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

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 Gd45Co50−xFexAl5 (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.


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 (−∆S 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 −∆S m of materials are closely related to angular momentum J, rare-earth (RE)based AM usually show excellent −∆S 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 Gd 50 Co 50 amorphous alloy reaches 267.5 K, and the peak of magnetic entropy change (−∆S m peak ) and refrigeration capacity (RC = −∆S m peak × δT FWHM means the comprehensive MCE of the material, which can be approximated by the product of the −∆S m peak and the temperature interval (δT FWHM ) that corresponds to the full width temperature range at the half value of the −∆S m peak in the −∆S 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 Gd 50 Co 50−x Fe x (x = 2, 5) and Co 50 Gd 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 (Gd 50 Co 45 Fe 5 ), 289 K (Co 50 Gd 47 Ni 3 ) and 297 K (Co 50 Gd 48 Ni 2 ), 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. Gd 45 Co 50 Al 5 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 Gd 45 Co 50 Al 5 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 Gd 45 Co 50−x Fe x Al 5 (x = 2, 5, 10) AM based on a Gd 45 Co 50 Al 5 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 −∆S m peak ) of the full AM, and the mechanism involved, were studied.

Materials and Methods
Samples with the nominal composition of Gd 45 Co 50−x Fe x Al 5 (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.

Results and Discussion
The XRD patterns of the Gd 45 Co 50−x Fe x Al 5 (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 Gd 45 Co 50−x Fe x Al 5 (x = 2, 5, 10) ribbons are amorphous structures.   The high Tc of the Gd45Co50-xFexAl5 (x = 5, 10) AM, which is comparable to the Gdbased 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 Tc 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 Tc 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 µB) itself is much larger than that of Co (=4.8 µB). The    The high Tc of the Gd45Co50-xFexAl5 (x = 5, 10) AM, which is comparable to the Gdbased 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 Tc 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 Tc 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 µB) itself is much larger than that of Co (=4.8 µB). The The high T c of the Gd 45 Co 50−x Fe x Al 5 (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 Gd 45 Co 50 Al 5 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 Gd 45 Co 50−x Fe x Al 5 (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 Gd 45 Co 50−x Fe x Al 5 (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 Gd 45 Co 50−x Fe x Al 5 (x = 2, 5, 10) alloy.
The  Figure 4a-c, it can be seen that all the curves exhibit a broad characteristic of SOMPT around the −∆S m peak . As shown in Table 1 [18][19][20][21][22][23][26][27][28][29][30][31], the relatively large RC of the Gd 45 Co 50−x Fe x Al 5 (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 Gd 50 Co 50−x Fe x (x = 2, 5) (~700 J/kg) and Co 50 Gd 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 Gd 45 Co 50 Al 5 , the −∆S 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 Gd 45 Co 40 Fe 10 Al 5 alloy, and its comprehensive RC is improved due to its wider temperature variation range, its magnetic entropy change property is seriously affected.  Figure 3. The slope of the Arrott curve of Gd45Co50-xFexAl5 (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 −ΔSm to produce a relatively large RC.
The  Figure 4a-c, it can be seen that all the curves exhibit a broad characteristic of SOMPT around the −ΔSm peak . As shown in Table 1

Samples
T c (K) −∆S m peak * (J/(kg·K)) RC (J/kg) Ref. 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 −ΔSm 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 Tc 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.

Samples
Tc (K) −ΔSm peak * (J/(kg·K)) RC (J/kg) Ref.  This phenomenon may be related to the variation tendency of M−T and −∆S 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 Gd 45 Co 40 Fe 10 Al 5 that the −∆S m −T curve of the alloy displays a nearly platform-like feature, and the peak position of its −∆S m is shifted, which is different from that of other typical amorphous alloys. By constructing the −∆S m peak~T c −2/3 curve and their linear fitting, as shown in Figure 5a, it is easy to find that Gd 45 Co 50−x Fe x Al 5 (x = 0, 2, 5) AM follows the −∆S m peak~T c −2/3 relationship [32]. Meanwhile, Gd 45 Co 40 Fe 10 Al 5 deviates from the ln(−∆S 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.

T 1.5 T 2 T 3 T 4 T 5 T 2 T 5 T
* The maximum magnetic entropy change (−ΔSm) value in the −ΔSm-T curves. This phenomenon may be related to the variation tendency of M−T and −ΔSm−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 −ΔSm−T curve of the alloy displays a nearly platform-like feature, and the peak position of its −ΔSm is shifted, which is different from that of other typical amorphous alloys. By constructing the −ΔSm peak~T c −2/3 curve and their linear fitting, as shown in Figure 5a, it is easy to find that Gd45Co50-xFexAl5 (x = 0, 2, 5) AM follows the −ΔSm peak~T c −2/3 relationship [32]. Meanwhile, Gd45Co40Fe10Al5 deviates from the ln(−ΔSmpeak )~ln(Tc) path, which may be caused by the presence of some heterogeneous structuresthe 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 −∆S m characteristics of the alloy. The −∆S m ∝H n 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(−∆S m peak ) and ln(H) in the inset of Figure 5, Gd 45 Co 48 Fe 2 Al 5 AM fits well with the actual situation, indicating the fully amorphous structure of the alloy. Meanwhile, in the Gd 45 Co 50−x Fe x Al 5 (x = 5, 10) alloy, the exponent n reaches 0.833 and 0.905, respectively, which indicates that the Materials 2023, 16, 4571 8 of 10 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 Gd 45 Co 45 Fe 5 Al 5 alloy showed a small amount of clusters, with a size limit below 5 nm, embedded in the disordered matrix, while the Gd 45 Co 40 Fe 10 Al 5 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 −∆S m , while the appearance of the intermediate range order is not conducive to the improvement in the −∆S m . Therefore, the obvious deterioration of −∆S m of Gd 45 Co 40 Fe 10 Al 5 in this paper may be related to the coarsening of nanocrystals in the structure. Combined with the above results, only the Gd 45 Co 45 Fe 5 Al 5 alloy shows excellent comprehensive magnetic properties with T c of about 281 K, −∆S 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 Gd 45 Co 45 Fe 5 Al 5 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.

Conclusions
In conclusion, by the addition of Fe to the base component of the Gd 45 Co 50 Al 5 ternary alloy, the Gd 45 Co 50−x Fe x Al 5 (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 −∆S m -T curve of the alloy showed a broader curve with the increase in Fe content. The relation of −∆S m peak~T c −2/3 deviated from the linear relationship and the −∆S m ∝H n 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 Gd 45 Co 40 Fe 10 Al 5 . 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 Gd 45 Co 40 Fe 10 Al 5 alloy with short range order showed better magnetic properties with T c of about 281 K, −∆S 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.   Data Availability Statement: The data of this study are available from the corresponding author upon reasonable request.