| 1. |
- Capriata, Corrado Carlo Maria
(författare)
-
Dynamics and Intrinsic Variability of Spintronic Devices
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Spintronics is a scientific domain focusing on utilizing electron spin for information processing. This is the element that distinguishes it from electronics, which only utilizes the charge of electrons. A common purpose of spintronic devices is to implement additional functionalities to state-of-the-art Complementary Metal-Oxide Semiconductor (CMOS) technology. The aim of this work was to assess the intrinsic variabilities of Nano-Constriction Spin Hall Nano-Oscillators (NC-SHNOs) and the dynamics of Perpendicular Magnetic Tunnel Junctions (pMTJs). The first part of the thesis focuses on NC-SHNO and two-dimensional arrays. They are nanometer-sized microwave oscillators, allowing for a wide frequency tuning range, and are compatible with CMOS Back End Of Line (BEOL). These devices are based on a heavy metal/ferromagnetic bilayer. Environmental conditions during processing, fabrication techniques, and temperature of operation can all create variabilities in the device's functioning. Crystallization grains naturally form during the sputtering of the metals. Atomic Force Microscope (AFM) characterization showed the grains being of different shapes, about 30 nm in size. Here, the aim was to develop a simulation technique based on importing the measured grain structure into micromagnetic simulations. Their results match the device-to-device variability and multi-modal behavior found in microwave measurements. Moreover, the presence of grains influences the synchronization of the arrays.The second part of this work focuses on pMTJ. These non-volatile memory elements have two metastable states, parallel (P) and antiparallel (AP), separated by an energy barrier Eb. Here, the aim was to show their potential as True Random Number Generators (TRNGs). A pulse-activated measurement set-up was used to realize random bitstreams. The randomness was confirmed by the National Institute of Standards and Technology Statistical Testing Suite (NIST-STS). After one whitening Exclusive OR (XOR) stage, all tests were successfully passed.The assessment was completed with the development of a model describing both macrospin and domain wall-mediated magnetization reversals, i.e. switching between P and AP. The analysis of the reversal dynamics was carried out with micromagnetic simulations and String Method calculations. As expected, Eb is lowered by the field and by decreasing the device size. This allows for faster fluctuations, marking the device as a potential TRNG. Both the switching attempt frequency and the energy barrier were explored by finite-temperature micromagnetic simulations.This thesis shows the potential of realistic simulations combined with measurements to assess oscillators. It also shows the efficacy of spintronic devices as 10s-MHz TRNG.
|
|
| 2. |
- Capriata, Corrado Carlo Maria, et al.
(författare)
-
Energy Barriers for Thermally Activated Magnetization Reversal in Perpendicularly Magnetized Nanodisks in a Transverse Field
- 2023
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Thermally-induced transitions between bistable magnetic states of magnetic tunnel junctions (MTJ) are of interest for generating random bitstreams and for applications in stochastic computing. An applied field transverse to the easy axis of a perpendicularly magnetized MTJ (pMTJ) can lower the energy barrier (Eb) to these transitions leading to faster fluctuations. In this study, we present analytical and numerical calculations of Eb considering both coherent (macrospin) reversal and non-uniform wall-mediated magnetization reversal for a selection of nanodisk diameters and applied fields. Non-uniform reversal processes dominate for larger diameters, and our numerical calculations of Eb using the String method show that the transition state has a sigmoidal magnetization profile. The latter can be described with an analytical expression that depends on only one spatial dimension, parallel to the applied field, which is also the preferred direction of profile motion during reversal. Our results provide nanodisk energy barriers as a function of the transverse field, nanodisk diameter, and material characteristics, which are useful for designing stochastic bitstreams.
|
|
| 3. |
- Capriata, Corrado Carlo Maria, et al.
(författare)
-
Enhanced Stochastic Bit Rate for Perpendicular Magnetic Tunneling Junctions in a Transverse Field
- 2023
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Perpendicular magnetic tunneling junctions(pMTJs) as true random number generators (TRNGs) have been investigated by means of high-temperature micromagnetic simulations using MuMax3. An in-plane applied field, which lowers the energy barrier for thermally activated reversal, can be used to control and increase the bitrates. We study the attempt rate and the energy barrier for 10 and 40 nm diameter devices in various applied magnetic fields. At room temperature, the presence of the field leads to orders of magnitude increase in the bitrate, up to ∼ 100 MHz.
|
|
| 4. |
- Capriata, Corrado Carlo Maria, et al.
(författare)
-
Grain structure influence on synchronized two-dimensional spin-Hall nano-oscillators
- 2023
-
Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226. ; 13:5
-
Tidskriftsartikel (refereegranskat)abstract
- Nanoconstriction spin-Hall nano-oscillators (NC-SHNOs) are excellent devices for a wide variety of applications, from RF communication to bio-inspired computing. NC-SHNOs are easy to fabricate in large arrays, are CMOS compatible, and feature a narrow linewidth and high output power. However, in order to take full advantage of the device capabilities, a systematic analysis of the array behavior with respect to the number and dimensions of oscillators, the temperature of operation, and the influence of layer quality is needed. Here, we focus on micromagnetic simulations of 2 x 2 and 4 x 4 NC-SHNO arrays with single oscillators separated by up to 300 nm. We observe a synchronization scheme that allows for column-wise selection of the oscillation frequency for a larger pitch. However, for smaller pitches, a coherent oscillation volume was observed, and this volume included both the constrictions and extended beyond that region. A local variation in the exchange coupling in the active oscillator region was investigated by placing physical grains in the free magnetic layer, and it was shown to influence both the stable current range and the resulting frequency and output power. De-coupling the oscillators along rows or columns could provide higher power due to more favorable phase shifts between oscillators. Our investigation helps in achieving a deeper understanding of the intrinsic working principles of NC-SHNO arrays and how they reach fully synchronized states, and this will help to expand non-conventional computing capabilities.
|
|
| 5. |
- Capriata, Corrado Carlo Maria, et al.
(författare)
-
Impact of Random Grain Structure on Spin-Hall Nano-Oscillator Modal Stability
- 2022
-
Ingår i: IEEE Electron Device Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 0741-3106 .- 1558-0563. ; 43:2, s. 312-315
-
Tidskriftsartikel (refereegranskat)abstract
- Spin-Hall nano-oscillators are a promising class of microwave spintronic devices with potential applications in RF/microwave communication and neuromorphic computing. The nano-constriction spin-Hall nano-oscillators (NC-SHNO) have relatively high power, narrow linewidth, and low drive current. Several synchronization schemes e.g. arrays of spin-wave coupled oscillators have been proposed for more stable operation and higher output power. For such arrays, it is crucial to have good oscillator stability and small device-to-device variability. Here, a micromagnetic simulation technique is proposed that includes realistic material properties and hence enables variability and modal stability to be investigated. It is demonstrated, using both measurements and simulation, that the presence of physical grains in the free magnetic layer can induce multiple oscillation modes or frequency sidebands. Our investigation could help in the development of more stable NC-SHNOs that would enable oscillator arrays with stronger synchronization. Author
|
|
| 6. |
- Rehm, L., et al.
(författare)
-
Stochastic Magnetic Actuated Random Transducer Devices Based on Perpendicular Magnetic Tunnel Junctions
- 2023
-
Ingår i: Physical Review Applied. - : American Physical Society (APS). - 2331-7019. ; 19:2
-
Tidskriftsartikel (refereegranskat)abstract
- True random number generators are of great interest in many computing applications, such as cryptography, neuromorphic systems, and Monte Carlo simulations. Here, we investigate perpendicular magnetic-tunnel-junction nanopillars (pMTJs) activated by short-duration (nanosecond) pulses in the ballistic limit for such applications. In this limit, a pulse can transform the Boltzmann distribution of initial free-layer magnetization states into randomly magnetized down or up states, i.e., a bit that is 0 or 1, easily determined by measurement of the tunnel resistance of the junction. It is demonstrated that bit streams with millions of events: (1) are very well approximated by a normal distribution; (2) pass multiple statistical tests for true randomness, including all the National Institute of Standards and Technology tests for random number generators with only one XOR operation; (3) can be used to create a uniform distribution of 8-bit random numbers; and (4) can have no drift in the bit probability with time. The results presented here show that pMTJs operated in the ballistic regime can generate true random numbers at around 50-MHz bit rates, while being more robust to environmental changes, such as their operating temperature, compared to other stochastic nanomagnetic devices.
|
|
