Reversal asymmetry and anomalous magnetic viscosity in exchange-bias systems
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Reversal asymmetry and anomalous magnetic viscosity in exchange-bias systems V. Z. C. Paes1, R. F. Lopes1,2 , J. Geshev1,a 1 Instituto de Física, UFRGS, Porto Alegre, Rio Grande do Sul 91501-970, Brazil 2 Instituto Federal Sul-rio-grandense, Câmpus Sapiranga, Sapiranga, Rio Grande do Sul 93804-870, Brazil
Received: 5 July 2020 / Accepted: 2 September 2020 © Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Employing vector magnetometry, interaction plots and model simulations, we probe the magnetization reversal in Co/Cu/IrMn exchange-biased trilayers. In the films with thin or no Cu spacer, the magnetization reversal is governed by domain-wall motion along one branch of the easy-axis hysteresis loop and by coherent rotation along the other. For thicker Cu spacers, both magnetization paths are dominated by coherent rotation. The latter also appears to be responsible for the reversal when the magnetic field is applied away from the exchange-bias direction for all films. We reveal a delayed domain-wall reversal along the descending-field path as compared to that predicted for coherent rotation. By exploiting in-field interaction plots, we evidenced a correlation between the asymmetric reversal and the exchange-bias coupling. Anomalous viscosity along recoil curves is detected and attributed to strong ferromagnetic coupling into the ferromagnet. The anomaly tends to fade away with the increase in the bias.
1 Introduction Heterostructures presenting interfaces between materials with unlike magnetic anisotropy parameters display a series of attractive properties. Among them, one finds the exchangebias (EB) phenomenon [1–3], typically observed in systems composed by a ferromagnet (FM) in atomic contact with an antiferromagnet (AF). Its most known manifestations are the in-field displacement of the magnetization (M) versus the external magnetic field (H ) hysteresis loop, i.e., the so-called EB field Heb , and the coercivity enhancement. Another signature of EB structures is the asymmetry of the hysteresis loop, i.e., the difference in shape observed when comparing its ascending and descending branches. This feature has been studied in a variety of EB systems in the past, using distinct approaches [4–8]. The hysteresis loops’ asymmetry of CoFe/IrMn bilayers has been associated with differences in the nucleation process [8]. It has been obtained that the magnetization reversal in Fe/MnF2 and Fe/FeF2 thin films occurs via either domain-wall (DW) motion or coherent rotation (CR) on the opposite sides of the same hysteresis loop [5]. A similar study on Co/CoO multilayers [7] has shown that, above the blocking temperature, magnetization rotation is observed in both branches; at lower temperatures, however, after the EB is set, the reversal of the FM spins toward the direction opposite to that of the bias field is strongly affected,
a e-mail: [email protected] (corresponding author)
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