BCMaterials Fortnightly Seminars #24

“Amphiphilic diblock copolymers for POCHBA” María San Sebastián (BCMaterials)        “Exchange bias of spring-like domain walls” Rafael Morales (UPV/EHU) The exchange coupling between antiferromagnetic (AFM) and ferromagnetic (FM) materials yields a shift of the hysteresis loop along the magnetic field axis. The magnitude of this shift is known as exchange bias field, Heb. It was experimentally and theoretically demonstrated that Heb decreases with the FM thickness, following an inverse proportionality law, which is assumed to be valid for any FM thickness thinner than the ferromagnetic correlation length [1]. We demonstrate that AFM/FM bilayers with spring-like domain walls can break the inverse proportionality law even for FM thicknesses smaller than the ferromagnetic correlation length. FeF2/NiFe bilayers with NiFe thicknesses between 10 and 140 nm were deposited by electron beam evaporation. This magnetic system shows spring-like domain walls, parallel to the AFM/FM interface, at low temperature. The thickness dependence of Heb significantly deviates from the inverse proportionality law as the NiFe thickness increases. The exchange bias magnitude of spring-like domain walls is always lower than the predicted by the inverse proportionality law. A model of magnetization planes parallel to the interface accounts for the experimental results. Both the shape of the hysteresis loops and the magnitude of Heb were reproduced in excellent agreement with the experiment [2]. Moreover, the thermal evolution of the FM spin structure leads to an anomalous temperature dependence of Heb. These findings reveal the importance of the FM domain structure on exchange bias.   Work supported by Spanish MINECO FIS2013-45469 and MAT2012-33037, EU FP7- IRSES-2012-318901, Catalan DUSI 2014SGR220, EU FEDER (Una manera de hacer Europa), University of Barcelona, and the U.S. Office of Basic Energy Science, U.S. Department of Energy, BES-DMS funded by the Department of Energy’s Office of Basic Energy Science, DMR under Grant No. DE FG02 87ER-45332.   [1] J. Nogués and I. K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999) [2] R. Morales et al. Phys. Rev. Lett. in-press