Journal of Magnetism and Magnetic Materials 249 (2002) 117–121 Microwave response of amorphous microwires: magnetoimpedance and ferromagnetic resonance M. Dominguez a, *, J.M. Garcia-Beneytez b , M. Vazquez b , S.E. Lofland c,d , S.M. Bhagat c a Dept. de Fisica de la Materia Condensada, Universidad de Cadiz, 11510 Puerto Real, Cadiz, Spain b Instituto de Magnetismo Aplicado and Instituto de Ciencia de Materiales (CSIC), 28049 Cantoblanco, Madrid, Spain c Department of Physics, University of Maryland, College Park, MD 20742, USA d Department of Chemistry and Physics, Rowan University, Glassboro, NJ 08028, USA Abstract It has been established that giant magnetoimpedance (GMI) in amorphous wires is due to a rapid change in the skin depth, caused by the low-field sensitivity of the azimuthal dynamic permeability (a classical electromagnetic effect). In 5 mm diameter glass-covered amorphous wires, GMI may be observed at the microwave range. A correlation between GMI and ferromagnetic resonance (FMR) was proposed in this microwave range. We have measured the microwave response of amorphous microwires for several alloys from the system (Co 100x Fe x Þ 75 Si 15 B 10 (x ¼ 2; 6; 10) with positive, zero, and negative magnetostriction, respectively. Our main results indicate: (i) GMI and FMR effects are well separated at different fields, (ii) GMI follows the magnetization process, confirming its classical electromagnetic origin, and (iii) FMR fields are also affected by the skin effect. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.20.En; 75.50.Kj; 76.50.+g Keywords: Amorphous wires; Magnetoimpedance; Ferromagnetic resonance 1. Introduction Giant magnetoimpedance (GMI) has attracted a considerable attention during the last years mainly because of its potential use in magnetic sensors [1], but it also raises some fundamental questions that remain unanswered about the physical process. There is general agreement [2,3] that the phenom- enon in the MHz regime is a classical electro- magnetic effect caused by the rapid reduction in skin depth d ¼ð2r=m 0 m f oÞ 1=2 due to the low-field sensitivity of the azimuthal (or circumferential) dynamic permeability m f ; in a material with DC resistivity r exposed to an azimuthal AC field of angular frequency o: According to these models, the smaller the radius of the wire, the higher the frequency range needed for the GMI effect to occur. With wire diameters in the range of a few mm, the effect can be observed at the microwave (GHz) range. It was suggested [4,5] that in this microwave frequency range, a correlation between *Corresponding author. Tel.: +34-956-016324; fax: +34- 956-016288. E-mail address: manolo.dominguez@uca.es (M. Dominguez). 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0304-8853(02)00517-6