IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000 3015 High Coercivity of Partially Devitrified Glass-Coated Finemet Microwires: Effect of Geometry and Thermal Treatment J. Gonzalez, A. Zhukov, V. Zhukova, A. F. Cobeño, J. M. Blanco, A. R. de Arellano-Lopez, S. Lopez-Pombero, J. Martinez-Fernandez, V. Larin, and A. Torcunov Abstract—The effect of annealing (at 300–700 C) on coercivity of finemet-type Fe Cu Nb Si B = 0 and 1.1) microwires has been studied. Strong magnetic hardening without deterioration of mechanical properties has been found at annealing temperatures around 500 C for one of the sample = 0). The exact value of coercivity, , and annealing temperature, , corresponding to such maximum of depend on the sample di- ameter and the glass coating thickness (ratio . Observed dependencies have been interpreted in terms of effect of magnetoe- lastic anisotropy in partially devitrified two-phase samples. Index Terms—Amorphous microwires, magnetoelastic anisotropy, devitrification, nanocrystallization. I. INTRODUCTION A noticeable interest has been recently exhibited in soft mag- netic microwires owing to their elevated technological potential [1]–[3]. These microwires are obtained by the Taylor-Ulitovski technique [4], [5], which allows to fabricate a broad spectrum of compositions of tiny ferromagnetic wires (in the order of 1 to 30 m in diameter) of Fe, Co or Ni-rich alloys coated by an insulating glass cover (of 1–10 m in thickness). On the other hand, Fe-base nanocrystalline materials (most typical of composition Fe Cu Nb Si B with trademark finemet) are widely studied mainly because of their excellent magnetic softness related to the nature of their microstructure which consists basically of small crystallites (mean grain size of 10 nm) embedded in an amorphous residual matrix [6]. These materials are obtained by devitrification of the precursor amorphous alloy in the range of 500–600 C for 1 hour (i.e., at temperatures between the first and second crystallization peaks). In this way, materials with different microstructures and magnetic properties can be obtained Manuscript received March 5, 2000. This work was supported in part by the Basque Country Government under Project PI-1997-33 and Spanish Science and Culture Ministry Project PB96/0899. J. Gonzalez is with the Dipartimento Física de Materiales, Facultad. Quimica, UPV, 20018 San Sebastián, Spain. A. Zhukov is with the Dipartimento Física de Materiales, Facultad. Quimica, UPV, and Fundación Donostia International Physics Centre, P. M. De Lardiz- abal, 4, 20018, San Sebastián, Spain. V. Zhukova, A. F. Cobeño, and J. M. Blanco are with the Department of Electr. y Telecom. and Department Física Aplicada I Department Electr. y Telecom, E.U.I.T.I., 20011, San Sebastián, Spain. A. R. de Arellano-Lopez, S. Lopez-Pombero, and J. Martinez-Fernandez are with the Department of Fisica de la Materia Condensada, Universidad de Sevilla, Spain. V. Larin and A. Torcunov are with the “AmoTec” Ltd., Kishinev, Moldova. Publisher Item Identifier S 0018-9464(00)08411-9. depending on the annealing parameters as well as on the chemical composition. Particularly a strong hardening has been observed in Fe Cu Nb Si B = 11.5; 13.5 and 16.5) microwires at annealing temperatures, , above 650 C [7]. Such magnetic hardening irrespectively to the amorphous nature of the alloy can be very useful for different technological applications such as information storage. Unfortunately such magnetic hardening observed in conventional finemet compo- sitions during the second recrystallization process has been accompanied by strong deterioration of mechanical properties. The aim of the present work is to study the effect of an- nealing conditions on the coercivity, , of finemet-type glass- coated microwires annealed at 300–700 C in order to develop a fine nanostructure leading to a remarkable magnetic hardening without deterioration of their mechanical properties. The influ- ence of the geometry (ratio ; = metallic nucleus di- ameter and = total diameter of the microwire) on the coer- civity is investigated considering the effect of the different in- ternal stresses induced by the difference in the thermal expan- sion coefficient of the metallic nucleus and the glass coating. II. EXPERIMENTAL RESULTS AND THEIR DISCUSSION The investigations were performed on glass-coated Fe Cu Nb Si B = 0 and 1.1) microwires fabricated by the Taylor-Ulitovski method. The diameter of the metallic nucleus, , was varied from 2 to 6 m, and the total di- ameter, , from 12 to 42.6 m. Conventional furnace annealing (300–700 C, 1 hour) has been carried out in order to investigate the devitrificaion process. The microstructural character of the samples was analyzed by X-ray diffraction (XRD) and by transmission electron microscopy (TEM) techniques. Magnetic characterization of the as-quenched and treated samples has been performed by a conventional induction method and using a vibrating sample magnetometer. Like in early studied microwires of Fe Cu Nb Si B = 11.5; 13.5 and 16.5) [6], a strong dependence of the magnetic properties on the sample’s geometry in the as-prepared state has been observed for all compositions. Hysteresis loops, typically of rectangular shape, have been observed in the as-prepared state. As it can be seen from Fig. 1, the coercivity of the sample Fe Cu Nb Si B strongly increases as the ratio decreases. These results of versus could be understood taking into account that the strength of internal stresses, , acting on the metallic nucleus 0018–9464/00$10.00 © 2000 IEEE