The Effect of Fe Addition on the Curie Temperature and Magnetic Entropy of the Gd₄₅Co₅₀Al₅ Amorphous Alloy

Abstract

The new magnetic refrigeration (MR) technology, which uses the magnetocaloric effect (MCE) of materials for refrigeration, has shown apparent advantages over the compression refrigeration of freon and other gases. Therefore, how to obtain materials with excellent magnetic entropy change near room temperature is of great significance for the realization of MR. In order to achieve high Tc of a Gd-based amorphous alloy, Gd₄₅Co₅₀Al₅ amorphous alloy with good room temperature MCE was selected, and a series of Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) amorphous alloys were prepared by adding Fe instead of Co. In this paper, the effect of Fe addition on the Curie temperature, and the magnetic entropy change in the alloys, were studied thoroughly. The results show that the Curie temperature is increased to 281 K by adding 5% Fe elements, which is mainly related to the enhanced 3d-3d interaction of transition elements caused by Fe addition, and the maximum value of magnetic entropy change is 3.24 J/(kg·K) under a field of 5 T. The results are expected to provide guidance for further improving the room temperature MCE of Gd-based amorphous alloys.

1. Introduction

Nowadays, clean and green energy has been touted in many fields due to its advantages of high efficiency and energy saving. In the field of refrigeration, the traditional refrigerator, which is characterized by compressed fluorocarbons and fluorine refrigeration gases, has been unable to meet the needs in the current era of high-quality development due to the disadvantages such as high energy consumption and environmental destruction [1,2,3,4]. By utilizing the magnetocaloric effect (MCE) of materials, that is, the phenomenon that the ferromagnet or paramagnet generate heat change when stimulated by an external magnetic field during adiabatic process, this new magnetic refrigeration (MR) pattern can realize a heat interaction with the exterior and the refrigeration efficiency can reach up to 60% of the Carnot cycle. Compared with the conventional gas compression refrigeration, MR shows the advantages of being more efficient, energy saving and environmentally friendly, therefore, it is considered to be a very advantageous new field [5,6,7,8].

The realization of MR depends on the MCE of materials [1,2]. Therefore, how to obtain large magnetic refrigeration capacity in a large temperature range, especially in the temperature range of room temperature, so as to obtain as much refrigeration efficiency as possible, is the topic issue regarding the application of MR at room temperature [9]. Among the prepared magnetic refrigerants at present, the materials with second-order magnetic phase transition (SOMPT), which are characterized by a continuous magnetic phase transition process, were considered to be the best choice because of their wide range of magnetic entropy change (−ΔS_m) region. Among these SOMPT MR materials, amorphous alloys (AM) have gained wide attention [10,11]. AM is in an energy metastable state due to its long-range disorder of the atomic arrangement, thus showing many unique performance advantages, such as good machinability, high corrosion resistance and large resistance to reduce eddy current loss [10,11,12,13,14,15,16,17,18,19,20,21,22]. In addition, AM can be formed within a certain composition range and the magnetic properties can be adjusted continuously with the change in composition, so it is possible for us to design a refrigerant with good MCE performance within a certain temperature range.

Since the −ΔS_m of materials are closely related to angular momentum J, rare-earth (RE)-based AM usually show excellent −ΔS_m properties [6,7]. However, the Curie temperature (T_c) of RE-based amorphous alloys is generally in a low temperature range, mostly below the cooling temperature of liquid nitrogen [11,12,13,14,15,16,17]. Meanwhile, the AM that are prepared by Gd have the T_c closest to room temperature due to the half-full structure of their 4f layer. Among them, the Curie temperature of Gd₅₀Co₅₀ amorphous alloy reaches 267.5 K, and the peak of magnetic entropy change (−ΔS_m^peak) and refrigeration capacity (RC = −ΔS_m^peak × δT_FWHM means the comprehensive MCE of the material, which can be approximated by the product of the −ΔS_m^peak and the temperature interval (δT_FWHM) that corresponds to the full width temperature range at the half value of the −ΔS_m^peak in the −ΔS_m curve) under a field of 5 T, are approximately 4.6 J/(kg·K) and 685.9 J/kg [18,19]. Afterwards, the T_c of Gd-based AM was successfully raised to near room temperature by the addition of the transition elements, Fe and Ni, among which Gd₅₀Co_50−xFe_x (x = 2, 5) and Co₅₀Gd_50−x(Fe/Ni)_x (x = 1, 2, 3) AM showed the highest T_c of the RE-based AM, which can reach 289 K (Gd₅₀Co₄₅Fe₅), 289 K (Co₅₀Gd₄₇Ni₃) and 297 K (Co₅₀Gd₄₈Ni₂), respectively [20,21,22,23]. However, with the increase in Fe and Ni content, the glass-forming ability (GFA) of these alloys deteriorates significantly, which makes the preparation of AM with higher T_c slightly difficult. The addition of Al can further ameliorate this situation. Gd₄₅Co₅₀Al₅ AM not only reduces Gd content to 45%, but also further improves the forming capacity of the AM. However, the only issue is that the T_c of Gd₄₅Co₅₀Al₅ AM is only 253 K.

Therefore, in order to further obtain Gd-based AM with T_c at room temperature and a better GFA, a series of Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) AM based on a Gd₄₅Co₅₀Al₅ sample were prepared by modulation of the proportion of transition elements through adding Fe instead of Co. The effect of Fe addition on the magnetic properties (T_c and −ΔS_m^peak) of the full AM, and the mechanism involved, were studied.

2. Materials and Methods

Samples with the nominal composition of Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) were prepared by melting the mixture of pure Gd, Co, Fe and Al (purity > 99.9 at.%) metals in an arc furnace protected with high-purity argon. Then, the amorphous ribbons were produced by the single roller melt-spinning method with a spinning tangent speed of about 45 m/s. The ribbons with an average thickness about 35 μm were selected as the standard samples for the structure and magnetic test. The amorphous properties were characterized by a X-ray diffractometer with Cu Kα radiation (XRD, PANalytical, Almelo, Holland). The microstructure of the ribbon was observed on a JEM-2010F high resolution electron microscope (HREM) after electrolytic polishing the specimen under the protection of liquid nitrogen (HREM, model JEM-2010F, Tokyo, Japan). The magnetic features, such as Curie temperature and magnetization curves, were performed by the Physical Property Measurement System (PPMS, San Diego, CA, USA) 6000 of Quantum Design.

3. Results and Discussion

The XRD patterns of the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) as-spun ribbons are shown in Figure 1. The XRD curves do not show significant sharp crystal peaks at the whole measured angle, and only at 2θ = 35° show the representative diffuse scattering peaks that are unique to the disordered structure. The results preliminary judge that the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) ribbons are amorphous structures.

Figure 2a shows the M−T diagrams of the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) amorphous ribbons under a magnetic field of 0.03 T (the solid line). Since the magnetization of the alloy decreases dramatically near the T_c, the T_c of these samples, that were obtained by deriving their M−T curves, as shown by the dotted line in the figure, were about 262 K, 281 K and 317 K, respectively.

The high T_c of the Gd₄₅Co_50−xFe_xAl₅ (x = 5, 10) AM, which is comparable to the Gd-based AM with the highest Curie temperature available to date, and also within the Curie temperature range of Fe-based AM, means that it can be applied around room temperature. If excellent MCE can be obtained, these alloys will be very promising as magnetic refrigerants. Compared to Gd₄₅Co₅₀Al₅ AM, the T_c of the sample increased by 3.5%, 11.1% and 25.3%, with Fe content added, from 2% to 10%. With the increase in Fe content, the T_c of the alloy is obviously improved in a nonlinear increasing trend, although the proportion of TM content remains unchanged. This anomaly may be related to the stronger direct 3d-3d interactions (Fe-Fe and Fe-Co) that were introduced by the addition of Fe, because the magnetic moment of Fe (=5.4 μ_B) itself is much larger than that of Co (=4.8 μ_B). The effect of the magnetic moment of transition element on the Curie temperature of the alloy may be more complicated in practice. The nominal magnetic moments (µ_nom.), that were calculated by simply adding the magnetic moments in atomic proportions, are about 6.01 µ_B, 6.03 µ_B and 6.06 µ_B, respectively. Meanwhile, according to the Curie–Weiss law [24], the effective magnetic moments (µ_eff) of Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) AM are about 8.48 µ_B, 11.29 µ_B and 11.75 µ_B in turns, as shown in the inset curves in Figure 2a; the effective magnetic moment increases with increasing iron content. The larger effective magnetic moment indicates that the interaction of the alloy may be more complex (due to the appearance of nanocrystalline particles in the Gd₄₅Co_50−xFe_xAl₅ (x = 5, 10) alloy), but the magnetic moment of the alloy is enhanced in general, which further indicates that the addition of Fe enhances the magnetic moment of the alloy, thus enhancing the T_c of the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) alloy.

The hysteresis loops of the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) amorphous ribbons under the field of 5 T are shown in Figure 2b. The samples display ferromagnetic (FM) properties at 10 K and paramagnetic (PM) properties at 380 K. At a low temperature of 10 K, the saturation magnetization (M_s) of the alloy reaches 155.5 Am²/kg, 154.5 Am²/kg and 146.2 Am²/kg with the Fe content from 2% to 10%, as shown in the enlarged illustration in the upper right corner of Figure 2b, and the coercivity (H_c) is about 66.3 Oe for x = 2, 41.2 Oe for x = 5 and 24.8 Oe for x = 10, all of which are less than 100 Oe, as the details show in the bottom right illustration in Figure 2b. The low coercivity fields characteristic of soft magnetic materials and relatively high M_s indicate that the alloy can be easily magnetized and demagnetized, which is very beneficial to the acquisition of reversible MCE and improvements in refrigeration efficiency.

The isothermal M−H curves of the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) AM under 5 T are shown in the inset of Figure 3. At low temperatures that are under the T_c, the alloys show obvious FM characteristics, and the curves have high magnetic susceptibility, which can increase rapidly and reach saturation under a small external magnetic field. Near the T_c, the FM to PM transition occurs, and the magnetization decreases obviously. When the temperature exceeds the T_c, the alloys are almost completely changed to PM, and the curves produce similar linear relations.

The Arrott curves (H/M−M²) of the alloy can be obtained by transforming the isothermal M−H curves, and the type of MPT in the magnetic field can be further verified by the Arrott curves in Figure 3. The slope of the Arrott curve of Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) AM is positive in the picture (a), (b), (c). According to the Banerjee rule [25], the slope of the curve is related to the thermal spin perturbation; that is, only the positive slope of the curve indicates that the alloy is undergoing a SOMPT. Therefore, these alloys are typical SOMPT materials, which allows them to have smaller magnetic and thermal hysteresis loss in MCE. At the same time, their wide-phase transition regions make the alloys have a continuous change in −ΔS_m to produce a relatively large RC.

The temperature dependence of isothermal −ΔS_m curves (−ΔS_m−T) of the alloy can also be obtained by applying Maxwell’s equation—∆S_m(T, H) = S_m(T, H) − S_m(T, 0) = ∫₀^H(∂M/∂T)_H dH—to the isothermal M−H curves. Figuring out the −ΔS_m under 1 T–5 T field, at different temperatures of the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) AM, as the curves show in Figure 4a–c, it can be seen that all the curves exhibit a broad characteristic of SOMPT around the −ΔS_m^peak. As shown in

Table 1. Curie temperature T_c(K), −ΔS_m^peak (J/(kg·K)) and RC(J/kg) data of the Gd₄₅Co_50−xFe_xAl₅ (x = 0, 2, 5, 10) amorphous alloy and other Gd-Co-based or Fe-Zr-B-based amorphous samples with similar T_c.

Samples	Tc (K)	−ΔSmpeak * (J/(kg·K))						RC (J/kg)		Ref.
		1 T	1.5 T	2 T	3 T	4 T	5 T	2 T	5 T
Gd45Co50Al5	253	1.35	1.83	2.28	3.09	3.85	4.55	260	728	Present work
Gd45Co48Fe2Al5	262	1.17	1.60	2.00	2.73	3.40	4.04	274	723
Gd45Co45Fe5Al5	281	0.83	1.17	1.49	2.10	2.68	3.24	~300	~740
Gd45Co40Fe10Al5	317	0.56	0.83	1.10	1.60	2.33	2.57	~310	~770
Gd50Co50	267	1.38		2.36	3.16	3.92	4.6	212.8	685.9	[18]
Gd50Co48Fe2	277			2.24			4.44		~700	[21]
Co50Gd49Fe1	283			2.32			4.4	226.7	680	[22]
Co50Gd48Fe2	297			1.98			3.96	~241	~674
Co50Gd49Ni1	273			2.53			4.90	263.2	684.6	[23]
Co50Gd48Ni2	280			2.26			4.46	239.3	685.8
Co50Gd47Fe2Ni1	302			1.85			3.73	~211	~620	[31]
Co50Gd46Fe2Ni2	310			1.59			3.28	~231	~670
Fe88Zr8B4	291			1.5			2.81	200	551	[27]
Fe87Co1Zr8B4	317			1.61			3.24		~600	[28]
Fe87Zr8B4Sm1	308			1.65			3.27		>550	[29]

* The maximum magnetic entropy change (−ΔS_m) value in the −ΔS_m–T curves.

, the −ΔS_m^peak of all samples under the field of 5 T are about 4.04 J/(kg·K) for Gd₄₅Co₄₈Fe₂Al₅, 3.24 J/(kg·K) for Gd₄₅Co₄₅Fe₅Al₅ and 2.57 J/(kg·K) for Gd₄₅Co₄₀Fe₁₀Al₅, respectively. Another important indicator of measuring the comprehension MCE of the alloy is the refrigeration capacity (RC = −∆S_m^peak × δT_FWHM), and the RC of the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) AM is calculated as 723 J/kg when x = 2, ~740 J/kg when x = 5 and ~770 J/kg when x = 10 under the field of 5 T, respectively. Compared with other results in the literature [18,19,20,21,22,23,26,27,28,29,30,31], the relatively large RC of the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) AM are comparable to that of the Gd-Co binary AM (~720 J/kg), and are obviously better than that of the Fe-Zr-B-based AM (~600 J/kg). They are even superior than those of Gd₅₀Co_50−xFe_x (x = 2, 5) (~700 J/kg) and Co₅₀Gd_50−x(Fe/Ni)_x (x = 1, 2, 3) (~680 J/kg) AM with the same Curie temperature. The RC results show that the addition of Fe can obviously improve the RC of the alloys. Unfortunately, compared to the original composition of Gd₄₅Co₅₀Al₅, the −ΔS_m^peak of the samples is decreased by 11.2%, 28.8% and 43.5% with Fe content from 2% to 10%, respectively. Although the highest T_c among Gd-based AM is obtained in Gd₄₅Co₄₀Fe₁₀Al₅ alloy, and its comprehensive RC is improved due to its wider temperature variation range, its magnetic entropy change property is seriously affected.

This phenomenon may be related to the variation tendency of M−T and −ΔS_m−T curves. Compared with x = 2%, it is not hard to see, from the M−T curves, that the changes are more slow at Curie temperature when x = 5% and 10%, which is more conducive to the alloy to obtain larger RC. Furthermore, it can also be found from the curve of Gd₄₅Co₄₀Fe₁₀Al₅ that the −ΔS_m−T curve of the alloy displays a nearly platform-like feature, and the peak position of its −ΔS_m is shifted, which is different from that of other typical amorphous alloys. By constructing the −ΔS_m^peak~T_c^−2/3 curve and their linear fitting, as shown in Figure 5a, it is easy to find that Gd₄₅Co_50−xFe_xAl₅ (x = 0, 2, 5) AM follows the −ΔS_m^peak~T_c^−2/3 relationship [32]. Meanwhile, Gd₄₅Co₄₀Fe₁₀Al₅ deviates from the ln(−ΔS_m^peak)~ln(T_c) path, which may be caused by the presence of some heterogeneous structures—the nanoclusters—in the alloy that maintain the magnetization and do not decrease significantly at the Curie temperature of the alloy.

This can be further explained by the field dependence of the −ΔS_m characteristics of the alloy. The −ΔS_m∝Hⁿ relationship was established according to the Arrott–Noakes equation [33,34], as shown in Figure 5b. For the SOMPT materials, especially for typical amorphous alloys, the variation of exponent n is roughly as follows: under the T_c region, the alloy exhibits ferromagnetism, and the exponent n is close to 1 and decreases with increasing temperature around the T_c region; then, the exponential n is gradually closer to 2 with the temperature reaching the fully paramagnetism state of the alloy. In the mean field theory, the exponent n near T_c is roughly 0.67; however, due to the inhomogeneity structure in practice, which may be due to the existence of atomic clusters, such as amorphous matter, free volume and even nanocrystalline and so on, the exponent n is roughly between 0.75 and 0.78 in the amorphous alloy. As can be seen in the linear fitting of the ln(−ΔS_m^peak) and ln(H) in the inset of Figure 5, Gd₄₅Co₄₈Fe₂Al₅ AM fits well with the actual situation, indicating the fully amorphous structure of the alloy. Meanwhile, in the Gd₄₅Co_50−xFe_xAl₅ (x = 5, 10) alloy, the exponent n reaches 0.833 and 0.905, respectively, which indicates that the nanocrystals exist in the alloys of these two components [35,36,37,38,39]. As shown in Figure 5c,d, the HRTEM image of the Gd₄₅Co₄₅Fe₅Al₅ alloy showed a small amount of clusters, with a size limit below 5 nm, embedded in the disordered matrix, while the Gd₄₅Co₄₀Fe₁₀Al₅ alloy showed local nanocrystallization in the amorphous matrix, and the diameter of the cluster is slightly larger than 5 nm. Previous results have shown that the presence of short range order [37,38], with the limits of 5 nm, will lead to an slight increase in exponential n around 0.8. This is beneficial to the improvement in −ΔS_m, while the appearance of the intermediate range order is not conducive to the improvement in the −ΔS_m. Therefore, the obvious deterioration of −ΔS_m of Gd₄₅Co₄₀Fe₁₀Al₅ in this paper may be related to the coarsening of nanocrystals in the structure. Combined with the above results, only the Gd₄₅Co₄₅Fe₅Al₅ alloy shows excellent comprehensive magnetic properties with T_c of about 281 K, −ΔS_m^peak of about 3.24 J/(kg·K) at 5 T and 1.49 J/(kg·K) under 2 T, and RC of nearly 740 J/kg at 5 T and about 300 J/kg under 2 T, respectively. The relatively excellent magnetic performance of the composite Gd₄₅Co₄₅Fe₅Al₅ amorphous alloy can provide a good basic alloy and experimental guidance for further improving the magnetic entropy change performance of a Gd-based amorphous alloy.

4. Conclusions

In conclusion, by the addition of Fe to the base component of the Gd₄₅Co₅₀Al₅ ternary alloy, the Gd₄₅Co_50−xFe_xAl₅ (x = 2, 5, 10) amorphous alloys, with an average thickness of about 35 μm, were prepared, and the effect of Fe addition on the Curie temperature and magnetic entropy change proprieties of the alloys, as well as the mechanism involved, were systematically studied. The XRD results show that all the prepared alloys are amorphous. The addition of Fe significantly improved the T_c of the alloy from 253 K to 262 K, 281 K and 317 K, corresponding to the Fe content increasing from 0% to 2%, 5% and 10%, respectively. This was mainly caused by the introduction of stronger 3d-3d direct interactions between the TM atoms with the addition of Fe, so that the µ_eff of the alloy increased gradually from 8.48 µ_B to 11.29 µ_B and 11.75 µ_B with the increase in Fe content and the T_c was obviously enhanced. The alloys also showed good soft magnetic proprieties at low temperature, and the coercivity was negligible. The similar Arrott plots and typical magnetocaloric behaviors indicated the SOMPT feature of the alloy, and the −ΔS_m–T curve of the alloy showed a broader curve with the increase in Fe content. The relation of −ΔS_m^peak~T_c^−2/3 deviated from the linear relationship and the −ΔS_m∝Hⁿ relationship was slightly different from that of other amorphous alloys, with the exponential n values being about 0.771, 0.833 and 0.905 around the T_c. This abnormal linearity was mainly associated with the intermediate range order nanocrystals (usually larger than 5 nm) in Gd₄₅Co₄₀Fe₁₀Al₅. The intermediate range order caused the magnetization properties of the alloy to be affected near the T_c, thus deviating from these empirical formulas. The Gd₄₅Co₄₀Fe₁₀Al₅ alloy with short range order showed better magnetic properties with T_c of about 281 K, −ΔS_m^peak about 3.24 J/(kg·K) and RC of nearly 740 J/kg under the field of 5 T. These results are expected to provide the foundation and guidance for further improving the room temperature MCE of Gd-based amorphous alloys.