Advances in the Fabrication and Magnetic Properties of Heusler Alloy Glass-Coated Microwires with High Curie Temperature

Abstract

This review article provides an in-depth analysis of recent advancements in the fabrication, structural characterization, and magnetic properties of Heusler alloy glass-coated microwires, focusing on Co₂FeSi alloys. These microwires exhibit unique thermal stability, high Curie temperatures, and tunable magnetic properties, making them suitable for a wide range of applications in spintronics, magnetic sensing, and biomedical engineering. The review emphasizes the influence of geometric parameters, annealing conditions, and compositional variations on the microstructure and magnetic behavior of these materials. Detailed discussions on the Taylor–Ulitovsky fabrication technique, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM) provide insights into the structural properties of the microwires. The magnetic properties, including room-temperature behavior, temperature dependence, and the effects of annealing, are thoroughly examined. The potential applications of these microwires in advanced spintronic devices, magnetic sensors, and biomedical technologies are explored. The review concludes with future research directions, highlighting the potential for further advancements in the field of Heusler alloy microwires.

1. Introduction

In recent years, ferromagnetic materials have attracted growing interest owing to various applications in magnetic sensors [1,2,3,4,5], magnetic refrigeration [6,7,8], microwave engineering [9,10], and spintronics [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32] because of their distinctive magnetic properties, which allow for the control and manipulation of spin currents. Among various ferromagnetic materials, micro/nano-structured materials have shown great potential for improving the performance of spintronic devices and communication engineering [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. Spintronics is one of the most promising multidisciplinary research fields, facilitating the development of next-generation nano- and microdevices that offer enhanced processing and memory capabilities while using less power [33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56]. To meet various essential criteria, such as high spin polarization and high Curie temperature (T_c), a new generation of multifunctional materials needs to be developed. Heusler alloys based on Co₂YZ are notable for their high spin polarization (P) and potential half-metallicity (P 100%). However, both theoretical and experimental studies indicate that spin polarization is highly sensitive to structural instability. The L2₁ crystalline phase provides the highest structural order, essential for achieving the desired spin-polarization levels. While the exchange of atoms at the Y–Z positions (B2 disorder) has a minimal impact on spin polarization, disorders involving X–Y or X–Y–Z positions (D03 or A2) can significantly reduce it. Additionally, these Heusler alloys have complex crystalline structures that require very high temperatures (typically over 1000 K in bulk form and over 650 K in thin-film form) for proper crystalline ordering. Consequently, a major challenge in producing X₂YZ full-Heusler low-dimensional materials is achieving the chemically ordered L2₁ phase, as the superior properties of Co₂-based Heusler compounds are generally expected in this phase (see Figure 1).

Heusler compounds are particularly well-suited for spintronic and magnetoelectronic applications [31,40]. Their advantages include excellent lattice matching with common substrates, T_c above room temperature, and the potential to achieve nearly 100% spin polarization near the Fermi level [40]. Co-based Heusler compounds are highly promising for multifunctional applications due to their low magnetic damping coefficients, high Curie temperature (T_c > 1200 K), adjustable band structure, and high magnetic moment. Additionally, these alloys exhibit remarkable and unusual physical properties both above and below room temperature, attributed to the significant Berry curvature associated with their band structure.

The aforementioned evidence highlights why Co-based full-Heusler alloys have garnered significant interest within the scientific community [58,59,60,61,62,63]. Consequently, these alloys are extensively studied in various forms, including nanoparticles, thin films, and nano/microwires [63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84]. However, it is important to note that producing Heusler alloy nanoparticles and thin films presents several challenges for practical applications. These challenges include the high cost of fabrication methods, chemical composition inhomogeneity, time consumption, and susceptibility to oxidation [52,53,54,55,72]. Additionally, the diffusion of substrate atoms into the films often leads to atomic disorder and phase separations, which are commonly observed [82,84]. The mismatch between the lattice structures of the alloy and the substrate further complicates fabrication. Moreover, to achieve the necessary structural ordering, Heusler alloys produced by arc melting or thin-film deposition require prolonged annealing at high temperatures [60,82]. The primary method for producing Heusler alloys is arc melting, followed by additional thermal treatment [40,55,60,63]. This technique allows for the fabrication of Heusler alloys in bulk form. However, miniaturization has been explored as an alternative strategy to enhance the properties of these alloys [84]. Reducing the dimensions of Heusler alloys can significantly improve their performance. For example, in magnetic cooling applications, decreasing the size of these alloys can increase the surface-to-volume ratio, thereby substantially enhancing the heat-exchange rate.

Since the 1960s, rapid melt quenching has been widely recognized by scientists as an effective method for producing innovative materials with diverse morphological characteristics, with either amorphous and nano/microcrystalline structures or metastable phases with reduced dimensions [85,86,87,88,89]. This technique enables the fabrication of alloys with precise chemical compositions through rapid solidification. Such materials with amorphous structure can present enhanced mechanical, magnetic, and corrosion properties [89,90,91,92,93,94,95]. Rapid melt quenching methods have been refined to produce a range of materials, such as ribbons, wires, flakes, microwires, and composite microwires. The final structure of these materials is significantly influenced by factors such as the phase diagram of the selected alloy, quenching conditions, and the geometry of the produced materials.

As previously noted, crystalline materials obtained through rapid quenching generally exhibit inferior mechanical and corrosion properties compared to their amorphous counterparts [89,96,97,98,99]. However, rapidly quenched crystalline materials can possess other desirable properties for various applications, such as Giant magnetoresistance or magnetocaloric effect [86,88]. Additionally, the challenge of miniaturizing rapidly quenched materials has become a key focus for numerous applications. As a result, considerable attention has been directed in recent years toward developing preparation methods capable of meeting these requirements.

The Taylor–Ulitovsky method has become a highly promising technique for miniaturizing rapidly quenched materials while simultaneously improving their magnetic, corrosion, and mechanical properties [89,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113]. This technique allows the production of thin metallic microwires, with metallic nucleus diameters ranging from 0.02 to 100 µm (with a difference of almost 4 orders of magnitude), coated by insulating glass [104,109]. The resulting glass-coated microwires, which may possess either amorphous or nanocrystalline structures, exhibit excellent magnetic softness. In addition, the thin glass coating imparts several advantages, such as enhanced mechanical strength, corrosion resistance, improved adhesion to polymer matrices, and biocompatibility [89,97]. Several successful efforts have been made to fabricate such wires using the in-rotating-water technique [102] or by producing glass-coated microwires from Heusler alloys via the Taylor–Ulitovsky method [78,80,88,89,92,104,105,106,107,108]. A key feature of the Taylor–Ulitovsky technique is its ability to simultaneously solidify metallic alloy and coat them with insulating glass [89,95,96,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127]. Such process creates internal stresses due to the mismatch in the thermal expansion coefficients between the glass and the metallic alloy [95,100]. The magnitude of these internal stresses, σ_i, is related to the ratio (ρ) between the diameter of the metallic nucleus, d_metal and the total, D_total diameters. Therefore, these internal stresses can be controlled by adjusting the ρ-ratio [95,100,113,115].

This review article focuses on the fabrication, structural characterization, and magnetic properties of Heusler alloy glass-coated microwires, specifically Co₂FeSi alloys. These microwires exhibit remarkable thermal stability, high Curie temperatures, and tunable magnetic properties, which are influenced by factors such as geometric parameters, annealing conditions, and compositional variations.

2. Materials and Methods

The experimental conditions for synthesizing bulk and glass-coated microwires of Co₂FeSi have been comprehensively detailed in previous studies [75,78]. The primary objective of this study is to fabricate these samples with different geometric parameters to explore how internal stresses induced by the glass coating influence their magnetic properties and microstructure in different X₂YZ-based full-Heusler microwire series.

To prepare the Co₂FeSi alloys, arc melting was employed. The precursor elements—Co (99.99%), Fe (99.9%), and Si (99.99%)—were weighed according to the nominal composition ratio (X)₂:(Y)₁:(Z)₁ and placed in a graphite crucible (see Figure 2). The elements were melted together to form ingots of the respective alloys. To ensure homogeneity, the melting process was repeated five times. Before proceeding with glass coating, the chemical compositions and nominal ratios of the X₂YZ alloys were verified.

Using the Taylor–Ulitovsky technique, we produced a variety of Heusler-based glass-coated microwires with precisely controlled dimensions and lengths, tailored to specific applications and research objectives [86,88,93,100,101,102,103,104,105,108]. By regulating the casting rate during the melting process, we successfully obtained microwires with a consistent metallic nucleus diameter and a uniform glass coating thickness, ensuring fixed geometric parameters.

After fabricating the Co₂FeSi microwires (MWs), we determined their geometrical parameters—including metal core diameter (d_metal), total diameter (D_total), and aspect ratio (ρ = d_metal/D_total)—using optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) (JEOL-6610LV, JEOL Ltd., Tokyo, Japan). The nominal chemical compositions of the samples were also analyzed. Once the chemical composition and nominal ratio were confirmed, microstructural characterization was performed at room temperature using X-ray diffraction (XRD) (D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany).

Magnetic characterization was conducted in several steps. First, hysteresis (M-H) loops were measured at room temperature under magnetic fields applied parallel and perpendicular to the metallic core axis to evaluate magnetic anisotropy and confirm the easy axis of magnetization. Next, the magnetic behavior of the samples was examined over a broad temperature range (5–400 K) by recording M-H loops along the wire’s axis, representing the easy magnetization direction. Finally, thermal magnetization curves—specifically, field cooling (FC) and field heating (FH) curves—were analyzed under a low external magnetic field to investigate potential irreversibility or magnetic phase transitions in the Co₂FeSi MWs. All magnetization measurements were conducted using a PPMS (Physical Property Measurement System, Quantum Design Inc., San Diego, CA, USA) vibrating-sample magnetometer.

3. Results

3.1. Effect of Annealing Conditions

In this section, we highlighted the effect of the thermal treatment conditions on the magnetic and structure properties of Heusler-based glass-coated microwires.

3.1.1. Effect of Time Annealing in Co₂FeSi Glass-Coated Microwires

(a)

XRD analysis

Figure 3 presents the morphological, compositional, and structural characterization of as-prepared and annealed Co₂FeSi glass-coated microwires, analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The microwires exhibit a uniform cylindrical cross-section with a homogeneous elemental distribution (Figure 3a). EDX analysis (Figure 3b) confirms that the metallic nucleus composition, measured across 10 points (B1–B10), closely matches the nominal Co₅₀Fe₂₅Si₂₅ composition despite minor deviations from the stoichiometric Co₂FeSi. Annealed samples also maintain this composition, with a consistent Co:Fe atomic ratio of 2:1 and an average composition of Co₄₅Fe₂₂Si₃₃, aligning with previous findings on similar Co₂FeSi microwires [66]. The elevated Si content detected is attributed to an interfacial layer between the metallic core and glass coating, a characteristic feature of the fabrication process, as previously reported [66,75,78,103]. This interfacial layer, typically ~0.5 µm thick, contributes significantly to the overall Si content, given the microwire diameter of approximately 4.36 µm.

XRD analysis (Figure 3c) reveals a broad peak centered at 2θ ≈ 22.5°, attributed to the amorphous glass coating [126,127]. Additionally, all samples exhibit a narrower peak at 2θ ≈ 46°, corresponding to a high-order L2₁ cubic metallic phase (space group: Fm-3m). Notably, the intensity of this peak increases significantly in annealed samples compared to the as-prepared ones, indicating recrystallization—a process consistent with the devitrification of amorphous microwires [103]. The measured lattice parameter of 5.640 Å aligns with values reported for similar compositions [75,78], further confirming the structural integrity of the material.

Figure 3d presents the calculated nanocrystalline grain size (Dg) using the Debye-Scherrer formula [103]:

D_g = K λ/β cos2θ

(1)

where K is a shape factor (~0.94), λ is the XRD wavelength (CuKα, λ = 1.54 Å), and β is the full width at half maximum (FWHM) of the peak at 2θ ≈ 46° (Figure 1c).

The crystalline phase content (C) is determined from the total diffraction peak area, which includes both crystalline and amorphous contributions, using the following equation [48]:

$$C={\displaystyle \frac{I_c}{I_c+I_a(0.1+e^{\raisebox{1ex}{$-Dg$}\!\left/ \!\raisebox{-1ex}{$25$}\right.})}}$$

(2)

where I_c and I_a are the integrated intensities of the crystalline and amorphous peaks, respectively, and 25 nm is a fitting constant [128].

Figure 3d shows a significant increase in Dg and C, from 17.8 nm and 52% (as prepared) to 31.6 nm and 97% (annealed at 873 K for 1 h). However, after 6 h of annealing, Dg slightly decreases to 29.7 nm, while C increases to 99%. The Dg values obtained for annealed Co₂FeSi are approximately four times higher than those reported for Co₂-based Heusler alloy thin films [74,129]. This increase in Dg after annealing is commonly observed in most of materials due to a decrease in the energy of grain boundary through the grain boundary strengthening, the boundary migration, or changes in the dislocation density [129,130]. In particular, enhanced atomic diffusion at elevated temperatures can promote grain growth [95,131,132,133,134]. Additionally, the transformation of the amorphous phase into a bcc structure further contributes to grain growth with prolonged annealing.

As commonly observed, grain size increases during the devitrification of amorphous materials [132,133,134]. Crystallization involves grain nucleation and subsequent growth, which can be controlled either by atomic transfer at grain boundaries or by diffusion-driven nucleation mechanisms [133,134]. In nanocrystalline materials obtained via rapid quenching, the crystallization process is often more complex, sometimes leading to grain refinement [135]. A similar effect occurs here, where Dg decreases from 31.6 nm to 29.7 nm, possibly due to the nucleation of new nanograins or the dissolution of metastable grains followed by the formation of new nanocrystals [135]. This variation in grain size plays a crucial role in influencing the magnetization behavior of the samples, which will be discussed further.

(b)

Magnetic properties

(i)

Magnetic properties of as-prepared sample

Figure 4 presents the magnetic properties of as-prepared Co₂FeSi glass-coated microwires, showing the dependence of the magnetic moment (M) on the applied magnetic field (H) across a temperature range of 5–400 K with fields up to ±50 kOe. As seen in Figure 4a,b, all M-H loops exhibit ferromagnetic behavior throughout the entire temperature range. The highest values of magnetic moment and remanence (M_r) are observed at 5 K, while the lowest values appear at 400 K. The hysteresis loops display a rectangular M-H shape, consistent with Co₂FeSi alloys produced by various deposition techniques and in different forms [56,136,137,138]. In the 200–400 K range, the loops show reduced saturation field (H_s), anisotropy field (H_k), remanence (M_r), and coercivity (H_c), as illustrated in Figure 4c,d.

The structure of the Co₂FeSi glass-coated microwires consists of a mixture of amorphous and crystalline phases. Due to this mixed structure, accurate determination of the anisotropy constant is challenging. In this study, the magnetic anisotropy field (H_k) was estimated from the hysteresis loops in Figure 4a,b and presented alongside with the coercivity (H_c) in Figure 4c. Notably, the nearly rectangular hysteresis loops observed here have also been reported in fully amorphous microwires and thin films [115,139,140], as well as in mixed amorphous–crystalline and nanocrystalline microwires [135]. Such rectangular shape of microwires has been attributed to shape magnetic anisotropy and the axial stresses induced by the difference in thermal expansion coefficients between the metallic nucleus and the glass coating.

Analysis of the M-H curves for Co₂FeSi glass-coated microwires (G-CMMWs) reveals a soft magnetic behavior, with the lowest coercivity (H_c = 9 Oe) detected at 200 K and the highest (H_c = 25 Oe) at 5 K, resulting in a variation of approximately 16 Oe. Similarly, the anisotropy field (H_k) exhibits its lowest value at 200 K and its highest at 5 K.

An unusual trend in H_c and H_k is observed (Figure 4c), where both parameters first increase as the temperature decreases from 400 K to 300 K, then decrease between 300 K and 200 K, and, finally, increase again, reaching their maximum at 5 K. This anomalous magnetic behavior has not been previously reported for Co₂FeSi alloys in other forms. It can be attributed to the internal stresses induced by the glass coating, which is temperature dependent and can affect the phase changes. These stresses, primarily caused by the thermal expansion mismatch between the metallic nucleus and the glass layer, can modify both the micromagnetic and crystalline structures, significantly affecting H_c and H_k [141].

The normalized remanence (M_r), defined as M_r = M_r/M₅ₖ (where is M_r at 5K), follows a more regular temperature-dependent trend (Figure 4d). It increases sharply from 0.28 to 0.81 as the temperature decreases from 400 K to 200 K, then stabilizes between 200 K and 100 K before rising again to reach its maximum at 5 K.

The temperature dependencies of M_r and H_c confirm the strong sensitivity of the magnetic properties of Co₂FeSi microwires to thermal variations. The anomalous behavior of H_c and H_k, alongside the expected trend of M_r, highlights the potential of these microwires for further studies on the effects of annealing and geometric factors. These findings could pave the way for utilizing Co₂FeSi microwires in spintronic devices based on thermo-magnetic switching.

Understanding the thermal stability of ferromagnetic materials is crucial for their application in spintronic devices, particularly in determining their performance at, below, or above room temperature (RT). To assess this, the temperature dependence of magnetization (M vs. T) was measured under different conditions, zero-field cooling (ZFC), field cooling (FC), and field heating (FH), using both low (H = 50 Oe) and high (H = 50 kOe) magnetic fields over a temperature range of 4 to 400 K (Figure 4e–h). To facilitate comparison, the M vs. T curves were normalized to the maximum magnetic moment at 5 K.

In the FC protocol, the Co₂FeSi G-CMMWs were cooled to 4 K under an applied magnetic field, aligning the magnetic moments parallel to the field. In contrast, in the ZFC system, the moments remained randomly oriented. As the temperature increased under a low external field, the magnetic moments aligned with the applied field, leading to an increase in magnetization until relaxation effects became dominant, causing ZFC to decrease and eventually merge with FC at higher temperatures [142].

A significant magnetic irreversibility was observed at low fields (H = 50 Oe), with a blocking temperature (T_B) of ~205 K (Figure 4e). However, this irreversibility disappeared under a high magnetic field (H = 50 kOe) (Figure 4h), confirming that it is strongly dependent on the field strength. Such behavior, commonly reported in magnetic materials, results from the coexistence of re-entrant ferromagnetism and spin glass-like behavior [143,144].

Additionally, the disordered structure and chemical composition of Co₂FeSi G-CMMWs contribute to this phenomenon. The magnetic ground state is not purely ferromagnetic; instead, a random spin-disordered B2 phase coexists with the ferromagnetic L2₁ phase [142,143]. This mixed-phase structure is induced by internal stresses from the glass coating during fabrication, leading to the formation of both disordered (B2) and ordered (L2₁) phases, alongside the amorphous phase, as confirmed by XRD analysis. Notably, applying a high magnetic field (50 kOe) suppressed the B2 phase, eliminating the irreversibility behavior.

    (ii)

Magnetic properties of annealed samples

Figure 5 presents the hysteresis loops of as-prepared and annealed Co₂FeSi glass-coated microwires at different annealing temperatures (T_ann) measured at room temperature with the magnetic field applied parallel to the microwire axis. All samples exhibit ferromagnetic behavior, with the transition to the paramagnetic phase occurring above 1100 K [64]. The as-prepared sample (Figure 5a) shows soft magnetic behavior with a narrow, non-squared loop. The relatively soft magnetic properties of the as-prepared sample can be linked to its nanocrystalline structure with Dg below 20 nm. In contrast, the annealed samples (Figure 5b,c) display harder magnetic behavior with more rectangular hysteresis loops. These shape of the hysteresis loops, with a reduced remanent magnetization (M/M_max ≈ 0.97) and higher Hc, suggest either the development of axial magnetic anisotropy, with the axial easy magnetization axis aligned with the direction of the applied field, or magnetic hardening due to increase in Dg. An increase in Dg up to 30–35 nm (see Figure 3d) and the induction of strong cubic magnetocrystalline anisotropy (CMA) along the (220) direction parallel to the microwire axis after annealing have been previously reported [139]. In contrast, the as-prepared sample predominantly exhibits uniaxial magnetic anisotropy (UMA), with a smaller contribution from CMA. The annealing conditions enhance the CMA, leading to modification in key magnetic parameters such as remanence (M_r), anisotropy field (H_k), and coercivity (H_c), as shown in Figure 5d.

The thermal stability of ferromagnetic materials is crucial for their potential use in spintronic devices. Notably, we observed a significant increase in coercivity (H_c), remanence (M_r), and anisotropy field (H_k) after annealing (Figure 6), confirming that cubic magnetocrystalline anisotropy (CMA) dominates in the annealed samples up to 400 K. Analyzing the M-H curves at different temperatures (5–400 K) for both as-prepared and annealed Co₂FeSi microwires, we observed an interesting temperature dependence of H_c and M/M_max-values, with respect to the highest magnetic moment at 5 K (see Figure 6a). The unusual behavior of the H_c of the as-prepared sample between 400 K and 200 K could be attributed to the competition between uniaxial magnetic anisotropy (UMA) and cubic magnetocrystalline anisotropy (CMA).

For the annealed samples, H_c and M_r are higher across the full temperature range compared to the as-prepared sample. Notably, H_c and M_r exhibit matching trends, indicating uniform magnetic anisotropy for all temperatures, unlike in the as-prepared sample (Figure 6). For the annealed sample at 1 h, H_c and M_r show a maximum at 150 K, followed by a flipping temperature point (T_f), where both parameters decrease as temperature decreases, reaching their lowest values at 5 K. A similar behavior is observed for the 6 h annealed sample but with the flipping point occurring at 55 K (Figure 6b–d). Moreover, we noticed a change in the M-H loops for the annealed samples at 1 h and 6 h below T_f. Above T_f, the hysteresis loops exhibit a single-step magnetic reversal with square loops. Below T_f, the loops show a multi-step magnetization reversal process, with a distorted shape, as indicated in Figure 7. This behavior has been observed in various magnetic materials, including magnetic nanostructured thin films, nanoparticles, and magnetic microwires with mixed amorphous–crystalline structure [145,146,147]. To our best knowledge, this is the first time that such magnetic behavior has been observed in annealed Co₂FeSi alloy glass-coated microwires. This suggests that there is a critical temperature at which this unusual behavior arises, likely related to the nanocrystalline structure and grain size. We can deduce that even small changes in the grain size (D_g) can significantly affect the magnetic behavior.

Multi-Step Magnetic Behavior and the Role of Nanocrystallinity in Co₂FeSi Microwires

An unusual multi-step magnetic behavior emerges in glass-coated Co₂FeSi microwires, strongly influenced by the superposition effect of an external magnetic field and the microwire’s stray field. This phenomenon, reported in previous studies [22,25,35,145,148,149], can be attributed to the mixed structure of studied samples [145], fluctuations in the metallic nucleus diameter, or the magnetostatic interaction of the magnetic microwires [150]. The strength of this superposition effect depends on the difference between the stray and switching fields, intensifying when this difference increases. A remarkable finding is that at low temperatures, a single microwire behaves as if composed of two distinct magnetic microwires due to the pronounced difference between the two fields. This results in the multi-step magnetization process, which is absent in as-prepared samples due to their small nanocrystalline grain size (D_g). This observation strongly suggests that annealing significantly affects the nanocrystalline structure (D_g-value), leading to the formation of two magnetic phases with distinct behaviors, ultimately triggering the multi-step magnetic response. Below the critical temperature (T_f), the switching fields of these magnetic phases diverge. Once one phase undergoes magnetization switching, the combined effect of the external field and the demagnetizing field from the already re-magnetized phase is insufficient to switch the second phase, which possesses a larger moment. This effect is further amplified by the presence of 50% Co (soft phase) and 25% Fe (hard phase) in the annealed samples, which contributes to hysteresis loop distortions and the multi-step magnetization reversal process.

Magnetic Phase Transitions and Thermal Effects

To explore the thermo-magnetic behavior and potential magnetic phase transitions, field-cooling (FC) and field-heating (FH) curves were measured under low magnetic fields. As-prepared samples (Figure 8a) exhibit a stable ferromagnetic state between 400 K and 190 K, with FC and FH curves coinciding. However, below 190 K, a small gap emerges between them, with FC rising above FH before they realign at 15 K. This suggests a magnetic phase transition, consistent with previous findings [66,75,78,151,152,153]. Notably, changes in H_c and M_r below 200 K (Figure 6) indicate a strong correlation between these parameters and the observed magnetic phase transition. Annealed samples at 873 K (Figure 8b,c) display more complex behavior. For the 1 h annealed sample, a sharp drop in magnetization occurs under a 50 Oe external field, suggesting a pronounced magnetic phase transition (Figure 8b). In contrast, for the 6 h annealed sample, this drop disappears, and a separation between FC and FH emerges only below 105 K, with the curves matching again at 60 K—another indication of a magnetic phase transition (Figure 8c). The blocking temperature (T_b) for the 1 h sample is 150 K, aligning precisely with the critical flipping temperature (T_f), reinforcing the idea that the nanocrystalline structure significantly influences magnetic transitions. These results demonstrate that Co₂FeSi Heusler alloys are highly sensitive to annealing conditions, which directly affect their lattice structure, local atomic arrangement, and stoichiometric composition [74]. The annealing at 873 K for 1 h and 6 h promotes recrystallization, accompanied by atomic ordering, the reduction of internal stresses, and the formation of two distinct magnetic phases with different magnetic responses, leading to the observed anomalous magnetic behavior in annealed Co₂FeSi microwires.

3.1.2. Effect of Annealing Temperature in Co₂FeSi Glass-Coated Microwires

Figure 9a–f presents the M-H loops of annealed Co₂FeSi microwires measured at varying temperatures, from 305 K to 5 K. Regardless of temperature, all samples exhibit ferromagnetic behavior, both above and below room temperature. Above room temperature, the hysteresis loops maintain a rectangular shape, with a gradual reduction in coercivity (H_c), saturation magnetization (M_s), and remanent magnetization (Mr) as the temperature increases, following the same trend as the as-prepared Co₂FeSi samples. However, for Co₂FeSi microwires annealed at 873 K and 973 K, a drastic shift in magnetic behavior emerges below room temperature. At critical temperatures of 105 K and 255 K, respectively, significant distortions appear in the hysteresis loops, evolving into a distinctive “kink” or “wasp-waisted” magnetic behavior, characterized by multi-step magnetic switching. Above these critical temperatures, the loops maintain a regular shape, but below them, the multi-step hysteresis loops are observed.

Interestingly, this type of magnetic response is well-documented in magnetic nanoparticles, nanostructured thin films, and amorphous microwires, yet it is first observed in Co₂FeSi alloy glass-covered microwires. Several mechanisms have been proposed to explain such behavior:

• Strong magnetic coupling between distinct magnetic phases (in this case, hard Co and soft Fe regions), leading to an oxidation-induced imbalance.
• Reordering of ferromagnetic spins below the critical temperature under an applied magnetic field, causing domain wall pinning and influencing hysteresis loop distortions.
• Superposition of an external magnetic field and the stray field from the microwire array, induced by factors such as metallic nucleus diameter fluctuations or mixed crystalline structure.

A particularly fascinating aspect of this study is that multi-step behavior is absent in as-prepared samples—it only emerges in samples annealed at 873 K and 973 K, disappearing again at 1073 K. This suggests that annealing induces two distinct magnetic phases with different responses, tightly linked to the critical temperature where multi-step switching occurs. Below this temperature, the coercive field of each magnetic phase varies. When one phase switches, the combined effect of its demagnetizing field and the external magnetic field is insufficient to trigger magnetization switching in the second phase, leading to the multi-step process. Moreover, the presence of 50% Co (responsible for magnetically hard phase) and 25% Fe (magnetically soft phase) in annealed samples can play a relevant role in shaping the “kink” or “wasp-waisted” hysteresis loops and reinforcing the multi-step magnetic behavior.

An unusual coercive field trend with temperature has been observed while analyzing the magnetic hysteresis loops of as-prepared and annealed Co₂FeSi samples (Figure 9g). All annealed samples exhibit a higher H_c compared to the as-prepared one. For the as-prepared sample, a soft magnetic behavior is evident, with only a slight variation in H_c between 305 K (18 Oe) and 5 K (21 Oe), indicating a minimal change of just 3 Oe over the temperature range. However, for samples annealed at 873 K and 973 K, an anomalous coercivity trend emerges. Initially, H_c increases as the temperature decreases, reaching a maximum at 105 K and 155 K, respectively. Below these critical points, H_c unexpectedly decreases with further cooling, reaching its lowest value at 5 K. Interestingly, this anomalous coercivity behavior disappears in samples annealed at 1073 K, suggesting a direct link between annealing conditions and the magnetic phase transitions. This peculiar coercivity trend aligns perfectly with the critical temperature of the multi-step magnetic switching observed in Figure 9h. Specifically, above the critical temperature, the samples exhibit regular ferromagnetic behavior with a single-step switching mechanism. Below the critical temperature, the hysteresis loops transform, displaying the characteristic multi-step magnetization reversal. In typical crystalline and polycrystalline ferromagnetic materials, decreasing the temperature generally enhances magnetic anisotropy, leading to a natural increase in Hc. However, in this case, the shift in coercivity at low temperatures is strongly tied to micromagnetic structure changes, driven by transformations in the magnetic phases (as discussed in Figure 9a–f).

Figure 10 displays the temperature dependence of magnetization for Co₂FeSi microwires. As shown in Figure 10a, the as-prepared sample exhibits strong ferromagnetic behavior across the entire temperature range: The magnetization changes from its maximum values at 5 K to its minimum at 400 K, which is typical for ferromagnetic materials. This M vs. T behavior explains the regular magnetic behavior observed in the H_c and M-H loops at different temperatures for the as-prepared Co₂FeSi sample. Additionally, the Curie temperature (T_c) of the as-prepared sample could not be directly measured from the M vs. T curve, as it is expected to be above 1100 K, which is beyond the maximum temperature (400 K) of the employed PPMS device. For the annealed samples at 873 K and 973 K, a magnetic phase transition is observed, as indicated by a sharp drop in magnetization when external magnetic fields of 50 Oe and 200 Oe are applied during field-cooled (FC) and field-heated (FH) measurements, as shown in Figure 10b,c. Notably, this sharp drop disappears when a higher field of 1 kOe is applied to both annealed samples. These magnetic phases, observed in the annealed samples, are linked to the anomalous magnetic behavior described in Figure 9. The blocking temperatures (TB) for the samples annealed at 873 K and 973 K are found to be 150 K and 205 K, respectively, which aligns with the critical temperature suggested earlier for these samples. The as-prepared sample and the sample annealed at 1073 K do not show any irreversible behavior (Figure 10a,d). Thus, Co₂FeSi Heusler alloys are sensitive to annealing conditions, which affect both the lattice structure and the local atomic environment, as well as their stoichiometric composition. The anomalies observed in the M vs T curves for magnetic Heusler alloys are often explained by three main phase transition temperatures: the Curie temperature of the martensitic (TCM) phase, the martensitic transition temperature (TM), and the Curie temperature of the austenitic cubic phase at high temperatures. In this case, the martensitic transition temperature is more relevant since Tc for the annealed samples at 873 K and 973 K is expected to be much higher than TM. Moreover, the TM range coincides with the critical temperature where the “kink” or “wasp-waisted” hysteresis loops and multi-step magnetic behavior are observed. We suggest that the annealing at 873 K and 973 K induces recrystallization, atomic ordering, and the reduction of internal stresses, leading to the formation of two distinct magnetic phases. The martensitic phase with a different magnetic response is responsible for the observed anomalous magnetic behavior in the annealed Co₂FeSi.

4. Potential Applications

Heusler alloy glass-coated microwires, with their unique combination of properties including thermal stability, high Curie temperatures, tunable magnetic behavior, and enhanced mechanical and chemical resistance due to the glass coating, hold significant promise for a variety of advanced applications.

4.1. Spintronic Devices

The unique combination of high spin polarization, tunable magnetic properties, and thermal stability makes Heusler alloy glass-coated microwires promising candidates for various spintronic devices. Spintronics leverages the spin of electrons in addition to their charge, enabling the development of faster, more power-efficient, and non-volatile memory and logic devices. Co₂-based Heusler alloys, in particular, are of great interest due to their predicted half-metallicity, where electrons of one spin orientation are metallic while others are insulating, leading to nearly 100% spin polarization at the Fermi level. Achieving and maintaining this high spin polarization, particularly in the ordered L2₁ phase, is crucial for spintronic applications [154,155,156,157,158,159,160,161,162,163,164].

Specifically, these microwires are attractive for the following applications:

• Spin valves and magnetic tunnel junctions (MTJs): Spin valves and MTJs are fundamental building blocks of spintronic devices. They consist of ferromagnetic layers separated by a non-magnetic metal (spin valve) or an insulating layer (MTJ). The resistance of these structures depends on the relative orientation of the magnetization in the ferromagnetic layers, a phenomenon known as giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR). Heusler alloys, with their high spin polarization, are excellent candidates for the ferromagnetic electrodes in MTJs, leading to high TMR ratios, which are essential for device performance. The ability to control the magnetic properties of microwires through annealing and compositional variations allows for the fine-tuning required for optimal spin valve and MTJ performance.
• Magnetic random access memory (MRAM): MRAM is a non-volatile memory technology that stores data using magnetic states. Different MRAM technologies exist, including Spin-Transfer Torque MRAM (STT-MRAM) and Spin-Orbit Torque MRAM (SOT-MRAM). In STT-MRAM, the magnetization of a free layer is switched by a spin-polarized current. SOT-MRAM utilizes spin–orbit interactions in a heavy metal layer to generate a spin current that switches the magnetization. Heusler alloys with high spin polarization and low damping are highly desirable for the magnetic layers in both STT-MRAM and SOT-MRAM, offering potential for high-speed and low-power operation. The thermal stability and potential for miniaturization offered by Heusler alloy microwires are advantageous for the development of high-density MRAM [160,161,162,163,164,165].
• Domain wall devices: The controlled manipulation of magnetic domain walls in ferromagnetic nanostructures is another promising avenue for spintronic applications, including logic and memory devices. The observed magnetic behavior in Heusler alloy microwires, such as the multi-step magnetization reversal, suggests their potential for use in domain wall-based devices. The geometry and internal stress distribution in microwires can influence domain wall dynamics, offering possibilities for tailoring their behavior for specific functionalities [112].

4.2. Magnetic Sensors

The sensitivity of the magnetic properties of Heusler alloy microwires to external stimuli like magnetic fields and temperature makes them highly suitable for sensor applications. Their soft magnetic behavior, which can be tuned by controlling factors such as geometric parameters and annealing conditions, allows for the development of sensitive magnetic field sensors. The ability to tailor parameters like coercivity and anisotropy is crucial for optimizing sensor performance. Furthermore, the distinct temperature dependence of the magnetic properties, including the observed magnetic phase transitions, can be effectively exploited for temperature-sensing applications. Glass-coated magnetic microwires, in general, have shown promise in various sensing applications due to their specific magnetic domain structure and high magnetoimpedance effect, offering advantages in terms of sensitivity and miniaturization compared to some other materials [101,115,117,166,167,168,169].

4.3. Biomedical Engineering

Heusler alloy glass-coated microwires hold significant potential in biomedical engineering, owing to their unique magnetic properties and the inherent biocompatibility provided by the glass coating. One key application is in magnetic hyperthermia, where the ability of these materials to generate heat upon exposure to an alternating magnetic field can be utilized for targeted cancer therapy. The localized heating can selectively destroy cancer cells with minimal damage to surrounding healthy tissue. Additionally, the sensitivity of the magnetic properties to biological molecules suggests their potential for developing novel biosensors. While specific in vivo or in vitro studies on Co₂FeSi microwires are subjects for future research, the broader class of glass-coated magnetic microwires has been explored for biomedical applications, including magnetic hyperthermia for in vitro cancer cell treatment [101,115,117,118,170].

4.4. Other Potential Applications

Beyond spintronics and sensing, Heusler alloy glass-coated microwires present exciting possibilities for other advanced applications. The magnetocaloric effect, where a temperature change is induced by a changing magnetic field, is observed in some Heusler alloys and suggests their potential for energy-harvesting devices. This effect could be utilized in solid-state cooling or for converting waste heat into usable energy. Furthermore, the magnetic field-induced shape memory effects observed in certain Heusler alloys could be harnessed for the development of microactuators. These micro-scale devices could find applications in various fields, including minimally invasive surgery or microfluidics. The tunable properties of Heusler alloy microwires make them versatile materials for exploring these and other novel functionalities.

5. Challenges and Future Works

While significant progress has been made in the fabrication and characterization of Heusler alloy glass-coated microwires, several challenges remain, and future research directions can be identified.

5.1. Precise Control of Microstructure

• Achieving precise control over the microstructure, particularly the degree of L2₁ ordering and the grain size distribution, remains a challenge.
• Future work should focus on optimizing fabrication parameters, such as annealing conditions and cooling rates, to enhance structural order and tailor the microstructure for specific applications.

5.2. Understanding Complex Magnetic Behavior

• The complex magnetic behavior observed in these microwires, including the multi-step magnetization reversal and the interplay between different magnetic anisotropies, requires further investigation.
• Future studies should aim to develop a deeper understanding of the underlying mechanisms governing these phenomena, potentially through advanced micromagnetic modeling and simulation.

5.3. Integration into Devices

• While the potential applications of Heusler alloy microwires are promising, their integration into actual spintronic devices, sensors, and biomedical technologies presents challenges.
• Future research should focus on developing reliable methods for device fabrication, addressing issues such as microwire alignment, electrical contacting, and compatibility with other device components.

5.4. Exploring New Materials and Compositions

• The current review primarily focuses on Co₂FeSi alloys. Future work should explore other Heusler alloy compositions and even quaternary or quinary alloys to discover new materials with enhanced properties.
• Investigating the effects of doping or introducing other elements into the microwires could also lead to novel functionalities.

5.5. In Situ Characterization

• In situ characterization techniques, such as real-time monitoring of microstructure evolution during annealing or magnetic measurements under applied stress, would provide valuable insights into the structure–property relationships in these materials.
• Developing and utilizing such techniques should be a priority for future research.

5.6. Modeling and Simulation

• Computational modeling and simulation can play a crucial role in complementing experimental studies.
• Future efforts should focus on developing accurate models to predict the structural, magnetic, and electronic properties of Heusler alloy microwires, aiding in the design of materials with tailored properties.

6. Conclusions

This review has provided a comprehensive overview of recent advancements in the fabrication, structural characterization, and magnetic properties of Co₂FeSi Heusler alloy glass-coated microwires. The Taylor–Ulitovsky method has proven to be a versatile technique for producing microwires with controlled dimensions and enhanced properties. The ability to tune the magnetic properties of these microwires through variations in composition, annealing conditions, and geometric parameters opens up a wide range of potential applications, particularly in spintronics, magnetic sensing, and biomedical fields. However, challenges such as achieving precise control over the L2₁ ordering, understanding complex magnetic phenomena like multi-step magnetization reversal, and integrating microwires into devices need to be addressed. Future research should focus on these areas, as well as exploring new Heusler alloy compositions and developing advanced characterization techniques, to fully realize the potential of these remarkable materials.