# Investigation of Energy Harvesting Capabilities of Metglas 2605 SA1
## 1 Introduction  
The number of wireless sensors and sensor networks is increasing with the rapid growth of the Internet of Things [1]. Powering such devices typically relies on primary batteries, leading to a short lifespan due to self-discharge, for instance [2], which becomes even more critical than the actual used energy. In this framework, energy harvesting systems are a great alternative to batteries and their associated drawbacks, such as environmental concerns, in the framework of self-powered sensors and sensor networks [3]. The harvester could be classified according to the energy source [4] and corresponding conversion material, such as magnetostrictive alloys for vibrational sources, thermoelectric modules for thermal energy [5, 6], and photovoltaic cells for solar sources [7]. Among the available energy sources, vibrations remain one of the most attractive options due to their ubiquity, availability even in confined environments, and good power densities [8]. A complete electromechanical energy harvesting system [9] consists of three main components: conversion material [10], mechanical structure [11, 12], and electrical interface [13]. From the conversion material [14] point of view, most of the existing vibrational energy harvesters use piezoelectric, magnetodynamic, or magnetostrictive coupling [15]. Piezoelectric materials are interesting because of their significant voltage output and active nature, but they also show several disadvantages such as potentially high voltage, low current, and high impedance at low frequency, typical to vibrational systems [16]. On the otherhand, magnetodynamic systems show several shortcomings such as low voltage output and predominant resistive losses at low frequencies. Magnetostrictive materials allow addressing such issues due to stress-dependent magnetic characteristics. The solid-state aspect of the magnetomechanical coupling in magnetostrictive materials also yields more compact devices compared to magnetodynamic systems, as the latter requires significant dimensional changes to allow the conversion to take place. [PERSON] discovered the effect of magnetostriction in 1842 [17], consisting of a change in the sample shape when subjected to a magnetic field. [PERSON] [18] observed the inverse magnetostrictive effect, where the sample magnetization changes when mechanical stresses are applied to it. Energy harvesting systems using magnetostrictive materials exploit the [PERSON] effect [19] and convert the variation of the mechanical excitation during vibration into a change in magnetic flux density, therefore inducing currents in the coils around the specimen. The magnetization of ferromagnetic material in response to an applied magnetic field originates from two related mechanisms [20]: the domain wall motions, and the rotation of magnetic moments within domains towards the field direction. The effect of magnetostriction and inverse magnetostriction could be mostly explained, as a first approximation, by the rotation of magnetic moments [21]. Generally, in a magnetostrictive specimen, numerous magnetic domains exist, each having a local magnetization while the total magnetization of the material is the sum of each local contribution. As depicted in Figure 1, in the initial state, all the domains are assumed to be randomly distributed, yielding a null magnetization. In an energy harvesting process, the application of a bias magnetic field first leads to a rotation of magnetic moments [22] following the magnetic force and makes the sample longer. When a tensile stress is superimposed along the same direction, the sample becomes elongated, and the magnetic domain continues rotating. This external tensile stress is imposed through mechanical excitation. The magnetic flux in the specimen then varies with the rotation of the magnetic domains. Materials such as Terfenol-D [23] and Galfenol [24] have been extensively used in magnetostrictive energy harvesters, along with Fe-Co alloys [25] and Metglas 2605 SA1 [26; 27] to some extent.