Stress-Induced Magnetic Anisotropy Enabling Engineering of Magnetic Softness and GMI Effect of Amorphous Microwires

Abstract

Stress-annealing enabled a considerable improvement in the GMI effect in both Fe-and Co-rich glass-coated microwires. Additionally, a remarkable magnetic softening can be achieved in stress-annealed Fe-rich microwires. Observed stress-annealing induced magnetic anisotropy is affected by annealing conditions (temperatures and stresses applied during annealing). The highest GMI ratio up to 310% was obtained in stress-annealed Co-rich microwires, although they presented rectangular hysteresis loops. A remarkable magnetic softness and improved GMI ratio over a wide frequency range were obtained in stress-annealed Fe-rich microwires. Irregular magnetic field dependence observed for some stress-annealing conditions is attributed to the contribution of both the inner axially magnetized core and outer shell, with transverse magnetic anisotropy.Appl. Sci. 2020, 10, 981 2 of 11 with respect to the varying of external stimuli, such as magnetic field, mechanical load and heat [30][31][32]. Consequently, metamaterials incorporating arrays of magnetic wires in fiber-reinforced polymer composites are potentially suitable for engineering of electromagnetic functionalities and stress/temperature monitoring.Industrial applications based on the GMI effect demand a size reduction of the magnetic element [16,17]. Therefore, the development of thin wires exhibiting GMI effect has become a topic of intensive research [4,5,[11][12][13][14][15]. Among different preparation techniques of magnetic wires, the so-called Taylor-Ulitovsky method allows for the thinnest wires' fabrication and reduced dimensions [11][12][13][14][15][16][31][32][33]. In order to observe the GMI effect in thin wires, the skin depth should be lower than the radius of the wire. As such, a decrease in diameter should be associated with an increase in the frequency range for observation of the GMI effect [34,35]. Besides these technical features, the overall cost should be also considered for applications (e.g., smart composites with wire inclusions) where substantial amounts of wires are required.Excellent soft magnetic properties with enhanced GMI effects are usually reported for as-prepared Co-rich [11][12][13][14][15] and nanocrystalline Fe-rich magnetic microwires [36,37]. The latter are often accompanied with an inherent brittleness of samples over the course of the nanocrystallization process, and therefore alternative stress-annealing approaches have been proposed, to enhance the GMI effect retaining the amorphous structure [38,39]. Stress-annealing can be performed in a temperature range well below the crystallization onset. In addition, the degree of stress-induced anisotropy can be selectively tuned through optimal annealing temperature, time and/or different values of applied stresses. Thus, this contribution aims at providing comparative studies on the effect of stress-induced anisotropy on magnetic softness and GMI effect in Fe-and Co-rich magnetic microwires.

Optimization of Magnetic Properties of Magnetic Microwires by Post-Processing

“…Accordingly, from Rc(tann) evaluated from Mr/Mo, we can again observe a gradual increase of the inner axially magnetized domain radius upon annealing. Such evolution of hysteresis loops upon annealing is confirmed in various Co-based microwires with low and negative λs-values [93][94][95][96]. One more example for another Co-rich (Co69.2Fe4.1B11.8Si13.8C1.1) microwire with low negative λs-values is shown in Figure 14.…”

“…Accordingly, R c (T ann ) dependence and observed evolution of the hysteresis loops upon annealing must be related to the relaxation of internal stresses as discussed elsewhere [74,[93][94][95]. [93][94][95][96]. One more example for another Co-rich (Co69.2Fe4.1B11.8Si13.8C1.1) microwire with low negative λs-values is shown in Figure 14.…”

“…Observed modification of the hysteresis loops shape and obtained R c (T ann ) dependence are consistent with the evolution of the hysteresis loop shape upon glass-coating removal by chemical etching (see Figures 3 and 4). Accordingly, R c (T ann ) dependence and observed evolution of the hysteresis loops upon annealing must be related to the relaxation of internal stresses as discussed elsewhere [74,[93][94][95]. [93][94][95][96].…”

Optimization of Magnetic Properties of Magnetic Microwires by Post-Processing

“…One of the most promising and effective methods for tuning of the magnetic properties of magnetic microwires is stress-annealing. In the case of magnetic microwires with positive magnetostriction coefficient, this post-processing allows remarkable magnetic softness improvement [43,52,86,87,94,95].…”

“…Observed magnetic softening is related to the presence of magnetic field that can considerably affect the magnetic anisotropy of amorphous materials [105]. It was reported that the macroscopic magnetic anisotropy of amorphous materials is originated by a preferred magnetization direction during the annealing and was discussed in terms of either the directional ordering of atomic pairs or compositional and topological short-range ordering [83][84][85][93][94][95]105]. Adapted from ref [102].…”