Spin-torque nano-oscillators (STNOs) are a type of nanoscale microwave auto-oscillators utilizing spin-torque to generate magnetodynamics with great promise for applications in microwaves, magnetic memory, and neuromorphic computing. Here, we report the first demonstration of exchange-spring STNOs, with an exchange-spring ([Co/Pd]-Co) reference layer and a perpendicular ([Co/Ni]) free layer. This magnetic configuration results in high-frequency (>10 GHz) microwave emission at a zero magnetic field and exchange-spring dynamics in the reference layer and the observation of magnetic droplet solitons in the free layer at different current polarities. Our demonstration of bipolar and field-free exchange-spring-based STNOs operating over a 20 GHz frequency range greatly extends the design freedom and functionality of the current STNO technology for energy -efficient high-frequency spintronic and neuromorphic applications.

Magnetic droplets are a type of non-topological magnetic soliton, which are stabilised and sustained by spin-transfer torques for instance. Without this, they would collapse. Here Ahlberg et al show that by decreasing the applied magnetic field, droplets can be frozen, forming a static nanobubble Magnetic droplets are non-topological magnetodynamical solitons displaying a wide range of complex dynamic phenomena with potential for microwave signal generation. Bubbles, on the other hand, are internally static cylindrical magnetic domains, stabilized by external fields and magnetostatic interactions. In its original theory, the droplet was described as an imminently collapsing bubble stabilized by spin transfer torque and, in its zero-frequency limit, as equivalent to a bubble. Without nanoscale lateral confinement, pinning, or an external applied field, such a nanobubble is unstable, and should collapse. Here, we show that we can freeze dynamic droplets into static nanobubbles by decreasing the magnetic field. While the bubble has virtually the same resistance as the droplet, all signs of low-frequency microwave noise disappear. The transition is fully reversible and the bubble can be thawed back into a droplet if the magnetic field is increased under current. Whereas the droplet collapses without a sustaining current, the bubble is highly stable and remains intact for days without external drive. Electrical measurements are complemented by direct observation using scanning transmission x-ray microscopy, which corroborates the analysis and confirms that the bubble is stabilized by pinning.

Magnetic droplets, a class of highly nonlinear magnetodynamic solitons, can be nucleated and stabilized in nanocontact spin-torque nano-oscillators. Here we experimentally demonstrate magnetic droplets in magnetic tunnel junctions (MTJs). The droplet nucleation is accompanied by power enhancement compared with its ferromagnetic resonance modes. The nucleation and stabilization of droplets are ascribed to the double-CoFeB free-layer structure in the all-perpendicular MTJ, which provides a low Zhang-Li torque and a high pinning field. Our results enable better electrical sensitivity in fundamental studies of droplets and show that the droplets can be utilized in MTJ-based applications and materials science.

Manipulation and detection of spins at the nanoscale is of considerable contemporary interest as it may not only facilitate a description of fundamental physical processes but also plays a critical role in the development of spintronic devices. Here, we describe the application of a novel combination of transient grating excitation with Lorentz ultrafast electron microscopy to control and detect magnetization dynamics with combined nanometer and picosecond resolutions. Excitation of Ni80Fe20 thin film samples results in the formation of transient coherently precessing magnetic gratings. From the time-resolved results, we extract detailed real space information of the magnetic precession, including local magnetization, precession frequency, and relevant decay factors. The Lorentz contrast of the dynamics is sensitive to the alignment of the in-plane components of the applied field. The experimental results are rationalized by a model considering local demagnetization and the phase of the precessing magnetic moments. We envision that this technique can be extended to the study of spin waves and dynamic behavior in ferrimagnetic and antiferromagnetic systems.

We investigate the origin of the experimentally observed varying current-frequency nonlinearity of the propagating spin wave mode in nanocontact spin torque oscillators. Nominally identical devices with 100 nm diameter are characterized by electrical microwave measurements and show large variation in the generated frequency as a function of drive current. This quantitative and qualitative device-to-device variation is described in terms of continuous and discontinuous nonlinear transitions between linear current intervals. The thin-film grain microstructure in our samples is determined using atomic force and scanning electron microscopy to be on the scale of 30 nm. Micromagnetic simulations show that the reflection of spin waves against the grain boundaries results in standing wave resonance configurations. For a simulated device with a single artificial grain, the frequency increases linearly with the drive current until the decreased wavelength eventually forces another spin wave antinode to be formed. This transition results in a discontinuous step in the frequency versus current relation. Simulations of complete, randomly generated grain microstructures additionally shows continuous nonlinearity and a resulting device-to-device variation in frequency that is similar to the experimental levels. The impact of temperature from 4 to 300 K on the resonance mode-transition nonlinearity and frequency noise is investigated using simulations and it is found that the peak levels of the spectral linewidth as a function of drive current agree quantitatively with typical levels found in experiments at room temperature. The impact of the grain microstructure on the localized oscillation modes is also investigated.

We demonstrate an approach for improving the spectral linewidth of a spin torque nano-oscillator (STNO). Using He + ion irradiation, we tune the perpendicular magnetic anisotropy (PMA) of the STNO free layer such that its easy axis is gradually varied from strongly out-of-plane to moderate in-plane. As the PMA impacts the non-linearity N of the STNO, we can, in this way, control the threshold current, the current tunability of the frequency, and, in particular, the STNO linewidth, which dramatically improves by two orders of magnitude. Our results are in good agreement with the theory for nonlinear auto-oscillators, confirm theoretical predictions of the role of the nonlinearity, and demonstrate a straightforward path toward improving the microwave properties of STNOs.

We demonstrate field and current controlled magnetodynamics in nanocontact spin-torque nano-oscillators based on orthogonal magnetic tunnel junctions. We systematically analyze the microwave properties (frequency f, linewidth Delta f, power P, and frequency tunability df/dI) with their physical origins-perpendicular magnetic anisotropy, dampinglike and fieldlike spin transfer torque (STT), and voltage-controlled magnetic anisotropy (VCMA). These devices present several advantageous characteristics: high emission frequencies (f>20 GHz), high frequency tunability (df/dI=0.25 GHz/mA), and zero-field operation (f similar to 4 GHz). Furthermore, detailed investigation of f(H, I) reveals that df/dI is mostly governed by the large VCMA [287 fJ/(V m)], while STT plays a negligible role.

The mechanism of the magnetic properties on the Zn2+ substituted Fe3O4 film have been investigated based on first principle calculations. It is found that the surface effect plays an important role in the occupation of Zn ion, and in turn changes the magnetic moment. It may also destroy the half metallic behavior of Fe3O4 film even if the Zn2+ concentration only is one Zn2+ per unit cell (4%), which is different from that in bulk material.

As an important parameter for characterizing the magnetization dynamics, Gilbert damping constant a in a thin film or a multilayer is generally extracted from the linear fitting of the frequency-dependence of the ferromagnetic resonance linewidth, sometimes accompanied with a tiny deviation of the linewidth to a smaller value at the low-frequency or high-frequency region due to the two-magnon scattering with an in-plane-field configuration, in which an in-plane magnetic field H perpendicular to a microwave field h was applied in film plane during measurement. In contrast, here we report, in ultrathin Gd/Cr/Fe multilayers, an anomalously large linewidth in the film plane at the low-frequency region. For the first time, we have successfully extracted the Gilbert damping constant from perfect theoretical fitting to the experimental data, by considering the effective direction of the magnetization around in precession staying out of the film plane when the in-pane H at which the precession starts is below the saturation field. This magnetization deviation from the film plane is found to have an obvious contribution to the enhanced linewidth caused by two magnon scattering, while slightly reduce the intrinsic linewidth. Under the same resonance frequency, the deviation angle reaches the maximum values at t(Cr) = 1.0 nm while decreases when t(Cr) increases to 1.5 nm, which coincides with the trend of the surface perpendicular anisotropy constant K-perpendicular to. A reduced intrinsic damping constant alpha is obtained as the introduction of Gd layer and Cr layer as a result of the competition between the spin pumping effect and the interfacial effects at the Fe/Gd and Fe/Cr interfaces. While the decreasing alpha for film with Cr layer thickness increasing to 1.5 nm might means the contribution of the electron density of states at the Fermi energy n(E-F). This study offers an effective way to accurately obtain the intrinsic damping constant of spintronic materials/devices, which is essential for broad applications in spintronics.

We demonstrate low-operational-current W/Co20Fe60B20/MgO spin Hall nano-oscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates. Thanks to a record high spin Hall angle of the beta-phase W (theta(SH) = -0.53), a very low threshold current density of 3.3 x 10(7) A/cm(2) can be achieved. Together with their very wide frequency tunability (7-28GHz), promoted by a moderate perpendicular magnetic anisotropy, HiR-Si/W/CoFeB based SHNOs are potential candidates for wide-band microwave signal generation. Their CMOS compatibility offers a promising route towards the integration of spintronic microwave devices with other on-chip semiconductor microwave components.