Superradiance, the cooperative spontaneous emission of photons from an ensemble of identical atoms, provides valuable insights into the many-body physics of photons and atoms. We show that an ensemble of resonant atoms embedded in the center of a planar cavity can be collectively excited by synchrotron radiation into a purely superradiant state. The collective coupling of the atoms via the radiation field leads to a substantial radiative shift of the transition energy, the collective Lamb shift. We simultaneously measured the temporal evolution of the superradiant decay and the collective Lamb shift of resonant 57Fe nuclei excited with 14.4-kilo-electron volt synchrotron radiation. Our experimental technique provides a simple method for spectroscopic analysis of the superradiant emission.

Current-induced spin-transfer torques (STT) and spin-orbit torques (SOT) enable the electrical switching of magnetic tunnel junctions (MTJs) in nonvolatile magnetic random access memories. In order to develop faster memory devices, an improvement of the timescales underlying the currentdriven magnetization dynamics is required. Here we report all-electrical time-resolved measurements of magnetization reversal driven by SOT in a three-terminal MTJ device. Single-shot measurements of the MTJ resistance during current injection reveal that SOT switching involves a stochastic two-step process consisting of a domain nucleation time and propagation time, which have different genesis, timescales, and statistical distributions compared to STT switching. We further show that the combination of SOT, STT, and voltage control of magnetic anisotropy (VCMA) leads to reproducible sub-ns switching with a spread of the cumulative switching time smaller than 0.2 ns. Our measurements unravel the combined impact of SOT, STT, and VCMA in determining the switching speed and efficiency of MTJ devices. Switching nanomagnets by current injection offers unparalleled scalability, as well as low power and high speed operation compared to control via external magnetic fields 1-3. Spin-transfer torques (STT) 1,4 are presently employed in memory and spin logic applications 5,6 to control the state of magnetic tunnel junctions (MTJ) via an electric current passing through the reference and free magnetic layers, which allows also for efficient readout of the MTJ through the tunnel magnetoresistance (TMR). Time-resolved studies of magnetization reversal in spin valve 7,8 and MTJ devices 9-12 have shown that STT enables switching on a timescale of 100 to 1 ns, depending on the driving current 13 and external field 14. However, STT switching is characterized by nonreproducible dynamic paths and incubation times up to several tens of ns long, which limit the reliability and speed of the reversal process to about 10-20 ns, even when mitigation strategies based on large driving currents or noncollinear spin injection are employed 13,15,16. These limitations may be overcome by magnetization reversal driven by spin-orbit torques (SOT) 3,17-19 , which has been recently demonstrated in three-terminal MTJs with in-plane 20,21 as well as out-of-plane magnetization 22-25. SOT switching combines an in-plane current injection geometry with charge-to-spin conversion due to the spin Hall effect and interfacial spin scattering 3. Such a geometry decouples the write and read current paths, improving the MTJ endurance and operation speed by minimizing electrical stress of the tunnel barrier and allowing for tuning the barrier thickness for high TMR, fast read-out, and minimal read disturbances. Moreover, in devices with perpendicular magnetization, the injected spin current is orthogonal to the quiescent magnetization of the free layer, thus providing an "instant on" torque that is expected to minimize the switching incubation time 25-27 .

The growth of a thin gold film on a conducting polymer surface from nucleation to formation of a continuous layer with a thickness of several nanometers is investigated in situ with grazing incidence small-angle X-ray scattering (GISAXS). Time resolution is achieved by performing the experiment in cycles of gold deposition on poly(N-vinylcarbazole) (PVK) and subsequently recording the GISAXS data. The 2D GISAXS patterns are simulated, and morphological parameters of the gold film on PVK such as the cluster size, shape, and correlation distance are extracted. For the quantitative description of the cluster size evolution, scaling laws are applied. The time evolution of the cluster morphology is explained with a growth model, suggesting a cluster growth proceeding in four steps, each dominated by a characteristic kinetic process: nucleation, lateral growth, coarsening, and vertical growth. A very limited amount of 6.5 wt % gold is observed to be incorporated inside a 1.2-nm-thick enrichment layer in the PVK film.

We demonstrate for the first time full-scale integration of top-pinned perpendicular MTJ on 300 mm wafer using CMOS-compatible processes for spin-orbit torque (SOT)-MRAM architectures. We show that 62 nm devices with a Wbased SOT underlayer have very large endurance (> 5x10 10 ), sub-ns switching time of 210 ps, and operate with power as low as 300 pJ.Introduction: The introduction of non-volatility (NV) at the cache level in advance logic nodes is sought as it would lead to a large decrease of the power consumption of microprocessors. Among NV memory technologies, spin-transfer torque (STT) MRAM has gained a lot of attention due to its scalability, low power and high speed, as well as compatibility with scaled CMOS processes and voltages. Despite all these advantages, STT-MRAM cannot operate reliably at ns and sub-ns scales due to large incubation delays [1,2], making it an unsuitable solution to tackle L1/2 SRAM cache replacement. In addition, the shared read/write path can impair the read reliability, while the write current can impose severe stress on the MTJ, leading to time dependent degradation of the memory cell. To mitigate these issues, spin-orbit torque (SOT)-MRAM has been recently proposed [2,3]. SOT induces switching of the free layer (FL) of the MTJ by injecting an in-plane current in an adjacent SOT layer, typically with the assistance of a static in-plane magnetic field [2]. This enables a three terminal MTJ-based concept that isolates the read/write path (Fig. 1), significantly improving the device endurance and read stability. Moreover, due to SOT spin transfer geometry, incubation time is negligible which allows for reliable switching operation at sub-ns timescales [4,5]. Here, we report the first successful integration of SOT-MTJ cells on 300 mm wafers using CMOS-compatible processes. We demonstrate low power sub-ns switching and pathways for further optimization. Finally, excellent endurance and absence of electro-migration effect of ultrathin SOT layers are shown.Integration flow: We used a SOT dedicated mask set in the imec 300 mm fab. The main steps of the integration process are summarized in Fig. 2: a SOT-MTJ stack is deposited on smooth bottom electrodes (BE), which are fabricated using a tungsten (W) damascene process. The MTJ is top pinned and consist of SOT/CoFeB/MgO/CoFeB/SAF perpendicularly magnetized (PMA) stack, where the SOT layer is W-based. Specific stop etch conditions have been developed to leave the SOT layer intact while patterning the MTJ pillar without producing sidewall shorts across the MgO barrier (Fig. 2c,d). Subsequently, the SOT layer is etched to form the three terminal device and a dual damascene Cu top electrode (TE) was fabricated to complete the electrical connection ( Fig. 2a).Stack development: SOTs possess a damping-like term (τDL) attributed to spin Hall and a field-like term (τFL) attributed to interface interactions [2]. Recent work indicates that τDL triggers switching while τFL accelerates it [5]. Charge-to-spin conversion efficiency parameters θDL and...

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Non-volatile magnetic random access memories such as spin-transfer torque (STT)-MRAM and next generation spin-orbit torque (SOT)-MRAM are emerging as key enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional (2D) van der Waals heterostructures bring ultra-compact multilayer compounds with unprecedented material-engineering capabilities. Here, we first provide an overview of the current developments and challenges in the field, and then outline the opportunities which can arise by implementing 2D materials into spin-based memory technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes.

We study ns scale spin-torque-induced switching in perpendicularly magnetized tunnel junctions (pMTJ). Although the switching voltages match with the macrospin instability threshold, the electrical signatures of the reversal indicate the presence of domain walls in junctions of various sizes. In the antiparallel (AP) to parallel (P) switching, a nucleation phase is followed by an irreversible flow of a wall through the sample at an average velocity of 40 m/s with back and forth oscillation movements indicating a Walker propagation regime. A model with a single-wall locally responding to the spin-torque reproduces the essential dynamical signatures of the reversal. The P to AP transition has a complex dynamics with dynamical back-hopping whose probability increases with voltage. We attribute this back-hopping to the instability of the nominally fixed layers.The spin-transfer-torque (STT) manipulation of the magnetization is a cornerstone of modern spintronics. In magnetic tunnel junctions (MTJ), the interplay between magnetizationdependent transport properties 1,2 and the spin torques results in a rich variety of phenomena 3 . After the discovery of STT, it was soon realized 4,5 that the cylindrical symmetry of the magnetic properties in Perpendicular Magnetic Anisotropy (PMA) systems and the resilience to thermal fluctuations that the anisotropy provides would make PMA systems ideal playgrounds to explore STT-induced dynamics. However MTJs with relevant properties became available only a decade after 6 and relied on ultrathin systems where strong interfacial effects can be present 7 ; besides, efficient spin-torque generation requires complex embedding stacks 8,9 in which each additional layers can be a fluctuator strongly coupled to the layer of main interest in a non uniform 8,10 and non local 11 manner. As a consequence the STT-induced magnetization switching in PMA MTJ systems exhibits rich features 12,13 that deserve to be studied, especially as it opens opportunities in information technologies.In this letter, we report single-shot time-resolved measurements of ns-scale STT switching events in PMA MTJs. We detail the electrical signature of the switching and account for its main features using a simple formalism. After an observable nucleation, the reversal proceeds in a non uniform manner with the motion of a domain wall (DW) in a Walker regime; this comes together with intensified excitations in the nominally fixed layers that can result in dynamical back-hopping. This complex dynamics calls for a revisit of the models describing the stability of magnetization and its switching under STT in perpendicularly magnetized confined systems. Our findings are also important for the understanding of other spin torque devices like spin majority gates 14 where the degree of coherence of the magnetization -the occurrence or non occurrence of domain walls-is crucial. The paper is organized as follows. We first describe in detail the properties of the thin films from which the samples are fabricated (section I). The d...

Microphase-separation structures in mixed diblock-triblock copolymer thin films are used for the incorporation of gold atoms inside the polymer matrix via sputtering of gold. Polystyrene (PS) spheres are arranged in a liquidlike type with a well defined nearest neighbor distance inside a polyisoprene matrix acting as a template for directing the gold atoms. Sputtering conditions are selected with a very low sputtering rate to avoid clustering in the atmosphere so that gold reaches the polymer surface in its atomic state. Due to the mobility of the gold atoms and the selective interaction with the PS parts of the microphase separation structure, gold is accumulated inside the polymer film in the PS spheres, as probed in situ with grazing incidence small-angle X-ray scattering (GISAXS). Nominally 4.3 A of gold is deposited, which by diffusion is spread out vertically over a thickness of 280 nm. UV-vis spectroscopy reveals a small blue shift for the gold sputtered polymer film. Atomic force microscopy proves the absence of gold clusters on the film surface. For low sputtering rate, GISAXS proves good sensitivity for gold migration inside the polymer film and opens new possibilities for studying polymer-metal interaction.