3.1. Structural Properties
Table 1 shows the results of an EDX/SEM examination done to determine the chemical composition of Co
2MnSiC glass-coated microwires and compared with Co
2MnSi sample. The composition of the metallic nucleus evaluated by EDX/SEM is somewhat different from the stoichiometric one (Co
2MnSiC). This slight variation caused by the preparation procedure's characteristics, which comprised alloy melting and casting. We evaluated at the actual composition of ten different places to see how much the variation is. The atomic average composition of
Co50.4Mn23.6Si25.6C0.4 for Co
2MnSiC validated for all sites. An elevated Si content attributed to the interfacial layer between the glass coating and the metallic nucleus [
37]. The Co, Mn, and Si at. % are almost similar for Co
2MnSiC and Co
2MnSi alloys (see
Table 1).
To confirm the chemical structure composition and their distribution at Co
2MnSiC glass-coated microwires we performed the elements mapping by using the high-resolution transmission electron microscopy (TEM) supported by EDX.
Figure 1 illustrates the homogenous distribution of Co, Mn, Si and C elements at a single Co
2MnSiC glass-coated microwire. The image cross section does not show a perfect circular shape due to the distortion due to not exactly perpendicular cutting process, which results in an oval image shape. In addition, at the edge of the image, the color shows more contracted color due to the either distortion or the interfacial layer, but at the rest of microwire the perfect homogenous distribution is obtained. The fine details appeared in
Figure 1d and 1e come from small pieces of glass coating, as evidenced by the increase in the Si percentage content.
Figure 2 illustrates the X-ray diffraction (XRD) patterns of the Co
2MnSiC glass-coated microwires measured at room temperature (RT). All Miller indices are labelled on the patterns. As illustrated in
Figure 1, there is a wide halo at 2θ ≈ 22º, commonly attributed to the presence of an amorphous glass-coating layer, also observed in our previous works [
19,
20,
21]. The presence of (220), (400), and (422) peaks in the XRD pattern must be attributed to the cubic structure [
38]. Accordingly, it is expected the presence of the austenite phase at room temperature in studied Co
2MnSiC samples.
As a result, the entire diffraction pattern has been successfully identified by the existence of the cubic austenite structure. We should state that the lack of (111) and (200) superlattice diffraction peaks confirms the presence of an A2-type cubic structure [
39]. Indeed, no secondary phase was detected in all the XRD patterns. To evaluate the grain size, Dg, related to each peak, we used the Debye Scherrer formula [
39,
40]:
where, K is a dimensionless form factor with a value of roughly 0.94 (which might vary depending on the actual shape of the crystallite), the experimental XRD wavelength (Cu-K (alpha) = 1.54), and β present the whole width at half maximum of the XRD peaks.
Table 2 summarizes the differences of the microstructure between the Co
2MnSiC and Co
2MnSi glass-coated microwires, where a notable reduction in D
g and lattice parameters are observed.
The average Dg is about 29.2 nm, which is lower than that we reported for Co
2MnSi-based glass-coated microwires (Dg = 46 nm). The reduced Dg –value can be related to several factors, such as doping by small amount of Carbon or higher quenching rate due to thinner glass-coating thickness (0.4 µm for Co
2MnSiC microwire versus 5 µm for Co
2MnSi microwire). As discussed elsewhere, the average grain size substantially affects the magnetic properties of nanocrystalline materials [
41]. Accordingly, such reduced Dg - value can substantially affect the magnetic properties of Co
2MnSiC glass-coated microwires as will be illustrated in the magnetic characterization part.
3.3. Magnetic Properties
This section deals with the magnetic behavior studied between 300 K and 5 K. As described in the experimental section, we employed the PPMS to explore the magnetic properties of Co
2MnSiC and Co
2MnSi glass-coated microwires over a wide temperature, T, and magnetic field, H, ranges.
Figure 4 depicts the M/M
5K (H) curves, measured at various temperatures. The M/M
5K (H) loops exhibit ideal saturated curves between 300 and 50 K, however at T 50 K; a noticeable deviation from the saturation begins to occur. Such deviation increases with T decreasing (see the inset of
Figure 4a). The peculiarity of the Co
2MnSiC microwires with respect to at Co
2MnSi is the presence of a fully disordered microstructure with A2-type as described in
Figure 2 and
Figure 3. This A2-type microstructure breaks the antiferromagnetic order of Mn-Mn and enhances the paramagnetic effect at temperature below 50 K. For sample without Carbon, strong antiferromagnetic coupling Mn-Mn has detected (see
Figure 4b) for more details see ref. [
21].
The complete M/M
5K (H) curves for Co
2MnSi and Co
2MnSiC glass-coated microwires are shown in
Figure 5. Such M/M
5K (H) loops, measured at magnetic field ±30 kOe almost perfectly match at temperatures 300-5K. Small differences were observed only at the saturation part of the M-H loops, as discussed in the previous paragraph. As illustrated in
Figure 5a and 5b, the Co
2MnSiC sample show higher coercivity and lower normalized remanent compared to the Co
2MnSi sample at low and high temperature. These variations is due to the changing of the microstructure, which affect the magnetic microstructure of the sample and its response with variation of the temperature and the magnetic field.
The main magnetic parameters, such as coercivity, Hc, and magnetic remanence, M
r, extracted from low field M/M
5K (H) loops measured at different temperature are shown in
Table 3. From M/M
5K (H) loops, we can deduce low Hc -values showing average Hc ≈ 19.4 Oe for Co
2MnSiC sample and the average of Hc ≈ 6.9 Oe for Co
2MnSi sample at all range of measuring temperature, illustrating soft magnetic properties of studied microwire. The temperature dependence of Hc and M
r show unique stability with temperature (see
Table 3). The in-plane coercivity of Co
2MnSiC glass-coated microwires show rather stable Hc –value, where the differences between the lowest and the highest value of Hc, i.e., ΔHc is around 0.3 Oe (compared to 4 Oe for sample without Carbon). In addition, the differences between the normalized M
r (max) and normalized M
r (mini), ΔM
r is about 0.03 as shown in
Table 3. Observed unusual high temperature stability of Hc and M
r makes this new alloy, i.e. Co
2MnSiC glass-coated microwires, as a promising for application in magnetic sensing. For Co
2MnSi-based glass coating microwires, i.e. without Carbon doping, the Hc and M
r temperature dependencies also show a quite stable behavior, but ΔHc is around 4 Oe and ΔM
r = 0.05. Therefore, studied Co
2MnSiC microwire present better thermal stability of Hc, that can be attuited by Carbon doping of the Co
2MnSi-glass coated microwires or higher quenching rate associated with thinner glass-coating. Accordingly, the energy loss of the ferromagnetic materials becomes stable for a temperature range 300 to 5 K, which is very important for magnetic storage media, sensors, and energy-efficient motors devices.
It is critical to analyze the completely magnetic behavior with temperature in order to examine its thermal stability, which is a critical physical quality in determining its potential for spintronics applications. Furthermore, the temperature dependence of magnetization can provide important information on magnetic phase transformation. The magnetization dependence versus temperature (M vs. T), i.e., zero field cooling, ZFC, and field cooling, FC, throughout a wide range of magnetic field (H = 50 Oe to 20 kOe) and temperature range (5 to 300 K) are shown in
Figure 6 and
Figure 7. The as-prepared Co
2MnSiC and Co
2MnSi glass-coated microwires were cooled down from 300 K to 5 K under an applied low magnetic field (H = 50 Oe) in the Field cooling protocol, causing the random magnetic moment vectors to freeze parallel to the applied field at low temperatures.
Figure 6 shows the ZFC, FC and FH measuring at low magnetic field. For Co
2MnSiC sample, all magnetization curves show perfect ferromagnetic behavior without any magnetic phase transition, where the M/M
5K ratio has a monotonic increase by decreasing the temperature from 300 K to 5 K. The differences between the M/M
5K (300 k) and M/M
5K (5K), i.e., (ΔM/M
5K) ZFC = 0.16, (ΔM/M
5K) FC = 0.19 and (ΔM/M
5K) FH = 0.18. Such a small differences in the (ΔM/M
5K) between the ZFC, FC and FH magnetization curves must be related to the changing in the internal stresses originated by the glass-coating under changing the magnetic field and the temperature. Meanwhile, for Co
2MnSi glass-coated microwires large irreversibility with a blocking temperature T = 150 K has observed as show in
Figure 6b. This irreversibility is stable at applying external magnetic field from 50 Oe to 20 kOe. This behavior illustrates the strong influence of Carbon to change the magnetic properties and it behaves at different magnetic fields and temperature.
Figure 7 depicts FC and FH applied at various magnetic fields ranging from 50 Oe to 20 kOe. All FC and FH magnetization curves exhibit ferromagnetic behavior over the entire temperature range. Magnetization curves, measured at low magnetic fields, such as 50 Oe and 200 Oe, present strong modification with temperature. The slope on M/M
5K (T) vanished when the applied external magnetic field was increased up to 1kOe, and the FC and FH curves became almost straight (see
Figure 7a).
Figure 7b shows how the FC and FH magnetization curves behave when an external magnetic field is applied. The M/M
5K (T) dependencies measured at different H illustrate the sensitivity of Co
2MnSiC glass-coated microwires on the temperature and the external magnetic field.
From the FC and FH magnetization curves of Co
2MnSiC glass-coated microwires measured at different magnetic fields, we can estimate the magnetization thermal stability (ΔM) of each FC and FH magnetization curves of Co
2MnSiC glass-coated microwires. We proposed a phenomenal formula of (ΔM) which it depends on the difference between the maximum value of the magnetization and the minimum value of the magnetization at a specific range of temperature. As all FC and FH shows a ferromagnetic behavior and the maximum value of M/M5K measured at 5 K and the lowest value of M/M
5K measured at T = 300 K. Therefore, we can estimate the ΔM (%) for ΔT (the range of measuring temperature i.e. 300 – 5 K) by using this formula;
All calculated values are summarized at
Table 4.
As illustrated in
Table 4, the minimum thermal magnetization stability detected for FC and FH magnetization curves at H = 50 Oe, at which it is over 80 %. The highest ΔM is observed at H = 1 kOe, at which ΔM is near 98 % i.e. the changing in the M/M
5K magnetization ratio with temperature is only 2 %, which mean very high magnetization thermal stability. In addition, the average magnetization thermal stability for all range of magnetic field is about 92 %. Such behavior was not observed in Co
2MnSi glass-coated microwires, as the FC and FH magnetization curves of Co
2MnSi microwires show a large irreversibility magnetic behavior at low temperatures. Thus, ΔM for Co
2MnSi glass-coated microwires has a low temperature stability as- compared to the Co
2MnSiC glass-coated microwires. Therefore, studied Co
2MnSiC glass-coated microwires is a suitable candidate for micro motors and generators devices based glass-coating microwires.