Browsing by Author "Zhou, Yifan"

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Bioinspired multisensory neural network with crossmodal integration and recognition
(2021-02-18) Tan, Hongwei; Zhou, Yifan; Tao, Quanzheng; Rosen, Johanna; van Dijken, Sebastiaan
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The integration and interaction of vision, touch, hearing, smell, and taste in the human multisensory neural network facilitate high-level cognitive functionalities, such as crossmodal integration, recognition, and imagination for accurate evaluation and comprehensive understanding of the multimodal world. Here, we report a bioinspired multisensory neural network that integrates artificial optic, afferent, auditory, and simulated olfactory and gustatory sensory nerves. With distributed multiple sensors and biomimetic hierarchical architectures, our system can not only sense, process, and memorize multimodal information, but also fuse multisensory data at hardware and software level. Using crossmodal learning, the system is capable of crossmodally recognizing and imagining multimodal information, such as visualizing alphabet letters upon handwritten input, recognizing multimodal visual/smell/taste information or imagining a never-seen picture when hearing its description. Our multisensory neural network provides a promising approach towards robotic sensing and perception.
Driven gyrotropic skyrmion motion through steps in magnetic anisotropy
(2019-04-25) Zhou, Yifan; Mansell, Rhodri; van Dijken, Sebastiaan
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The discovery of magnetic skyrmions in ultrathin heterostructures has led to great interest in possible applications in memory and logic devices. The non-trivial topology of magnetic skyrmions gives rise to a gyrotropic motion, where, under an applied energy gradient a skyrmion gains a component of motion perpendicular to the applied force. So far, device proposals have largely neglected this motion or treated it as a barrier to correct operation. Here, we show that skyrmions can be efficiently moved perpendicular to an energy step created by local changes in the perpendicular magnetic anisotropy. We propose an experimentally-realizable skyrmion racetrack device which uses voltage-controlled magnetic anisotropy to induce a step in magnetic anisotropy and drive a skyrmion unidirectionally using alternating voltage pulses.
Electronic and magnetic characterization of epitaxial VSe2 monolayers on superconducting NbSe2
(2020-06-26) Kezilebieke, Shawulienu; Huda, Md Nurul; Dreher, Paul; Manninen, Ilkka; Zhou, Yifan; Sainio, Jani; Mansell, Rhodri; Ugeda, Miguel M.; van Dijken, Sebastiaan; Komsa, Hannu Pekka; Liljeroth, Peter
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
There has been enormous recent interest in heterostructures of two-dimensional van der Waals materials. Integrating materials with different quantum ground states in vertical heterostructures is predicted to lead to novel electronic properties that are not found in the constituent layers. Here, we present direct synthesis of a superconductor-magnet hybrid heterostructure by combining superconducting niobium diselenide (NbSe2) with the monolayer vanadium diselenide (VSe2). Molecular-beam epitaxy growth in ultra-high vacuum yields clean and atomically sharp interfaces. Combining different characterization techniques and density-functional theory calculations, we investigate the electronic and magnetic properties of VSe2 on NbSe2. Low temperature scanning tunneling microscopy measurements show an absence of the typical charge density wave on VSe2 and demonstrate a reduction of the superconducting gap of NbSe2 on the VSe2 layer. This suggests magnetization of the VSe2 sheet, at least on the local scale. Our work demonstrates superconducting-magnetic hybrid materials with potential applications in future electronics devices.
Optically controlled large-coercivity room-temperature thin-film magnets
(2021-12-23) Philip, Anish; Zhou, Yifan; Tewari, Girish C.; van Dijken, Sebastiaan; Karppinen, Maarit
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Photo-controlled room-temperature hard magnets could open new horizons for high-density information storage. For this, the material should be fabricated as device-integrable (conformal, stretchable, transparent,etc.) thin films and preferably from readily available and innocuous chemical constituents. Here we report a viable material candidate to address all these criteria. The material basis is the ferrimagnetic ε-Fe 2O 3polymorph of trivalent iron oxide with an exceptionally high room-temperature magnetic coercivity. To bring the photo-controllability, azobenzene moieties withtrans-cisisomerization tendency are embedded into this matrix as regular monomolecular layers. The strongly emerging atomic/molecular layer deposition (ALD/MLD) technique offers us a scientifically elegant yet industrially feasible tool to fabricate these superlattice thin films with nanoscale precision. We demonstrate reversible changes in both coercivity and magnetization values with alternating irradiations of the films with UV and visible light. Interestingly, the azobenzene layers not only add this switching functionality but also enhance the overall magnetic performance of the ε-Fe 2O 3matrix.
Random thermal motion and voltage control of magnetic skyrmions
(2021) Zhou, Yifan
School of Science | Doctoral dissertation (article-based)
Digital data permeates all aspects of daily life, and magnetic materials provide the dominant medium of the digital data storage. However, current magnetic storage devices suffer from high energy consumption. In order to improve the energy efficiency, magnetic skyrmions are being intensively investigated as a promising candidate for future magnetic devices. Magnetic skyrmions show advantage of being stable at room temperature with a small size and high mobility. Previous research on skyrmions has focused on devices driven by electric currents, which still require large energy consumption per bit due to the Joule heating. In this thesis, two alternatives to manipulate skyrmions in thin magnetic films are explored: by thermal fields and by applying voltages. In both cases, the heating by electric currents is mitigated. The results of this thesis, based on magnetic thin films hosting skyrmions, cover four aspects: the methodology of extracting magnetic parameters at different temperatures, the random thermal motion of skyrmions in granular films, the voltage control of the creation and annihilation of skyrmions, and skyrmion dynamics guided by anisotropy steps. In the first topic, the correlations between magnetic parameters that determine the behavior of skyrmions are investigated. By performing experiments as a function of temperature and utilizing different models, linear correlations between the strength of the Dzyaloshinskii-Moriya interaction, the perpendicular magnetic anisotropy, and the exchange stiffness are demonstrated. Combined with a model of skyrmion stability, the results imply the skyrmions should be thermally stable over a wide range of temperature. Secondly, using micromagnetic simulations, it is shown that interactions between skyrmions in dense skyrmion arrays increase the skyrmion diffusion coefficient in thin films without grains. Meanwhile, grains in thin magnetic films, which are omnipresent in experiments, reduce the skyrmion diffusion coefficient most when the grain size and skyrmion diameter are similar. For larger grains, random thermal motion leads to the clustering of skyrmions in grains with lower magnetic anisotropy. Thirdly, voltage control of the creation and annihilation of skyrmions in a GdOx/Gd/Co/Pt heterostructure is demonstrated. Micromagnetic simulations are used to explore how changes in the magnetic anisotropy and Dzyaloshinskii-Moriya interaction affect the skyrmion density. The voltage effects are shown to occur on a few-second time scale, which suggests voltage-induced changes in the orbital filling at the Co/GdOx interface as the most likely origin. Finally, by exploiting voltage control of magnetic anisotropy, the motion of skyrmions at magnetic anisotropy steps is simulated. Based on these results, a voltage-controlled device utilizing the gyrotropic motion of skyrmions is proposed.
Spontaneous creation and annihilation dynamics of magnetic skyrmions at elevated temperature
(2021-08-13) Wang, Junlin; Strungaru, Mara; Ruta, Sergiu; Meo, Andrea; Zhou, Yifan; Deák, András; Szunyogh, László; Gavriloaea, Paul Iulian; Moreno, Roberto; Chubykalo-Fesenko, Oksana; Wu, Jing; Xu, Yongbing; Evans, Richard F.L.; Chantrell, Roy W.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Skyrmions are topologically protected nanoscale magnetic structures with a wide range of potential applications. Here we determine the life cycle of skyrmions from their creation to their intrinsic dynamics and thermal stability to their eventual thermodynamic demise. Using atomistic simulations of Ir/Co/Pt, parameterized from ab initio calculations, we demonstrate the thermal phase transition to a skyrmion state under application of a perpendicular magnetic field. The created skyrmions exhibit Brownian particlelike dynamics driven by the underlying thermal spin fluctuations. At an elevated temperature window well below the Curie temperature, the skyrmions are metastable and can collapse to a uniform magnetic state. With application of an external magnetic field, the intrinsic local thermal spin fluctuations at this elevated temperature window are sufficiently large to allow the spontaneous formation of new skyrmions in thermodynamic equilibrium analogous to a spontaneous skyrmion gas. Such a system could be used to implement a skyrmion-based true random number generator.
Tactile sensory coding and learning with bio-inspired optoelectronic spiking afferent nerves
(2020-03-13) Tan, Hongwei; Tao, Quanzheng; Pande, Ishan; Majumdar, Sayani; Liu, Fu; Zhou, Yifan; Persson, Per O.Å.; Rosen, Johanna; van Dijken, Sebastiaan
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The integration and cooperation of mechanoreceptors, neurons and synapses in somatosensory systems enable humans to efficiently sense and process tactile information. Inspired by biological somatosensory systems, we report an optoelectronic spiking afferent nerve with neural coding, perceptual learning and memorizing capabilities to mimic tactile sensing and processing. Our system senses pressure by MXene-based sensors, converts pressure information to light pulses by coupling light-emitting diodes to analog-to-digital circuits, then integrates light pulses using a synaptic photomemristor. With neural coding, our spiking nerve is capable of not only detecting simultaneous pressure inputs, but also recognizing Morse code, braille, and object movement. Furthermore, with dimensionality-reduced feature extraction and learning, our system can recognize and memorize handwritten alphabets and words, providing a promising approach towards e-skin, neurorobotics and human-machine interaction technologies.
Temperature dependence of the Dzyaloshinskii-Moriya interaction in ultrathin films
(2020-02-24) Zhou, Yifan; Mansell, Rhodri; Valencia, Sergio; Kronast, Florian; van Dijken, Sebastiaan
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The Dzyaloshinskii-Moriya interaction gives rise to a chiral exchange between neighboring spins in the technologically relevant class of perpendicularly magnetized ultrathin film materials. In this paper, we study the temperature dependence of the Dzyaloshinskii-Moriya interaction based on extensive characterization of a thin film which hosts a skyrmion state using both bulk magnetometry and x-ray magnetic circular dichroism photoemission electron microscopy. A version of the Bloch law explicitly for thin film geometries is derived to extract the exchange stiffness. The strength of the Dzyaloshinskii-Moriya interaction, D, is found to have a dependence on the saturation magnetization, M-s of D proportional to M-s(1.86 +/- 0.16). Further, by extracting the uniaxial anisotropy K-u and the exchange stiffness A, we find that D proportional to K-u(1.02 +/- 0.11) and D proportional to A(0.95 +/- 0.07). Skyrmion radii are also used to extract the strength of the Dzyaloshinskii-Moriya interaction which is compared to that derived from measurements of stripe domains. The origins of the correlations between material parameters are discussed and consequences of these relationships for skyrmion devices are considered.
Thermal motion of skyrmion arrays in granular films
(2021-10-21) Zhou, Yifan; Mansell, Rhodri; Ala-Nissila, Tapio; Van Dijken, Sebastiaan
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Magnetic skyrmions are topologically distinct swirls of magnetic moments which display particlelike behavior, including the ability to undergo thermally driven diffusion. In this paper we study the thermally activated motion of arrays of skyrmions using temperature dependent micromagnetic simulations where the skyrmions form spontaneously. In particular, we study the interaction of skyrmions with grain boundaries, which are a typical feature of sputtered ultrathin films used in experimental devices. We find the interactions lead to two distinct regimes. For longer lag times the grains lead to a reduction in the diffusion coefficient, which is strongest for grain sizes similar to the skyrmion diameter. At shorter lag times the presence of grains enhances the effective diffusion coefficient due to the gyrotropic motion of the skyrmions induced by their interactions with grain boundaries. For grain sizes significantly larger than the skyrmion diameter clustering of the skyrmions occurs in grains with lower magnetic anisotropy.
Voltage control of skyrmions: Creation, annihilation, and zero-magnetic field stabilization
(2021-04-26) Zhou, Yifan; Mansell, Rhodri; van Dijken, Sebastiaan
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Voltage manipulation of skyrmions is a promising path toward low-energy spintronic devices. Here, voltage effects on skyrmions in a GdOx/Gd/Co/Pt heterostructure are observed experimentally. The results show that the skyrmion density can be both enhanced and depleted by the application of an electric field, along with the ability, at certain magnetic fields to completely switch the skyrmion state on and off. Further, a zero magnetic field skyrmion state can be stabilized at a negative bias voltage using a defined voltage and magnetic field sequence. The voltage effects measured here occur on a few-second timescale, suggesting an origin in voltage-controlled magnetic anisotropy rather than ionic effects. By investigating the skyrmion nucleation rate as a function of temperature, we extract the energy barrier to skyrmion nucleation in our sample. Further, micromagnetic simulations are used to explore the effect of changing the anisotropy and Dzyaloshinskii-Moriya interaction on skyrmion density. Our work demonstrates the control of skyrmions by voltages, showing functionalities desirable for commercial devices.
Weakly pinned skyrmion liquid in a magnetic heterostructure
(2022-08-10) Mansell, Rhodri; Zhou, Yifan; Kohvakka, Kassius; Ying, See Chen; Elder, Ken R.; Granato, Enzo; Ala-Nissila, Tapio; Van Dijken, Sebastiaan
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Magnetic skyrmions are topologically distinct particles whose thermally activated motion could be used to implement probabilistic computing paradigms. While solid-liquid phase transitions in skyrmion lattices have been demonstrated, the behavior of a skyrmion liquid and the effects of pinning are largely unknown. Here we demonstrate the formation of a weakly pinned skyrmion liquid in a magnetic heterostructure. By inserting a Ru wedge layer at the ferromagnet/heavy metal interface we evaluate the dependence of skyrmion dynamics on the skyrmion size and density. Our experiments demonstrate that the diffusion of skyrmions is largest in dense liquids with small skyrmions. The thermal motion of skyrmions at room temperature easily overcomes the narrow distribution of pinning site energies in the granular film structure, satisfying a key requirement of probabilistic device architectures. Micromagnetic simulations support the findings and also reveal the existence of a thermally activated high-frequency collective oscillation.