Preparation and magnetic properties of Co₂-based Heusler alloy glass-coated microwires with high Curie temperature

Co₂-based Heusler alloys, (Co₂FeSi) in particular, are considering promising smart materials for multifunction applications, especially in the spintronics, due to unique physical properties such as extraordinary electronic structure, high Curie point, near to 100% spin polarization at the Fermi level position, high saturation magnetization, very low Gilbert damping constant (α = 0.004) and unusual anomalous Hall Effect.^1–5 Up until now, Co₂FeSi Heusler alloys have been used to design superconductor (SC)-based devices,⁶ ultra-high-performance giant magnetoresistance devices,⁷ semiconductor-based spintronic devices⁸ and magnetic tunnel junctions.⁹ 

Due to its unique combination of (electronic, electrical, mechanical, magnetic, and anticorrosive) properties, Co₂-based Half-Metallic Heusler alloys made using the Taylor–Ulitovsky technique provide significant advantages over conventional Co₂-based Heusler alloys.^10–13 Additionally, the tunable microstructure of the microwires prepared by the Taylor–Ulitovsky method has led to the fabrication of Heusler-based glass-coated microwires with a variety of microstructures, including amorphous, poly/monocrystalline, and granular structures. The growing interest in this method has increased significantly in last decades due to their technological applications, especially in sensing and biomedical applications.^14–19 In addition, the Taylor–Ulitovsky provide a simple, fast (up to a few hundred meters per minute) and low cost preparation technique to fabricate thin\thick Heusler-based glass-coated microwires alloys without the necessity of additional long thermal treatments.^14,15 One of the advantages of the Taylor–Ulitovsky technique is the ability to produce microwires with metallic nucleus with a wide range of diameters (from 200 nm to 100 µm) and several kilometers long.^19,20 Moreover, the glass coating add additional flexibility to prevent metallic core nuclei from the oxidation and enhance the mechanical and biocompatibility properties.^11,19,21,22 Therefore, Co₂FeSi Heusler alloy glass coated microwires are expected to be useful in a variety of applications.

In current study, we will focus on the magnetic behavior of as-prepared and annealed (600 °C for 1 h) of Co₂FeSi glass-coated microwires under a wide range to temperature (5 to 400 K) and magnetic field (50 Oe to 20 kOe). Different magnetic behavior has observed depending on the external applied magnetic field and the temperature. The annealed sample show gradual uniform magnetic dependence by increasing the applied magnetic field.

Starting from melting high purities of Co (99.99%), Fe (99.99%) and Si (99.99%) with aspect ratio (2:1:1) using conventional arc furnace in argon atmosphere to prevent oxide formation during melting process. The melting process repeated for five time to obtain high homogeny alloy. Then we checked the chemical composition using the EDX/SEM, as reported in our previous work.^12,15,23 After confirming the chemical composition, we proceeded to fabricate the glass-coated microwires by using Taylor–Ulitovsky method. For more details about the method of glass-coated microwire fabrication was reported and described elsewhere.^{14,15,23–25} The inner metallic nucleus diameter, d, of Co₂FeSi glass-coated microwires has final geometric parameters of 4.4 µm, while the total diameter of the exterior Pyrex coating, D, is around 17.6 µm. After fabrication, the Co₂FeSi microwire was subsequently annealed at 600 °C for 1 h in a protective helium atmosphere. The structural analysis and EDX/SEM evaluation of the metallic nucleus’ chemical composition were previously examined and described elsewhere.^11–13 The magnetic properties were studied by using PPMS (Physical Property Magnetic System, Quantum Design Inc., San Diego, CA) at temperatures, T, between 5 and 400 K and wide range of applied magnetic field (H = 50 Oe to 20 kOe). The results are provided in terms of the normalized magnetization, M/M_5K, where M_5K is the magnetic moment measured at 5 K with a magnetic field equal to 20 kOe. The microwire bunch was employed for magnetic measurements revealing relative variations of magnetization.

Figure 1 shows the axial hysteresis loops of Co₂FeSi as-prepared and annealed glass-coated microwires, measured by the PPMS with an applied magnetic field of 20 kOe parallel to the microwires’ axis and a temperature range of 300 to 5 K. Every loop was adjusted to the maximum magnetic moment measured at 5 K. As shown in Fig. 1, all annealed samples show perfectly squared hysteresis loops with higher coercivity and M/M_5K ratio as-compared to the as-prepared sample. The M/M_5K ratio show a notable increase by decreasing the temperature from 300 to 5 K. The coercivity increases as the temperature decreases from 300 K, reaching a maximum at 150 K, and then starts to drop by decreasing the temperature to 5 K. This is an intriguing finding in magnetic M–H loops of annealed material. In contrast, the M/M_5K ratio for the annealed sample just slightly changes with temperature, while the as-prepared sample shows a significant variation.

Figure 2 summarized the temperature dependence of magnetization, M/M_5K, of the as-prepared sample. In Fig. 1 is illustrated that the samples exhibit ferromagnetic behavior over the whole measurement temperature range of 5 to 400 K. Additionally, when the temperature decreases, the magnetization rises to its maximum at 5 K. A more substantial magnetization dependence was found for field cooling (FC) and field heating (FH) protocols when an applied magnetic field is 50 Oe. Additionally, there is a discrepancy between FC and FH magnetization curves. This mismatching can be attributed to the variation of the internal stresses in glass-coated microwires stress with temperature and the applied external magnetic field. Additionally, the magnetic field dependence does not exhibit a consistent trend with temperature; this is because disordered crystalline phases coexist with ordered ones in the same sample. As reported recently by the XRD analysis, as-prepared Co₂FeSi glass-coated microwires show a mixed structure (amorphous + crystalline [with L2₁ (ordered) and B2 (disordered)].^10–12 An increase of average grain size (from 18 to 30 nm) and content of crystalline phase (from 50 to almost 100%) was observed upon annealing of Co₂FeSi glass-coated microwires.^11,12 Softer magnetic properties and the non-squared hysteresis loops shape (illustrated in Fig. 1) can be attributed to the contribution of an amorphous precursor and non-uniform magnetic behavior with variation the magnetic field and temperature (seen in Fig. 2). Such substantial dependence of the hysteresis loops on the average grain size and the crystalline phase content is reported for nanocrystalline materials and, in particular, for magnetic microwires with nanocrystalline structure.^26–28 

The M/M_5K vs (T) dependence of Co₂FeSi glass-coated microwires annealed for 1 h at 600 °C is completely different from that of the as-prepared sample. Several anomalous features have been observed in the temperature and magnetic field dependencies. The magnetic phase transition with considerable irreversibility has been demonstrated with a blocking temperature of 150 K in the M/M_5K vs (T) dependence with applied low magnetic field, i.e. 50 and 200 Oe. Such behavior does not appear in as-prepared sample when the same magnetic field is applied. This difference (see Figs. 2 and 3) can be explained by the mixed amorphous/crystalline structure of as-prepared sample, as recently reported elsewhere.^11,12 The irreversible magnetic behavior vanished when the applied magnetic field is above 200 Oe. In this case the M/M_5K vs (T) curves exhibit similar to the as-prepared Co₂FeSi glass-coated microwire behavior. The most intriguing aspect is that the annealed Co₂FeSi glass-coated microwire becomes extremely sensitive to temperature and the applied magnetic field (see Fig. 3). The anomalous magnetic behavior of annealed Co₂FeSi is due to the annealing-induced recrystallization process, which is accompanied by an increase in crystalline phase content, atomic order and a decrease in internal stresses. Additionally, this process induces two different magnetic phases (the martensitic phase), each of, which has a different magnetic response.^11,12

In conclusion, we illustrate the effect of annealing on hysteresis loops and on the magnetic field and temperature effect on the magnetic properties of Co₂FeSi glass-coated microwires with nanocrystalline structure where L2₁ is the dominant structure order. Annealing leads to a change in the magnetic properties of Co₂FeSi glass coated microwires, such as the appearance of a perfectly squared hysteresis loop shape with M/M_5K close to unity. In addition, an improvement in the magnetic response from non-uniform for as-prepared sample to a uniform response to an applied magnetic field for an annealed sample is observed. At low field, the annealed sample, shows a martensitic phase transition with a large irreversible magnetic behavior, while this magnetic transition did not appear in the as-prepared sample. The result obtained confirms the strong effect of annealing on the magnetic properties and their temperature and magnetic field dependence of Co₂FeSi glass-coated microwires.

This work was supported by the Spanish MCIU, under PGC2018-099530-B-C31 (MCIU/AEI/FEDER, UE), by EU under “INFINITE” (Horizon Europe) project, by the Government of the Basque Country, under PUE_2021_1_0009 and Elkartek (MINERVA and ZE-KONP) projects and under the scheme of “Ayuda a Grupos Consolidados”(ref. IT1670-22), by the Diputación Foral de Gipuzkoa in the frame of Programa “Red guipuzcoana de Ciencia, Tecnología e Innovación 2021” under 2021-CIEN-000007-01 project and by the University of Basque Country under COLAB20/15 project. The authors are thankful for the technical and human support provided by SGIker of UPV/EHU (Medidas Magnéticas Gipuzkoa) and European funding (ERDF and ESF). We wish to thank the administration of the University of the Basque Country, which not only provides very limited funding, but even expropriates the resources received by the research group from private companies for the research activities of the group. Such interference helps keep us on our toes.

The authors have no conflicts to disclose.

M. Salaheldeen: Data curation (equal); Formal analysis (equal); Investigation (equal); Validation (equal); Writing – original draft (equal). M. Ipatov: Data curation (equal); Formal analysis (equal); Methodology (equal); Validation (equal). V. Zhukova: Data curation (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Validation (equal). A. García-Gomez: Data curation (equal); Formal analysis (equal); Investigation (equal). J. Gonzalez: Funding acquisition (equal); Project administration (equal); Resources (equal). A. Zhukov: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Project administration (equal); Resources (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.