Enhancing the Squareness and Bi-Phase Magnetic Switching of Co2FeSi Microwires for Sensing Application

Citation: Salaheldeen, M.; Wederni, A.; Ipatov, M.; Zhukova, V.; Lopez Anton, R.; Zhukov, A. Enhancing the Squareness and Bi-Phase Magnetic Switching of Co2FeSi Microwires for Sensing Application. Sensors 2023,23, 5109. https://doi.org/10.3390/ s23115109 Academic Editor: Christer Johansson Received: 11 April 2023 Revised: 13 May 2023 Accepted: 25 May 2023 Published: 26 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sensors Article Enhancing the Squareness and Bi-Phase Magnetic Switching of Co2FeSi Microwires for Sensing Application Mohamed Salaheldeen 1,2,3,4,* , Asma Wederni 1,2,4 , Mihail Ipatov 1,2, Valentina Zhukova 1,2,4 , Ricardo Lopez Anton 5,* and Arcady Zhukov 1,2,4,6 1Department of Polymers and Advanced Materials, Faculty of Chemistry, University of the Basque Country, UPV/EHU, 20018 San Sebastián, Spain 2Department of Applied Physics I, EIG, University of the Basque Country, UPV/EHU, 20018 San Sebastián, Spain 3Physics Department, Faculty of Science, Sohag University, Sohag 82524, Egypt 4EHU Quantum Center, University of the Basque Country, UPV/EHU, 20018 San Sebastián, Spain 5Department of Applied Physics, Regional Institute for Applied Scientific Research (IRICA), University of Castilla-La Mancha, 13071 Ciudad Real, Spain 6IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain *Correspondence: [email protected] (M.S.); [email protected] (R.L.A.) Abstract: In the current study we have obtained Co 2 FeSi glass-coated microwires with different geometrical aspect ratios, ρ = d/D tot (diameter of metallic nucleus, d and total diameter, D tot ). The structure and magnetic properties are investigated at a wide range of temperatures. XRD analysis illustrates a notable change in the microstructure by increasing the aspect ratio of Co 2 FeSi-glasscoated microwires. The amorphous structure is detected for the sample with the lowest aspect ratio ( ρ = 0.23), whereas a growth of crystalline structure is observed in the other samples (aspect ratio ρ = 0.30 and 0.43). This change in the microstructure properties correlates with dramatic changing in magnetic properties. For the sample with the lowest ρ -ratio, non-perfect square loops are obtained with low normalized remanent magnetization. A notable enhancement in the squareness and coercivity are obtained by increasing ρ -ratio. Changing the internal stresses strongly affects the microstructure, resulting in a complex magnetic reversal process. The thermomagnetic curves show large irreversibility for the Co 2 FeSi with low ρ -ratio. Meanwhile, if we increase the ρ -ratio, the sample shows perfect ferromagnetic behavior without irreversibility. The current result illustrates the ability to control the microstructure and magnetic properties of Co 2 FeSi glass-coated microwires by changing only their geometric properties without performing any additional heat treatment. The modification of geometric parameters of Co 2 FeSi glass-coated microwires allows to obtain microwires that exhibit an unusual magnetization behavior that offers opportunities to understand the phenomena of various types of magnetic domain structures, which is essentially helpful for designing sensing devices based on thermal magnetization switching. Keywords: Heusler alloys; glass-coated microwires; multi-step magnetic behavior; sensing applications 1. Introduction The use of ferromagnetic materials in spintronic applications has garnered increasing attention in recent years due to their unique magnetic properties that enable the control and manipulation of spin currents. Among the different types of ferromagnetic materials, micro/nano-structured materials have emerged as promising candidates for enhancing spintronic devices’ performance [ 1 – 9 ]. One of the most promising multidisciplinary research fields is spintronics, which enables the creation of the next generation of nano & microdevices with improved processing and memory capability while consuming less power [ 10 ]. To address the various required criteria, such as high spin polarization or high Sensors 2023,23, 5109. https://doi.org/10.3390/s23115109 https://www.mdpi.com/journal/sensors Sensors 2023,23, 5109 2 of 14 Curie temperature, Tc, a new generation of materials with multifunction uses has to be created [ 11 ]. These Heusler compounds are well suited for spintronic and magneto-electronic applications [ 12 ]. Between the advantages of these compounds, we can highlight several ones: good lattice matching with the most typical substrates, Tc above room temperature, and the possibility of obtaining close to 100% of spin polarized near the Fermi level [ 11 – 15 ]. In particular, Co 2 -based full-Heusler compounds are among the most promising half-metallic alloys due to their high thermal stability, high Curie temperatures ( Tc ≈1100 K ) in bulk form, high magnetic moment (~6 µB /f.u.), and low Gilbert damping constant ( α = 0.004) [ 14 , 16 , 17 ]. Additionally, they exhibit interesting transport properties and high magnetic moments. It is noteworthy that these Co 2 -based Heusler alloys present a significant anomalous Hall linked to the enormous Berry curvature associated with their band structure [ 16 , 18 ]. All the precedent evidence why the scientific community is so interested nowadays in Co-based full-Heusler alloys. Hence, these alloys are extensively researched in several configurations: nanoparticles [ 19 ], thin films [ 15 , 17 , 20 ], and nano/microwires [ 21 – 24 ]. It is relevant to note that the fabrication of Heusler alloys nanoparticles and thin films faces several difficulties for application purposes, including the high cost of preparation methods, chemical composition inhomogeneity, and ease of oxidation [ 20 ]. The diffusion of substrate atoms into the film results in the existence of the atomic disorder and phase separations, which are commonly observed [ 25 ], in addition to the lattice mismatch between the alloy and the substrate. Furthermore, in order to start the requisite structural ordering, the arc-melted or thin-film-formed Heusler alloys need lengthy, high-temperature annealing procedures [26]. Magnetic wire research has received a lot of interest during the last several decades [ 27 ]. The focus is on amorphous magnetic wires, which can exhibit unusual magnetic features such as spontaneous magnetic bistability or the Giant magnetoimpedance phenomenon [ 27 , 28 ]. Several manufacturing processes involving fast solidification can be used to create magnetic wires containing amorphous and/or nanocrystalline phases [ 27 , 28 ]. Nevertheless, only the Taylor-Ulitovsky manufacturing approach allows the preparation of magnetic microwires with the widest diameter range (from 0.2 to 100 µ m) [ 27 , 28 ]. Such microwires are composites consisting of metallic nuclei (with 0.2 ≤ d ≤ 100 µ m) usually comprised of iron, cobalt, nickel, or their alloys, covered by thin, flexible, and insulating glass (typically Pyrex or Duran) coating (typically with thickness from 0.5 to 10 µ m) [ 27 , 28 ]. As a result, the prospective applications of glass-coated microwires in sensing, actuation, and biomedical engineering have been expanded. The insulating and flexible glass coating protects the microwires from oxidation, corrosion, and other environmental factors while simultaneously giving them outstanding mechanical stability. Moreover, the glass layer and the magnetically flexible amorphous metallic nucleus provide high sensitivity to external stimuli including magnetic fields, temperature fluctuations, and mechanical stress [ 27 – 35 ]. Such sensitivity is connected to the ferromagnetic origin of the metallic nucleus, which responds to the applied stimulus. Innovative sensors that monitor magnetic fields, temperature, and stress have been developed using glass-coated microwires for a range of applications [ 27 – 29 ]. Additionally, they have shown potential characteristics for actuators and in medical applications, including cancer treatment (through magnetic hyperthermia) [ 29 ] and medicine administration. Future technological advancements can use glass-coated microwires because of their distinctive combination of properties [27,29]. In this article, we report an attempt to prepare Co2FeSi glass-coated microwires with variable geometrical aspect ratios ρ = d/D tot (being d-diameter of the metallic nucleus and D tot —total diameter). The fabrication method was chosen because of the interesting relationship between the magnetic and structural properties in the case of Heusler alloys in the form of glass-coated microwires, coupled with the properties provided by this fabrication method: excellent mechanical properties, insulating behavior, thin and flexible glass-coating, and small dimensionality [ 27 , 29 – 35 ]. As a result, we have prepared Co 2 FeSi glass-coated microwires using the Taylor-Ulitovsky procedure, which is detailed previously [ 27 , 36 , 37 ]. The Taylor-Ulitovsky method, which has been fairly utilized since Sensors 2023,23, 5109 3 of 14 the 1960s [ 37 ], is probably the most used fabrication method to make Heusler alloys glasscoated microwires with a wide variety of geometric characteristics [ 21 , 22 , 24 , 27 – 35 ]. The primary benefit of this non-expensive technique is that it allows the production of thin and long (up to several kilometers long) microwires with a wide diameter range (from 0.2 µ m up to 100 µ m) at a high rate (up to some hundred meters per minute) [ 36 – 38 ]. This process is also used to prepare glass-coated microwires with excellent mechanical properties [ 21 , 39 – 41 ]. Additional benefits of glass coating on microwires are better isolation and protection from the surroundings. Moreover, the fact that the glass coating is biocompatible, coupled with the commented properties, make this approach well-suited for biological applications [ 29 , 42 , 43 ]. Therefore, Heusler microwires of Co 2 FeSi are an interesting material for a broad range of applications and devices. As far as we know, there is no report up to date on the production and structural, mechanical, or magnetic properties of Co 2 FeSi-based glass-covered Heusler microwires with varied ρ -ratios, as well as the investigation of its influence on magneto-structure behavior. 2. Materials and Methods Arc melting is a method of manufacturing Co 2 FeSi alloys that involves melting the precursor components together in an electric arc furnace. Typically, the following procedures are used to create Co 2 FeSi alloys by arc melting: (i) preparing the precursor ingredients. The precursor elements for the Co 2 FeSi alloy are weighed and deposited in a graphite crucible, containing cobalt (powder) (99.99%), iron (powder) (99.9%), and silicon (powder) (99.99%) supplied by Technoamorf S.R.L. Co. (Cisineu, Moldavia). (ii) The materials melting. The crucible containing the precursor materials is put in an electric arc furnace, and an electrical current is fed through the materials to start the melting process in a vacuum and argon atmosphere. The furnace temperature is precisely regulated to ensure that the ingredients melt and mix equally. (iii) The cooling and solidification processes. The crucible is withdrawn from the furnace and allowed to cool once the components have melted and combined. The Co 2 FeSi alloy (ingot) is created as the ingredients consolidate. This process was then repeated five times to achieve perfect homogeneity and a homogeneous microstructure. Once the Co 2 FeSi alloy has solidified and formed an ingot, the ingot is used to prepare Co 2 FeSi glass-coated microwires using the Taylor-Ulitovsky process. As described in the introduction, the Taylor-Ulitovsky preparation technique offers significant benefits over alternative procedures for manufacturing glass-coated microwires. One advantage is that it enables the fabrication of microwires with rather thin glass coatings, generally up to a few micrometers thick. This thin insulating coating permits the electrical and magnetic characteristics of the microwire metallic nucleus to be preserved, making the resultant microwires valuable for a wide range of applications. Many prior publications [ 21 , 22 , 24 , 27 – 35 ] explain the manufacturing method in detail. To summarize it, a glass capillary was filled with Co 2 FeSi alloy, molten using a high frequency inductor for heating an ingot over its melting temperature. The variation of the speed of wire drawing, alloy temperature, glass tube feed rate and of the rotation of the pick-up bobbin were the parameters used to control the diameter of the metallic nuclei, dmetal (µm), and total diameter Dtotal (µm) as explained in detail elsewhere [ 38 ]. Finally, the microwire is cooled with a coolant stream to complete the rapid melt quenching process. All geometric parameters of samples investigated in current study are listed in Table 1, also including a similar sample previously studied (in ref. [ 24 ]), whose results will be also discussed in the following section. Sensors 2023,23, 5109 4 of 14 Table 1. The geometrical parameters d metal ( µ m), D total ( µ m), aspect ratio, and average (Av.) of atomic percentage of Co, Fe, and Si elemental composition in Co2FeSi glass-coated microwires. Sample dmetal (µm) Dtotal (µm) Aspect Ratio (ρ) Chemical Composition GCMWA5.1 ±0.1 22.2 ±0.1 0.23 ±0.01 Co44Fe23Si33 GCMW** 4.4 ±0.1 17.6 ±0.1 0.25 ±0.01 Co44Fe23Si33 GCMWB6.4 ±0.1 21.3 ±0.1 0.30 ±0.01 Co44Fe23Si33 GCMWC7.7 ±0.1 17.9 ±0.1 0.43 ±0.01 Co44Fe23Si33 GCMW**: Co2FeSi glass-coated microwires with (ρ= 0.25) [24]. We used Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) (JEOL-6610LV, JEOL Ltd., Tokyo, Japan) to determine the aspect ρ -ratio of Co 2 FeSi glasscoated microwires samples and its related nominal chemical composition. The XRD structure analysis was carried on by using X-ray diffraction (XRD) BRUKER (D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany). The magnetic behavior was studied in two different ways: hysteresis loops at temperatures between 5 and 350 K, and thermomagnetic curves following three different protocols, zero field cooling (ZFC), field cooling (FC), and field heating (FH) at the low magnetic field (H = 200 Oe). All magnetization curves were measured using a PPMS (Physical Property Magnetic System, Quantum Design Inc., San Diego, CA, USA) vibrating-sample magnetometer at temperatures, T, between 5 and 400 K for ZFC, FC, and FH magnetic curves. For the hysteresis loops, we only focus on the in-plane configuration where the applied magnetic field is parallel to the wire axis. The results are provided in terms of the normalized magnetization, M/M 5K , where M 5K is the magnetic moment obtained at 5 K to avoid misleading of the estimation of the errors in the estimation of the magnetization saturation values. 3. Results 3.1. Analysis of Chemical and Structural Data The geometries ( ρ -ratios) and chemical compositions of prepared samples are shown in Table 1. The variation of the microwires diameters (d metal and D total ) is achieved by controlling the drawing rate, alloy temperature, glass tube feed rate, and the receiving bobbin rotation speed [ 38 ]. Using the EDX data from Table 1, it was revealed that the metallic nucleus composition differed considerably from the stoichiometric one (Co 2 FeSi). The features of the preparation process, which involved alloy melting and drawing, were the cause of this slight variation. To quantify the difference, we checked the nominal composition for eight sites as illustrated in Figure 1a. An atomic average of Co 44 Fe 23 Si 33 was used to confirm that the true 2:1 ratio of Co and Fe was applied in all sites. A high Si ratio was found because of the interfacial layer that exists between the metallic nucleus and the glass covering. Sensors 2023, 23, 5109 4 of 14 Table 1. The geometrical parameters d metal (µm), D total (µm), aspect ratio, and average (Av.) of atomic percentage of Co, Fe, and Si elemental composition in Co 2 FeSi glass-coated microwires. Sample d metal (µm) D total (µm) Aspect Ratio (ρ) Chemical Composition GCMW A 5.1 ± 0.1 22.2 ± 0.1 0.23 ± 0.01 Co 44 Fe 23 Si 33 GCMW** 4.4 ± 0.1 17.6 ± 0.1 0.25 ± 0.01 Co 44 Fe 23 Si 33 GCMW B 6.4 ± 0.1 21.3 ± 0.1 0.30 ± 0.01 Co 44 Fe 23 Si 33 GCMW C 7.7 ± 0.1 17.9 ± 0.1 0.43 ± 0.01 Co 44 Fe 23 Si 33 GCMW**: Co 2 FeSi glass-coated microwires with (ρ = 0.25) [24]. We used Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) (JEOL-6610LV, JEOL Ltd., Tokyo, Japan) to determine the aspect ρ-ratio of Co 2 FeSi glasscoated microwires samples and its related nominal chemical composition. The XRD structure analysis was carried on by using X-ray diffraction (XRD) BRUKER (D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany). The magnetic behavior was studied in two different ways: hysteresis loops at temperatures between 5 and 350 K, and thermomagnetic curves following three different protocols, zero field cooling (ZFC), field cooling (FC), and field heating (FH) at the low magnetic field (H = 200 Oe). All magnetization curves were measured using a PPMS (Physical Property Magnetic System, Quantum Design Inc., San Diego, CA, USA) vibrating-sample magnetometer at temperatures, T, between 5 and 400 K for ZFC, FC, and FH magnetic curves. For the hysteresis loops, we only focus on the in-plane configuration where the applied magnetic field is parallel to the wire axis. The results are provided in terms of the normalized magnetization, M/M 5K , where M 5K is the magnetic moment obtained at 5 K to avoid misleading of the estimation of the errors in the estimation of the magnetization saturation values. 3. Results 3.1. Analysis of Chemical and Structural Data The geometries (ρ-ratios) and chemical compositions of prepared samples are shown in Table 1. The variation of the microwires diameters (d metal and D total ) is achieved by controlling the drawing rate, alloy temperature, glass tube feed rate, and the receiving bobbin rotation speed [38]. Using the EDX data from Table 1, it was revealed that the metallic nucleus composition differed considerably from the stoichiometric one (Co 2 FeSi). The features of the preparation process, which involved alloy melting and drawing, were the cause of this slight variation. To quantify the difference, we checked the nominal composition for eight sites as illustrated in Figure 1a. An atomic average of Co 44 Fe 23 Si 33 was used to confirm that the true 2:1 ratio of Co and Fe was applied in all sites. A high Si ratio was found because of the interfacial layer that exists between the metallic nucleus and the glass covering. Figure 1. The cross section of selected Co 2 FeSi glass-coated microwires with aspect ratio 0.30 images (a) and the chemical composition spectra of EDX of one of the points (b). Figure 1. The cross section of selected Co 2 FeSi glass-coated microwires with aspect ratio 0.30 images (a) and the chemical composition spectra of EDX of one of the points (b). Sensors 2023,23, 5109 5 of 14 In order to study the order state of our produced Co 2 FeSi glass-coated microwires, and to elucidate the effect of the aspect ratio modification on the crystalline structure, XRD structure analysis was carried on by using X-ray diffraction (XRD). As illustrated in Figure 2, the change in the geometric ρ -ratio has a strong influence on the structure of Co 2 FeSi glass-coated microwires. For the sample with the lowest ρ -ratio, i.e., ρ = 0.25, the sample shows an amorphous structure where no crystalline peaks are detected. The wide halo at 2 θ = 22.3 ◦ is related to the glass coating layer, as reported in our previous works [ 21 , 22 , 24 , 27 – 35 ]. By increasing the geometric aspect ratio, a crystalline structure of the metallic nucleus becomes evident with a notable peak at 2 θ = 46.2 ◦ , attributing to the (220) reflection. Further increase of geometric ρ -ratio results in the perfect crystalline structure of samples studied, where the crystalline peak intensity increases and an additional peak appears at 2 θ = 85.4 ◦ , corresponding to the (422) reflection. The analysis of XRD profiles of the two crystalline Co 2 FeSi samples, i.e., GCMW C ( ρ = 0.30) and GCMW B ( ρ = 0.43), indicates an A2 single-phase structure with a small tetragonal distortion (traces of tetragonal martensite phase), and a broadened peak around 22 ◦ attributed to an amorphous state for GCMW C and mixed L2 1 or B2 phases with the amorphous state for GCMWBsample [34,35]. Sensors 2023, 23, 5109 5 of 14 In order to study the order state of our produced Co 2 FeSi glass-coated microwires, and to elucidate the effect of the aspect ratio modification on the crystalline structure, XRD structure analysis was carried on by using X-ray diffraction (XRD). As illustrated in Figure 2, the change in the geometric ρ-ratio has a strong influence on the structure of Co 2 FeSi glass-coated microwires. For the sample with the lowest ρratio, i.e., ρ = 0.25, the sample shows an amorphous structure where no crystalline peaks are detected. The wide halo at 2Ө = 22.3° is related to the glass coating layer, as reported in our previous works [21,22,24,27–35]. By increasing the geometric aspect ratio, a crystalline structure of the metallic nucleus becomes evident with a notable peak at 2Ө = 46.2°, attributing to the (220) reflection. Further increase of geometric ρ-ratio results in the perfect crystalline structure of samples studied, where the crystalline peak intensity increases and an additional peak appears at 2Ө = 85.4°, corresponding to the (422) reflection. The analysis of XRD profiles of the two crystalline Co 2 FeSi samples, i.e., GCMW C (ρ = 0.30) and GCMW B (ρ = 0.43), indicates an A2 single-phase structure with a small tetragonal distortion (traces of tetragonal martensite phase), and a broadened peak around 22 ⁰ attributed to an amorphous state for GCMW C and mixed L2 1 or B2 phases with the amorphous state for GCMW B sample [34,35]. Figure 2. XRD analysis of Co 2 FeSi glass-coated microwires with different aspect ratio measured at room temperature. The inset of Figure 2 indicates the A2-type cubic structure. The (220) and (422) reflections in GCMW C sample are split due to some tetragonal distortions of the crystal lattice. A similar phenomenon was seen and discussed elsewhere [44]. It is known that a split in the brag diffraction patterns leads to a small distortion of the crystalline structure [45]. The absence of a (400) peak around 85°, which is expected to be present in the A2 structure, increases the possibility that the crystallites are too fine to be detected by X-rays, as reported elsewhere [46]. In addition, the absence of some peaks can be caused by a similar scattering factor of the constituent elements (Co, Fe, and Si) Figure 2. XRD analysis of Co 2 FeSi glass-coated microwires with different aspect ratio measured at room temperature. The inset of Figure 2indicates the A2-type cubic structure. The (220) and (422) reflections in GCMW C sample are split due to some tetragonal distortions of the crystal lattice. A similar phenomenon was seen and discussed elsewhere [ 44 ]. It is known that a split in the brag diffraction patterns leads to a small distortion of the crystalline structure [ 45 ]. The absence of a (400) peak around 85 ◦ , which is expected to be present in the A2 structure, increases the possibility that the crystallites are too fine Sensors 2023,23, 5109 6 of 14 to be detected by X-rays, as reported elsewhere [ 46 ]. In addition, the absence of some peaks can be caused by a similar scattering factor of the constituent elements (Co, Fe, and Si) [ 47 ]. Otherwise, according to the theoretical outcomes of Zhang et al., the disordered A2 state is more energetically preferable than those of the ordered L2 1 or B2 phases [ 48 , 49 ]. Nevertheless, the well-defined and sharp diffraction patterns in this sample (GCMW C sample) indicate a high crystallinity, as compared with the other two XRD spectra. As the development of traces of the secondary phase (tetragonal martensite) can affect magnetic behavior, this will be explored in more detail in the following sections. We estimated the lattice parameters of the two crystalline Co 2 FeSi glass-coated microwires, and then we employed the Debye-Scherrer’s equation, as presented in our previous work [ 23 ], to investigate the microstructure of Co 2 FeSi in greater depth. Using this methodology, we can estimate the average grain size, D g , associated with the principal peaks, which is approximately 17.8 m, 37.6 nm, and 45.8 nm for GCMW**, GCMW B , and GCMW C of Co 2 FeSi microwires, respectively, as illustrated in Table 2. Thus, D g has a monotonic increase with increasing the aspect ratio. Table 2. The average grain size and lattice parameters of Co 2 FeSi glass-coated microwires with different aspect ratios. Sample Average Grain Size (nm) Lattice Parameters (Å) GCMWA- - GCMW** 17.8 ±0.1 5.64 ±0.01 GCMWB37.6 ±0.1 5.63 ±0.01 GCMWC45.8 ±0.1 2.81 ±0.01 GCMW**: Co2FeSi glass-coated microwires with (ρ= 0.25) [24]. 3.2. Magnetic Characterization 3.2.1. Room Temperature Magnetic Properties Figure 3shows the magnetic hysteresis loops of Co 2 FeSi-glass-coated microwires with different ρ -ratios, obtained at room temperature with an applied magnetic field parallel to the microwire axis. All samples exhibit typical ferromagnetic behaviour, due to the high Curie point of Co 2 FeSi alloy greater than 1100 K [ 46 ]. The sample with a low ρ -ratio exhibits soft magnetic properties with coercivity, H c , around 14 Oe, and a non-square hysteresis loop shape (Figure 3a). However, the sample with the largest ρ -ratio shows almost perfectly square hysteresis loops with higher H c (about 87 Oe), than Co 2 FeSi with a low ρ -ratio (see Figure 3b,c). In addition, the hysteresis loop shows multistep magnetic behavior (indicated with arrows in Figure 3c). The almost square hysteresis loops for the GCMW C microwire with normalized remanent, M r , near 0.96 indicates the axial character of magnetic anisotropy with the easy axis of magnetization along the direction of the applied magnetic field. Thus, the increase in ρ -ratio affects the magnetocrystalline anisotropy, and its direction has the same direction of (220) and (420), as illustrated in the structural section. However, in the sample GCMW B with a crystalline structure, non-perfectly square loops are observed. Such change in the hysteresis loop shape must be related to the presence of a considerable amount of amorphous phase beside the disordered B2 or little-ordered L2 1 structures. In our previous work at the same alloys, but with a low ρ -ratio ( ρ = 0.26), the enhancement of the magnetocrystalline anisotropy, the squareness, and coercivity of Co 2 FeSi glass-coated microwires after annealing was observed [ 21 , 22 , 24 ]. As we illustrated in our previous work, the two main factors affecting the magnetic anisotropy behavior in Heusler-based glass-coated microwires are uniaxial magnetic anisotropy and cubic magnetocrystalline anisotropy [ 21 , 22 , 24 ]. By increasing the ρ -ratio, an enhancement in the crystalline phase content correlates with the magnetic property modification, i.e., the main factor controlling the magnetic anisotropy is the cubic magnetocrystalline anisotropy. Unfortunately, currently, we are not able to measure this type of anisotropy experimentally, but the perfectly square loop indicates its strong effect on the GCMW B and GCMW C samples. As seen in Figure 4, the GCMWc sample shows the highest anisotropy field Sensors 2023,23, 5109 7 of 14 H k , coercivity H c , and normalized remnant M r . The odd behavior of H k is likely related to the big amorphous phase present in GCMW A and GCMW B samples, as the behavior of Hc for these two samples is also quite similar (there is not a decrease, as in the case of the H k , but the values are almost the same) and maybe the growth of the crystalline structure eases initially the reduction of the anisotropy field. In addition, the different types of microstructures (L21, B2, and A2) can also strongly affect the Hkbehavior. Sensors 2023, 23, 5109 7 of 14 field Hk, coercivity Hc, and normalized remnant Mr. The odd behavior of Hk is likely related to the big amorphous phase present in GCMWA and GCMWB samples, as the behavior of Hc for these two samples is also quite similar (there is not a decrease, as in the case of the Hk, but the values are almost the same) and maybe the growth of the crystalline structure eases initially the reduction of the anisotropy field. In addition, the different types of microstructures (L21, B2, and A2) can also strongly affect the Hk behavior. Figure 3. Room temperature hysteresis loops for Co2FeSi glass-coated microwires (a) GCMWA, (b) GCMWB, and (c) GCMWC. The arrows in (c) pinpoints the multistep magnetic behavior. -200 -100 0 100 200 -1 0 1 M/M5K Magnet ic Fi eld ( Oe) GCMW C (c) -200 -100 0 100 200 -1 0 1 M/M5K Magnet ic Fi eld ( Oe) GCMW B (b) -200 -100 0 100 200 -1 0 1 M/M5K Magnet ic Fi eld ( Oe) GCMW A (a) Increasing Aspect ratio 0.3 0.4 0 26 52 78 0.51 0.68 0.85 1.02 70 84 98 112 Hc (Oe) Hc Mr Mr Hk (Oe) H k Aspect ratio Figure 3. Room temperature hysteresis loops for Co 2 FeSi glass-coated microwires ( a ) GCMW A , (b) GCMWB, and (c) GCMWC. The arrows in (c) pinpoints the multistep magnetic behavior. Sensors 2023,23, 5109 8 of 14 Sensors 2023, 23, 5109 7 of 14 field Hk, coercivity Hc, and normalized remnant Mr. The odd behavior of Hk is likely related to the big amorphous phase present in GCMWA and GCMWB samples, as the behavior of Hc for these two samples is also quite similar (there is not a decrease, as in the case of the Hk, but the values are almost the same) and maybe the growth of the crystalline structure eases initially the reduction of the anisotropy field. In addition, the different types of microstructures (L21, B2, and A2) can also strongly affect the Hk behavior. Figure 3. Room temperature hysteresis loops for Co2FeSi glass-coated microwires (a) GCMWA, (b) GCMWB, and (c) GCMWC. The arrows in (c) pinpoints the multistep magnetic behavior. -200 -100 0 100 200 -1 0 1 M/M5K Magnet i c Field ( Oe) GCMW C (c) -200 -100 0 100 200 -1 0 1 M/M5K Magnet i c Field ( Oe) GCMW B (b) -200 -100 0 100 200 -1 0 1 M/M5K Magnet i c Field ( Oe) GCMW A (a) Increasing Aspect ratio 0.3 0.4 0 26 52 78 0.51 0.68 0.85 1.02 70 84 98 112 Hc (Oe) Hc Mr Mr Hk (Oe) H k Aspect ratio Figure 4. Aspect ratio dependence on coercivity (H c ), normalized remanence (M r ), and in –plane anisotropy field (Hk) of Co2FeSi glass-coated microwires (lines for eye guide). 3.2.2. Thermomagnetic Properties It is worth noting that the ferromagnetic materials temperature stability is a crucial characteristic for their possible applications in spintronic and sensing devices. Hence, for a wide range of measurement temperatures, 5–350 K, we investigated the magnetic behavior of Co 2 FeSi glass-coated microwires with different ρ -ratios. The shape of the loops follows the same trend observed at room temperature: non-square for the GCMWAsample, quite square for the GCMW B one, and almost square for the GCMW C one (loops not shown). In Figure 5, the evolution of H c and M r with the temperature is shown. This behavior demonstrates that for the GCMW C sample, cubic magnetocrystalline anisotropy prevails up to 350 K. Sensors 2023,23, 5109 9 of 14 Sensors 2023, 23, 5109 8 of 14 Figure 4. Aspect ratio dependence on coercivity (Hc), normalized remanence (Mr), and in –plane anisotropy field (Hk) of Co2FeSi glass-coated microwires (lines for eye guide). 3.2.2. Thermomagnetic Properties It is worth noting that the ferromagnetic materials temperature stability is a crucial characteristic for their possible applications in spintronic and sensing devices. Hence, for a wide range of measurement temperatures, 5–350 K, we investigated the magnetic behavior of Co2FeSi glass-coated microwires with different ρ-ratios. The shape of the loops follows the same trend observed at room temperature: non-square for the GCMWA sample, quite square for the GCMWB one, and almost square for the GCMWC one (loops not shown). In Figure 5, the evolution of Hc and Mr with the temperature is shown. This behavior demonstrates that for the GCMWC sample, cubic magnetocrystalline anisotropy prevails up to 350 K. Figure 5. Temperature dependence of the coercivity (a) and normalized remanence (b) of Co2FeSi glass-coated microwires with different aspect ratio (lines for eye guide). The error bar is as big or smaller than the size of the symbols. By analyzing the hystersis loops measured at temperature range, 5–350 K of Co2FeSi glass-coated microwires with different ρratio, an interesting magnetic behavior is found for both the temperature dependence of Hc and of the normalized remanence, Mr. GCMWC sample shows the highest value of the coercivity at the all measuring range of temperature range, with an average value of Hc six times higher than those of the GCMW**, GCMWA and GCMWB samples. GCMW**, GCMWA, and GCMWB samples show quite similar values of the Hc where the difference between the average value of coercivity is about 2 Oe. By estimating the differences in the coercivity (ΔHc) between the maximum value of coercivity (Hc (max)) and the lowest value of the coercivity (Hc (min)) for all samples, we pretend to show its stability with temperature. The samples with a clear crystalline phase, GCMW**, GCMWB, and GCMWC samples, show higher temperature stability than the amorphous GCMWA sample. Hence, the ΔHc is 11 Oe, 3.5, and 9 Oe for GCMW**, GCMWB, and GCMWC samples, respectively, whereas ΔHc is 15 Oe for the GCMWA one. The magnetic stability is clearer in the case of Mr tendency with the temperature of Co2FeSi glass-coated microwires with different ρ-ratios. As shown in Figure 5b, both GCMWB and GCMWC samples show high stability with temperature, with ΔMr 0.05 and 0.06, respectively (see Table 3). Meanwhile, the behavior of Mr of GCMWA is rather different, compared to the other samples with higher ρ-ratios, where a monotonic increase with decreasing the temperature has been observed. 0 50 100 150 200 250 300 350 0 20 40 60 80 100 Hc (Oe) T (K) GCMWA GCMWB GCMWC (a) 0 50 100 150 200 250 300 350 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Mr T (K) GCMWA GCMWB GCMWC (b) Figure 5. Temperature dependence of the coercivity ( a ) and normalized remanence ( b ) of Co 2 FeSi glass-coated microwires with different aspect ratio (lines for eye guide). The error bar is as big or smaller than the size of the symbols. By analyzing the hystersis loops measured at temperature range, 5–350 K of Co 2 FeSi glass-coated microwires with different ρ -ratio, an interesting magnetic behavior is found for both the temperature dependence of H c and of the normalized remanence, M r . GCMW C sample shows the highest value of the coercivity at the all measuring range of temperature range, with an average value of H c six times higher than those of the GCMW**, GCMW A and GCMW B samples. GCMW**, GCMW A , and GCMW B samples show quite similar values of the H c where the difference between the average value of coercivity is about 2 Oe. By estimating the differences in the coercivity ( ∆ Hc) between the maximum value of coercivity (H c (max) ) and the lowest value of the coercivity (H c (min) ) for all samples, we pretend to show its stability with temperature. The samples with a clear crystalline phase, GCMW**, GCMW B, and GCMW C samples, show higher temperature stability than the amorphous GCMW A sample. Hence, the ∆ Hc is 11 Oe, 3.5, and 9 Oe for GCMW**, GCMW B, and GCMW C samples, respectively, whereas ∆ Hc is 15 Oe for the GCMW A one. The magnetic stability is clearer in the case of Mr tendency with the temperature of Co 2 FeSi glass-coated microwires with different ρ -ratios. As shown in Figure 5b, both GCMW B and GCMW C samples show high stability with temperature, with ∆ M r 0.05 and 0.06, respectively (see Table 3). Meanwhile, the behavior of Mr of GCMW A is rather different, compared to the other samples with higher ρ -ratios, where a monotonic increase with decreasing the temperature has been observed. Table 3. The geometrical parameters and average (Av.) of Co 2 FeSi glass-coated microwires with different aspect ratios. Sample ∆Hc(Hc (max) −Hc (min))∆Mr(Mr (max) −Mr (min)) GCMWA15 ±2 Oe 0.7 ±0.1 GCMW** 11 ±1 Oe 0.6 ±0.1 GCMWB3.5 ±0.5 Oe 0.06 ±0.01 GCMWC9±2 Oe 0.05 ±0.01 GCMW**: Co2FeSi glass-coated microwires with (ρ= 0.25) [24]. Figure 6shows the complete thermomagnetic behavior of Co 2 FeSi glass-coated microwires with different ρ -ratios. We performed the ZFC, FC, and FH magnetic temperature dependence to check any possible phase transition. Thus, the measurements were performed at a low magnetic field of 200 Oe. For the GCMW A sample, the ZFC, FC, and FH magnetizations curves show non-homogonous behavior, besides an irreversible magnetic behavior at T = 150 K. Such irreversibility has been observed in our previous work deal-