Giant rectified voltages from magnetization dynamics of an atomically sharp domain wall.

Magnetization dynamics is of great interest in the aim of using spins in nanoscale information technology, which ultimately should reach the atomic size. In the present work, we explore magnetization and spin dynamics in atomic ferromagnetic contacts both experimentally and theoretically. We demonstrate that domain walls induce a giant rectification effect as the DC voltages measured across the contacts are greatly enhanced by the presence of a domain wall. This effect is understood using multiscale dynamic simulations showing that the atomic sized walls oscillate, both in position and size, when submitted to the radio-frequency excitation. This leads to an increase by three orders of magnitude due to the large atomic scale spin excursion at resonance in the presence of an atomic sized domain wall. Beside the interesting amplified rectification, this could also be used as a unique tool to measure dynamical properties at the atomic scale.

Propagation of magnetic vortices using nanocontacts as tunable attractors.

Magnetic vortices in thin films are in-plane spiral spin configurations with a core in which the magnetization twists out of the film plane. Vortices result from the competition between atomic-scale exchange forces and long-range dipolar interactions. They are often the ground state of magnetic dots, and have applications in medicine, microwave generation and information storage. The compact nature of the vortex core, which is 10-20 nm wide, makes it a suitable probe of magnetism at the nanoscale. However, thus far the positioning of a vortex has been possible only in confined structures, which prevents its transport over large distances. Here we show that vortices can be propagated in an unconstrained system that comprises electrical nanocontacts (NCs). The NCs are used as tunable vortex attractors in a manner that resembles the propelling of space craft with gravitational slingshots. By passing current from the NCs to a ferromagnetic film, circulating magnetic fields are generated, which nucleate the vortex and create a potential well for it. The current becomes spin polarized in the film, and thereby drives the vortex into gyration through spin-transfer torques. The vortex can be guided from one NC to another by tuning attractive strengths of the NCs. We anticipate that NC networks may be used as multiterminal sources of vortices and spin waves (as well as heat, spin and charge flows) to sense the fundamental interactions between physical objects and fluxes of the next-generation spintronic devices.

Large RF susceptibility of transverse domain walls.

Changes in domain wall resistance under radio-frequency (RF) irradiation are experimentally studied for transverse walls. An original experimental technique is applied to the measurement in a permalloy nano-stripe with a notch, where the walls are found to provide a largely enhanced resistive response as compared to saturated domains. Their susceptibility is found to be an order of magnitude larger than that of the domains in a frequency range between 5 and 20 GHz. We argue that the RF fields induce an internal distortion of the magnetization profile that depends on the shape of the domain wall.

Understanding nanoscale temperature gradients in magnetic nanocontacts.

We have determined the temperature profile in magnetic nanocontacts under applied current densities typical of spin-torque oscillators (∼10(8) A/cm2). The study combines experimental measurements of the electrical and magnetic properties of the nanocontacts and full three-dimensional simulations of the heat and current flow in these systems. It is found that the quadratic current-induced increase of the resistance due to Joule heating is independent of the applied temperature from 6 to 300 K. In terms of magnetization dynamics, the measured current-induced vortex nucleation, a thermally activated process, is found to be consistent with local temperatures increases of between 147 and 225 K. Simulations reproduce the experimental findings and show that significant thermal gradients exist out to 450 nm from the nanocontact.