Large quantities of random numbers are crucial in a wide range of applications. We have recently demonstrated that perpendicular nanopillar magnetic tunnel junctions (pMTJs) can produce true random bits when actuated with short pulses. However, our implementation used high-end and expensive electronics, such as a high bandwidth arbitrary waveform generator and analog-to-digital converter, and was limited to relatively low data rates. Here, we significantly increase the speed of true random number generation (TRNG) of our stochastic actuated pMTJs (SMART-pMTJs) using Field Programmable Gate Arrays (FPGAs), demonstrating the generation of over $10^{12}$ bits at rates exceeding 10Mb/s. The resulting bitstreams pass the NIST Statistical Test Suite for randomness with only one XOR operation. In addition to a hundred-fold reduction in the setup cost and a thousand-fold increase in bitrate, the advancement includes simplifying and optimizing random bit generation with a custom-designed analog daughter board to interface an FPGA and SMART-pMTJ. The resulting setup further enables FPGA at-speed processing of MTJ data for stochastic modeling and cryptography.

Nanoscale magnetic tunnel junction (MTJ) devices can efficiently convert thermal energy in the environment into random bitstreams for computational modeling and cryptography. We recently showed that perpendicular MTJs actuated by nanosecond pulses can generate true random numbers at high data rates. Here, we explore the dependence of probability bias-the deviations from equal probability (50/50) 0/1 bit outcomes-of such devices on temperature, pulse amplitude, and duration. Our experimental results and device model demonstrate that operation with nanosecond pulses in the ballistic limit minimizes variation of probability bias with temperature to be far lower than that of devices operated with longer-duration pulses. Furthermore, operation in the short-pulse limit reduces the bias variation with pulse amplitude while rendering the device more sensitive to pulse duration. These results are significant for designing true random number generator MTJ circuits and establishing operating conditions.

True random number generators (TRNGs) are fundamental building blocks for many applications, such as cryptography, Monte Carlo simulations, neuromorphic computing, and probabilistic computing. While perpendicular magnetic tunnel junctions (pMTJs) based on low-barrier magnets (LBMs) are natural sources of TRNGs, they tend to suffer from device-to-device variability, low speed, and temperature sensitivity. Instead, medium-barrier magnets (MBMs) operated with nanosecond pulses - denoted, stochastic magnetic actuated random transducer (SMART) devices - are potentially superior candidates for such applications. We present a systematic analysis of spin-torque-driven switching of MBM-based pMTJs (Eb ~ 20 - 40 kBT) as a function of pulse duration (1 ps to 1 ms), by numerically solving their macrospin dynamics using a 1-D Fokker-Planck equation. We investigate the impact of voltage, temperature, and process variations (MTJ dimensions and material parameters) on the switching probability of the device. Our findings indicate SMART devices activated by short-duration pulses (< 1 ns) are much less sensitive to process-voltage-temperature (PVT) variations while consuming lower energy (~ fJ) than the same devices operated with longer pulses. Our results show a path toward building fast, energy-efficient, and robust TRNG hardware units for solving optimization problems.

Magnetic tunnel junctions (MTJs), which are the fundamental building blocks of spintronic devices, have been used to build true random number generators (TRNGs) with different trade-offs between throughput, power, and area requirements. MTJs with high-barrier magnets (HBMs) have been used to generate random bitstreams with ≲ 200 Mb/s throughput and pJ/bit energy consumption. A high temperature sensitivity, however, adversely affects their performance as a TRNG. Superparamagnetic MTJs employing low-barrier magnets (LBMs) have also been used for TRNG operation. Although LBM-based MTJs can operate at low energy, they suffer from slow dynamics, sensitivity to process variations, and low fabrication yield. In this paper, we model a TRNG based on medium-barrier magnets (MBMs) with perpendicular magnetic anisotropy. The proposed MBM-based TRNG is driven with short voltage pulses to induce ballistic, yet stochastic, magnetization switching. We show that the proposed TRNG can operate at frequencies of about 500 MHz while consuming less than 100 fJ/bit of energy. In the short-pulse ballistic limit, the switching probability of our device shows robustness to variations in temperature and material parameters relative to LBMs and HBMs. Our results suggest that MBM-based MTJs are suitable candidates for building fast and energy-efficient TRNG hardware units for probabilistic computing.

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 (ns) 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 junction's tunnel resistance. It is demonstrated that bitstreams with millions of events: 1) are very well described by the binomial distribution; 2) can be used to create a uniform distribution of 8-bit random numbers; 3) pass multiple statistical tests for true randomness, including all the National Institute of Standards tests for random number generators with only one XOR operation; 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 GHz bitrates, while being more robust to environmental changes, such as their operating temperature, compared to other stochastic nanomagnetic devices.

Temperature plays an important role in spin torque switching of magnetic tunnel junctions causing magnetization fluctuations that decrease the switching voltage but also introduce switching errors. Here we present a systematic study of the temperature dependence of the spin torque switching probability of state-of-the-art perpendicular magnetic tunnel junction nanopillars (40 to 60 nm in diameter) from room temperature down to 4 K, sampling up to a million switching events. The junction temperature at the switching voltage---obtained from the thermally assisted spin torque switching model---saturates at temperatures below about 75 K, showing that junction heating is significant below this temperature and that spin torque switching remains highly stochastic down to 4 K. A model of heat flow in a nanopillar junction shows this effect is associated with the reduced thermal conductivity and heat capacity of the metals in the junction.

Spin-transfer magnetic random access memory is of significant interest for cryogenic applications where a persistent, fast, low-energy consumption and high device density is needed. Here we report the low-temperature nanosecond duration spin-transfer switching characteristics of perpendicular magnetic tunnel junction (pMTJ) nanopillar devices (40 to 60 nm in diameter) and contrast them to their room temperature properties. Interestingly, at fixed pulse voltage overdrive the characteristic switching time decreases with temperature, in contrast to macrospin model predictions, with the largest reduction in switching time occurring between room temperature and 150 K. The switching energy increases with decreasing temperature, but still compares very favorably to other types of spin-transfer devices at 4 K, with < 300 fJ required per switch. Write error rate (WER) measurements show highly reliable (WER <= 5x10^-5 with 4 ns pulses at 4 K) demonstrating the promise of pMTJ devices for cryogenic applications and routes to further device optimization.

We present a study of precessional magnetization switching in orthogonal spin-torque spin-valve devices at low temperatures. The samples consist of a spin-polarizing layer that is magnetized out-of-the film plane and an in-plane magnetized free and reference magnetic layer separated by non-magnetic metallic layers. We find coherent oscillations in the switching probability, characterized by high speed switching (~200 ps), error rates as low as 10 −5 and decoherence effects at longer timescales (~1 ns). Our study, which is conducted over a wide range of parameter space (pulse amplitude and duration) with deep statistics, demonstrates that the switching dynamics are likely dominated by the action of the out-of-plane spin polarization, in contrast to in-plane spin-torque from the reference layer, as has been the case in most previous studies. Our results demonstrate that precessional spin-torque devices are well suited to a cryogenic environment, while at room temperature they have so far not exhibited coherent or reliable switching.

We present a study of pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperatures. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy (Ni0.83Fe0.17) free layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin-torque asymmetry parameter, reduced spin polarization, and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.

A focused ion beam was used to obtain cross-sectional specimens from both magnetic multilayer and Nb/Al-AlOx/Nb Josephson junction devices for characterization by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX). Automated multivariate statistical analysis of the EDX spectral images produced chemically unique component images of individual layers within the multilayer structures. STEM imaging elucidated distinct variations in film morphology, interface quality, and/or etch artifacts that could be correlated to magnetic and/or electrical properties measured on the same devices.

We present experimental results and macrospin simulations of the switching characteristics of orthogonal spin-transfer devices incorporating an out-of-plane magnetized polarizing layer and an in-plane magnetized spin valve device at cryogenic temperatures. At $T=4$ K we demonstrate: high speed deterministic switching at short pulse lengths - down to 100 ps - with sufficient measurement statistics to establish a switching error rate of $10^{-5}$; coherent precessional switching at longer times; and ensemble decoherence effects at even longer times. Finite temperature macrospin models capture the precessional switching well but fail to fully reproduce all the decoherence and switching error behavior.

Spin-orbit interaction derived spin torques provide a means of reversing the magnetization of perpendicularly magnetized ultrathin films with currents that flow in the plane of the layers. A basic and critical question for applications is the speed and efficiency of switching with nanosecond current pulses. Here, we investigate and contrast the quasistatic (slowly swept current) and pulsed current-induced switching characteristics of micrometer scale Hall crosses consisting of very thin (<;1 nm) perpendicularly magnetized CoFeB layers on β-Ta. While complete magnetization reversal occurs at a threshold current density in the quasistatic case, short duration (≤10 ns) larger amplitude pulses (≏10 times the quasistatic threshold current) lead to only partial magnetization reversal and domain formation. We associate the partial reversal with the limited time for reversed domain expansion during the pulse.

We present the quasi-static and dynamic switching characteristics of orthogonal spin-transfer devices incorporating an out-of-plane magnetized polarizing layer and an in-plane magnetized spin valve device at cryogenic temperatures. Switching at 12 K between parallel and anti-parallel spin-valve states is investigated for slowly varied current as well as for current pulses with durations as short as 200 ps. We demonstrate 100% switching probability with current pulses 0.6 ns in duration. We also present a switching probability diagram that summarizes device switching operation under a variety of pulse durations, amplitudes, and polarities.

We present a study of the response to pulsed infrared radiation of Fe layer shunted pnictide thin film microstructures. The thin film multilayer consisting of 20-nm-thick Fe buffer, 50-nm-thick Ba(Fe, Co)2As2 film, and gold protection layer were deposited on heated MgO and MgAl2O4 substrates by pulsed-laser deposition. The multilayers were patterned into 5- to 8-μm-wide and 5-μm-long microbridges by electron-beam lithography and ion-milling technique. The microbridges show Tc ≈ 20 K and a critical current density up to 2.56 MA/cm2 at T = 10 K. The photo-response of Fe-shunted Ba(Fe, Co)2As2 thin film microbridges to infrared radiation was studied in a wide range of incident optical power, operation temperature, and bias current. We have found that the electron energy relaxation in studied multilayers is dependent on substrate material and is 1.75 times faster in the case of MgAl2O4 characterized by lattice matching to the pnictide film in comparison to the MgO substrate.

We investigate the detection efficiency of a spiral layout of a superconducting nanowire single-photon detector. The design is less susceptible to the critical current reduction in sharp turns of the nanowire than the conventional meander design. Detector samples with different nanowire widths from 100 to 300 nm are patterned from a 4-nm-thick NbN film deposited on sapphire substrates. The critical current I C at 4.2 K for spiral, meander, and single-bridge structures is measured and compared. On the 100-nm-wide samples, the detection efficiency is measured in the wavelength range 400-1700 nm and the spectral bandwidth of the intrinsic detection efficiency is determined. In the optical range, the spiral detector reaches a detection efficiency of 27.6%, which is ~ 1.5 times the value of the meander. In the infrared range the detection efficiency of the spiral superconducting nanowire single-photon detector is more than doubled.

For the development of superconducting electronic devices made from iron-arsenide superconductors, thin films have to be patterned into sub-micrometer sized structures while retaining high superconducting transition temperature (TC) and critical current (IC) values. To investigate current transport properties, we structure Ba(Fe,Co)2As2 thin films with and without Fe buffer layers into μm- and sub-μm-bridges and measure the temperature- and field-dependent IC. The process of penetration and de-pinning of self-generated and externally excited magnetic vortices in such bridges is discussed.