Noel D'Souza

Four-state nanomagnetic logic using multiferroics

The authors theoretically demonstrate the implementation of a low-power 4-state universal logic gate (NOR) using a linear array of three dipole-coupled magnetostrictive-piezoelectric multiferroic nanomagnets (e.g. Ni/PZT) with biaxial magnetocrystalline anisotropy. The two peripheral nanomagnets in the array encode the 4-state input bits in their magnetization orientations and the central nanomagnets magnetization orientation represents the output bit. Numerical simulations are performed to confirm that the 4-state output bit is the Boolean NOR function of the two 4-state inputs bits when the array reaches its ground state. A voltage pulse alternating between ?0.2 and +0.2?V, applied to the piezoelectric layer of the central nanomagnet, generates alternating tensile and compressive stress in its magnetostrictive layer. This drives the array to the correct ground state where dipole interaction between the magnets ensures that the output is the NOR function of the input. For the system considered, the gate operation is executed while dissipating only ~33?000?kT (0.138?fJ) of energy.

Giant voltage manipulation of MgO-based magnetic tunnel junctions via localized anisotropic strain: A potential pathway to ultra-energy-efficient memory technology

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.

Experimental demonstration of acoustic wave induced magnetization switching in dipole coupled magnetostrictive nanomagnets for ultralow power computing

Dipole-coupled cobalt nanomagnet pairs of elliptical shape (with their major axes parallel) are delineated on 128° Y-cut lithium niobate. Each pair is initially magnetized along the major axis with a magnetic field forming the (↑↑) state. When an acoustic wave (AW) is launched in the substrate from interdigitated electrodes, the softer nanomagnet in the pair flips to produce the (↑↓) state since the AW modulates the stress anisotropy. This executes the logical NOT operation because the output bit encoded in the magnetization state of the softer nanomagnet becomes the logic complement of the input bit encoded in the magnetization of the harder one. The AW acts as a clock to trigger the NOT operation and the energy dissipated is a few tens of aJ. Such AW clocking can be utilized to flip nanomagnets in a chain sequentially to steer logic bits unidirectionally along a nanomagnetic logic wire with miniscule energy dissipation.

Hybrid spintronics-straintronic nanomagnetic logic with two-state elliptical and four-state concave magnetostrictive nanomagnets

Recently, nanomagnetic logic has emerged as a promising alternative to transistor based logic because it offers both non-volatility and energy-efficiency. In particular, if the switching of the nanomagnets employs “straintronics” [1], whereby the magnetization of a multiferroic magnet is switched with a tiny voltage generating strain in a magnetostrictive-piezoelectric composite, the energy dissipated per bit flip can be reduced to a few hundred kT at room temperature. We had shown, in prior work, that a multiferroic nanomagnet with biaxial magnetocrystalline anisotropy has four stable magnetization orientations that can encode four states (Fig. 1a). Besides doubling the logic density (four-state versus two-state) for logic applications [2, 3], these four-state nanomagnets can be exploited for higher order applications such as image reconstruction and recognition in the presence of noise, associative memory and neuromorphic computing [4].

Experimental demonstration of strain-clocked Boolean nanomagnetic logic and information propagation

Nanomagnetic logic has emerged as a promising alternative to transistor based logic because it offers both non-volatility and energy-efficiency. Recent experiments by Bhowmik et al. [1] demonstrate energy-efficient magnetization switching in nanomagnets using the Spin Hall effect. Another switching paradigm claiming unprecedented energy-efficiency involves magnetization switching of the nanomagnets via “straintronics” [2], whereby the magnetization of a multiferroic magnet is switched with a tiny voltage generating strain in a magnetostrictive-piezoelectric composite (Fig. 1a). This scheme, proposed by our group, was previously shown to reduce the energy dissipated per bit flip to a few hundred kT at room temperature [2-4]. In this work, we show for the first time experimental results implementing some of these schemes, using elliptical magnetostrictive nanomagnets of nominal lateral dimensions ~200 nm and thickness ~12 nm that possess shape anisotropy and are grown on a (001) PMN-PT substrate (Fig. 1b). A voltage is applied along the length of the PMN-PT substrate to generate mechanical strain, via d33 coupling, along the nanomagnets easy axis of magnetization. The resulting strain-induced magnetization switching is investigated for single-domain nanomagnets and for clocking of dipole-coupled magnet arrays to implement Boolean logic using the schemes illustrated in Fig. 2. The strain clocking schemes used in our experiments are studied with Magnetic Force Microscopy (MFM) that is used to image the single domain magnetization switching and demonstrate strain clocked nanomagnetic logic for the first time [5]. These experimental results will be highlighted in this talk. Preliminary results are included here that show ferromagnetic (Fig. 3b) and anti-ferromagnetic ordering (Fig. 3c) in such nanomagnets.