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-Temperature-resilient random number generation with stochastic actuated magnetic tunnel junction devices, Laura Rehm, Md Golam Morshed, Shashank Misra, Ankit Shukla, Shaloo Rakheja, Mustafa Pinarbasi, Avik W. Ghosh, and Andrew D. Kent, Applied Physics Letters 124, 052401 (2024).
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.
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-Hybrid spin Hall nano-oscillators based on ferromagnetic metal/ferrimagnetic insulator heterostructures, Haowen Ren, Xin Yu Zheng, Sanyum Channa, Guanzhong Wu, Daisy A. O’Mahoney, Yuri Suzuki & Andrew D. Kent, Nature Communications 14, 1406 (2023).
Spin-Hall nano-oscillators (SHNOs) are promising spintronic devices to realize current controlled GHz frequency signals in nanoscale devices for neuromorphic computing and creating Ising systems. However, traditional SHNOs devices based on transition metals have high auto-oscillation threshold currents as well as low quality factors and output powers. Here we demonstrate a new type of hybrid SHNO based on a permalloy (Py) ferromagnetic-metal nanowire and low-damping ferrimagnetic insulator, in the form of epitaxial lithium aluminum ferrite (LAFO) thin films. The superior characteristics of such SHNOs are associated with the excitation of larger spin-precession angles and volumes. We further find that the presence of the ferrimagnetic insulator enhances the auto-oscillation amplitude of spin-wave edge modes, consistent with our micromagnetic modeling. This hybrid SHNO expands spintronic applications, including providing new means of coupling multiple SHNOs for neuromorphic computing and advancing magnonics.
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-Stochastic Magnetic Actuated Random Transducer Devices Based on Perpendicular Magnetic Tunnel Junctions, L. Rehm, C.C.M. Capriata, S. Misra, J.D. Smith, M. Pinarbasi, B.G. Malm, and A.D. Kent, Physical Review Applied 19, 024035 (2023).
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.
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-Brillouin light scattering from quantized spin waves in nanowires with antisymmetric exchange interactions, Jun-Wen Xu, Grant A. Riley, Justin M. Shaw, Hans T. Nembach, and Andrew D. Kent, Physical Review B 107, 054402 (2023).
Antisymmetric exchange interactions lead to nonreciprocal spin-wave propagation. As a result, spin waves confined in a nanostructure are not standing waves; they have a time-dependent phase, because counterpropagating waves of the same frequency have different wavelengths. We report on a Brillouin light scattering (BLS) study of confined spin waves in Co/Pt nanowires with strong Dzyaloshinskii-Moriya interactions. Spin-wave quantization in narrow (≲200-nm-wide) wires dramatically reduces the frequency shift between BLS Stokes and anti-Stokes lines associated with the scattering of light incident transverse to the nanowires. In contrast, the BLS frequency shift associated with the scattering of spin waves propagating along the nanowire length is independent of nanowire width. A model that considers the chiral nature of modes captures this physics and predicts a dramatic reduction in frequency shift of light scattered from higher-energy spin waves in narrow wires, which is confirmed by our experiments.
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-Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium, Jonathan Gibbons, Takaaki Dohi, Vivek P. Amin, Fei Xue, Haowen Ren, Jun-Wen Xu, Hanu Arava, Soho Shim, Hilal Saglam, Yuzi Liu, John E. Pearson, Nadya Mason, Amanda K. Petford-Long, Paul M. Haney, Mark D. Stiles, Eric E. Fullerton, Andrew D. Kent, Shunsuke Fukami, and Axel Hoffmann, Physical Review Applied 18, 024075 (2022).
Spin torque is a promising tool for driving magnetization dynamics for computing technologies. These torques can be easily produced by spin-orbit effects, but for most conventional spin source materials, a high degree of crystal symmetry limits the geometry of the spin torques produced. Magnetic ordering is one way to reduce the symmetry of a material and allow exotic torques, and antiferromagnets are particularly promising because they are robust against external fields. We present spin torque ferromagnetic resonance (ST-FMR) measurements and second harmonic Hall measurements characterizing the spin torques in antiferromagnetic iron rhodium alloy. We report extremely large, strongly temperature-dependent exotic spin torques with a geometry apparently defined by the magnetic ordering direction. We find the spin torque efficiency of iron rhodium to be (207 ± 94)% at 170 K and (88 ± 32)% at room temperature. We support our conclusions with theoretical calculations showing how the antiferromagnetic ordering in iron rhodium gives rise to such exotic torques.
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-Spin-Transfer-Torque Oscillator with an Antiferromagnetic Exchange-Coupled Composite Free Layer, I. Volvach, A.D. Kent, E.E. Fullerton and V. Lomakin, Physical Review Applied 18, 024071 (2022).
We present an antiferromagnetically exchange-coupled composite (soft and hard layers) spin torque oscillator (AF-ECC STO) and demonstrate its operation via both analytical and micromagnetic modeling. The operation exploits the exchange field due to the antiferromagnetic coupling between soft and hard sublayers of the free layer as well as on the easy-plane anisotropy of the soft sublayer. Optimized AFECC STO structures can generate large-amplitude magnetization oscillations, which can be tuned over a broad frequency range with precessions mostly generated by the soft layer. We demonstrate that the AF-ECC STO structure offers flexibility in current control of the oscillation frequency and magnetization angle for realistic material parameters.
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-Field-free current-induced magnetization switching in GdFeCo: A competition between spin–orbit torques and Oersted fields, Jean-Loïs Bello, Yassine Quessab, Jun-Wen Xu, Maxime Vergès, Héloïse Damas, Sébastien Petit-Watelot, Juan-Carlos Rojas Sánchez, Michel Hehn, Andrew D. Kent, and Stéphane Mangin, Journal of Applied Physics 132, 083903 (2022).
Switching of perpendicular magnetization via spin–orbit torque (SOT) is of particular interest in the development of non-volatile magnetic random access memory (MRAM) devices. We studied current-induced magnetization switching of Ir/GdFeCo/Cu/Pt heterostructures in a Hall cross geometry as a function of the in-plane applied magnetic field. Remarkably, magnetization switching is observed at zero applied field. This is shown to result from the competition between SOT, the Oersted field generated by the charge current, and the material’s coercivity. Our results show a means of achieving zero-field switching that can impact the design of future spintronics devices, such as SOT-MRAM.
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-Zero-Field Nucleation and Fast Motion of Skyrmions Induced by Nanosecond Current Pulses in a Ferrimagnetic Thin Film, Yassine Quessab, Jun-Wen Xu, Egecan Cogulu, Simone Finizio, Jörg Raabe, and Andrew D. Kent, ACS Nano Letters 2c01038 (2022).
Skyrmion racetrack memories are highly attractive for next-generation data storage technologies. Skyrmions are noncollinear spin textures stabilized by chiral interactions. To achieve a fastoperating memory device, it is critical to move skyrmions at high speeds. The skyrmion dynamics induced by spin−orbit torques (SOTs) in the commonly studied ferromagnetic films is hindered by strong pinning effects and a large skyrmion Hall effect causing deflection of the skyrmion toward the racetrack edge, which can lead to information loss. Here, we investigate the current-induced nucleation and motion of skyrmions in ferrimagnetic Pt/CoGd/(W or Ta) thin films. We first reveal field-free skyrmion nucleation mediated by Joule heating. We then achieve fast skyrmion motion driven by SOTs with velocities as high as 610 m s−1 and a small skyrmion Hall angle |θSkHE| ≲ 3°. Our results show that ferrimagnets are better candidates for fast skyrmion-based memory devices with low risk of information loss.
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-Third harmonic characterization of antiferromagnetic heterostructures, Yang Cheng, Egecan Cogulu, Rachel D. Resnick, Justin J. Michel, Nahuel N. Statuto, Andrew D. Kent & Fengyuan Yang, Nature Communications 13, 3659 (2022).
Electrical switching of antiferromagnets is an exciting recent development in spintronics, which promises active antiferromagnetic devices with high speed and low energy cost. In this emerging field, there is an active debate about the mechanisms of current-driven switching of antiferromagnets. For heavy-metal/ferromagnet systems, harmonic characterization is a powerful tool to quantify current-induced spin-orbit torques and spin Seebeck effect and elucidate current-induced switching. However, harmonic measurement of spin-orbit torques has never been verified in antiferromagnetic heterostructures. Here, we report harmonic measurements in Pt/α-Fe2O3 bilayers, which are explained by our modeling of higher-order harmonic voltages. As compared with ferromagnetic heterostructures where all current-induced effects appear in the second harmonic signals, the damping-like torque and thermally-induced magnetoelastic effect contributions in Pt/α-Fe2O3 emerge in the third harmonic voltage. Our results provide a new path to probe the current-induced magnetization dynamics in antiferromagnets, promoting the application of antiferromagnetic spintronic devices.
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-Quantifying Spin-Orbit Torques in Antiferromagnet–Heavy-Metal Heterostructures, Egecan Cogulu, Hantao Zhang, Nahuel N. Statuto, Yang Cheng, Fengyuan Yang, Ran Cheng, and Andrew D. Kent, Physical Review Letters 128, 247204 (2022).
The effect of spin currents on the magnetic order of insulating antiferromagnets (AFMs) is of fundamental interest and can enable new applications. Toward this goal, characterizing the spin-orbit torques (SOTs) associated with AFM–heavy-metal (HM) interfaces is important. Here we report the full angular dependence of the harmonic Hall voltages in a predominantly easy-plane AFM, epitaxial c-axis oriented α-Fe2O3 films, with an interface to Pt. By modeling the harmonic Hall signals together with the α-Fe2O3 magnetic parameters, we determine the amplitudes of fieldlike and dampinglike SOTs. Out-of-plane field scans are shown to be essential to determining the dampinglike component of the torques. In contrast to ferromagnetic–heavy-metal heterostructures, our results demonstrate that the fieldlike torques are significantly larger than the dampinglike torques, which we correlate with the presence of a large imaginary component of the interface spin-mixing conductance. Our work demonstrates a direct way of characterizing SOTs in AFM-HM heterostructures.
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-Interplay between Spin-Orbit Torques and Dzyaloshinskii-Moriya Interactions in Ferrimagnetic Amorphous Alloys, Yassine Quessab, Jun-Wen Xu, Md Golam Morshed, Avik W. Ghosh, and Andrew D. Kent, Advanced Science 2021, 2100481 (2021).
Ferrimagnetic thin films are attractive for low-power spintronic applications because of their low magnetization, small angular momentum, and fast spin dynamics. Spin orbit torques (SOT) can be applied with proximal heavy metals that also generate interfacial Dzyaloshinskii–Moriya interactions (DMI), which can stabilize ultrasmall skyrmions and enable fast domain wall motion. Here, the properties of a ferrimagnetic CoGd alloy between two heavy metals to increase the SOT efficiency, while maintaining a significant DMI is studied. SOT switching for various capping layers and alloy compositions shows that Pt/CoGd/(W or Ta) films enable more energy-efficient SOT magnetization switching than Pt/CoGd/Ir. Spin-torque ferromagnetic resonance confirms that Pt/CoGd/W has the highest spin-Hall angle of 16.5%, hence SOT efficiency, larger than Pt/CoGd/(Ta or Ir). Density functional theory calculations indicate that CoGd films capped by W or Ta have the largest DMI energy, 0.38 and 0.32 mJ m−2, respectively. These results show that Pt/CoGd/W is a very promising ferrimagnetic structure to achieve small skyrmions and to move them efficiently with current.
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-A quantum material spintronic resonator, Jun‑Wen Xu, Yizhang Chen, Nicolás M. Vargas, Pavel Salev, Pavel N. Lapa, Juan Trastoy, Julie Grollier, Ivan K. Schuller & Andrew D. Kent, Scientific Reports 11, 15082 (2021).
In a spintronic resonator a radio-frequency signal excites spin dynamics that can be detected by the spin-diode efect. Such resonators are generally based on ferromagnetic metals and their responses to spin torques. New and richer functionalities can potentially be achieved with quantum materials, specifcally with transition metal oxides that have phase transitions that can endow a spintronic resonator with hysteresis and memory. Here we present the spin torque ferromagnetic resonance characteristics of a hybrid metal-insulator-transition oxide/ ferromagnetic metal nanoconstriction. Our samples incorporate V2O3, with Ni, Permalloy (Ni80Fe20) and Pt layers patterned into a nanoconstriction geometry. The frst order phase transition in V2O3 is shown to lead to systematic changes in the resonance response and hysteretic current control of the ferromagnetic resonance frequency. Further, the output signal can be systematically varied by locally changing the state of the V2O3 with a dc current. These results demonstrate new spintronic resonator functionalities of interest for neuromorphic computing.
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-Thermal Effects in Spin-Torque Switching of Perpendicular Magnetic Tunnel Junctions at Cryogenic Temperatures, L. Rehm, G. Wolf, B. Kardasz, E. Cogulu, Y. Chen, M. Pinarbasi and A.D. Kent, Physical Review Applied 15, 034088 (2021).
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 (40–60 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.
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-Spin-torque switching mechanisms of perpendicular magnetic tunnel junction nanopillars, J. Beik Mohammadi and A. D. Kent, Applied Physics Letters 118, 132407 (2021).
Understanding the spin-transfer magnetization switching mechanisms of perpendicular magnetic tunnel junction nanopillars is critical to optimizing their performance in memory devices. Here, we use micromagnetics to study how the free layer’s exchange constant affects its switching dynamics. Switching is shown to generally occur by (1) growth of the magnetization precession amplitude in the element center; (2) an instability in which the reversing region moves to the element edge, forming magnetic domain wall(s); and (3) the motion of the domain wall(s) across the element. For small exchange and large element diameters, step 1 leads to a droplet with a fully reversed core that experiences a drift instability (step 2). While in the opposite case (large exchange and small diameters), the central region of the element is not fully reversed before step 2 occurs. The origin of the micromagnetic structure is shown to be the free layer’s non-uniform demagnetization field. More coherent, energy-efficient, and faster switching is associated with larger exchange, showing that increasing the exchange interaction strength leads to improvements in device performance.
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-Direct imaging of electrical switching of antiferromagnetic Néel order in α-Fe2O3 epitaxial films, Egecan Cogulu, Nahuel N. Statuto, Yang Cheng, Fengyuan Yang, Rajesh V. Chopdekar, Hendrik Ohldag, and Andrew D. Kent, Physical Review B 103, L100405 (2021).
We report the direct observation of switching of the Néel vector of antiferromagnetic (AFM) domains in response to electrical pulses in micron-scale Pt/α-Fe2O3 Hall bars using photoemission electron microscopy. Current pulses lead to reversible and repeatable switching with the current direction determining the final state, consistent with Hall effect experiments that probe only the spatially averaged response. Current pulses also produce irreversible changes in domain structure, in and even outside the current path. In both cases only a fraction of the domains switch in response to pulses. Furthermore, the analysis of images taken with different xray polarizations shows that the AFM Néel order has an out-of-plane component in equilibrium that is important to consider in analyzing the switching data. These results show that—in addition to effects associated with spin-orbit torques from the Pt layer, which can produce reversible switching—changes in AFM order can be induced by purely thermal effects.
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-Micromagnetic instabilities in spin-transfer switching of perpendicular magnetic tunnel junctions, Nahuel Statuto, Jamileh Beik Mohammadi and Andrew D. Kent, Physical Review B 103, 014409 (2021).
Micromagnetic instabilities and nonuniform magnetization states play a significant role in spin-transfer induced switching of nanometer scale magnetic elements. Here we model domain wall mediated switching dynamics in perpendicularly magnetized magnetic tunnel junction nanopillars. We show that domain wall surface tension always leads to magnetization oscillations and instabilities associated with the disk shape of the junction. A collective coordinate model is developed that captures aspects of these instabilities and illustrates their physical origin. Model results are compared to those of micromagnetic simulations. The switching dynamics are found to be very sensitive to the domain wall position and phase, which characterizes the angle of the magnetization in the disk plane. This sensitivity is reduced in the presence of spin torques, and the spin current needed to displace a domain wall can be far less than the threshold current for switching from a uniformly magnetized state. A prediction of this model is conductance oscillations of increasing frequency during the switching process.
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-Magnetic droplet solitons, Ferran Macià and Andrew D. Kent, J. Appl. Phys. 128, 100901 (2020).
Magnetic droplet solitons are dynamical magnetic textures that form due to an attractive interaction between spin waves in thin films with perpendicular magnetic anisotropy. Spin currents and the spin torques associated with these currents enable their formation as they provide a means to excite non-equilibrium spin-wave populations and compensate their decay. Recent years have seen rapid advances in experiments that realize and study magnetic droplets. Important advances include the first direct x-ray images of droplets, determination of their threshold and sustaining currents, measurement of their generation and annihilation time, and evidence for drift instabilities, which can limit their lifetime. This perspective discusses these studies and contrasts these solitons to other types of spin-current excitations, such as spin-wave bullets, and static magnetic textures, including magnetic vortices and skyrmions. Magnetic droplet solitons can also serve as current controlled microwave frequency oscillators with potential applications in neuromorphic chips as nonlinear oscillators with memory.
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-Charge-To-Spin Conversion Efficiency in Ferromagnetic Nanowires by Spin Torque Ferromagnetic Resonance: Reconciling Lineshape and Linewidth Analysis Methods, Jun-Wen Xu and Andrew D. Kent, Physical Review Applied 14, 014012 (2020).
Spin orbit torques are of great interest for switching the magnetization direction in nanostructures, moving skyrmions and exciting spin waves. The standard method of determining their efficiency is by spin torque ferromagnetic resonance (ST-FMR), a technique that involves analyzing the resonance linewidth or lineshape. On microstuctures these two analysis methods are quite consistent. Here we present ST-FMR results on permalloy (Ni80Fe20) nanowires—with widths varying from 150 to 800 nm—that show that the standard model used to analyze the resonance linewidth and lineshape give different results; the efficiency appears greatly enhanced in nanowires when the lineshape method is used. A ST-FMR model that properly accounts for the sample shape is presented and shows much better consistency between the two methods. Micromagnetic simulations are used to verify the model. These results and the more accurate nanowire model presented are of importance for characterizing and optimizing charge-to-spin conversion efficiencies in nanostructures.
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-Planar Hall Driven Torque in a Ferromagnet/Nonmagnet/Ferromagnet System, Christopher Safranski, Jonathan Z. Sun, Jun-Wen Xu and Andrew D. Kent, Physical Review Letters 124, 197204 (2020).
An important goal of spintronics is to covert a charge current into a spin current with a controlled spin polarization that can exert torques on an adjacent magnetic layer. Here we demonstrate such torques in a two ferromagnet system. A CoNi multilayer is used as a spin current source in a sample with structure CoNi=Au=CoFeB. Spin torque ferromagnetic resonance is used to measure the torque on the CoFeB layer. The response as a function of the applied field angle and current is consistent with the symmetry expected for a torque produced by the planar Hall effect originating in CoNi. We find the strength of this effect to be comparable to that of the spin Hall effect in platinum, indicating that the planar Hall effect holds potential as a spin current source with a controllable polarization direction.
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-Tuning interfacial Dzyaloshinskii-Moriya interactions in thin amorphous ferrimagnetic alloys, Y.Quessab, J-W.Xu, C. T. Ma, W. Zhou, G.A. Riley, J. M. Shaw, H. T. Nembach, S. J. Poon and A. D. Kent, Scientific Reports 10, 7447 (2020).
Skyrmions can be stabilized in magnetic systems with broken inversion symmetry and chiral interactions, such as Dzyaloshinskii-Moriya interactions (DMI). Further, compensation of magnetic moments in ferrimagnetic materials can significantly reduce magnetic dipolar interactions, which tend to favor large skyrmions. Tuning DMI is essential to control skyrmion properties, with symmetry breaking at interfaces offering the greatest flexibility. However, in contrast to the ferromagnet case, few studies have investigated interfacial DMI in ferrimagnets. Here we present a systematic study of DMI in ferrimagnetic CoGd films by Brillouin light scattering. We demonstrate the ability to control DMI by the CoGd cap layer composition, the stack symmetry and the ferrimagnetic layer thickness. The DMI thickness dependence confirms its interfacial nature. In addition, magnetic force microscopy reveals the ability to tune DMI in a range that stabilizes sub-100 nm skyrmions at room temperature in zero field. Our work opens new paths for controlling interfacial DMI in ferrimagnets to nucleate and manipulate skyrmions.
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-Reduced Exchange Interactions in Magnetic Tunnel Junction Free Layers with Insertion Layers, Jamileh Beik Mohammadi, Bartek Kardasz, Georg Wolf, Yizhang Chen, Mustafa Pinarbasi, and Andrew D. Kent, ACS Appl. Electron. Mater 1, 2025-2029 (2019).
Perpendicularly magnetized CoFeB layers with ultrathin nonmagnetic insertion layers are very widely used as the electrodes in magnetic tunnel junctions for spin-transfer magnetic random access memory devices. Exchange interactions play a critical role in determining the thermal stability of magnetic states in such devices and their spin torque switching efficiency. Here, the exchange constant of free layers incorporated in full magnetic tunnel junction layer stacks, specifically CoFeB free layers with W insertion layers, is determined by magnetization measurements in a broad temperature range. A significant finding is that the exchange constant decreases significantly and abruptly with W insertion layer thickness. The perpendicular magnetic anisotropy shows the opposite trend; it initially increases with the W insertion layer thickness and shows a broad maximum for approximately one monolayer (0.3 nm) of W. These results highlight the interdependencies of magnetic characteristics required to optimize the performance of magnetic tunnel junction devices.
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-Sub-nanosecond spin-torque switching of perpendicular magnetic tunnel junction nanopillars at cryogenic temperatures, L. Rehm, G. Wolf, B. Kardasz, M. Pinarbasi, and A. D. Kent, Appl. Phys. Lett. 115, 182404 (2019).
Spin-transfer magnetic random access memory devices are of significant interest for cryogenic computing systems where a persistent, fast, low-energy consuming, and nanometer scale device operating at low temperature is needed. Here, we report the low-temperature nanosecond duration spin-transfer switching characteristics of perpendicular magnetic tunnel junction (pMTJ) nanopillar devices (40–60 nm in diameter) and contrast them to their room temperature properties. Interestingly, the characteristic switching time decreases with temperature, with the largest reduction occurring between room temperature and 150 K. The switching energy increases with decreasing temperature, but still compares very favorably with other types of spin-transfer devices at 4 K, with <300 fJ required per switch. Write error rate (WER) measurements show highly reliable switching with WER ≤ 5 × 10–5 with 4 ns pulses at 4 K. Our results demonstrate the promise of pMTJ devices for cryogenic applications and show routes to further device optimization.
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-Spin transport in an insulating ferrimagnetic-antiferromagnetic-ferrimagnetic trilayer as a function of temperature, Yizhang Chen, Egecan Cogulu, Debangsu Roy, Jinjun Ding, Jamileh Beik Mohammadi, Paul G. Kotula, Nancy A. Missert, Mingzhong Wu, and Andrew D. Kent, AIP Advances 9, 105319 (2019).
We present a study of the transport properties of thermally generated spin currents in an insulating ferrimagnetic-antiferromagneticferrimagnetic trilayer over a wide range of temperature. Spin currents generated by the spin Seebeck effect (SSE) in a yttrium iron garnet (YIG) YIG/NiO/YIG trilayer on a gadolinium gallium garnet (GGG) substrate were detected using the inverse spin Hall effect (ISHE) in Pt. By studying samples with different NiO thicknesses, the spin diffusion length of NiO was determined to be ∼3.8 nm at room temperature. Surprisingly, a large increase of the SSE signal was observed below 30 K, and the field dependence of the signal closely follows a Brillouin function for an S=7/2 spin. The increase of the SSE signal at low temperatures could thus be associated with the paramagnetic SSE from the GGG substrate. Besides, a broad peak in the SSE response was observed around 100 K. These observations are important in understanding the generation and transport properties of spin currents through magnetic insulators and the role of a paramagnetic substrate in spin current generation.
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-Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer, L. Rehm, V. Sluka, G. E. Rowlands, M.-H. Nguyen, T. A. Ohki, and A. D. Kent, Appl. Phys. Lett 114, 212402 (2019).
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.
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-Multiple magnetic droplet soliton modes, Nahuel Statuto, Christian Hahn, Joan Manel Hernàndez, Andrew D. Kent, and Ferran Macià, Phys. Rev. B 99, 174436 (2019).
Droplet solitons are large amplitude localized spin-wave excitations that can be created in magnetic thin films with uniaxial anisotropy by a spin-polarized current flowing through an electrical nanocontact. Here, we report a low-temperature (4 K) experimental study that shows there are multiple and, under certain conditions, combinations of droplet modes, each mode with a distinct high-frequency spin precession (tens of gigahertz). Low-frequency (≲1 GHz) voltage noise is used to assess the stability of droplet modes. It is found that droplets are stable only in a limited range of applied field and current, typically near the current and field at which they nucleate, in agreement with recent predictions. Applied fields in the film plane favor multiple droplet modes, whereas fields perpendicular to the film plane tend to stabilize a single droplet mode. Micromagnetic simulations are used to show that spatial variation in the energy landscape in the nanocontact region (e.g., spatial variation of magnetic anisotropy or magnetic field) can lead to quantized droplet modes and low-frequency mode modulation, characteristics observed in our experiments.
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-Voltage-Controlled Topological Spin Switch for Ultralow-Energy Computing: Performance Modeling and Benchmarking, Shaloo Rakheja, Michael E. Flatté, and Andrew D. Kent, Physical Review Applied 11, 054009 (2019).
A voltage-controlled topological spin switch (VTOPSS) that uses a hybrid topological insulator– magnetic insulator multiferroic material is presented that can implement Boolean logic operations with sub-10-aJ energy per bit and an energy-delay product on the order of 10−26 J s. The device uses a topological insulator, which has the highest efficiency of conversion of the electric field to spin torque yet observed at room temperature, and a low-moment magnetic insulator that can respond rapidly to a given spin torque. We present the theory of operation of the VTOPSS, develop analytic models of its performance metrics, elucidate performance scaling with dimensions and voltage, and benchmark the VTOPSS against existing spin-based and CMOS devices. Compared with existing spin-based devices, such as allspin logic and charge-spin logic devices, the VTOPSS offers 10–70 times lower energy dissipation and 70–1700 times lower energy-delay product. With experimental advances and improved material properties, we show that the energy and energy-delay product of the VTOPSS can be lowered to a few attojoules per bit and 10−28 J s, respectively. As such, the VTOPSS technology offers competitive metrics compared with existing CMOS technology. Finally, we establish that interconnect issues that dominate the performance in CMOS logic are relatively less significant for the VTOPSS, implying that highly resistive materials can indeed be used to interconnect VTOPSS devices.
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-Asymmetric Magnetization Switching in Perpendicular Magnetic Tunnel Junctions: Role of the Synthetic Antiferromagnet’s Fringe Field, M. Lavanant, P. Vallobra, S. Petit Watelot, V. Lomakin, A.D. Kent, J. Sun, and S. Mangin, Physical Review Applied 11, 034058 (2019).
The field- and current-induced magnetization reversal of a Co-Fe-B layer in a perpendicular magnetic tunnel junction (pMTJ) is studied at room temperature. The magnetization switching probability from the parallel (P) state to the antiparallel (AP) state and from AP to P is found to be asymmetric. We observe that this asymmetry depends on the magnetic configuration of the synthetic antiferromagnetic (SAF). The state diagram and the energy landscape are compared for two SAF configurations. We conclude that the asymmetry is due to the inhomogeneity of the fringe field generated by the SAF. Our study highlights the role of the reference-layer fringe field on the free layer’s energy barrier height, which has to be carefully controlled for memory applications.
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-A cryogenic spin-torque memory element with precessional magnetization dynamics, G. E. Rowlands, C.A. Ryan, L.Ye, L. Rehm, D. Pinna, A. D. Kent and T.A. Ohki, Scientific Reports 9, 803 (2019).
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 outof-the flm plane and an in-plane magnetized free and reference magnetic layer separated by nonmagnetic metallic layers. We fnd coherent oscillations in the switching probability, characterized by high speed switching (~200ps), error rates as low as 10−5 and decoherence efects 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.
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-Increased energy efficiency spin-torque switching of magnetic tunnel junction devices with a higher order perpendicular magnetic anisotropy, Marion Lavanant, Sebastien Petit-Watelot, Andrew D. Kent, and Stephane Mangin, Applied Physics Letters 114, 012404 (2019).
We study the influence of a second order magnetic anisotropy on magnetization reversal by spin transfer torque in perpendicularly magnetized magnetic tunnel junctions (pMTJs). Using a macrospin model to describe the dynamics of the free layer, analytical solutions for the switching voltage and the voltage threshold for precession are determined as a function of the first and second order magnetic anisotropies. To compare the spin-transfer-torque energy efficiency to that of a classical pMTJ, a junction without the second order anisotropy term, we compare these cases at a fixed energy barrier to thermally activated reversal. We show that the critical voltage for switching can be reduced by a factor 0.7 when the ratio of the second to the first order magnetic anisotropy is 1/3. Importantly, the switching time can be reduced by nearly a factor of two for this magnetic anisotropy ratio. These results highlight an important and practical method to increase the spin-torque efficiency, while reducing the energy dissipation and switching time in magnetic random access memory devices.
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-First harmonic measurements of the spin Seebeck effect, Yizhang Chen, Debangsu Roy, Egecan Cogulu, Houchen Chang, Mingzhong Wu, and Andrew D. Kent, Applied Physics Letters 113, 202403 (2018).
We present measurements of the spin Seebeck effect (SSE) by a technique that combines alternating currents (AC) and direct currents (DC). The method is applied to a ferrimagnetic insulator/heavy metal bilayer, Y3Fe5O12 (YIG)/Pt. Typically, SSE measurements use an AC current to produce an alternating temperature gradient and measure the voltage generated by the inverse spin-Hall effect in the heavy metal at twice the AC frequency. Here, we show that when Joule heating is associated with AC and DC bias currents, the SSE response occurs at the frequency of the AC current drive and can be larger than the second harmonic SSE response. We compare the first and second harmonic responses and show that they are consistent with the SSE. The field dependence of the voltage response is used to distinguish between the damping-like and field-like torques. This method can be used to explore nonlinear thermoelectric effects and spin dynamics induced by temperature gradients.
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-Ferromagnetic resonance linewidth in coupled layers with easy-plane and perpendicular magnetic anisotropies, Jun-Wen Xu, Volker Sluka, Bartek Kardasz, Mustafa Pinarbasi, and Andrew D. Kent, Journal of Applied Physics 124, 063902 (2018).
Magnetic bilayers with different magnetic anisotropy directions are interesting for spintronic applications as they offer the possibility to engineer tilted remnant magnetization states. We investigate the ferromagnetic resonance (FMR) linewidth of modes associated with two interlayer exchange-coupled ferromagnetic layers, the first a CoNi multilayer with a perpendicular magnetic anisotropy, and the second a CoFeB layer with an easy-plane anisotropy. For antiferromagnetic interlayer exchange coupling, a local maximum in the FMR linewidths is observed below a characteristic field. This is in contrast to what is found in uncoupled, ferromagnetically coupled and single ferromagnetic layers in which the FMR linewidth increases monotonically with field. We show that the existence of the local maximum as well as the characteristic field below which there is a dramatic increase in FMR linewidth can be understood using a macrospin model with Heisenberg-type exchange coupling between the layers.
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-Generation and stability of dynamical skyrmions and droplet solitons, Nahuel Statuto, Joan Manel Hernàndez, Andrew D Kent and Ferran Macià, Nanotechnology 29, 325302 (2018).
A spin-polarized current in a nanocontact to a magnetic film can create collective magnetic oscillations by compensating the magnetic damping. In particular, in materials with uniaxial magnetic anisotropy, droplet solitons have been observed—a self-localized excitation consisting of partially reversed magnetization that precesses coherently in the nanocontact region. It is also possible to generate topological droplet solitons, known as dynamical skyrmions (DSs). Here, we show that spin-polarized current thresholds for DS creation depend not only on the material’s parameters but also on the initial magnetization state and the rise time of the spin-polarized current. We study the conditions that promote either droplet or DS formation and describe their stability in magnetic films without Dzyaloshinskii–Moriya interactions. The Oersted fields from the applied current, the initial magnetization state, and the rise time of the injected current can determine whether a droplet or a DS forms. DSs are found to be more stable than droplets. We also discuss electrical characteristics that can be used to distinguish these magnetic objects.
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-Generation and annihilation time of magnetic droplet solitons, Jinting Hang, Christian Hahn, Nahuel Statuto, Ferran Macià & Andrew D. Kent, Scientific Reports 8, 6847 (2018).
Magnetic droplet solitons were first predicted to occur in materials with uniaxial magnetic anisotropy due to a long-range attractive interaction between elementary magnetic excitations, magnons. A non-equilibrium magnon population provided by a spin-polarized current in nanocontacts enables their creation and there is now clear experimental evidence for their formation, including direct images obtained with scanning x-ray transmission microscopy. Interest in magnetic droplets is associated with their unique magnetic dynamics that can lead to new types of high frequency nanometer scale oscillators of interest for information processing, including in neuromorphic computing. However, there are no direct measurements of the time required to nucleate droplet solitons or their lifetime– experiments to date only probe their steady-state characteristics, their response to dc spin-currents. Here we determine the timescales for droplet annihilation and generation using current pulses. Annihilation occurs in a few nanoseconds while generation can take several nanoseconds to a microsecond depending on the pulse amplitude. Micromagnetic simulations show that there is an incubation time for droplet generation that depends sensitively on the initial magnetic state of the nanocontact. An understanding of these processes is essential to utilizing the unique characteristics of magnetic droplet solitons oscillators, including their high frequency, tunable and hysteretic response.
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