PHYSICAL REVIEW B 96, 054421 (2017) Magnetic aftereffects in CoFeB/Ta/CoFeB spin valves of large area R. Morgunov, 1, 2 , * Y. Lu, 3 M. Lavanant, 3 T. Fache, 3 X. Deveaux, 3 S. Migot, 3 O. Koplak, 1 A. Talantsev, 1, 4 and S. Mangin 3 1 Institute of Problems of Chemical Physics, 142432, Chernogolovka, Moscow, Russia 2 Tambov State Technical University, 392000, Tambov, Russia 3 Institut Jean Lamour, UMR 7198 CNRS, Université de Lorraine, 54506 Vandoeuvre, Nancy, France 4 Department of Emerging Materials Science, DGIST, 42988, Daegu, South Korea (Received 5 February 2017; revised manuscript received 10 July 2017; published 15 August 2017) Magnetic aftereffect measurements on a synthetic antiferromagnet with strong perpendicular anisotropy are presented. Two types of magnetic transitions are observed in the CoFeB/Ta/CoFeB bilayer where the two ferromagnetic CoFeB layers are antiferromagnetically coupled: the transition from a parallel to an antiparallel CoFeB layers magnetization alignment and the transition between two possible antiparallel magnetization states. Magnetic relaxation measurements show a complete reversal in the case of the transition between the two antiferromagnetic configurations. The experimental data can be well fitted in the frame of extended exponential relaxation using the Fatuzzo-Labrune model. Consequently, the domain nucleation and domain-wall propagation can be studied as a function of temperature and applied field. DOI: 10.1103/PhysRevB.96.054421 I. INTRODUCTION Synthetic antiferromagnets (SAFs) with perpendicular anisotropy (p-SAF) have been designed to be used for applications in magnetic memory and data storage elements engineering. Domain-wall dynamics in systems with perpen- dicular magnetic anisotropy (PMA) attracts much attention for fundamental studies and potential applications [1,2]. Indeed, domain-wall motion in thin magnetic films provides interesting opportunities for the design of high-performance spintronics devices [1] such as racetrack memories [2], domain-wall logic circuits [3], and domain-wall nano-oscillators [4]. One of the physical constraints for fast dynamics is the presence of the Walker breakdown. The Walker limit corresponds to the magnetic field threshold value leading to the decrease of domain-wall velocity under increasing magnetic field [5]. Quite recently, it has been shown through numerical and analytical modeling that the Walker breakdown limit could be extended or completely eliminated in antiferromagnetically coupled magnetic nanowires such as SAF. SAF nanowires would allow high domain-wall velocity driven by field and/or current as compared to conventional nanowires. In this context, we focused on the analyses of magnetization dynamics in a model CoFeB/Ta/CoFeB bilayer. It consists of two ultrathin antiferromagnetically exchange-coupled ferro- magnetic CoFeB layers with strong perpendicular anisotropy. The multilayer stack was grown directly on a GaAs substrate. Quasistatic magnetization versus field hysteresis loops were measured for different temperatures ranging from 5 to 300 K. It allowed a field-temperature (H -T ) magnetization switching diagram to be constructed [6]. Now we would like to focus on the magnetization dynamics during the transition from one magnetic state to the other. Domain-wall dynamics in p-SAF systems is very unusual [1,2] and becomes more complicated for multilayered samples. Another feature of thin spin valves is very unusual exponen- tial relaxation [7] indicating single-potential barrier relief for * Corresponding author: morgunov2005@yandex.ru domain walls. Observation of reversal magnetization dynamics in single ferromagnetic layers and its comparison with that in the double-ferromagnetic-layer system allows one to judge the contribution of the interlayer interaction to domain-wall dynamics. Many papers are focused on time-dependent switching of the multilayered heterostructure in the nanosecond scale [1–4]. The damping parameter governs effectiveness of switching of Giant Magnetoresistance devices and their convenience for high-frequency readout in the microwave frequency range. The results mentioned above relate to nanosized spin valves designed for memory cells. Reversal magnetization of these small devices obeys the macrospin approach. Meanwhile, magnetic nanoparticles sensor technology requires larger areas of devices for better resolution and sensitivity to magnetic field. The magnetic aftereffect in these struc- tures has a larger time scale and magnetization reversal cannot be considered in terms of the macrospin model only. In the present paper, we will pay attention to another problem, which dramatically affects the quality of the large (∼1–10 mm 2 ) area of CoFeB multilayered structures. Thin ferromagnetic layers as well as CoFeB multilayered struc- tures manifest slow magnetic relaxation due to domain-wall dynamics [8]. In [8], magneto-optic Kerr effect (MOKE) microscopy of those samples studied in this paper was reported. This process is very complicated due to the ther- mally activated character of the domain-wall unpinning, peculiar interaction between ferromagnetic layers, and inho- mogeneous distribution of magnetization through the layer [Fig. 1(a)]. In this paper, we present a detailed study of the time- resolved (for time ranging from seconds) response of mag- netization in the CoFeB/Ta/CoFeB bilayer and the CoFeB single-layer samples as a function of temperature and applied magnetic field. Similarities and differences between single- layer magnetization reversal and double-layer simultaneous reversal (direct transition between the antiparallel magnetic states: AP + and AP −) in the CoFeB/Ta/CoFeB bilayer will be discussed. The goals of the paper are as follows: 2469-9950/2017/96(5)/054421(10) 054421-1 ©2017 American Physical Society