Current-induced Domain Wall Dynamics Probed by Electrical Transport Measurements

Abstract

During the last decades, the rising need for data storage devices exhibiting high storage density, fast access times, long data retention and a low power consumption inspired research on new materials and novel memory concepts [PHT08; Cow+07; Xu+08; Pav+97]. Amongst other approaches the magnetic hard disk drive [MFC06] is probably the best known data storage device. Its basic concept is the interpretation of information (bits 0 and 1) encoded in magnetic domains whose magnetization is pointing in either the left/right or up/down direction. The information is read and written using read/write heads, which are mounted on cantilever arms moving above the spinning magnetic disks. Based on the same storage concept, the MRAM and racetrack memory technology introduced novel mechanisms of manipulating the magnetic domains and especially the domain walls using electric currents, which are directly injected into the magnetic material. Hereby, the access time and power consumption could be drastically reduced since no mechanical components are required. Furthermore, the virtually unlimited endurance and data retention is kept for which the magnetic hard disk drive is well known.This opened a new field of research, called spintronics, focusing on the interaction of spin polarized currents with magnetic domains and especially their interaction with domain walls separating the domains from each other. For applications, a number of domain walls need to be shifted at the same time by the same distance in order to keepthe size of the domains and therefore the stored information. However, this turned out to be experimentally challenging since several mechanisms such as Joule heating, Oersted field effects and spin torque are involved in the current-induced domain wall motion.Until now, two main spin torques, the adiabatic [Ber78] and non-adiabatic [ZL04; Thi+05] spin torque, acting on the domain walls have been theoretically and experimentally identified allowing for an efficient manipulation of the magnetization on the nanoscale. In experiments a strong dependence of these torques on the material composition and the magnetization configuration (in-plane or out-of-plane) has been found. First results on in-plane magnetized nanostructures exhibiting wide and complex domain wall structures revealed current-induced domain wall motion requiring large critical current densities (> 1 · 1012 A/m2) [Kl¨a+05; Hey+09], which is accompanied by Joule heating effects. Thus the interest shifted to out-of-plane magnetized materials for which large non-adiabatic contributions due to narrow domain walls were predicted. These non-adiabatic effects are expected to significantly lower the necessary current densities and should lead to higher domain wall velocities. For a quantitative comparison of the experiments the dimensionless non-adiabaticity parameter was introduced being directly proportional to the domain wall velocity. Nevertheless, a vast variety of different and partly contradicting values has been found, highlighting the importance of detailed and fundamental studies of the mechanisms involved in current-induced domain wall motion and their dependence on the material compositions.Besides Joule heating effects, the local injection of a charge current into nanowire structures also leads to the creation of an concentric magnetic field (Biot-Savart-law), the Oersted field, which is commonly used to manipulate the local magnetization [Ilg+08; You08] and nucleate domain walls [Koy+11; Ued+11; Koy+08; NIA11; Fuk+10; Tan+11;Chi+10; Ohs+11]. However, on the nanoscale, high currents are needed to create sufficiently large magnetic fields. These injected current densities are drastically increasing when the wire dimensions are further reduced thus causing unwanted Joule heating effects.In this thesis, measurements for the characterization of different magnetic materials suitable for spintronic applications are presented. Furthermore, new methods for separating the contribution of spin torque, Oersted field and Joule heating effects to the current-induced domain wall dynamics are introduced. Finally, the non-adiabaticity parameter is determined in various systems. The last part of this thesis deals with experiments using low current densities to significantly lower the effects of Joule heating. At room temperature, the investigation of thermally activated magnetization dynamics allows to test the validity and robustness of two independent approaches to extract the non-adiabaticity parameter at the same time on one sample.