.1 Nanomagnetic logic (NML) is an emerging paradigm for low-power computing 1-4 where logic bits 0 and 1 are encoded in two stable magnetization directions parallel to the easy axis of a single domain nanomagnet with uniaxial shape anisotropy. Bits are switched by flipping the magnetization of a nanomagnet with an external agent that acts like a clock.The ultimate energy-efficiency of NML is determined by the clocking methodology, i.e. how bits are flipped. If a magnetic field generated with a current 5 , or spin transfer torque 6 , is used to flip a nanomagnet's magnetization, then the energy consumed can be exorbitant, which defeats the very purpose of NML. However, stress-induced rotation of magnetization in multiferroic nanomagnets seems to be able to switch magnets with much higher energy efficiency 7-9 . This is only now beginning to attract attention 7-10 .The magnetization vector of a multiferroic nanomagnet, consisting of a piezoelectric and a magnetostrictive layer, can be flipped with an electrostatic potential applied to the piezoelectric layer. The voltage generates a strain that is elastically transferred to the magnetostrictive layer causing its magnetization to flip. We can embellish the functionality of the multiferroic element by introducing biaxial magnetocrystalline anisotropy in the magnetostrictive layer, giving it four possible stable magnetization directions ('up', 'right' ,'down', 'left') that are chosen to encode four possible 2-bit combinations (00, 01, 11, 10), illustrated in Fig. 1(a). The choice is motivated by allowing only 1 bit to change for every 90º rotation of magnetization. For single crystal Ni, with magnetocrystalline anisotropy constant K 1 < 0, the "easy" directions that encode these states in the (001) plane are the [110], [11 − 0], [1 − 1 − 0] and [1 − 10] directions, as shown in the energy curves in Fig. 1(a) (saddle-shaped curve, with the four easy directions of magnetization having lower energy levels than the other principal crystallographic axes).

The need for increasingly powerful computing hardware has spawned many ideas stipulating, primarily, the replacement of traditional transistors with alternate 'switches' that dissipate miniscule amounts of energy when they switch and provide additional functionality that are beneficial for information processing. An interesting idea that has emerged recently is the notion of using two-phase (piezoelectric/magnetostrictive) multiferroic nanomagnets with bistable (or multi-stable) magnetization states to encode digital information (bits), and switching the magnetization between these states with small voltages (that strain the nanomagnets) to carry out digital information processing. The switching delay is ∼1 ns and the energy dissipated in the switching operation can be few to tens of aJ, which is comparable to, or smaller than, the energy dissipated in switching a modern-day transistor. Unlike a transistor, a nanomagnet is 'non-volatile', so a nanomagnetic processing unit can store the result of a computation locally without refresh cycles, thereby allowing it to double as both logic and memory. These dual-role elements promise new, robust, energy-efficient, high-speed computing and signal processing architectures (usually non-Boolean and often non-von-Neumann) that can be more powerful, architecturally superior (fewer circuit elements needed to implement a given function) and sometimes faster than their traditional transistor-based counterparts. This topical review covers the important advances in computing and information processing with nanomagnets, with emphasis on strain-switched multiferroic nanomagnets acting as non-volatile and energy-efficient switches-a field known as 'straintronics'. It also outlines key challenges in straintronics.

We report nanomagnetic switching with Acoustic Waves (AW)