Studies of magnetization dynamics have incessantly facilitated the discovery of fundamentally novel physical phenomena, making steady headway in the development of magnetic and spintronics devices. The dynamics can be induced and detected electrically, offering new functionalities in advanced electronics at the nanoscale. However, its scattering mechanism is still disputed. Understanding the mechanism in thin films is especially important, because most spintronics devices are made from stacks of multilayers with nanometer thickness. The stacks are known to possess interfacial magnetic anisotropy, a central property for applications, whose influence on the dynamics remains unknown. Here, we investigate the impact of interfacial anisotropy by adopting CoFeB/MgO as a model system. Through systematic and complementary measurements of ferromagnetic resonance (FMR) on a series of thin films, we identify narrower FMR linewidths at higher temperatures. We explicitly rule out the temperature dependence of intrinsic damping as a possible cause, and it is also not expected from existing extrinsic scattering mechanisms for ferromagnets. We ascribe this observation to motional narrowing, an old concept so far neglected in the analyses of FMR spectra. The effect is confirmed to originate from interfacial anisotropy, impacting the practical technology of spin-based nanodevices up to room temperature.

No abstract

Energy-efficient spintronic technology holds tremendous potential for the design of next-generation processors to operate at terahertz frequencies. Femtosecond photoexcitation of spintronic materials generates sub-picosecond spin currents and emission of terahertz radiation with broad bandwidth. However, terahertz spintronic emitters lack an active material platform for electric-field control. Here, we demonstrate a nonlinear electric-field control of terahertz spin current-based emitters using a single crystal piezoelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) that endows artificial magnetoelectric coupling onto a spintronic terahertz emitter and provides 270% modulation of the terahertz field at remnant magnetization. The nonlinear electric-field control of the spins occurs due to the strain-induced change in magnetic energy of the ferromagnet thin-film. Results also reveal a robust and repeatable switching of the phase of the terahertz spin current. Electric-field control of terahertz spintronic emitters with multiferroics and strain engineering offers opportunities for the on-chip realization of tunable energy-efficient spintronic-photonic integrated platforms.

Neuromorphic computing (NC) is gaining wide acceptance as a potential technology to achieve lowpower intelligent devices. To realize NC, researchers investigate various types of synthetic neurons and synaptic devices, such as memristors and spintronic devices. In comparison, spintronics-based neurons and synapses have potentially higher endurance. However, for realizing low-power devices, domain wall (DW) devices that show DW motion at low energies�typically below pJ/bit�are favored. Here, we demonstrate DW motion at current densities as low as 10 6 A/m 2 by engineering the β-W spin−orbit coupling (SOC) material. With our design, we achieve ultralow pinning fields and current density reduction by a factor of 10 4 . The energy required to move the DW by a distance of about 18.6 μm is 0.4 fJ, which translates into the energy consumption of 27 aJ/bit for a bit-length of 1 μm. With a meander DW device configuration, we have established a controlled DW motion for synapse applications and have shown the direction to make ultralow energy spin-based neuromorphic elements.

Nanometer‐thick Co–Fe–B/MgO based structures have been widely accepted as the preferred system for immediate and long‐term goals in magnetic random access memory (MRAM) devices because of excellent spin‐torque efficiency and promise for high‐density MRAM. To realize next‐generation ultra‐low‐power MRAM, further lowering of power consumption in these structures is a crucial ongoing effort. Gilbert damping is one critical material parameter toward lowering energy consumption but is traditionally large (≈10−2) in these Co–Fe–B/MgO systems. Here, Gilbert damping of (1.3 ± 0.3) × 10−3 from a perpendicular double Co–Fe–B/MgO interface system engineered at different boron compositions is reported. Remarkably, this value is achieved with ≈1 nm of Co–Fe–B thickness while maintaining magnetic anisotropy of 0.4 Merg cc−1. An unusual damping trend that scales with layer thickness in high‐boron content films established from both experiments and first‐principles calculations is reported.

We report a comprehensive study on the role of the free layer thickness (tF) in electric-field controlled nanoscale perpendicular magnetic tunnel junctions (MTJs), comprising of free layer structure Ta/Co40Fe40B20/MgO, by using dc magnetoresistance and ultra-short magnetization switching measurements. Focusing on MTJs that exhibits positive effective device anisotropy (Keff), we observe that both the voltage-controlled magnetic anisotropy (ξ) and voltage modulation of coercivity show strong dependence on tF. We found that ξ varies dramatically and unexpectedly from ∼−3 fJ/V-m to ∼−41 fJ/V-m with increasing tF. We discuss the possibilities of electric-field tuning of the effective surface anisotropy term, KS as well as an additional interfacial magnetoelastic anisotropy term, K3 that scales with 1/tF2. Voltage pulse induced 180° magnetization reversal is also demonstrated in our MTJs. Unipolar switching and oscillatory function of switching probability vs. pulse duration can be observed at higher tF, and agrees well with the two key device parameters — Keff and ξ.

Multi‐state spin‐orbit torque (SOT) switching, particularly in magnetic tunnel junction (MTJ) geometry, is promising hardware for artificial intelligence due to its potential to form artificial neurons and to increase storage density per bit. Here, multi‐state switching is demonstrated in Pt/Co40Fe40B20/MgO‐based elliptical MTJ, fabricated on a 200 mm wafer platform, using nanosecond spin‐orbit torque pulses. Multi‐state switching is achieved using MTJs composed of a single ferromagnetic storage layer and without resorting to any domain‐wall pinning step. The genesis of the multi‐states, rather than the expected bistable states in MTJs, come from the formation and stabilization of metastable domains by the voltage pulse. The persistence and the probability of the mid‐states are investigated by two different spin‐orbit torque pulse sequences. Additionally, it is reported that size (>domain‐wall width), exchange constant (<20 × 10−12 J m‐1), current density amplitude (≈threshold values), and pulse sequence are important considerations for realizing multi‐state SOT MTJs. These industry‐grade MTJs can be leveraged to accelerate the development of spintronics‐based neuromorphic devices.

Through a 3D diffraction method combined with HRTEM images, we have successfully determined the specific phase of each FexGe island grown on the Ge substrate.