Supervised learning of an opto-magnetic neural network with ultrashort laser pulses

Abstract

The explosive growth of data and its related energy consumption is pushing the need to develop energyefficient brain-inspired schemes and materials for data processing and storage. Here, we demonstrate experimentally that Co/Pt films can be used as artificial synapses by manipulating their magnetization state using circularly-polarized ultrashort optical pulses at room temperature. We also show an efficient implementation of supervised perceptron learning on an opto-magnetic neural network, built from such magnetic synapses. Importantly, we demonstrate that the optimization of synaptic weights can be achieved using a global feedback mechanism, such that the learning does not rely on external storage or additional optimization schemes. These results suggest there is high potential for realizing artificial neural networks using optically-controlled magnetization in technologically relevant materials, that can learn not only fast but also energy-efficient.The rapid growth of modern information and communication technology (ICT) has led to an enormous increase in energy consumption, which is already now around 7% of the global electrical energy production. Owing to the inherently energy-efficient brain-inspired computing principles, implementing such neuromorphic architectures offers enormous potential to dramatically reduce the energy consumption of ICT. Magnetic materials are already at the center of computing today, due to their ability to store information within the direction of magnetic moments in a non-volatile and rewritable way. In recent years, tremendous progress has been made in controlling magnetism with femtosecond optical pulses, including demonstrations of record-breaking fast write-read events 1 , operation in technologically relevant materials such as Co/Pt 2 and enabling magnetic recording that is not only much faster but also exhibits a projected heat load of only 22 aJ per magnetic bit 3 . These demonstrations suggest that all-optical manipulation of magnetism offers great potential to realize neural networks that can be trained not only much faster but also much more efficiently than all-electrical material implementations under development today 4-9 . In particular, similar as for purely optical neural networks 10-12 operation proceeds ultrafast and at very low energy cost. Moreover, combining this with magnetism integrates inherent non-volatility and optical adaptability, which potentially yields fast and energy-efficient learning as well. However, so far it has not been demonstrated that it is possible to exploit optical control of magnetism for brain-inspired computing. Here, we study the control of the magnetization state of Co/Pt thin films in response to picosecond optical pulses. We experimentally demonstrate that the cumulative all-optical switching process in these materials 13,14 provides an energy-efficient mechanism for realizing artificial synapses, in which the internal state of the synapse is stored with non-volatility and can be controlled continuously and reversibly using the h...

Time-Resolved XUV Absorption Spectroscopy and Magnetic Circular Dichroism at the Ni M2,3-Edges

“…The problem is interesting from a fundamental perspective-to date, it is not clear how angular momentum is transferred between the spin and electron systems and the crystalline lattice on sub-picosecond time scales-but it is also technologically relevant, as using light to steer magnetization on sub-ps timescales might pave the way for ultrafast all-optical spintronics and data storage [8][9][10]. Recent developments illustrating the application potential of ultrafast optical excitations include, for example, the creation of artificial neural networks [11], or the demonstration that focusing grating couplers can be used for photonic-electronic integration with magnetoresistive random access memory technology [12].…”

Efficient All-Optical Helicity Dependent Switching of Spins in a Pt/Co/Pt Film by a Dual-Pulse Excitation

“…We prepared a Ta (4 nm)/Pt (3.0 nm)/Co (0.6 nm)/Pt (3.0 nm)/ MgO (2.0 nm)/Ta (1.0 nm) multilayer on a synthetic quartz glass substrate by magnetron sputtering. Pt/Co/Pt systems are relevant for testing the dual-pulse approach because they are typical candidates for spintronic devices (Brataas et al, 2012) as well as for all-optical helicity dependent switching (Lambert et al, 2014;Hadri et al, 2016a;Hadri et al, 2016b;Medapalli et al, 2017;Parlak et al, 2018;Quessab et al, 2018;Chakravarty et al, 2019;Kichin et al, 2019;Cheng et al, 2020). DC and RF sources were used for depositing Ta, Pt, and Co, and MgO, respectively.…”

“…This is the reason why alloptical switching in ferromagnets attracts great attention. Ferromagnets, such as Pt/Co/Pt multilayers (Lambert et al, 2014;Hadri et al, 2016a;Hadri et al, 2016b;Medapalli et al, 2017;Parlak et al, 2018;Quessab et al, 2018;Chakravarty et al, 2019;Kichin et al, 2019;Cheng et al, 2020) and granular FePt alloys (Lambert et al, 2014;Takahashi et al, 2016;John et al, 2017), have recently shown all-optical helicity-dependent switching (AO-HDS), magnetization switching depending on the circular polarization of the optical excitation pulses. However, AO-HDS generally requires at least tens and usually hundreds of laser pulses for full switching (Kichin et al, 2019).…”

Ultrafast optical manipulation of magnetic order in ferromagnetic materials

“…Taking advantage of laser-induced contributions, such as harnessing spin-polarized hot electrons [ 107 , 108 ] and enhancing opto-magnetic effect [ 116 ], could help us to further push AO-HDS of ferromagnetic materials. On the other hand, the cumulative AOS may benefit the design of neuromorphic architectures [ 128 , 129 ]. To achieve a high-density recording, AOS of nanoscale magnetic nanostructures instead of continuous media should be considered.…”