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

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 a promising candidate for enhancing spintronic devices' performance [1,2,3,4,5,6,7,8,9]. One of the most promising multidisciplinary research fields is spintronics, which enables the creation of the next-generation of nano & micro devices with improved processing and memory capability while consuming less power [10]. To address the various required criteria, such as high spin polarization or high Curie temperature, Tc, a new generation of materials with multifunction uses has to be created [11]. The Heusler compounds are one of the potential smart materials with desirable characteristics for spintronic and magneto-electronic applications [12]. Perfect lattice matching with major substrates, high Tc above room temperature, and intermetallic controllability for spin density of states at the Fermi energy level—where approximately 100% of spin polarized near the Fermi level is reported [11,12,13,14,15]—are just a few of their advantages.

In terms of half-metallic alloys, Co₂-based full-Heusler compounds are among the most promising ones because of their high thermal stability, high Curie temperatures (Tc ≈1100 K) for the bulk sample, high magnetic moment (~ 6 µ_B/f.u.), and low Gilbert damping constant ( α = 0.004) [14,16,17]. They also have exotic transport properties, an electric structure determined by ab initio calculations [16], and high magnetic moments. The significant anomalous Hall effect that Co₂-based Heusler compounds exhibit because to the enormous Berry curvature associated with their band structure is also of current interest [16,18]. These factors have the scientific community interested in Co-based full-Heusler alloys. These types of alloys are therefore extensively researched in several configurations, including nanoparticles [19], thin films [15,17,20], and nano/micro wires [21,22,23,24]. It is important to note that the fabrication of Heusler alloys nanoparticles and thin films faces numerous difficulties for application purposes, including the high cost of preparation methods, chemical composition inhomogeneity, and ease of oxidation from the perspective of proper atomic ordering and material chemistry [20]. The diffusion of substrate atoms into the film results in the existence of 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 wires 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 µm) [27,28]. Such microwires are composites consisting of metallic nucleus (with 0.2 ≤d ≤100 µm) 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 µm) [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. [27,28,29,30,31,32,33,34,35]. 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 [27,28,29]. Additionally, they have shown potential characteristics for actuators and in medical applications, including as cancer treatment 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 metallic nucleus and D_tot - total diameter). The fabrication method was chosen because of the intriguing magneto-structural behavior of glass-coated microwires derived from Heusler alloys, as well as functional properties of glass-coated microwires such as superior mechanical properties, insulating, thin and flexible glass-coating, and thin dimensionality [27,29,30,31,32,33,34,35]. As a result, we have prepared Co₂FeSi glass-coated microwires using the Taylor-Ulitovsky procedure, which is detailed previously [27,36,37]. The Taylor-Ulitovsky approach, which has been utilized since the 1960s [36], is one of the current fabrication methods used to make Heusler alloys glass-coated microwires with a wide variety of geometric characteristics [21,22,24,27,28,29,30,31,32,33,34,35]. The primary benefit of this low-cost technique is that it allows the production of thin and long (a few kilometers long) microwires with a wide diameter range (d- values ranging from 0.2 to 100 µm) at high speeds (up to a few hundred meters per minute) [36,37,38]. This process is also used to prepare glass-coated microwires with excellent mechanical properties [21,39,40,41]. Glass coating on microwires can give us with extra benefits such as better insulation and environmental protection. Moreover, the availability of a biocompatible thin, flexible, insulating, and highly transparent glass coating may aid biological applications [29,42,43]. As a result, Heusler microwires based on Co₂FeSi are a potentially smart material with applications in a wide range of devices. To the best of our knowledge, up to date no one has reported on the production and structural, mechanical, or magnetic characterization of Co₂FeSi-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 Co₂FeSi alloys that involves melting the precursor components together in an electric arc furnace. Typically, the following procedures are used to create Co₂FeSi alloys by arc melting: i) preparing the precursor ingredients. The precursor elements for the Co₂FeSi alloy are weighed and deposited in a graphite crucible, containing cobalt (99.99%), iron (99.9%), and silicon (99.99%). 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 Co₂FeSi alloy (ingot) is created as the ingredients consolidate. This process were then repeated five times to achieve perfect homogeneity and a homogeneous microstructure. Once the Co₂FeSi alloy has solidified and formed an ingot, the ingot is used to prepare Co₂FeSi 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 very thin glass coatings, generally up to a few micrometers thick. This thin covering 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 [21,22,24,27,28,29,30,31,32,33,34,35] explain the manufacturing method in detail. A glass capillary was produced and filled with molten Co2FeSi alloy after a high frequency inductor heated an ingot over its melting temperature. The diameter of the metallic nuclei, d, was then determined by varying the speed of wire drowning and the rotation of the pick-up bobbin. The manufactured microwire is sent via a coolant stream to complete the fast melt quenching process.

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 Co₂FeSi glass-coated 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 through 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 low magnetic field ( H = 200 Oe). All magnetization curves were measured using a PPMS (Physical Property Magnetic System, Quantum Design Inc., San Diego, CA) vibrating-sample magnetometer at temperatures, T, between 5 and 400 K for ZFC, FC and FH magnetic curves. For the hysterics 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 the misleading of the errors estimation 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. Using the EDX data from Table 1, it was revealed that the metallic nucleus composition differed considerably from the stoichiometric one (Co₂FeSi). The features of the preparation process, which involved alloy melting and casting, were the cause of this slight variation. To quantify the difference, we checked at the nominal composition for 8 sites as illustrated in Figure 1a. An atomic average of Co₄₄Fe₂₃Si₃₃ was used to confirm that the true 2:1 ratio of Co and Fe 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.

In order to study the order state of our produced Co₂FeSi 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 changing in the geometric ρ-ratio has a strong influence on the structure of Co₂FeSi 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 is detected. The wide halo at 2Ө = 22.3° is related to the glass coating layer, as reported in our previous works [21,22,24,27,28,29,30,31,32,33,34,35]. By increasing the geometric aspect ratio a crystalline structure of metallic nucleus becomes evident with a notable peak at 2Ө = 46.2°, attributed to the (220) reflection. Further increase of geometric ρ-ratio results in the perfect crystalline structure of studied samples, where the crystalline peak intensity increases and additional peak appears at 2Ө = 85.4°, corresponding to the (422) reflection. The analysis of XRD profiles of the two crystalline Co₂FeSi samples, i.e. GCMW_C (P = 0.30) and GCMW_B (P = 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 L2₁ or B2 phases with amorphous state for GCMW_B sample [34,35].

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 lead to a small distortion of the crystalline structure [45]. The absence of a (400) peak around 85^⁰, 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) [47]. Otherwise, according to the theoretical outcomes of Zhang et al., the disordered A2 state is more energetically preferable than those of the ordered L2₁ 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 spectrum. As the the development of traces of secondary phase (tetragonal martensite) can affect the magnetic behavior, this will be explored in more details in the following sections.

We estimated the lattice parameters of the two crystalline Co₂FeSi glass-coated microwires and then we employed the Debye- Scherrer's equation, as presented in our previous work [23], to investigate the microstructure of Co₂FeSi in greater depth. Using this methodology, we can estimate the average grain size, D_g, associated to the principal peaks, which is approximately 37.6 nm and 45.8 nm for GCMW_B and GCMW_C of Co₂FeSi microwires, respectively as illustrated in Table 2.

3.2. Magnetic Characterization

3.2.1. Room Temperature Magnetic Properties

Figure 3 shows the magnetic hysteresis loops of Co₂FeSi 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 Co₂FeSi alloy greater than 1100 K [46]. The sample with low ρ -ratio exhibits soft magnetic properties with coercivity, Hc, around 14 Oe and non-square hysteresis loop shape (Figure 3a). However, the sample with the largest ρ -ratio shows almost perfectly square hysteresis loops with higher Hc (about 87 Oe), than Co₂FeSi with low ρ -ratio (see Figure 3b and 3c). 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, Mr, near to 0.96 indicates the axial character of magnetic anisotropy with easy axis of magnetization along the direction of applied magnetic field. Thus, the increase in ρ - ratio affects the magnetocrysttaline ansiotropy and its direction has the same direction of (220) and (420), as illustrated in the structural section. However, in the sample GCMW_B with crystaline structure, non-perfectly square loops is observed. Such change in the hysteresis loop shape must be related to the presence of the considerable amount of amorphous phase beside the disordered B2 or little ordered L2₁ structures. In our previous work at the same alloys, but with low ρ – ratio (ρ = 0.26), the enhancement of the magnetocrystlline anisotropy, the squareness and coercivity of Co₂FeSi glass coated microwires after annealling was observed [21,22,24]. As we illustated in our previous work, the main two factors affecting the magnetic anisotropy behavior in Heusler based glass- coated mixcrowires are unixial magnetic anisotropy and cubic magnetocrystalline anisotropy [21,22,24]. By increasing the ρ-ratio an enhacement in the crystalline phase content correlates with the magnetic properties modification, i.e. the main factor controlling the magnetic anistropy is the cubic magnetocrystalline anistropy. Unfortunately, currently we are not able to measure this type of anisotropy experimentaly, but the perfectly square loop indicates its stronge effect for the GCMW_B and GCMW_C samples. As seen in Figure 4, the GCMWc sample shows the highest anisotropy field H_k, coercivity H_c and normalized remenant M_r.

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 GCMW_A sample, 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.

By analyzing the hystersis loops measured at temperature range , 5–350 K of Co₂FeSi 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 6 times higher than those of the GCMW_A and GCMW_B samples. Both GCMW_A and GCMW_B samples show quite similer values of the H_c where the different between the average value of coercivity is about 2 Oe. By estimating the differences of the coercivity (ΔHc) 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_B and GCMW_C samples, show higher temperature stability than the amorphous GCMW_A sample: Hence the ΔHc is 3.5 and 9 Oe for GCMW_B and GCMW_C samples, respectivly, whereas ΔHc is 15 Oe for the GCMW_A one. The magnetic stability is more clear in the case of Mr tendency with temperature of Co₂FeSi 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 ΔM_r 0.05 and 0.06, respectivily (see Table 3). Meanwhile, the behavior of Mr of GCMW_A is rather different, comparing to the other samples with higher ρ -ratios, where a monotonic increase with decreasing the temperature has been observed.

Figure 6 shows the complete thermomagnetic behavior of Co₂FeSi 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 low magnetic field 200 Oe. For GCMW_A sample, the ZFC, FC and FH magnetizations curves show non-homogonous behavior, beside an irreversible magnetic behavior at T = 150 K. Such irreversibility has been observed in our previous work dealing with Co₂FeSi-based glass-coated microwires with aspect ratio ρ = 0.26, (see [21,22,24].). In this works we have illustrated that the irreversibility enhanced by preforming annealing at 873 K and 973 K for 1 h. The induced martensitic transition and the changing in the internal stresses associated with the glass-covering layer with temperature allow to control the irreversibility behavior. The interesting point for Co₂FeSi-glass-coated microwires with the lowest ρ- ratio, i.e., ρ = 0.23, is that its blocking temperature is observed at T = 150 K, like the Co₂FeSi-glass-coated microwires with ρ = 0.26 [21,22,24]. For GCMW_A sample, which is totally in amorphous state (see Figure 2), the main reason for the irreversibility behavior is the strong internal stress induced by the glass covering layer. For GCMW_B and GCMW_C sample, i.e., increasing ρ -ratio, the irreversibility behavior disappeared and the usual ferromagnetic behavior is observed with homogenous ZFC, FC and FH magnetic curves. The homogenous magnetization curves are due to the induced crystal structure with A2-type and B2 or L2₁ cubic structure for GCMW_C and GCMW_B, respectively.

We believe that increasing the ρ - ratio of Co₂FeSi glass-coating microwires affects recrystallization, atomic ordering, and stress reduction. Furthermore, for samples with a high ρ -ratio, the induced L2₁/B2 and A2 cubic structure types generate a strong magneto crystalline anisotropy, explaining the the behavior of magnetic properties such as H_c, M_r, H_k, and thermomagnetic curves with temperature. In fact, as shown in several previous publication, the internal stresses values are affected by the ρ – ratio: the lower the ρ – ratio, the higher the internal stresses related to the presence of the glass-coating [50,51,52]. On the other hand, the glass-coating thermal conductivity can affect the quenching rate of the metallic nucleus: lower quenching rate must be at the origin of higher crystallinity of the microwires with relatively thick glass-coating [27].

Resuming, Co₂FeSi-glass-coated microwires are an excellent candidate for a wide range of industrial applications, and specially for sensors, due to their perfect squared loops at a wide temperature range and homogenous thermomagnetic behavior with temperature.

5. Conclusions

In summary, we have fabricated Co₂FeSi glass-coated microwires with different geometrical aspect ratios. A strong influence of the geometric aspect ratio on the magnetic and structural properties is observed and discussed. The increase in the aspect ratio correlates with the increasing degree of crystallinity. Promising coercivity and normalized remnant stability with temperature are found for Co₂FeSi-glass coated microwires with a high aspect ratio. For the sample with the lowest aspect ratio, the thermomagnetic curves show large irreversibility with a blocking temperature of T = 150 K. The induced crystal structure of the A2-type gives rise to a high cubic magnetocrystalline anisotropy that controls the magnetic behavior of Co₂FeSi glass-coated microwires and makes it a suitable candidate for magnetic sensing.