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On-line SPICE-SPIN+X Seminars

On-line Seminar: 23.09.2020 - 15:00 (CET)

Spintronics Nanodevice
- How small can we make it and what else can we use it for -

Hideo Ohno, Tohoku University

Development of spintronics nonvolatile nanodevices and their integration with CMOS circuits has resulted in realizing low-energy, yet high performance integrated circuits suitable for a number of applications such as Internet-of-Things (IoT), high-performance computing and artificial intelligence. Magnetic tunnel junction (MTJ), a spintronics device, plays a central role here, which has been shown to scale down to 20 nm with the perpendicular-easy-axis CoFeB-MgO system [1, 2]. I will first discuss the factors that limit the scalability of such MTJs. Then show how one can extend its scalability to the range of 4-8 nm and below [3, 4] by employing a new (and yet not so new) concept. Current-induced switching of magnetization and high thermal stability of these devices are also shown. I will then describe how one can use less stable MTJs for a novel form of computation, probabilistic computing, to address optimization problems. I show that one can formulate integer factorization as an optimization problem in such a way that the most preferred state in terms of energy gives the factorized result [5]. If I have time I will touch upon proof-of-concept spintronics devices for artificial synapse as well as neuron for neuromorphic applications [6, 7].
Work done in collaboration with S. Fukami and the CSIS team. A portion of the work described here is a result of collaboration with A. Z. Pervaiz, K. Y. Camsari, and S. Datta of Purdue University. Supported in part by the ImPACT Program of CSTI, JST-OPERA JPMJOP1611 and Grant-in-Aid for Specially Promoted Research (17H06093).

References
[1] S. Ikeda, et al. Nature Materials, 9, 721 (2010).
[2] H. Sato, et al. IEDM 2013 and Appl. Phys. Lett. 105, 062403 (2014).
[3] K. Watanabe, et al. Nature Commun. 9, 663 (2018).
[4] B. Jinnai, et al. Appl. Phys. Lett. (Perspective), 116, 160501 (2020).
[5] W. A. Borders, et al. Nature 573, 390-393 (2019).
[6] W. A. Borders et al. Appl. Phys. Express 10, 013007 (2017).
[7] A. Kurenkov, et al. Advanced Materials 31, 1900636 (2019).

PDF file of the talk available here

Tuning the exchange and potential scattering strength of individual magnetic adsorbates on superconductors

Katharina Franke

Magnetic impurities in conventional superconductors induce a pair-breaking potential, which leads to bound states inside the superconducting energy gap. These states are called Yu-Shiba-Rusinov (YSR) states, and can be probed by scanning tunneling spectroscopy at the atomic scale. The energy of these states depends on the strength of both exchange and potential scattering. The individual YSR states can be regarded as the building blocks for topological superconductivity in adatom chains on conventional superconductors.
Here, we explore different strategies to tune the energy of the YSR states. In the first case, we tune the strength of the magnetic exchange scattering to the Cooper pairs. Upon tip approach we are able to continuously vary the energy of YSR states induced by Fe-porphin molecules on Pb(111) across the Fermi energy. This model system further allows to study the quantum phase transition between a screened and unscreened spin [1].
In the second case, we make use of the charge-density wave (CDW) of NbSe2, which coexists with superconductivity, to tune the energy of YSR states of individual Fe atoms. All atoms are placed in the same atomic adsorption site, but at different positions with respect to the CDW. The YSR states exhibit different energies and different oscillatory patterns. We ascribe the shift in energies to the variation of the density of states as well as to changes in the potential scattering strength [2]. These results are important for designing topological nanostructures.

[1] L. Farinacci, G. Ahmadi, G. Reecht, M. Ruby, N. Bogdanoff, O. Peters, B. W. Heinrich, F. von Oppen, K. J. Franke, Phys. Rev. Lett. 121, 196803 (2018).
[2] E. Liebhaber, S. Acero Gonzalez, R. Baba, G. Reecht. B. W. Heinrich, S. Rohlf, K. Rossnagel, F. von Oppen, K. J. Franke, Nano Lett. 20, 339 (2020).

Magnetic exchange through s- and d-wave superconductors

Sachio Komori

At a ferromagnet / superconductor interface, a magnetic exchange field can couple with the superconducting state. For the case of an s-wave (isotropic) superconductor, the coupling manifests as a spin-splitting of the superconducting density of states which decays in the superconductor over the Cooper pair coherence length which is tens of nanometers in Nb. In a d-wave (anisotropic) superconductor, the Cooper pair coherence length is spatially anisotropic and sub-nanometre in all directions meaning magnetic coupling is equivalently short ranged. In this talk I will present our recent experiments on investigating magnetic coupling at ferromagnet / superconductor interfaces with s-wave (Nb) and d-wave (YBa2Cu3O7) superconductors. For Nb, superconducting spin-transport based on triplet Cooper pairs is blocked in the singlet superconducting state and rapidly-suppressed in the normal state due to spin-orbit scattering [1]. In YBCO, magnetic exchange field are found to be long-ranged, penetrating tens of coherence lengths due to the quasiparticle nodal states [2]. The results demonstrate dynamic coupling between unconventional superconductivity and magnetism.

[1] S. Komori et al., arXiv:2006.16654
[2] A. Di Bernardo et al., Nat. Mater. 18, 1194 (2019)

Unconventional superconductivity and magnetic-related states induced in a conventional superconductor by nonmagnetic chiral molecules

Oded Millo

Hybrid ferromagnetic/superconducting systems are well known for hosting intriguing phenomena such as emergent triplet superconductivity at their interfaces and the appearance of in-gap, spin-polarized Yu-Shiba-Rusinov (YSR) surface-states bound to magnetic impurities. In this work we demonstrate that similar phenomena can be induced on a surface of a conventional superconductor upon chemisorbing non-magnetic chiral molecules. By applying scanning tunneling spectroscopy, we show that the singlet-pairing s-wave order parameter of Nb, NbN and NbSe2 is significantly altered upon the adsorption of chiral polyalanine alpha-helix molecules on the surface. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of a triplet-pairing component, corroborated by fits to theoretical spectra. Conductance spectra measured on devices comprising NbSe2 flakes over which these chiral molecules were adsorbed, exhibit, in some cases, in-gap states nearly symmetrically positioned around zero bias. These states shift apart with magnetic field, akin to YSR states, as corroborated by theoretical simulations. Other samples show evidence for a collective phenomenon of hybridized YSR-like states giving rise to unconventional, possibly triplet superconductivity, manifested in the conductance spectra by the appearance of a zero bias conductance peak that diminishes, but does not split, with magnetic field. The transition between these two scenarios appears to be governed by the density of adsorbed molecules. Chiral molecules were also found to have a unique signature on the TC of Nb and NbRe films when linking Au nanoparticles to them. Finally, low-energy muon spin rotation (LE-μSR) data demonstrate clear evidence for a strong modification of the screening supercurrent distribution deep inside a Nb film upon adsorption of chiral molecules, providing evidence for unconventional chiralinduced superconductivity. The adsorption-modified local magnetic field profile inside the (65 nm thick) Nb film monitored by LE-μSR, a measure of the screening modification, is well fitted to a model calculation where the chiral molecules layer is considered as an insulating spin-active interface that is proximity-coupled to the Nb film.

The work was done in collaboration with the following groups: Yossi Paltiel, Hen Alpern, Nir Sukenik, Tamar Shapira, Shira Yochelis (The Hebrew University of Jerusalem), Jason Robinson, Harry Bradshaw (University of Cambridge), Angelo Di Bernardo, Elke Scheer (Konstanz University), Jacob Linder (Norwegian University of Science and Technology).

On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.09.2020 - 15:00 (CET)

Spintronic devices for artificial neural networks

Saima Siddiqui, University of Illinois

Spintronics promises intriguing device paradigms where electron spin is used as the information token instead of its charge counterpart. In the future cognitive era, nonvolatile magnetic memories hold the key to solve the bottleneck in the computational performance due to data shuttling between the processing and the memory units. The application of spintronic devices for these purposes requires versatile, scalable device design that is adaptable to emerging material physics. We design, model and experimentally demonstrate spin orbit torque induced magnetic domain wall devices as the building blocks (i.e. linear synaptic weight generator and the nonlinear activation function generator) for in-memory computing, in particular for artificial neural networks. Spin orbit torque driven magnetic tunnel junctions show great promise as energy efficient emerging nonvolatile logic and memory devices. In addition to its energy efficiency, we take advantage of the spin orbit torque induced domain wall motion in magnetic nanowires to demonstrate the linear change in resistance of the synaptic devices. Modifying the spin-orbit torque from a heavy metal or utilizing the size dependent magnetoresistance of tunnel junctions, we also demonstrate a nonlinear activation function for thresholding signals (analog or digitized) between layers for deep learning. A complete neuromorphic hardware accelerator using embedded nonvolatile magnetic domain wall devices can revolutionize computer architectures by embedding memory into logic circuits in a fine grained fashion.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 09.09.2020 - 15:00 (CET)

Long-Range Phonon Spin Transport

Rembert Duine, Utrecht University

One of the goals of spintronics is to achieve dissipationless spin currents. In this talk, I will discuss phonon spin transport in an insulating ferromagnet-nonmagnet-ferromagnet heterostructure. I will discuss how the magnetoelastic interaction between the spins and the phonons leads to nonlocal spin transfer between the magnets. This transfer is mediated by a local phonon spin current and accompanied by a phonon spin accumulation. The spin conductance depends nontrivially on the system size, and decays over millimeter length scales for realistic material parameters, far exceeding the decay lengths of magnonic spin currents.

 

 

 

 

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 19.08.2020 - 15:00 (CET)

Antiferromagnetic Insulatronics: Spintronics without magnetic fields

Mathias Kläui, JGU Mainz

While known for a long time, antiferromagnetically ordered systems have previously been considered, as expressed by Louis Néel in his Nobel Prize Lecture, to be “interesting but useless”. However, since antiferromagnets potentially promises faster operation, enhanced stability with respect to interfering magnetic fields and higher integration due to the absence of dipolar coupling, they could potentially become a game changer for new spintronic devices. The zero net moment makes manipulation using conventional magnetic fields challenging. However recently, these materials have received renewed attention due to possible manipulation based on new approaches such as photons [1] or spin-orbit torques [2].

In this talk, we will present an overview of the key features of antiferromagnets to potentially functionalize their unique properties. This includes writing, reading and transporting information using antiferromagnets.

We recently realized switching in the metallic antiferromagnet Mn2Au by intrinsic staggered spin-orbit torques [3,4] and characterize the switching properties by direct imaging. While switching by staggered intrinsic spin-orbit torques in metallic AFMs requires special structural asymmetry, interfacial non-staggered spin-orbit torques can switch multilayers of many insulating AFMs capped with heavy metal layers.

We probe switching and spin transport in selected collinear insulating antiferromagnets, such as NiO [5-7], CoO [8,9] and hematite [10,11]. In NiO and CoO we find that there are multiple switching mechanisms that result in the reorientation of the Néel vector and additionally effects related to electromigration of the heavy metal layer can obscure the magnetic switching [5,7,9]. For the spin transport, spin currents are generated by heating as resulting from the spin Seebeck effect and by spin pumping measurements and we find in vertical transport short (few nm) spin diffusion lengths [6,8].

For hematite, however, we find in a non-local geometry that spin transport of tens of micrometers is possible [10,11]. We detect a first harmonic signal, related to the spin conductance, that exhibits a maximum at the spin-flop reorientation, while the second harmonic signal, related to the Spin Seebeck conductance, is linear in the amplitude of the applied magnetic field [10]. The first signal is dependent on the direction of the Néel vector and the second one depends on the induced magnetic moment due to the field. We identify the domain structure as the limiting factor for the spin transport [11]. We recently also achieved transport in the easy plane phase [12], which allows us to obtain long distance spin transport in hematite even at room temperature [12]. From the power and distance dependence, we unambiguously distinguish long-distance transport based on diffusion [10,11] from predicted spin superfluidity that can potentially be used for logic [13].
A number of excellent reviews are available for further information on recent developments in the field [14].

PDF file of the talk available here

References

[1] A. Kimel et al., Nature 429, 850 (2004).

[2] J. Zelezny et al., Phys. Rev. Lett. 113, 157201 (2014); P. Wadley et al., Science 351, 587 (2016).

[3] S. Bodnar et al., Nature Commun. 9, 348 (2018)

[4] S. Bodnar et al., Phys. Rev. B 99, 140409(R) (2019).

[5] L. Baldrati et al., Phys. Rev. Lett. 123, 177201 (2019)

[6] L. Baldrati et al., Phys. Rev. B 98, 024422 (2018); L. Baldrati et al. Phys. Rev. B 98, 014409 (2018)

[7] F. Schreiber et al., arxiv:2004.13374 (in press 2020)

[8] J. Cramer et al., Nature Commun. 9, 1089 (2018)

[9] L. Baldrati et al., Phys. Rev. Lett. 125, 077201 (2020)

[10] R. Lebrun et al., Nature 561, 222 (2018).

[11] A. Ross et al., Nano Lett. 20, 306 (2020).

[12] R. Lebrun et al., arxiv:2005.14414 (2020).

[13] Y. Tserkovnyak et al., Phys. Rev. Lett. 119, 187705 (2017).

[14] Rev. Mod. Phys. 90, 15005 (2018); Nat. Phys. 14, 200-242 (2018); Adv. Mater. 32, 1905603 (2020)

On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.08.2020 - 15:00 (CET)

Magnetic Matchmaking: Hybrid Magnon Modes

Axel Hoffmann, University of Illinois

Hybrid dynamic excitations have gained increased interest due to their potential impact on coherent information processing. Towards this end, magnons, the fundamental excitation quant of magnetically ordered systems, are of particular interest, since they can be easily tuned by external magnetic fields and interact with a wide range of other excitations, such as microwave and optical photons, phonons, and other magnons.1 We have explored recently the integration of permalloy (Ni80Fe20) thin film structures into hybrid magnon systems. By combining permalloy structures with high-quality superconducting microwave resonators, we demonstrated strong magnon-photon coupling in co-planar, on-chip geometry, which is readily scalable to more complex devices.2 Furthermore, we demonstrated strong coupling of permalloy magnons to standing magnon modes in yttrium iron garnet films, which revealed the importance of dampin-like torques originating from spin pumping.3 Lastly, we demonstrated how the coupling between magnons in Ni and surface acoustic waves in LiNbO3 can be used to modulate phonon propagation.4
This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division.

PDF file of the talk available here

References:
1. Y. Li, et al., arXiv:2006.16158.
2. Y. Li, et al., Phys. Rev. Lett. 123, 107701 (2019).
3. Y. Li, et al., Phys. Rev. Lett. 124, 117202 (2020).
4. C. Zhao, et al., Phys. Rev. Appl. 13, 054032 (2020).

New Opportunities for Charge and Spin in the 2D Magnet RuCl3

Kenneth Burch

 

Precise control of electronic charge at the nanoscale has been crucial in creating new phases of matter and devices. Here I will present results on the 2D magnet RuCl3 that demonstrate it is able to induce large charge on short length scales in other materials. I will discuss its ability to work with various systems, and potential for control via relative twist angle. I will also review the limitations of this technique in terms of ultimate charge doping and homogeneity. Time permitting I will briefly discuss the unique magnetic excitations in this system useful for topological computing, and implications for heterostructures of RuCl3 with other 2D magnets.