Fast domain wall dynamics in amorphous and nanocrystalline magnetic microwires R. Varga a,n , P. Klein a , K. Richter a , A. Zhukov b , M. Vazquez c a Institute of Physics, Faculty of Science, UPJS, Park Angelinum 9, 041 54, Kosice, Slovakia b Dept. Fisica de Materiales, Fac. Quimica, UPV/EHU, San Sebastian, Spain c Instituto de Ciencia de Materiales de Madrid, CSIC, Sor Juana Ine ´s de la Cruz 3, 28049 Cantoblanco, Madrid, Spain article info Available online 25 February 2012 Keywords: Domain structure Domain wall dynamics Magnetic microwires Magnetization processes abstract We have studied the effect of thermal treatment on the domain wall dynamics of FeSiB and FeCoMoB microwires. It was shown that annealing in transversal magnetic field increases the domain wall mobility as well as the domain wall velocity. Annealing under the tensile stress hinders the appearance of the monodomain structure but application of tensile stress leads to the magnetic bistability having the domain wall mobility twice higher that in as-cast state. Further increase of the tensile stress reduces the domain wall mobility but the domain wall velocity increases as a result of the decrease of critical propagation field. Annealing of the FeCoMoB microwire by Joule heating leads to introduction of the circular anisotropy that favors the vortex domain wall. Such treatment increases the domain wall mobility as well as the maximum domain wall velocity. & 2012 Elsevier B.V. All rights reserved. 1. Introduction The amorphous microwires are ideal materials to study the domain wall dynamics [1–3]. They consist of metallic nucleus (with diameter between 1 and 30 mm) surrounded by glass coating (with thickness of 2 to 20 mm) and they are prepared by Taylor–Ulitovsky technique [1]. Due to their amorphous nature, the main anisotropy that governs their magnetic properties is a magnetoelastic one that arise from the interaction of magnetic moments with the stress distribution introduced by quenching and drawing. Moreover, additional stresses are introduced due to different thermal expansion coefficient of metallic nucleus and glass coating [4,5]. In the case of positive magnetostrictive microwires, as a result of minimization magnetoelastic and shape energies, the domain structure consists of inner mono-domain with axis magnetization surrounded by radially magnetized domain structure (see Fig. 1). Moreover, closure domain appears at the end of the wire in order to decrease the stray fields. When the applied axial field reaches the switching field value, the closure domain wall depinns from its original position and starts to propagate along the wire in a single Barkhausen jump. This phenomenon is currently labeled as magnetic bistability. Extremely fast domain wall propagation has been found in amorphous glass-coated microwires [6,7]. In some cases the domain wall velocity even exceeds the sound speed [7] showing the effect of ‘‘supersonic boom’’ when the velocity approaches the sound speed [8]. In general there are at least three possible reasons for fast domain wall propagation in glass-coated magnetic microwires [8]. First, it is the low anisotropy that results in low domain wall damping. Second, it is the presence of two perpendicular aniso- tropies that can average out each other. Finally it is the existence of radial domain structure just below the surface of microwires that shields the domain wall from pinning on the surface of the wire and creates the stray fields that help the domain wall keep its velocity high [9]. In the given contribution, we show the way, how to increase the domain wall velocity by introducing the various kind of anisotropy either by field of by stress annealing. 2. Experimental Amorphous glass-coated Fe 77.5 Si 7.5 B 15 and Fe 40 Co 38 Mo 4 B 18 microwires were produced by the Taylor–Ulitovski method [4]. The diameter of metal core was 30 mm and thickness of glass coating was 15 mm. The length of all samples used in measure- ments was 10 cm. The microwires were annealed for 1 h at temperatures 200 1C in order to observe the influence of the annealing temperature. Field annealing has been performed in the transversal magnetic field of 1 T. Joule heating was performed by DC current of amplitude of 95 mA. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.02.091 n Corresponding author. Tel.: þ421 55 62 211 28; fax: þ421 55 62 221 24. E-mail address: rvarga@upjs.sk (R. Varga). Journal of Magnetism and Magnetic Materials 324 (2012) 3566–3568