Influence of Co on Structure and Magnetic Properties of Ni_50−xCo_xMn₂₉Ga₂₁ Shape Memory Alloy Ribbons

Abstract

In this work, we investigated the influence of Co on the structure and magnetic properties of Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, 4, 6, and 8) shape memory alloy ribbons fabricated by using the melt-spinning method. The addition of Co increases the formation of the austenitic crystalline phase in the alloy ribbons. The crystalline grains in rod-shapes with diameter of ∼2 µm and length of ∼10 µm are mostly oriented perpendicular to the ribbon surface. The martensitic-austenitic transformation was performed on both the thermomagnetization and differential scanning calorimetry methods. With the increase of Co concentration, the martensitic-austenitic structural phase transformation temperature (T_M-A) of the alloy gradually decreases while the Curie temperature of the austenite phase (T_C^A) of the alloy increases from 355 K (for x = 0) to 432 K (for x = 8). Besides, the martensitic-austenitic phase transformation is also significantly affected by the external magnetic field. The structural phase transformation temperature of the alloy tends to shift towards lower temperatures as the external magnetic field increases.

1. Introduction

Shape memory alloys (SMAs) are materials which can return to their original shape after being deformed when subjected to magnetic field or temperature. Recently, Heusler-type magnetic shape memory alloys (MSMAs) have been widely investigated as multifunctional materials due to great potential application in the fields of biomedical, aerospace, microelectronics, sensors, and actuators….^1–8) The advantage of the Heusler alloys is that their shape memory effect (SME) is controlled by both the magnetic field and temperature. Under the influence of the magnetic field, the SME in the alloy is larger than in the case of temperature impact. The magnetic shape memory effect (MSME) is presented in the Heusler alloys because magnetic interactions and structural phase transformations coexist in the material.^1–12) The MSMAs undergo the first-order martensitic-austenitic (M-A) phase transition from a tetragonal, orthorhombic or monoclinic disordered martensitic phase (low symmetry phase) to a cubic ordered austenitic phase (high symmetry phase) during applying heat or magnetic field.^8–12) In order to put the alloy into practical application, it is necessary to control both the magnetic phase transition and the structural phase transformation temperatures of the alloy. That means the materials which have the desired temperature and width of phase transition must be found.^12–17) Nowadays, many MSMAs with the large MSME have been discovered, such as Ni–Mn–Ga,^{7,9,11,13–19)} Ni–Mn–Sn (Sb, In),^2,20–26) Ni–Fe–Ga,^{18,19,27–30)} Ni–Mn–Al,^31–35) and Ni–Co–Al.^36–39)

Among these materials, Ni–Mn–Ga alloys are of interest because they exhibit a giant magnetic field-induced strain (MFIS) and fast response to magnetic stimulation.^{7,9,11,13–19)} The MFIS greater than 11% was reported in the Ni–Mn–Ga alloy at room temperature.⁸⁾ Therefore, there have been many studies on its structure, structural transformation, magnetic properties, and MFIS.^13–19) The structure, magnetic transition, and structural phase transformation of the alloy are very sensitive to the composition and number of valence electrons per atom ratio (e/a).^13–19) The magnetic orders, the M-A transformation temperature (T_M-A), and Curie temperatures of the Ni₅₀Mn_50−xGa_x bulk alloys with a wide composition range have been investigated.⁴⁰⁾ These Heusler alloys display the M-A transformation within a limited range of compositions, accompanied by different magnetic orders of the involved phases.⁴⁰⁾ It is possible to coexist different magnetic orders such as antiferromagnetic (AFM), ferromagnetic (FM), and paramagnetic (PM) in the alloys.^13–19,40) The M-A transformation temperature (T_M-A) decreases when Ga concentration increases.⁴⁰⁾ The M-A transformation in Ni–Mn–Ga Heusler alloys is a structural change between the low-temperature martensitic phase and the high-temperature austenite phase.³⁰⁾ Besides, intermartensitic transformations have also occurred from one martensitic phase to another.⁴¹⁾ Depending on the composition of Ni₅₀Mn_50−xGa_x Heusler alloys, intermartensitic transformations occur in the sequences 7M → 5M, L1₀ → 7M, and L1₀ → 7M → 5M with increasing temperature.⁴¹⁾ The substitution of Cu for Ga or Mn results in a significant increase in the T_M-A, and also increases the ductility and elasticity of the Ni–Mn–Ga alloys.^42–44) The T_M-A of the Ni₅₀Mn₂₅Ga_25−xCu_x alloys increases in the range of 202 K–691 K with increasing in Cu-concentration due to the increase of the e/a ratio from 7.50 to 8.17.⁴⁴⁾ Additionally, with Cu-addition, the type of martensitic transformation is changed when cooling from the L2₁ austenite phase. The structure transformations were found to be from the L2₁ austenitic phase to the martensitic phase with 14M, 2M, or 10M structural types depending on the Cu concentration.⁴⁴⁾ The Co addition of Mn-rich Ni–Mn–Ga alloys produces important changes in the structure and magnetic properties of the alloys.^45,46) Co enhances the ferromagnetic interaction in the austenitic phase while weakening the ferromagnetism in the martensitic phase leading to an increase of the M-A transformation for the alloys.^45,46) As a result, the Curie temperature of the martensitic phase (T_C^M) is reduced and the Curie temperature of the austenitic one (T_C^A) is increased with increasing Co concentration in the Ni–Mn–Co–Ga alloys.^45,46) Therefore, the addition of Co with a reasonable concentration in the Ni–Mn–Ga Heusler alloys has contributed to adjusting the working temperature range of the alloy to the most suitable temperature for magnetic field-induced M-A transformation applications.^45,46) On the other hand, the structure and magnetic properties of the alloy are significantly affected by fabrication conditions. Previous studies have often focused on bulk alloys.^40–47) In order to obtain the desired phase in the bulk alloys, complex heat treatment with high annealing temperature and long annealing time (over 1000 K for several days) is required.^42–47) However, the alloy ribbons prepared by using the melt-spinning method can have the desired structure and properties without heat treatment.^48–50) In addition, the ribbon shape can be very suitable for their use in practical applications as actuators or sensors.

In our previous study,⁵¹⁾ the influence of Ga concentration on the structure, structural transformation and magnetic transition of the Ni–Mn–Ga alloy ribbons were investigated. The results showed that the Ni₅₀Mn₂₉Ga₂₁ alloy ribbons have a sharp M-A phase transformation. In this work, we investigated the effect of Co on the structure, magnetic properties and martensitic-austenitic transformation of Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0–8) alloy ribbons fabricated by using the melt-spinning method.

2. Experimental

Five polycrystalline ingots with a nominal composition of Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, 4, 6, and 8) were prepared by arc-melting in an argon atmosphere, using high-purity elements (99.9%) of nickel, cobalt, manganese, and gallium. According to the previous studies,^44,51) because Mn was evaporated during the melting process, an additional 5 mass% of Mn was added to each sample. In the melting process, the samples were re-melted five times to ensure high homogeneity for ingots. A single copper wheel melt-spinning system was used to fabricate alloy ribbons from the bulk samples. The tangential speed of the copper wheel was 40 m/s. To avoid oxidation of the ribbons, this process is also performed in an Ar atmosphere. The structure of the alloy ribbons was checked at room temperature by an X-ray diffractometer (XRD) using Cu-K_α radiation source. Morphology and actual composition of the alloys were examined by using scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) methods, respectively. The temperature dependence of the magnetization at different magnetic fields and the field dependence of the magnetization were determined by using a vibrating sample magnetometer (VSM). The heating rate for the measurement of the thermomagnetization curves was about 20 K/min. The structural transformation temperatures were determined by differential scanning calorimetry (DSC) with the same heating and cooling rate of 20 K/min.

3. Results and Discussion

XRD patterns of the Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, 4, 6, and 8) ribbons measured at room temperature are displayed in Fig. 1. According to the data of the standard tags, the appeared diffraction peaks are determined to belong to the cubic-Ni₂MnGa structure ($\text{Fm}\bar{3}\text{m}$ space group) [JCPDS No. 00-050-1518], tetragonal-Ni₂MnGa structure (I4/mmm space group) [JCPDS No. 01-074-8832], and cubic-Ga₇Ni₃ structure ($\text{Im}\bar{3}\text{m}$ space group) [JCPDS No. 01-073-4714]. The cubic-Ni₂MnGa structure corresponds to the austenitic phase, whereas the martensitic phase is with the tetragonal-Ni₂MnGa structure.^9–12) Therefore, we can see that the formation of crystalline phases in the alloy depends on the concentration of Co. Without Co-addition, the tetragonal-Ni₂MnGa, cubic-Ni₂MnGa, and cubic-Ga₇Ni₃ structures co-exist in the alloy. The Ga₇Ni₃ crystalline phase is not detected in the ribbons with the addition of Co. Besides, as the concentration of Co increases, the intensity of diffraction peaks of the austenitic phase with the cubic-Ni₂MnGa structure increases while the martensitic phase with the tetragonal-Ni₂MnGa structure decreases. That means the addition of Co increases the formation of the austenitic phase in the alloy. These results are similar to those reported for some other alloys of Ni–Mn–Ga–Co such as Ni_56−xCo_xMn₂₅Ga₁₉, Ni_56−x/2Mn_25−x/2Co_zGa₁₉,⁵²⁾ Ni_53−xMn₂₅Ga₂₂Co_x.⁵³⁾ The percentage of individual phases was calculated by using Rietveld method (Table 1). The obtained results show that the Ga₇Ni₃ phase only appears in the x = 0 sample with a small fraction (4.6%). The fraction of the cubic-Ni₂MnGa phase increases while that of the tetragonal-Ni₂MnGa phase decreases with increasing Co concentration.

Fig. 1

XRD patterns of the Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, 4, 6 and 8) alloy ribbons.

Table 1 Percentage of the individual phases of the Ni_50−xCo_xMn₂₉Ga₂₁ alloy determined by the Rietveld analysis.

To investigate the microstructure of the ribbons, their cross-sections were observed by scanning electron microscopy (SEM). Figure 2 shows SEM images of the cross-section of the Ni_50−xCo_xMn₂₉Ga₂₁ ribbons with various concentrations of Co. The wheel surface (the surface in contact with the copper wheel) of the ribbons is at the bottom. The obtained results reveal that the ribbons have a thickness of about 15 µm. The grain size at the free surface of ribbons is bigger than that near the wheel surface. This is attributed to the heat transfer conditions of the melt-spinning process.⁴⁸⁾ The crystalline grains have an average diameter in the range of 1–2 µm. All the samples have good directional crystallization. The crystalline grains in rod-like shapes are almost parallel to each other and perpendicular to the surface of the ribbon. The good directional crystallization of the grains in the SMAs can be advantageous for some of their practical applications. Besides, it can be noted that the directional crystallization of the grains decreases with increasing Co-concentration.

Fig. 2

SEM images of the cross-section of Ni_50−xCo_xMn₂₉Ga₂₁ with x = 0 (a), x = 2 (b), x = 4 (c), x = 6 (d) and x = 8 (e).

The actual compositions of the Ni_50−xCo_xMn₂₉Ga₂₁ ribbons were checked by energy dispersive X-Ray spectroscopy (EDX). Figure 3 is the EDX spectrum of three representative samples with x = 0 (a), x = 4 (b) and x = 8 (c). The actual compositions of all the samples are listed in Table 2. The obtained results show that the atomic percentage of the elements specified by using EDX spectroscopy is almost consistent with that of the original composition. The peaks in the EDX spectroscopy are corresponding to the elements of Ni, Co, Mn, and Ga. With increasing the values of x, the concentration of Co increases while the Ni concentration decreases, and the concentrations of Ga and Mn are almost unchanged. From the compositions obtained from EDX analysis, the number of valence electrons per atom (e/a) ratio has been calculated (Table 2). The valence electronic configurations used for Ni, Mn, Ga, and Co are 3d⁸4s², 3d⁵4s², 3d²4s¹, and 3d⁷4s², respectively.^52,54) The obtained results show that the e/a ratio decreases as the Co concentration increases. According to previous studies, the magnetic transition and structural transformation temperatures strongly depend on the valence electron concentration per atom and Mn–Mn interatomic distances.^{48–50,52,54)} The substitution of Co for Ni reduces the e/a ratio of the ribbons. As a result, the fraction of the austenitic phase in the alloy ribbons increases as the Co concentrations increases. This is consistent with the above structural analysis. The decrease in the e/a ratio with increasing Co concentration has a direct effect on the magnetic properties of the alloy as shown below.

Fig. 3

EDX spectroscopy of the Ni_50−xCo_xMn₂₉Ga₂₁ ribbons with x = 0 (a), x = 4 (b), and x = 8 (c).

Table 2 Chemical compositions were determined by EDX analysis of the Ni_50−xCo_xMn₂₉Ga₂₁ alloy ribbons and the number of valence electrons per atom (e/a) ratio.

Previous studies showed that both the austenitic and martensitic phases can coexist in Heusler alloys.^48–54) The austenitic phase is strong ferromagnetic, while the martensitic phase is weak ferromagnetic or anti-ferromagnetic.^48–54) The austenitic phase exists at higher temperatures in comparison with the martensitic phase.^48–54) Most of the Heusler alloys undergo the first-order martensitic-austenitic (M-A) and reverse (A-M) transition at T_M-A and T_A-M temperatures, and the second-order ferromagnetic-paramagnetic (FM-PM) phase transition at T_C^A temperature. During heating, the austenitic phase begins to form at the austenite start temperature (A_s) and ends at the austenite finish temperature (A_f). As for during cooling, the martensitic phase begins to form at martensite start tempearture (M_s) and ends at the martensite finish temperature (M_f). Therefore, by investigating the temperature dependence of magnetization, M(T), both the magnetic and structural phase transitions in the alloys can be observed. Figure 4 shows M(T) curves of the Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, 4, 6, and 8) alloy ribbons in an applied magnetic field of 1 kOe. With increasing temperature, there is a jump-like change in magnetization from A_s to A_f. This is a characteristic of the M-A structural transformation that is commonly observed in the Heusler alloys because this transformation often takes place between the weak FM martensitic phase and FM austenitic phase.^48–55) Then, the magnetization on the M(T) curves decreases to near zero at the T_C^A transition temperature due to the PM nature of the material. Notably, there are two humps marked with asterisks in Fig. 4(a). This can be related to the presence of FM clusters or inhomogeneity in the crystal structure.⁵⁶⁾ This feature was observed only in the low magnetic field and disappeared as the magnetic field is high enough. Besides, we can see that the M-A phase transformation appears in all the samples, except the sample with x = 8. The T_M-A, which is determined from the maximum of the dM/dT(T) curves (see inset of Fig. 4(b)), decreases from 333 K to 205 K when Co concentration increases from 0 to 6 at%. Thus, the T_M-A of the alloy ribbons can be adjusted to room temperature with the addition of Co. The T_M-A strongly depends on the e/a ratio of the Heusler alloys.^50–54) According to the above-obtained results (Table 2), the replacement of Co for Ni reduces the e/a ratio of the alloy. Therefore, as the Co concentration increases, the M-A structural transformation shifts towards the lower temperature and does not appear in the measured temperature range for the x = 8 sample. The T_C^A is significantly enhanced with the addition of Co. The T_C^A value for the samples with x = 0, 2, 4, 6 and 8 is 355, 365, 390, 412 and 432 K, respectively. As a result, the T_C^A of the alloy ribbons is increased with increasing Co concentration. On the other hand, it is seen that the maximum magnetization of the austenitic phase increases significantly, whereas that of the martensite changes much less prominently with increasing the Co substitution for Ni (Fig. 4). From these M(T) data, the maximum magnetization ratio of austenitic and martensitic phases (M_A/M_M) can be calculated qualitatively. The M_A/M_M value is about 2, 3, 4 and 5 for the samples with x = 0, 2, 4 and 6, respectively. This proves that the addition of Co increases the ferromagnetic interactions in the austenitic phase of the alloy.

Fig. 4

Thermomagnetization curves of the Ni_50−xCo_xMn₂₉Ga₂₁ alloy ribbons with x = 0, 2, 4 (a) and x = 6, 8 (b) in an applied magnetic field of 1 kOe.

With an increasing magnetic field, the M-A phase transformation in the alloy is significantly affected while the FM-PM phase transition of the austenitic phase is almost unchanged (Fig. 5). Both the amplitude (ΔM) and width (ΔT) of the M-A phase transformation decrease at a high magnetic field of 10 kOe. The M-A phase transformation is hardly observed in the samples with x = 0 and 2. This result is consistent with previously published results in Refs. 51, 57).

Fig. 5

Thermomagnetization curves of the Ni_50−xCo_xMn₂₉Ga₂₁ alloy ribbons with x = 0, 2, 4 (a) and x = 6, 8 (b) in an applied magnetic field of 10 kOe.

To further investigate the influence of the magnetic field on the M-A phase transformation of the alloy ribbons, we measured a series of M(T) curves of the representative sample with x = 2 at various magnetic fields in the range of 0.1 to 10 kOe (Fig. 6). The amplitude of the M-A phase transition increases as the magnetic field increases from 0.1 to 4 kOe (Fig. 6(a)) and decreases in the magnetic field range from 6–10 kOe (Fig. 6(b)). This shows that the M-A phase transformation in the alloy is considerably improved with a suitable strength of the applied magnetic field. The A_s and A_f temperatures shift to the lower temperature region as the magnetic field increases. Thus, the M-A transformation is greatly affected by the external magnetic field. This result is similar to some results reported earlier.^51,57)

Fig. 6

Thermomagnetization curves of the Ni_50−xCo_xMn₂₉Ga₂₁ alloy ribbons with x = 2 in various magnetic fields in the range of 0.1–4 kOe (a) and 6–10 kOe (b).

Figure 7 shows the DSC curves of the Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, and 4) alloy ribbons during heating and cooling cycles. Endothermic and exothermic peaks that correspond to the austenitic-martensitic and martensitic-austenitic transformations are observed. The A_s, A_f, T_M-A during heating, and M_s, M_f, T_A-M during cooling temperatures are presented in Table 3. To compare the T_M-A values determined from M(T) and DSC curves, the samples with x = 0 and x = 2, are considered. The T_M-A values determined from M(T) curves for these two samples are also presented in Table 3. The T_M-A of the x = 2 sample determined from DSC curve (315 K) is in good agreement with that found on the thermomagnetic curve M(T) (316 K). Meanwhile, the T_M-A of the x = 0 sample is higher (17 K) than that obtained from the M(T) curve. This deviation can be explained as follows. The T_C^A of the sample with x = 0 is found to be 355 K (as mentioned above). Meanwhile, the A_f of this sample is 357 K, which is near to the T_C^A. That means the FM-PM magnetic transition of the austenitic phase takes place before the austenite finishes. At T_C^A, the x = 0 sample consists of mixture of both the martensitic and austenitic phases. Therefore, the T_M-A of vthe x = 0 sample obtained from the M(T) curve (333 K) is lower than that obtained from the DSC curves (350 K). Similar result was also observed in the Ni₅₅Mn_19.6Ga_25.4 alloy ribbons.⁵⁸⁾ The thermal hysteresis loss (T_M-A − T_A-M) between heating and cooling cycles in the DSC curves, displaying the first-order nature of the martensitic-austenitic transformation, are found to be 13, 15, and 16 K for the samples with x = 0, 2, and 4, respectively.

Fig. 7

Heating and cooling DSC curves of the Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, and 4) alloy ribbons.

Table 3 The A_s, A_f, M_s, M_f, T_M-A and T_A-M temparetures obtained from DSC curves of Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2 and 4) alloy ribbons.

The hysteresis loops of the Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, 4, 6, and 8) alloy at room temperature are presented in Fig. 8. The results show that all the samples exhibit soft magnetism with coercivity less than 100 Oe. The magnetization of the ribbons increases as the concentration of Co increases. Co can enhance exchange interactions in most of the ferromagnetic materials because it is a strong ferromagnetic element. Therefore, the substitution of Co for Ni increases the ferromagnetic interaction in the austenitic phase of the alloy. At applied magnetic field of 10 kOe, the magnetization (M_{10 kOe}) of the samples with x = 0, 2, 4, 6 and 8 is 22, 27, 45, 67 and 76 emu/g, respectively. The trend of Co concentration dependence of the M_{10 kOe} can be observed in the inset of Fig. 8(a). On the other hand, we also see that there is a small hysteresis on the hysteresis loops of the samples with x = 0, 2, and 4 (Fig. 8(b)). This can be due to the transformation of the metamagnetic phases under the influence of the external magnetic field.⁵⁹⁾ This hysteresis decreases as the Co concentration increases. This is great for materials as they are put into practical applications.

Fig. 8

Hysteresis loops (a) and enlarging hysteresis loops in the magnetic field range from 0 to 6 kOe (b) at room temperature of Ni_50−xCo_xMn₂₉Ga₂₁ (x = 0, 2, 4, 6 and 8) alloy ribbons.

4. Conclusions

The structure and magnetic properties of the Ni_50−xCo_xMn₂₉Ga₂₁ alloy ribbons were investigated with different concentrations of Co in the range of 0–8 at%. The austenitic phase is identified to be of cubic-Ni₂MnGa structure, whereas the martensitic phase is with tetragonal-Ni₂MnGa structure. The martensitic-austenitic transformation temperature, T_M-A, obtained from the thermomagnetization measurement is quite consistent with that determined by the differential scanning calorimetry method. The T_M-A temperature of the alloy can be adjusted to the desired values by appropriate substitution of Co for Ni. Co significantly raises both the magnetization and the Curie temperature of the austenitic phase of the ribbons with the highest value of 102 emu/g and 432 K, respectively, achieved in the sample with a Co concentration of 8 at%. The structure phase transformation in the alloy is also significantly affected by the external magnetic field. The austenitic start and finish temperatures tend to shift towards lower temperatures as the magnetic field increases. This makes the alloy ribbons of interest as a potential functional material.

Acknowledgments

This work was supported by Vietnam Academy of Science and Technology under grant number NVCC04.06/22-23, and the National Foundation for Science and Technology Development (NAFOSTED) of Viet Nam under grant number of 103.02-2019.344. Nguyen Hai Yen was funded by the Postdoctoral Scholarship Programme of Vingroup Innovation Foundation (VINIF), code VINIF.2023.STS.67. A part of the work was done in the Key Laboratory for Electronic Materials and Devices, Institute of Materials Science, VAST, Viet Nam.

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