Sohrab Redjai Sani

Spin-wave-beam driven synchronization of nanocontact spin-torque oscillators

The synchronization of multiple nanocontact spin-torque oscillators (NC-STOs) is mediated by propagating spin waves (SWs). Although it has been shown that the Oersted field generated in the vicinity of the NC can dramatically alter the emission pattern of SWs, its role in the synchronization behaviour of multiple NCs has not been considered so far. Here we investigate the synchronization behaviour in multiple NC-STOs oriented either vertically or horizontally, with respect to the in-plane component of the external field. Synchronization is promoted (impeded) by the Oersted field landscape when the NCs are oriented vertically (horizontally) due to the highly anisotropic SW propagation. Not only is robust synchronization between two oscillators observed for separations larger than 1,000 nm, but synchronization of up to five oscillators, a new record, has been observed in the vertical array geometry. Furthermore, the synchronization can no longer be considered mutual in nature.

Spin wave mode coexistence on the nanoscale: a consequence of the Oersted field-induced asymmetric energy landscape

It has been argued that if multiple spin wave modes are competing for the same centrally located energy source, as in a nanocontact spin torque oscillator, that only one mode should survive in the steady state. Here, the experimental conditions necessary for mode coexistence are explored. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field, which leads to a physical separation of the modes, and is further promoted by spin wave localization at reduced applied field angles. Finally, both simulation and experiment reveal a low frequency signal consistent with the intermodulation of two coexistent modes.

Nanostructured MnGa films on Si/SiO2 with 20.5 kOe room temperature coercivity

Nanostructured Mn67Ga33 films exhibiting high room temperature coercivity (HC = 20.5 kOe) have been prepared by sputtering onto thermally oxidized Si substrates. Both the morphology and the coercivity of the films can be tuned by varying the growth parameters. The low deposition rate film, sputtered at a reduced power and working pressure, demonstrates a discontinuous island-like growth and the highest HC. The large HC is linked to the presence of the high anisotropy DO22 Mn3Ga phase and the single domain character of the exchange isolated, dipolar interacting, single crystal islands.

Spin transfer torque generated magnetic droplet solitons (invited)

We present recent experimental and numerical advancements in the understanding of spin transfer torque generated magnetic droplet solitons. The experimental work focuses on nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has an easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The NC-STO resistance and microwave signal generation are measured simultaneously as a function of drive current and applied perpendicular magnetic field. Both exhibit dramatic transitions at a certain current dependent critical field value, where the microwave frequency drops 10 GHz, modulation sidebands appear, and the resistance exhibits a jump, while the magnetoresistance changes sign. We interpret these observations as the nucleation of a magnetic droplet soliton with a large fraction of its magnetization processing with an angle greater than 90°, i.e., around a direction opposite that of the applied field. This interpretation is corroborated by numerical simulations. When the field is further increased, we find that the droplet eventually collapses under the pressure from the Zeeman energy.

Current induced vortices in multi-nanocontact spin-torque devices

We demonstrate spin transfer torque (STT) switching in multi-nanocontact STT devices fabricated using hole mask colloidal lithography. We also study the STT device resistance and switching properties as a function of applied magnetic field and nanocontact current. At low nanocontact current, magnetoresistance measurements show sharp, single-step switching at low switching fields. When the current is increased, the switching becomes multistep, and the switching field increases dramatically. We explain these results as arising from a transition from a predominantly single domain like switching to switching involving a vortex state. Micromagnetic simulations corroborate this picture, indicating that a single magnetic vortex nucleates in between the nanocontacts through the influence from the total Oersted field generated by the nanocontact ensemble.

Recent Advances in Nanocontact Spin-Torque Oscillators

We present a comprehensive review of the most recent advances in nanocontact spin torque oscillators (NC-STOs). NC-STOs are highly tunable, with both applied magnetic field and dc, broadband microwave signal generators. As opposed to the nanopillar geometry, where the lateral cross section of the entire device has been confined to a typically <;100 nm diameter, in NC-STOs, it is only the current injection site that has been laterally confined on top of an extended magnetic film stack. Three distinct material combinations will be discussed: 1) a Co/Cu/NiFe pseudospin valve (PSV) where both the Co and NiFe have a dominant in-plane anisotropy; 2) a Co/Cu/[Co/Ni]4 orthogonal PSV where the Co/Ni multilayer has a strong perpendicular anisotropy; and 3) a single NiFe layer with asymmetric non-magnetic Cu leads. We explore the rich and diverse magnetodynamic modes that can be generated in these three distinct sample geometries.

Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators

Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current–field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field

The nano-scale spin torque oscillator (STO) is a compelling device for on-chip, highly tunable microwave frequency signal generation. Currently, one of the most important challenges for the STO is to increase its longer-time frequency stability by decreasing the 1/f frequency noise, but its high level makes even its measurement impossible using the phase noise mode of spectrum analyzers. Here, we present a custom made time-domain measurement system with 150 MHz measurement bandwidth making possible the investigation of the variation of the 1/f as well as the white frequency noise in a STO over a large set of operating points covering 18–25 GHz. The 1/f level is found to be highly dependent on the oscillation amplitude-frequency non-linearity and the vicinity of unexcited oscillation modes. These findings elucidate the need for a quantitative theoretical treatment of the low-frequency, colored frequency noise in STOs. Based on the results, we suggest that the 1/f frequency noise possibly can be decreased by improving the microstructural quality of the metallic thin films.

Microwave Signal Generation in Single-Layer Nano-Contact Spin Torque Oscillators

We demonstrate spin transfer torque (STT) driven microwave signal generation, from about 250 MHz to above 3 GHz, in single permalloy layers underneath a nano-contact with diameter of 100 nm. The threshold current for signal generation is found to be strongly hysteretic, the microwave signal shows a number of harmonics, zero-field operation is straightforward, and the microwave frequency increases quasi-linearly with drive current. All observations are consistent with STT driven motion of a vortex-antivortex pair nucleated by the Oersted field underneath the nano-contact. While the generated power is about 10 dB smaller than the best GMR based nanocontact spin torque oscillators, the linewidth of 6-100 MHz is of the same order.

Magnetic droplet solitons in orthogonal spin valves

We review the recent experimental advancements in the realization and understanding of magnetic droplet solitons generated by spin transfer torque in orthogonal nanocontact based spin torque nanooscillators (STNOs) fabricated on extended spin valves and spin valve nanowires. The magnetic droplets are detected and studied using the STNO microwave signal and its resistance, the latter both quasistatically and time-resolved. The droplet nucleation current is found to have a minimum at intermediate magnetic field strengths and the nature of the nucleation changes gradually from a single sharp step well above this field, mode-hopping around the minimum, and continuous at low fields. The mode-hopping and continuous transitions are ascribed to droplet drift instability and re-nucleation at different time scales, which is corroborated by time-resolved measurements. We argue that the use of tilted anisotropy fixed layers could reduce the nucleation current further, move the nucleation current minimum to lower fields, and potentially remove the need for an applied magnetic field altogether. Finally, evidence of an edge mode droplet in a nanowire is presented.