Glass Forming Ability, Magnetic Properties and Magnetocaloric Effect of the Tb₆₅Ni₁₅Co₂₀ Amorphous Alloy

Abstract

In the paper, Tb₆₅Ni_35−xCo_x (x = 0, 10, 20, 30) amorphous ribbons were successfully prepared, and the glass-forming ability (GFA) of these ribbons was investigated. Both the T_rg and γ of the Tb₆₅Ni_35−xCo_x amorphous ribbons were larger than those of the Tb₆₅Ni₃₅ alloy and reached the maximum when x = 20, indicating that the Tb₆₅Ni₁₅Co₂₀ amorphous alloy has an optimal GFA. The magnetic properties as well as magnetocaloric effect of the Tb₆₅Ni₁₅Co₂₀ amorphous alloy were studied. The Curie temperature (T_c) of the amorphous alloy was ~79 K and the typical spin-glass-like behavior was found in the alloy. The peak value of magnetic entropy change (−∆S_m^peak) for the amorphous alloy was up to 9.47 J kg⁻¹ K⁻¹ under 5 T. The mechanism for the increased T_c and enlarged −∆S_m^peak of the Tb₆₅Ni₁₅Co₂₀ amorphous alloy was analyzed.

1. Introduction

With the increasing problems of energy consumption and air pollution, it is very necessary to develop new refrigeration methods to replace the traditional refrigeration technology using freon as a refrigerant. Among these refrigeration technologies, the magnetic refrigeration (MR) method, which is based on the magnetocaloric effect (MCE), has attracted more interest over the past several decades [1,2]. Compared with traditional vapor compression cycle refrigeration technology, MR possesses the advantages of high efficiency (as high as 30–60%, but the traditional refrigeration efficiency is only 5–10%), free of greenhouse gas and more compactness due to the use of solid refrigerant [1,2,3,4,5]. The performance of a magnetic refrigeration equipment fundamentally depends on the MCE of its refrigerant; thus, it is significant to choose appropriate refrigeration materials.

At present, the magnetic materials that exhibit MCE can be divided into two categories: (1) Crystalline compounds undergoing a first-order magnetic phase transition (FOMPT) usually show ultra-high magnetic entropy change peak (−∆S_m^peak), but this ultra-high −∆S_m^peak only exists within a very narrow temperature range, such as Gd-Si-Ge-, La-Fe-Si- and Ni-Mn-based alloys [6,7,8,9,10]. (2) The MCE materials undergoing a second-order magnetic phase transition (SOMPT), represented by pure Gd metal and amorphous alloys (AAs), exhibit relatively lower −∆S_m^peak than FOMPT materials [11,12,13,14]. However, the magnetic entropy change (−∆S_m) curves of the SOMPT MCE materials are broader, which means they can operate in a wide temperature range and, thus, leads to a much larger refrigeration capacity. Therein, AAs can be formed within a wide compositional range and can easily tune their Curie temperature (T_c) and −∆S_m^peak by compositional adjustment [15,16,17]. In addition, compared with the crystalline alloys, AAs also have better mechanical properties, higher corrosion resistance and lower eddy current losses [18,19]. Therefore, amorphous MCE alloys may be more suitable candidates as magnetic refrigerants used in magnetic refrigeration.

Among amorphous MCE alloys, rare earth (RE)-transition metal (TM)-based AAs and TM-based AAs are the main two categories. The TM-based amorphous MCE alloys usually show very low −∆S_m^peak (not exceed 4.0 J kg⁻¹ K⁻¹ under 5 T) [3,20,21,22]. Instead, the magnetocaloric effect of the RE-TM-based AAs are quite excellent, especially in Gd-TM-based AAs [23,24,25,26,27,28,29]. For example, a −∆S_m^peak under 5 T of up to 11.06 J kg⁻¹ K⁻¹ was achieved in a ternary Gd₃₄Ni₃₃Al₃₃ metallic glass [24]; Gd_55-60Co_15-30Al_15-30 AAs exhibited the −∆S_m^peak of 8.6~9.6 J kg⁻¹ K⁻¹ under 5 T [25]. In recent reports, other RE-TM-based (such as Nd, Tb and Dy) AAs also showed rather high −∆S_m^peak. The −∆S_m^peak under 5 T of Nd₆₅Co₃₅ AA reached 7.59 J kg⁻¹ K⁻¹ [26]. The Tb/Dy-TM AAs even showed a −∆S_m^peak comparable to those of Gd-based metallic glasses [27,28,29]. Thus, it is important for the application of MR to develop and improve the −∆S_m^peak of the RE-TM-based AAs as high as possible.

In previous work, we have systematically investigated the glass-forming ability (GFA) and magnetocaloric properties of the Tb-Ni binary alloys, and a −∆S_m^peak under 5 T of 8.7 J kg⁻¹ K⁻¹ was obtained in the Tb₆₅Ni₃₅ alloy, which was the only fully amorphous sample in the binary alloys [30]. Frustratingly, the GFA of the Tb₆₅Ni₃₅ AA was very poor. Therefore, in order to increase the GFA and concurrently further improve the MCE of the Tb₆₅Ni₃₅ AA, the replacement of Ni atom with other TM atoms should be a valid way. In the present work, Co was used in the substitution of Ni to prepare Tb₆₅Ni_35−xCo_x (x = 0, 10, 20 and 30) amorphous ribbons, and the GFA of the ternary alloys was investigated. The best glass former in the ternary alloys was employed to study its magnetic properties and MCE in detail.

2. Materials and Methods

The Tb₆₅Ni_35−xCo_x (x = 0, 10, 20, 30) alloy ingots were produced by arc-melting the mixture of Tb, Ni and Co metals (purity > 99.9 at.%) in a high vacuum furnace under the protection of high-purity Ar atmosphere. Each master ingot was remelted in a quartz tube filled with Ar atmosphere and then the melt was injected on a copper wheel with a speed of 30 m/s to fabricate the Tb₆₅Ni_35−xCo_x ribbons. These ribbons with a width of ~3 mm and a uniform thickness of ~40 μm were selected for the structure and performance measurements. A Rigaku D/max-2550 X-ray diffractometer (XRD, Rigaku, Tokyo, Japan) with Cu K_α radiation was employed to detect the structural information of the Tb₆₅Ni_35−xCo_x as-spun ribbons. The thermal properties of the glassy sample were achieved from their differential scanning calorimetric (DSC) curves measured on a model 404 C calorimeter produced by NETZSCH Company (Selb, Germany). The GFA of these amorphous ribbons was evaluated according to their thermal parameters and the best glass former was determined to investigate its magnetic properties and MCE. The magnetic measurements of the amorphous ribbon with the best GFA, including magnetization vs. temperature (M-T) curves, hysteresis loops and isothermal magnetization (M-H) curves, were performed on a model 6000 Physical Property Measurement System (PPMS) produced by Quantum Design Company (San Diego, CA, USA).

3. Results and Discussion

The X-ray diffraction results of the Tb₆₅Ni_35−xCo_x (x = 0, 10, 20, 30) as-spun ribbons are displayed in Figure 1. There were no obviously sharp crystalline peaks and only smoothly broad diffraction diffusion in their XRD patterns, which indicates the typical amorphous characteristic of these ribbons.

Figure 2a shows the DSC curves of Tb₆₅Ni_35−xCo_x (x = 0, 10, 20, 30) amorphous ribbons. It can be seen that as the temperature rose, a faint upward endothermic peak first appeared, which corresponded to the glass transition behavior of the ribbon, following by downward exothermic crystallization peaks on each curve. This further proves the typical amorphous features of these samples. From their DSC curves and melting curves, we can obtain the temperatures of glass transition (T_g), primary crystallization temperatures (T_x) and liquidus temperatures (T_l) to evaluate the glass-forming ability of the Tb₆₅Ni_35-xCo_x (x = 0, 10, 20, 30) AAs, as listed in

Table 1. Thermal parameters, T_rg and γ of the Tb₆₅Ni_35−xCo_x (x = 0, 10, 20 and 30) amorphous alloys.

Tb65Ni35−xCox	Tg (K)	Tx (K)	Tl (K)	Trg	γ
x = 0	548	579	1137	0.482	0.344
x = 10	565	582	1022	0.553	0.367
x = 20	566	588	982	0.576	0.380
x = 30	567	581	987	0.574	0.374

. Two criteria used commonly (i.e., the reduced glass transition temperature (T_rg = T_g/T_l) [31] and parameter γ (= T_x/(T_g + T_l)) [32]) can be calculated accordingly. As shown in Figure 2b, both the T_rg and γ were larger than those of the Tb₆₅Ni₃₅ amorphous alloy [30], indicating that the replacement of Ni atom with Co atom can dramatically improve the GFA of the Tb₆₅Ni₃₅ binary alloy. In addition, the value of the T_rg and γ first increased and then decreased with the increase in Co content and reached the maximum value when x = 20, which implies the best glass former in the Tb₆₅Ni_35−xCo_x ternary alloys was Tb₆₅Ni₁₅Co₂₀.

Figure 3a shows the variation of magnetization with temperature under 0.03 T for the Tb₆₅Ni₁₅Co₂₀ glassy ribbon after two different cooling treatments from room temperature to 10 K, i.e., zero-field-cooling (ZFC) and field-cooling (FC). Obviously, as the temperature decreased, the ZFC and FC M-T curves were almost coincident at first, and until a certain temperature (~64 K), the two curves begin to deviate. The λ-shaped M-T curves usually occur in the spin-glass systems and other spin-glass-like metallic glass [26,28,29,30,33], indicating the typical spin-glass-like behavior of the Tb₆₅Ni₁₅Co₂₀ glassy alloy. The T_c and spin freezing temperature (T_f) were obtained to be ~79 K and ~64 K by derivating the M-T curves of the glassy sample. The increased T_c of the Tb₆₅Ni₁₅Co₂₀ glassy ribbon than the Tb₆₅Ni₃₅ amorphous ribbon (T_c = 64 K) may be closely related to the enhanced 3d-3d interaction between TM atoms because the magnetic moment of Co is larger than that of Ni. Similar to some RE (such as Nd, Tb and Dy)-based metallic glasses [26,27,28,30], the spin-glass-like behavior resulted in large coercivity (H_c) and magnetic hysteresis at low temperatures below T_f. Hence, the hysteresis loops of the Tb₆₅Ni₁₅Co₂₀ AA at 10, 70 and 160 K were measured as illustrated in Figure 3b. The amorphous sample shows hard magnetic with a H_c of 0.624 T at 10 K (well lower than T_f), soft magnetic at 70 K (between T_f and T_c) and paramagnetic at 160 K (well above T_c).

The large coercivity even inhibited the magnetization of the alloy at low temperature and, thus, brought about the abnormal magnetization phenomena. Figure 4 shows the M-H curves of the Tb₆₅Ni₁₅Co₂₀ glassy alloy at various temperatures from 0 to 5 T. At temperatures above T_f, the magnetization of the ribbon increased with the decrease in the temperature. However, the magnetization showed a positive correlation with the temperature under an extreme low magnetic field, when the temperature was below T_f, especially at 10 K, which also implies the spin-glass-like behavior of the Tb₆₅Ni₁₅Co₂₀ glassy sample.

It is known that the abnormal magnetization behavior induced by the inhibition of the coercivity will affect the magnetocaloric properties of the AAs [26,30,34,35]. As such, we obtained the −∆S_m of the Tb₆₅Ni₁₅Co₂₀ amorphous ribbon under different magnetic fields and temperatures according to Maxwell’s equation. Figure 5a displays the relationship between −∆S_m and temperature (−∆S_m-T curves) under 1 T, 1.5 T, 2 T, 3 T, 4 T and 5 T for the Tb₆₅Ni₁₅Co₂₀ glassy ribbon. Similar with the situation in the spin-glass-like AAs, the −∆S_m of the sample even decreases to a negative value at the temperatures below 30 K, which implies the irreversible magnetocaloric effect of the Tb₆₅Ni₁₅Co₂₀ glassy alloy. As the temperature rose, the −∆S_m first increased and then decreased to near zero and reached a maximum −∆S_m at the vicinity of T_c. The value of −∆S_m^peak for the Tb₆₅Ni₁₅Co₂₀ amorphous ribbon was 2.70 J kg⁻¹ K⁻¹ under 1 T, 3.79 J kg⁻¹ K⁻¹ under 1.5 T, 4.75 J kg⁻¹ K⁻¹ under 2 T, 6.48 J kg⁻¹ K⁻¹ under 3 T, 8.05 J kg⁻¹ K⁻¹ under 4 T and 9.47 J kg⁻¹ K⁻¹ under 5 T. On the other hand, according to the −∆S_m^peak under different magnetic fields (H), the ln(−∆S_m^peak) vs. ln(H) plots of the Tb₆₅Ni₁₅Co₂₀ amorphous ribbon can be constructed, which is proposed by V. Franco [36]. The slope (defined as n) of its linear fitting curve near T_c, as displayed in the inset of Figure 5a, was 0.776 and agreed well with the experimental value in some AAs experiencing SOMPT.

Compared with the binary Tb₆₅Ni₃₅ AA [30], the MCE of the TbNi(Co) glassy alloy was improved obviously by replacing Ni atoms with Co atoms. Figure 5b illustrates the −∆S_m-T curves under 1.5 T and 5 T for the Tb₆₅Ni₃₅ and Tb₆₅Ni₁₅Co₂₀ amorphous ribbons. The addition of 20 at.% Co atoms not only increased the −∆S_m^peak temperature of the Tb₆₅Ni₃₅ AA, but also made the −∆S_m^peak under 1.5 T and 5 T of the Tb₆₅Ni₃₅ glassy alloy enlarge by 15.9% and 8.2%, respectively. The enlarged −∆S_m^peak induced by the substitution of Ni atoms with Co atoms was likely due to the additional 3d-3d interaction between Ni and Co atoms [37].

The adiabatic temperature change as a function of temperature (∆T_ad-T curves) for the Tb₆₅Ni₁₅Co₂₀ amorphous ribbon under various magnetic fields were estimated from its −∆S_m-T and C_p(T) curve according to:

$$\Delta T_{\mathit{ad}}\left(T,0\to H\right)=-\frac{T}{C_p\left(T\right)}\Delta S_m(T,0\to H)$$

Figure 6 illustrates the ∆T_ad-T curves under 1 T to 5 T of the Tb₆₅Ni₁₅Co₂₀ amorphous ribbon, and the inset shows its C_p(T) curve. The maximum ∆T_ad of the sample is approximately 1.14 K under 1 T, 2.12 K under 2 T, 2.94 K under 3 T, 3.73 K under 4 T, and 4.47 K under 5 T, all of which are comparable to some crystal magnetic refrigeration materials with a giant MCE [10,38]. Furthermore, the large −∆S_m^peak and ∆T_ad (9.47 J kg⁻¹ K⁻¹ and 4.47 K under 5 T) of the Tb₆₅Ni₁₅Co₂₀ amorphous ribbon are higher than those of most Gd-based MCE AAs [12,24,25,39,40,41], and amorphous alloys possess better mechanical properties and corrosion resistance than intermetallic compounds [42], both of which jointly indicate the application perspective of the amorphous alloy as the magnetic refrigerants in magnetic refrigeration near the liquefaction temperature of nitrogen.

4. Conclusions

In summary, we prepared the Tb₆₅Ni_35−xCo_x (x = 0, 10, 20, 30) amorphous ribbons by replacing Ni atoms with Co atoms, and the GFA of the ternary alloys was studied. The results show that the addition of Co atoms obviously improves the GFA of the Tb₆₅Ni₃₅ AA, and the T_rg as well as γ reach to the maximum value when x = 20, which indicates the best glass former is Tb₆₅Ni₁₅Co₂₀. The magnetic properties and MCE of the Tb₆₅Ni₁₅Co₂₀ glassy ribbon were further investigated. The λ-shaped M-T curves as well as the anomalous M-H curves at low field and low temperature indicate the spin-glass-like behavior of the Tb₆₅Ni₁₅Co₂₀ AA, with a T_f of ~64 K and a T_c of ~79 K. The large coercivity (~0.624 T at 10 K) of the AA results in the irreversible −∆S_m at the temperatures well below T_f. The −∆S_m^peak and ∆T_ad under 5 T of the Tb₆₅Ni₁₅Co₂₀ amorphous ribbon reach to 9.47 J K⁻¹ kg⁻¹ and 4.47 K, both of which were larger than that of most Gd-based AAs, indicating a promising perspective in the application of magnetic refrigeration. Compared with the Tb₆₅Ni₃₅ glassy alloy, the increased T_c and enlarged −∆S_m^peak of the Tb₆₅Ni₁₅Co₂₀ amorphous ribbon may be closely related to the extra 3d-3d interaction between Ni and Co atoms due to the addition of Co atoms with larger magnetic moment.