Attila Kákay

Stacked topological spin textures as emitters for multidimensional spin wave modes

The investigation of propagating spin waves is a key topic of contemporary magnetism research. For the excitation of spin waves with short wavelengths, it was typically necessary to either use transducers with sizes on the order of the desired wavelengths (striplines or point-contacts) or to generate those spin waves parametrically by a double-frequency spatially uniform microwave signal. Only recently, a novel mechanism for the local excitation of spin waves, which overcomes the wavelength limit given by the minimum patterning size has been discovered. This method utilizes the translation of natural topological defects, namely the gyration of spin vortex cores. A spin vortex is characterized by a planar, flux-closing magnetization curl, which tilts out of the plane in the central nanoscopic core region [cf. Fig. 1(a)]. Both, the in-plane rotation sense of the curl (circulation) and the orientation of the perpendicular core (polarity), are independently either positive or negative. The initial study was carried out on a vortex pair system with opposite circulations and equal polarities, in which the two vortices were stacked via a nonmagnetic inter-layer [cf. Fig. 1(b) and 1(c)]. In such a system, spin waves can be generated by lateral magnetic field excitation at the vortex cores. Scanning transmission x-ray microscopy (STXM) was used to directly image these spin waves propagating to the rim of the sample in a spiraling manner [cf. Fig. 1(d)]. Thereby, the resulting spin wave length was found to be directly tunable by the excitation frequency. Moreover, the resulting spin waves were analytically calculated to exhibit a gapless, linear, and non-reciprocal dispersion relation with much shorter wave lengths compared to spin waves of the same frequency in corresponding single layer films.

Origin and Manipulation of Stable Vortex Ground States in Permalloy Nanotubes

We present a detailed study on the static magnetic properties of individual permalloy nanotubes (NTs) with hexagonal cross-sections. Anisotropic magnetoresistance (AMR) measurements and scanning transmission X-ray microscopy (STXM) are used to investigate their magnetic ground states and its stability. We find that the magnetization in zero applied magnetic field is in a very stable vortex state. Its origin is attributed to a strong growth-induced anisotropy with easy axis perpendicular to the long axis of the tubes. AMR measurements of individual NTs in combination with micromagnetic simulations allow the determination of the magnitude of the growth-induced anisotropy for different types of NT coatings. We show that the strength of the anisotropy can be controlled by introducing a buffer layer underneath the magnetic layer. The magnetic ground states depend on the external magnetic field history and are directly imaged using STXM. Stable vortex domains can be introduced by external magnetic fields and can be erased by radio-frequency magnetic fields applied at the center of the tubes via a strip line antenna.

Magnetic domain walls as controllable spin-wave nanochannels

The emerging research field of magnonics (i.e., combining waves and magnetism) provides a new paradigm for information technologies. That is, information can be encoded in coherent spin waves (rather than on moving electrons) that do not suffer from ohmic losses. In the context of transmitting and processing information, it is therefore necessary to find energy-efficient means to enable complex spin-wave circuits that have a high integration density. However, the anisotropic and magnetic-field-dependent propagation characteristics of spin waves mean that it remains a challenge to steer them on the nanometer scale and that many concepts, such as simple geometrical shaping of spin-wave conduits (e.g., bended waveguides), are hindered. To date various on-chip schemes have been explored for guiding spin waves. These methods include the use of waveguides with spatially rotating fields1, 2 or the use of magnetic field wells.3 Most of these concepts, however, are limited in their capacity for further optimization, and they rely on energyconsuming fields/currents. For magnonics-based devices to become fully competitive, novel schemes for channeling spin waves within a few nanometers are thus required. In our work, we make use of the fact that magnetic domain walls (also known as Néel walls)—which form at the interfaces between regions of different magnetic orientation (domains)— create magnetic field inhomogeneities. These inhomogeneities thus act as potential wells and can be used to effectively confine spin waves on very small length scales. Their width is a fundamental material parameter and is typically on the range of tens of nanometers (it may be possible to tune them to even smaller sizes). In this study,4, 5 we report an experimental observation Figure 1. Scanning electron microscope image of the experimentally realized domain wall nanochannel, a 180 Néel wall (yellow line) stabilized along the middle of the magnetic bottle-shaped carrier structure. Kerr microscopy was used to record the domain configuration (the magnetic orientation is indicated by white arrows). A microwave antenna on the top of the carrier structure was used to locally excite spin waves. hrf : Radio frequency in-plane magnetic field.4

Spin-transfer effects in MgO-based tunnel junctions with an out-of-plane free layer and an in-plane polarizer: Static states and steady-state precession

This paper aims to explore potential mechanisms for sustaining steady-state precession in MgO-based magnetic tunnel junctions (MTJ) with an in-plane polarizer and an out-of-plane free layer. The Landau-Lifshitz-Gilbert-Slonczewski equation is analytically and numerically solved for a nano-pillar MTJ with circular cross-section under constant perpendicular applied current and field. It is demonstrated that the spin torque angular asymmetry is sufficient to sustain the spin transfer torque-driven dynamics of spin-torque nano-oscillators.