Microwave assisted resonant domain wall nucleation in permalloy nanowires

Abstract

We have designed a system to study microwave assisted domain wall nucleation in permalloy nanowires. We find a substantial decrease in the nucleation field when microwave fields are applied, in comparison to pulse fields. A clear resonance peak is observed in the frequency dependence of the nucleation field, which coincides with the uniform mode ferromagnetic resonance frequency. Owing to the well-defined nucleation process, the switching field distribution is small in contrast to previous reports. Our results show that localized microwave field provides an efficient tool for injecting domain walls into magnetic nanowires. * E-mail address: hayashi.masamitsu@nims.go.jp 2 Recent progress in the understanding of spin transfer torque and its related phenomena enabled development of domain wall based memory 1 and logic devices 2 . In particular, a magnetic shift register mimicking the so called "Racetrack memory" 1 has been demonstrated using localized magnetic fields to inject domain walls and spin polarized current to move them [3][4][5] . One challenge of exploring materials for domain wall based device applications is the difficulty in injecting domain walls into the nanowire since it typically requires large localized magnetic field 3,6 . Solid understanding of the nucleation process is thus required for further development of such devices.Microwave assisted magnetization reversal (MAMR) provides a unique tool to lower the magnetization switching field 7,8 . The concept has been demonstrated in a number of systems 7,9-18 but the understanding of the underlying physics is still developing. In most of the systems, it is likely that the magnetization reversal process involves nucleation and propagation of domain walls (exception is the coherent rotation of magnetic moments 12 ).Both domain wall (reversed domain) nucleation and propagation can be enhanced by the application of microwave field. However, its response may differ depending on the applied microwave frequency and thus may cause broadening of the switching field 9,10,17,18 . Thus to gain further insight into MAMR, it is essential to distinguish the effect of microwave field application on domain wall nucleation 19 and domain wall propagation 20,21 .Here we design a system to study the effect of microwave field application on a well-defined domain wall nucleation process in permalloy naonwires. Substantial drop in the nucleation field is observed for a microwave field frequency of ~3 GHz. We find a clear resonance peak in the frequency dependence of the nucleation field, in which the peak position corresponds to the ferromagnetic resonance uniform excitation mode. Fig. 1(a)). The width of line A parallel to the permalloy nanowire is set to ~2 m, large enough to cover the 600 nm wide nanowire. A signal generator or a pulse generator is connected to field generation lines A and/or B to apply localized magnetic field. Note that the Oersted field generated at line A (B), when current is passed along the line, is pointing transverse (parallel) to the wire...

Magnetic domain walls: types, processes and applications

“…Specific choices of the polarity and duration of the current pulses and polarity of magnetic fields led to stochastic pinning of DWs in the nanowire region bounded by the current lines as well as the injection of vortex and transverse walls into the nanowire. In the same year, Hayashi et al [58] used short lived current pulses (with ∼ns lifetimes) to decrease the magnetic fields required to nucleate DWs in IM nanowires. They found that the nucleation field reduced by around half and the distribution of the switching fields decreased in width as well and the authors attributed this to using localised fields generated by the current lines.…”

Microwave assisted switching in two-phase nanowires

“…Micromagnetic simulations have shown that, in 16 nm composite nanopillars, consisting of ferromagnetically coupled soft and hard magnetic sections, MAS can be realized even if the damping constant of the hard magnetic section is large [11]. Several works have focused on the MAS in nanowires [12][13][14] but mainly in the form of permalloy thin film strip waveguides. The wall structure and dynamic response to an applied field (wall velocity and Walker fields) in nanostrips depend sensitively on the geometrical characteristics, due to the demagnetizing field arising from the precessional magnetization component perpendicular to the strip plane [15].…”

Current-driven periodic domain walls injection in a ferromagnetic nanostrip with a modified perpendicular magnetic anisotropy region

“…A number of alternatives to the use of magnetic fields, including electric fields or microwave-assisted generation of DWs in patterned nanowires, have been investigated [14,15]. It has been shown that a Slonczewski-like spin torque assisted by an Oersted field allows controlled injection of a series of DWs, providing a controlled method for writing binary information [16].…”