In the present paper, we report a systematic study of the magnetoelastic resonance of amorphous magnetic microwires of composition Fe 73 Si 11 B 13 Nb 3 . The study was performed for samples annealed at different temperatures. It was observed that such microwires present the key feature of performing magnetoelastic resonance in the absence of applied field. This fact, in addition to their small size, gives the microwires unique advantages over the widespreaded ribbons, currently in use as magnetoelastic sensors. Beyond the study of the resonance, magnetic properties of the samples were studied by means of Vibrating Sample Magnetometer (VSM) measurement in order to find an explanation to their bias-free resonance property. Finally, we show two possible applications of microwire based magnetoelastic sensors, a fluid density sensor and a mass-loading sensor. In recent years, much interest and effort has been devoted to develop soft magnetic materials due to their technological potential. 1 The main use of these materials can be found in the sensing industry which includes a broad spectrum of applications ranging from the automotive, mobile communication, chemistry and biochemistry industry among many others. [2][3][4][5] Amorphous microwires are one of the most widely studied soft materials. They are fabricated by means of extracting melt-spinning Taylor technique. 6 Those microwires are composed by a metallic core and a Pyrex cover both in the lm range. The metallic core provides the magnetic behaviour while the cover has a protective and stress-inducting function. 7 The ratio between the total diameter and the magnetic core, often called aspect ratio, is one of the key parameters of such microwires, since magnetic properties depend dramatically on it. For high values of the aspect ratio, the microwires present a bistable hysteresis loop, while this bistability vanishes for low ones. 8,9 In fact, amorphous or nanocrystalline magnetic microwires are among the softest materials. Many properties of these materials have been deeply studied both from the point of view of the basic physics and the applications. 10,11 This is the case of the giant magnetoimpedance effect, 12 biestability, 13 and ferromagnetic resonance. 14 However, the magnetoelastic resonance of those materials was only thoughtfully considered a few years ago. 15 It is well known that amorphous magnetostrictive alloys present magnetoelastic nature, so those materials may perform magnetoelastic resonance when exposed to a timevarying magnetic field. The physical parameters that describe this resonance, such as the resonance frequency, the amplitude, and the damping, are functions of the material environment. For this reason, their monitorization may be used to obtain information about the media. That is the working principle of a magnetoelastic sensor. [16][17][18][19] This kind of sensors allow a high degree of specificity, by functionalizing the sensing material to make it react with an specific agent of the media, as can be certain gases (e.g., humid...