Studies of thermal and magnetic properties of Fe-based amorphous and nanocrystalline glass coated microwires M. Churyukanova a , V. Zhukova b , A. Talaat b , J.J. del Val b , S. Kaloshkin a , E. Kostitcyna a , E. Shuvaeva a , V. Sudarchikova a , A. Zhukov b,c,⇑ a National University of Science and Technology «MISIS», Moscow, 119049, Russia b Dpto. Fisica de Materiales, UPV/EHU, San Sebastian, Spain c IKERBASQUE, Basque Foundation for Science, Bilbao, Spain article info Article history: Available online 9 December 2013 Keywords: Glass-coated amorphous microwire Curie temperature Heat capacity Nanocrystallization Giant Magnetoimpedance effect Magnetoelastic energy abstract We studied the influence of magnetoelastic energy on properties of nanostructured glass-coated micro- wires with soft magnetic behaviour (Finemet-type microwires of Fe 70.8 Cu 1 Nb 3.1 Si 14.5 B 10.6 and Fe 73.8 Cu 1- Nb 3.1 Si 13 B 9.1 compositions). The magnetoelastic energy, stored during the Finemet-type microwires fabrication, affected the hysteresis loops, coercivity and heat capacity of Finemet-type microwires. Hys- teresis loops of all as-prepared microwires showed rectangular shape, typical for Fe-rich microwires. As expected, coercivity, H C , of as-prepared microwires increases with decreasing of the ratio q defined as the ratio between the metallic nucleus-diameter, d to total microwire diameter, D. On the other hand, the change of heat capacity in T C , DCp, exhibits increasing with the ratio q. This relationship holds for micro- wires in the as-prepared state as well as after annealing. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Glass-coated ferromagnetic microwires (typically of 1–30 mm in metallic nucleus diameter) have attracted growing attention in the last few years mostly owing to their outstanding soft magnetic properties (magnetic bistability, enhanced magnetic softness, GMI effect, fast domain wall propagation) [1,2]. Recent studies have demonstrated that optimization of soft magnetic properties and GMI effect of amorphous glass coated microwires is possible choosing the appropriate chemical composition of metallic nucleus and adequate post-fabrication processing conditions [1]. Most attention has been paid to studies of amorphous microwires. But in some cases, nanocrystallization allows to achieving better mag- netic softness and enhanced GMI effect in ferromagnetic materials. Such soft magnetic character is thought to be originated because the magnetocrystalline anisotropy vanishes and the very small magnetostriction value when the grain size approaches 10 nm [3,4]. Like for conventional nanocrystalline materials, average anisotropy for randomly oriented a-Fe(Si) grains is negligibly small when the average grain is about 10–20 nm. Consequently, low val- ues of coercivity in the nanocrystalline microwires were ascribed to small effective magnetic anisotropy. In addition to the sup- pressed magnetocrystalline anisotropy, low magnetostriction val- ues provide the basis for the superior soft magnetic properties observed in particular compositions [3,4]. Low values of the mag- netostriction are essential to avoid magnetoelastic anisotropies arising from internal or external mechanical stresses. As regarding magnetic properties of amorphous ferromagnetic microwires the overall shape of hysteresis loops depends on the composition of the metallic nucleus as well as on the composition and thickness of the glass coating. As described elsewhere, shape of hysteresis loops changes from rectangular, typical for amorphous Fe-rich compositions, to inclined, typical for Co-rich compositions [5]. Amorphous microwires with vanishing magnetostriction exhi- bit quite soft magnetic properties. Aforementioned dependence of the hysteresis loops on these parameters should be attributed to the magnetoelastic energy gi- ven by: K me ¼ 3=2k s r i ð1Þ where k s is the saturation magnetostriction and r i is the internal stress. The magnetostriction constant depends mostly on the chem- ical composition and is vanishing in amorphous Fe–Co based alloys with Co/Fe 70/5 [5–7]. One of the peculiarities of the fabrication technique of glass-coated microwires is that it involves the simulta- neous solidification of composite microwire consisting of ferromag- netic nucleus surrounded by glass coating. Quite different thermal expansion coefficients of the glass and the metallic alloys introduce considerable internal stresses inside the ferromagnetic nucleus dur- ing simultaneous fast solidification of the composite microwire 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.12.030 ⇑ Corresponding author. Address: Dpto. Fisica de Materiales, UPV/EHU, San Sebastian, Spain. Tel.: +34 943018611; fax: +34 943017130. E-mail address: arkadi.joukov@ehu.es (A. Zhukov). Journal of Alloys and Compounds 615 (2014) S256–S260 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom