We study the behaviour of skyrmions and related topological states such as antiskyrmion, antiferromagnetic skyrmion, ferrimagnetic skyrmion, skyrmionium in magnetic thin films under the effect of torques generated by spin-polarized current or external magnetic field. We also explore the behaviour of skyrmions in constrained geometries such as nanopillars for their applications in Radio Frequency oscillators or sensors.
Related Publications:
R. Juge*, N. Sisodia* et al., “Skyrmions in synthetic antiferromagnets and their nucleation via electrical current and ultrafast laser illumination” (*equal contribution) Nat. Commun. 13, 4807 (2022)
N. Sisodia, P. K. Muduli, N. Papanicolaou and S. Komineas, “Chiral droplets and current-driven motion in ferromagnets” Phys. Rev. B 103, 024431 (2021)
N. Sisodia, S. Komineas, and P. K. Muduli, “Chiral skyrmion auto-oscillations in a ferromagnet under spin-transfer torque” Phys. Rev. B 99, 184441 (2019)
Skyrmions are topologically protected magnetic textures that can be driven at high velocities in magnetic heterostructures on the application of current. These skyrmions interact with each other in a magnetic thin film via exchange and magnetostatic interactions. Skyrmions have been proposed to be used as magnetic bits in racetrack memories where the data (skyrmions) is moved towards a read head (usually an MTJ) instead of the read head moving towards data. Additionally, the skyrmion-skyrmion interactions can also be exploited to perform some useful computations. We at SpinDev are working towards integrating the storage and the computing capability of the skyrmion to propose logic-in-memory devices where the data is stored as well as computed upon in a single platform/device. This is different from the conventional computing architecture where the storage and logic units are physically separate leading to high energy losses due to the constant shuttling of data between logic and storage units.
Related Publications:
N. Sisodia, J. Pelloux-Prayer, L. D. Buda-Prejbeanu, L. Anghel, G. Gaudin and O. Boulle, “Robust and programmable logic-in-memory devices exploiting skyrmion confinement and channeling using local energy barriers” Phys. Rev. Applied 18, 014025 (2022)
N. Sisodia, J. Pelloux-Prayer, L. D. Buda-Prejbeanu, L. Anghel, G. Gaudin and O. Boulle, “Programmable skyrmion demultiplexer based on skyrmion tunnelling” Phys. Rev. Applied 17, 064035 (2022)
R. Juge, K. Bairagi, K. G. Rana, M. Sall, D. Mailly, V. T. Pham, Q. Zhang, N. Sisodia et al. “Helium Ions Put Magnetic Skyrmions on the Track” Nano Lett. 21, 7, 2989–2996 (2021)
The Spin Torque Diode effect in Magnetic Tunnel Junctions converts an injected RF signal to a corresponding DC voltage. This voltage as a function of an external field gives a characteristic curve which is a combination of symmetric and anti-symmetric Lorentzian functions. As the resonance field of this response curve is dependent on the frequency of the injected signal, the device can be used as an RF detector. At SpinDev, we are working on enhancing the detection sensitivity and frequency of such RF detectors using various effects, such as voltage-controlled anisotropy modulation, injection locking, etc., to come up with devices/configurations which well surpass the sensitivity of conventional Schottky detectors.
Related Publications:
N. Sisodia and P. K. Muduli, "Frequency and sensitivity enhancement for signal detection" Indian Patent Application Number- 201911014379)
N. Sisodia and P. K. Muduli, “Simultaneous enhancement of spin-torque diode sensitivity and detection frequency by using voltage controlled magnetic anisotropy” Appl. Phys. Lett. 115, 102401 (2019)
P. K. Muduli, R. Sharma, D. Tiwari, N. Sisodia, A. Houshang, O. G. Heinonen and J. Åkerman, “Book Chapter : Microwave Oscillators and Detectors Based on Magnetic Tunnel Junctions” IN “Emerging Non-volatile Memory Technologies” Springer, Singapore (2021)
N. Sisodia and P. K. Muduli, “Enhancement of microwave detection sensitivity in un-biased perpendicular magnetic tunnel junctions using voltage controlled magnetic anisotropy” J. Magn. Magn. Mater. 515, 167301 (2020)
Magnetic Tunnel Junctions are popular spintronic devices essentially composed of a fixed magnetic layer that orients the magnetic spin of the incoming electrons along a given direction and a free layer whose magnetic moment is free to move under the influence of torque due to the electrons coming from the fixed layer. These two layers are separated by a non-magnetic oxide layer. Under optimal conditions, the torque due to the spin-polarized current on the free layer can counteract the damping forces in the system leading to a sustained magnetic precession. This continuous oscillation of magnetic moments is converted to resistance or voltage signal via the Tunnel Magnetoresistance effect. The frequency of this signal, which lies in the GHz range, can be tuned by varying the amplitude of the external magnetic field or current, giving us a tunable Radio Frequency oscillator converting DC current to RF signals. We at SpinDev are working on improving the output characteristics of these devices (power output, noise, sideband modulation) focusing on wireless communication applications.
Related Publications:
R. Sharma, N. Sisodia, J. Åkerman, and P. K. Muduli, “Enhanced modulation bandwidth of a magnetic tunnel junction based spin torque nano-oscillator under strong current modulation” IEEE Electron Device Lett. 42, 1886-1889 (2021)
R. Sharma, N. Sisodia, P. Dürrenfeld, J. Åkerman, and P. K. Muduli, “Time-domain stability of parametric synchronization in a spin-torque nano-oscillator based on a magnetic tunnel junction”, Phys. Rev. B 96, 024427 (2017)
R. Sharma, N. Sisodia, E. Iacocca, A. A. Awad, J. Åkerman and P. K. Muduli, “A high-speed single sideband generator using a magnetic tunnel junction spin torque nano-oscillator” Sci. Rep. 7, 13422 (2017)
The free layer of a Magnetic Tunnel Junction can be switched between parallel (P) and anti-parallel (AP) configurations relative to the fixed magnetic layer on the application of external current beyond a certain threshold value. Such a device is ideal to represent a data bit where the bits 0 and 1 can be encoded by the P and AP states. We are planning to commence work toward hybrid MTJ devices with multiple coupled magnetic layers working as the free layer to obtain energy-efficient and robust switching.
The operation of spintronic devices (MTJs, SHNOs, skyrmion-based heterostructures) is usually accompanied by the observation of strong non-linear phenomenas due to the inherent complexity and non-linear nature of the Landau-Lifshitz-Gilbert equation which is the driving framework behind all spintronic devices. The energy efficiency of these devices has also improved considerably in the past decade with the usage of Spin-Orbit torques and high TMR devices to obtain efficient conversion of charge current to spin-polarized current. Given their non-linear nature, these energy-efficient analog devices can be well-suited for performing neuromorphic computations. In our group, we are coming up with various ways in which a network of spintronic devices (MTJs/skyrmions) can be used to perform standard mathematical operations (Multiply and Accumulate (MAC), ReLU, etc. ) required in a neural network.
Related Publications:
U. Sahu, N. Sisodia, J. Sharda, P. K. Muduli, and D. Bhowmik, “Ferrimagnetic Synapse Devices for Fast and Energy-Efficient On-Chip Learning on An Analog-Hardware Neural Network” IEEE Trans. Electron Devices 69, 1713-1720 (2022)