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Ultrafast Magnetization Dynamics in High Magnetic Fields

Time: Tuesday, October 23rd, 12:00
Speaker: Anna POGREBNA, Nijmegen

The idea to change properties of media with the help of light has been intriguing people for a long time. Recently it was demonstrated that the laser induced ultrafast spin dynamics in ferrimagnetic GdFeCo can be dramatically influenced by the application of high magnetic fields. It was observed that across a critical field called the spin-flop field, the material shows distinct spin dynamics. The underlying mechanism was attributed to the spin dynamics associated with the two-different magnetic sublattices. However, these measurements were sensitive only to the iron magnetic sublattice. It has already been shown that, one can element specifically probe in the visible spectral range, the laser triggered spin dynamics in the ferrimagnetic TbFeCo. In this work we therefore consider TbFeCo to study the ultrafast spin dynamics associated with the individual magnetic sublattices across the spin-flop transition.
In our experiment, TbFeCo with out-of-plane magnetic anisotropy was studied in an out-of-plane magnetic field up to 28 T. Our study reveals anomalous hysteresis loops and strong field-dependence of the laser-induced dynamics in the vicinity of the compensation point in this ferrimagnet. In particular, the experiment shows that applying fields weaker than the exchange interaction between Tb and Fe, both sublattices show fast spin dynamics (≈ 200 fs), associated with laser-induced demagnetization. If the field is strong enough to trigger a spin-flop transition (≈ 13 T), both the sublattices show a dramatic slowing down of the spin dynamics. Here we explain these observations as manifestations of the first and second order phase transitions in this ferrimagnet.

Efficient all-optical helicity-dependent switching in Pt/Co/Pt with dual laser pulses

Time: Tuesday, October 23rd, 11:20
Speaker: Kihiro YAMADA, Nijmegen

Ultrafast magnetization switching with an ultra-short laser pulse attracts much attention for future applications for magnetic recording, operating with low energy consumption and at high speed. Since the pioneering work in ferrimagnetic GdFeCo [1], various systems have been tested, including metals [2], semiconductors [3], and insulators [4]. Recently, it has been demonstrated that also ferromagnetic multilayers display all-optical helicity-dependent switching (AO-HDS) [5], i.e., the magnetization can be controlled using circularly-polarized laser pulses. However, to observe switching, many pulses were required [6]. Here, we demonstrate that the number of pulses required for AO-HDS can be dramatically reduced by using a dual-pulse method. In the experiment, we used a single stack of Pt (3 nm)/Co (0.6 nm)/Pt (3 nm) with perpendicular magnetic anisotropy, which is very widely used for static as well as ultra-fast experiments in spintronics. In order to fully flip the magnetization, we used two pulses fulfilling different roles: the first short (70 fs) linearly-polarized laser pulse for demagnetizing and the second longer (3 ps) circularly-polarized laser pulse for switching. Five such pairs of pulses fully flip the magnetization for a time separation between the two laser pulses of 2 ps, and the fluence of the first and second laser pulse of 2.29 and 1.94 mJ/cm2, respectively. In stark contrast, using only long circularly polarized laser pulses, 100-150 pulses are still necessary. The results suggest that the dual-pulse method is a potential route towards realizing efficient AO-HDS in ferromagnetic metals on an ultrashort time scale.

[1] C. D. Stanciu et al., Phys. Rev. Lett. 99, 047601(2007).

[2] S. Mangin et al., Nat. Mater. 13, 286-292 (2014).

[3] A. J. Ramsay et. al., Phys. Rev. Lett. 114, 067202 (2015).

[4] A Stupakiewicz et. al., Nature 542, 71 (2017)

[5] C-H. Lambert et al., Science 345, 1337-1340 (2014).

[6] R. Medapalli et al., Phys. Rev. B 96, 224421 (2017).

On the subtle interplay of light induced magnetization, driven currents and heat in ultrafast magnetism

Time: Tuesday, October 23rd, 10:30
Speaker: Markus MÜNZENBERG, Greifswald

Based on many observations in a large number of experiments by now we have the different puzzle pieces to understand ultrafast magnetism and to develop new frontiers. THz spintronics and all-optical spin manipulation are becoming more and more feasible. We can build on first applications. Nevertheless the disentangling of different ultrafast processes is still a challenge. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces an delocalized, laser induced magnetization is more a local effect. But, in dependence of the sample this may be very different. I will go though some recent examples from our group to demonstrate the subtle interplay.

Merging femtomagnetism with spintronics– Laser-induced spin currents & all-optical switching of spintronic devices

Time: Tuesday, October 23rd, 9:20
Speaker: Bert KOOPMANS, Eindhoven

Novel schemes for controlling the ferromagnetic state at femtosecond time scales by pulsed laser excitation have received great interest. Driving systems into the strongly non-equilibrium regime, it has been shown possible not only to quench magnetic order, but also to switch the magnetization by single laser pulses – so-called all-optical switching (AOS). More recently, it has been proposed that pulsed laser excitation can also induce spin currents over several to tens of nanometers, which can act as an additional source of sub-picosecond magnetization dynamics. Thereby, an interesting link between the fields of ‘femtomagnetism’ and spintronic transport physics has emerged. In this presentation, further integration of all-optical magnetic control and spintronic will be explored in an attempt to develop hybrid spintronic-photonic devices.
After a general introduction into laser-induced magnetization dynamics, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address recent experiments that have demonstrated laser-induced spin transfer torqueon a free magnetic layer, using a collinear multilayer configuration consisting of a free in-plane layer on top of a PMA injection layer and separated by a nonmagnetic spacer [1]. As it will be shown, these non-collinear fs spin currents are absorbed within a few nanometers, and thereby provide ideal conditions for exciting THz spin waves. This allowed us to map out the dispersion of the frequency, , and the Gilbert damping, , of thin Co(B) layers [2,3].
In the second part, AOS in spintronically relevant structureswill be discussed. In particular, focus will be on highly efficient AOS and current-induced domain wall motion in the very same system: Pt/Co/Gd trilayers, displaying strong spin-orbit torques and Dzyaloshinskii-Moriya interaction. It will be shown that the magnetization of this synthetic ferrimagnetic thin film system can be reversed fully deterministically using single fs pulses. Threshold fluences are determined as a function of Co thickness and record-low efficiencies corresponding to below 50 fJ needed to switch a 50x50 nm²are found [3]. Simple modelling using a multi-layer version of the microscopic three-temperature model will be discussed. Moreover we quantitatively determined spin orbit torques and analyzed the coherent current-driven motion of opposite (up-down and down-up) domain walls. Finally, optically writing magnetic information ‘on-the-fly’ will be demonstrated. In these experiments, AOS is established while a constant electrical current is driving the magnetic information along a magnetic racetrack [4]. Such a scenario is envisioned to provide a route towards on-chip spintronic-photonic memories.

J. Schellekens, K.C. Kuiper, R.R.J.C. de Wit, and B. Koopmans, Nature Comm. 5, 4333 (2014).
L.M. Lalieu, P.L.J. Helgers, and B. Koopmans, Phys. Rev. B 96, 014417 (2017).
L.M. Lalieu et al., to be published.
L.M. Lalieu, M.J.G. Peeters, S.R.R. Haenen, R. Lavrijsen, and B. Koopmans, Phys. Rev. B 96, 220411(R) (2017).
L.M. Lalieu et al., to be published.

08.10.2018 – Spintronics meets Neuromorphics

Spintronics meets Neuromorphics

An entire suite of novel Neuromorphic computational paradigms, taking inspiration from the functional properties of the brain, has emerged over the past two decades to address the need to efficiently process and analyze the exponential amount of data produced in our Information Age. Research on neural networks, reservoir computers and Boltzmann machines, has demonstrated that it is possible to perform complex computational tasks such as image and pattern recognition at a level comparable to that of a human. All proof-of-concepts have however relied mostly on digital implementations of their respective computational scheme. Whereas this has justified the importance of such techniques, their implementation into scalable and energy efficient analog electronic devices is still much of an open problem. The workshop “Spintronics meets Neuromorphics” aims to show how the challenges posed by neuromorphic computing paradigms can be addressed effectively with spintronics. The low-current tunability, thermal susceptibility and rich dynamics of magnetic thin-film heterostructures offer an ideal toolbox for implementing novel neuromorphic devices. Furthermore, progress in their material science guarantees that promising proof-of-concepts will have a high chance of proving scalable enough to afford industrial production.

For videos of the talks and further information, please visit the workshop home page.

Spintronic devices for unconventional computing

Time: Friday, October 12th, 11:40
Speaker: Giovanni FINOCCHIO, UNIME

The recent availability of big data and computational power (Graphic Processing Unit) lunched the application of deep learning, machine learning and artificial intelligence (AI) in real-life applications. Most of the solutions are software oriented and the time required for the setting proper parameters of the neural networks need a lot of time. Among all the possible hardware implementations, spintronics seems to be a candidate for that having the potential to work at low power and the possibility to exhibits strong non-linearity that is fundamental for the performance of AI systems.
In this talk, I will discuss recent results achieved in Messina in the development of spintronic devices that can be used for unconventional computing, development of AI, and neuromorphic applications. The first part of the talk will be oriented to the presentation of spin-Hall random bit generators and their possible impact in computing and security. Finally, I will discuss some possible configurations to use antiferromagnetic devices and skyrmions for unconventional computing and for approaching problem that can be converted in a topological network.

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Realization of the Hopfield model in finite size Ising systems with RKKY type interactions

Time: Friday, October 12th, 11:00
Speaker: Alexander Ako KHAJETOORIANS, Radboud University

We demonstrate that a two-dimensional periodic array of spins coupled via RKKY-like exchange can exhibit tunable energy landscapes ranging from robust double-well toward spin glasses. We characterize the magnetic ground states and energy landscapes of such arrays, by the distribution of low energy states, energy barriers, aging relaxation dynamics and the size of the basins of attraction. We identify three phases resulting from singularly varying the RKKY period: a double well phase, a spin glass phase and a multi-well phase. The spin glass behavior results from self-induced glassiness, which is driven by the incommensurability of the RKKY period and the periodic array. We address these different phases considering scaling and aging arguments. Finally, we argue that the tunable complexity of these spin arrays can function as an associative memory (Hopfield network) at the atomic scale, and illustrate these memories for a given lattice and a scheme for pattern recognition.

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Perspectives on 3D spintronics

Time: Friday, October 12th, 10:10
Speaker: Amalio FERNANDEZ-PACHECO, University of Cambridge

Three-dimensional nanomagnetism is a new and exciting area of research focused on investigating nanomagnets that extend beyond the standard planar configuration [1]. In these systems, with unconventional geometries and spin interactions, new physical effects emerge, with geometry, topology and chirality becoming interlinked, which paves the way to novel devices with functionalities beyond the substrate plane. Specifically, future non-volatile 3D devices are expected to have ultra-high storage densities and very high interconnectivity, of great interest for future neuromorphic computing devices [2].
The leap to 3D is extremely complex from a fabrication and characterisation point of view, but it is starting to become possible thanks to new nanotechnology tools suitable to create and probe 3D geometries and spin configurations [3]. In this talk, I will review the state of the art of 3D nanomagnetism and will present some of our recent work on 3D magnetic nanostructures for applications in spintronics.
The introduction of novel 3D nano-printing methods based on focused electron beams is allowing us for the first time to prototype complex 3D nanomagnets [4-6]. In particular, the exploitation of auto-catalytic effects can be used to create diameter-modulated suspended nanowires [7], where advanced X-ray magnetic microscopy experiments reveal the presence of skyrmionic tubes during their magnetic reversal, in agreement with recent theoretical predictions [8]. Additionally, we have successfully injected domain walls from the substrate plane onto 3D Permalloy “ramped” nanowires interconnected to the substrate [9], a pioneering work comprising the development of a new 3D magneto-optical nano-magnetometry method exploiting dark-field effects. This work opens an exciting new route to advanced operation of 3D domain wall conduits.
I acknowledge funding from EPSRC grants EP/M008517/1 and EP/L015978/1, from the Cambridge Winton Program for the Physics of Sustainability, the Royal Society and EU funding via the COST action CELINA.
[1] Fernández-Pacheco et al, Nature Comm. 8, 15756 (2017).
[2] Torrejón et al, Nature 547, 428 (2017).
[3] Donnelly et al, Nature 547, 328 (2017).
[4] Sanz-Hernández et al, Beilstein J. Nanotechnol. 8, 2151 (2017).
[5] Fowlkes et al, ACS Appl. Nano Mater. 1, 1028 (2018).
[6] Sanz-Hernández et al, Nanomaterials 8, 483 (2018).
[7] Pablo-Navarro et al. J. Phys. D: Appl. Physics 50, 18LT01 (2017).
[8] Charilaou et al, arXiv:1711.03511 (2017).
[9] Sanz-Hernández et al, ACS Nano 11, 11066 (2017).

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Tutorial: p-bits for Probabilistic Spin Logic

Time: Friday, October 12th, 09:00
Speaker: Kerem CAMSARI, Purdue University

Digital computers are built out of bits that represent information in deterministic states 0 and 1. At the other end, quantum computers are built out of q-bits that represent information as delicate superpositions of 0 and 1. In this talk, I will introduce our body of work on probabilistic bits (p-bit) that randomly fluctuate between 0 and 1, placing p-bits conceptually in between deterministic and quantum bits. We have shown that hardware p-bits can be realized with present day, room temperature devices using magnetic and non-magnetic components and p-circuits built of interconnected p-bits can be useful for a wide variety of real world applications that are inherently probabilistic, just as quantum bits are naturally suited for inherently quantum mechanical problems.

We have shown that p-circuits can be broadly relevant for Quantum Computing and Machine Learning inspired problems. In the context of Machine Learning, p-bits can function as low-level representations of Binary Stochastic Neurons (BSN), therefore they can be used to realize efficient hardware implementations for Bayesian and Inference Networks, as well as hardware accelerators for BSN-based statistical learning algorithms. In the context of Quantum Computing, p-bits can be used for optimization problems, covering both classical and quantum annealing, to solve hard problems such as Traveling Salesman and Integer Factorization. Inverse problems such as Integer Factorization are enabled by the "invertibility" feature of p-circuits, where a logic gate not only finds the appropriate output for a set of fixed inputs but also the appropriate inputs for a fixed set of outputs. I will illustrate each application by a representative example either by using experimentally benchmarked device models simulated in circuit simulators such as SPICE or by hardware implementations of p-circuits using non-magnetic building blocks.

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Unconventional computing with stochastic magnetic tunnel junctions

Time: Thursday, October 11th, 15:30
Speaker: Alice MIZRAHI, NIST

Magnetic tunnel junctions are bi-stable nanodevices which magnetic state can be both read and written electrically. Their high endurance, reliability and CMOS-compatibility have made them flagship devices for novel forms of computing. While they are mostly used as non-volatile binary memories, they can be made unstable and thus behave as stochastic oscillators. Here, we show how stochastic magnetic tunnel junctions are promising elements for low energy implementations of unconventional computing. In this goal, we present several uses of these devices.
First, an analogy can be drawn between stochastic magnetic tunnel junctions and stochastic spiking neurons. We apply neuroscience computing paradigm to these devices and demonstrate that they can be the building blocks of low energy artificial neural networks capable of on-chip learning.
Then, we demonstrate that these stochastic oscillators are capable of harnessing noise to synchronize on a source at much lower energy consumption than deterministic oscillators. This opens the way to low energy implementations of synchronization based computing.
Finally, we show that these devices are low energy true random number generators, which can be used for various applications such as cryptography and stochastic computing.

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