Graded magnetic anisotropy in Co-rich microwires

Abstract

Magnetic wires can present outstanding magnetic properties such as giant magnetoimpedance (GMI) or magnetic bistability and related single domain wall (DW) propagation. Magnetically soft glass-coated magnetic microwires prepared by the Taylor-Ulitovsky technique with metallic nucleus covered by flexible, insulating and biocompatible glass are quite demanded for a great number of applications [1]. Recenly, we showed that stress-annealing allows magnetic softening, GMI effect improvement and even grading the magnetic anisotropy in Fe-rich microwires [2].Although generally thinner wires present certain advantages related to the industrial application, for some applications (like non-contact stress monitoring or magnetic tags) thicker wires are highly requested. Recently, preparation of thicker (above 50 μm in diameter, d) glass-coated Fe-rich microwires is reported [3].In this paper, we present our recent experimental results on preparation and processing of “thick” Co-rich magnetic microwires (d = 94 μm) with graded magnetic anisotropy. From previous publications related to the effect of stress-annealing on magnetic anisotropy of Fe-rich microwires, is known that the stress-annealing induced anisotropy depends on the annealing temperature, Tann [2]. Such graded magnetic anisotropy has been obtained by stress-annealing of Co-rich microwire at variable annealing temperature. The hysteresis loops for different sections of the microwire have been evaluated using a short (2 mm long) movable pick-up coil.A gradual change in the hysteresis loop (measured by the short pick-up coil) of the sample stress-annealed at variable Tann is observed (see Fig.1). Such microwire, presents also variable squareness ratio, Mr/M0, along the microwire length, L (see Fig.1). Obtained graded anisotropy has been attributed to a gradual modification of the domain structure along the microwire.Previously, for preparation of thin films with a graded magnetic anisotropy, rather sophisticated methods were used, including a change in the chemical composition during the deposition of thin films [4]. Accordingly, we propose a rather simple route to design graded magnetic anisotropy in a magnetic microwire.