Effect of Annealing Temperature on Microstructure and Magnetocaloric Properties of Gd-Based Metallic Microfibers

Abstract

In this paper, vacuum annealing has been adopted to introduce atomic cluster micro-regions inside Gd-based metallic microfibers to further explore the effect of the structural changes on the magnetocaloric properties and the mechanism which is systematically expressed. The experimental results indicate that the as-prepared Gd-based metallic microfibers have favorable amorphous formation ability and thermal stability. After annealing @ 380 °C, the maximum magnetic entropy change −ΔS_m^max, refrigerating capacity (RC), and relative cooling power (RCP) values of the Gd-based metallic microfibers are 7.20 J/kg·K, 459.4 J/kg, and 588.7 J/kg, respectively. Combined with the transmission electron microscopy analysis results, the internal organizational order of the annealed microfibers is significantly altered, and the atomic clusters formed in localized regions, which reduce the magnetocrystalline anisotropy of the microfibers. While under the uni-action of an external magnetic field, the magnetic moment rotation state and magnetic domain structure distribution of the micro-region atoms will be changed obviously, thereby changing the general magnetic properties and magnetocaloric properties of the metallic microfibers. The above research results can promote the engineering application of Gd-based metallic microfibers in the field of magnetic refrigeration.

1. Introduction

Magnetic metallic microfibers have been widely used in the fields of displacement sensors, magnetic sensitive sensors, magnetic refrigeration devices, etc. [1,2,3,4,5]. Magnetic refrigeration technology based on the magnetocaloric effect (MCE) has received a lot of attention due to its unique structure and electromagnetic properties, as well as its advantages in terms of efficiency and environmental protection [6,7,8,9,10,11,12].

The magnetocaloric effect [13,14] refers to the phenomenon that the temperature of a ferromagnet or paramagnet changes with the change in magnetic field intensity in the adiabatic process. MCE can be quantified by a ferromagnetic or paramagnetic spin system near the magnetic ordering temperature. The total entropy of the system is composed of lattice and magnetic entropy. The magnetic moment is arranged and the magnetic entropy is reduced by applying a magnetic field adiabatically. In order to keep the total entropy of the system unchanged, the lattice entropy increases, and an increase in the lattice entropy means an increase in the system temperature. Therefore, the heat is extracted by the heat transfer medium, and then the magnetic field is removed under adiabatic conditions to achieve system cooling.

In amorphous systems, due to the disorder of the atomic arrangement in amorphous materials, there is no macroscopic magnetic anisotropy, so the magnetization process is more uniform. There is no grain boundary in the amorphous alloy, which avoids the decrease in magnetic properties caused by the grain boundary. Therefore, the amorphous systems have high saturation magnetization, M_s, and magnetic permeability, μ_m, as well as low coercive force, H_c. Moreover, the Curie temperature, T_C [15,16], of amorphous magnetic materials is usually higher, which means that they can still maintain good magnetic stability at high temperatures. At the same time, because there are no grains and grain boundaries of crystalline alloys in amorphous materials, the magnetization process is easier to carry out. This makes the amorphous material have a smaller conductivity when magnetized, further reducing the energy loss.

The Gd-based magnetic alloys have been widely studied in terms of the unpaired electrons in the 4f orbital of the element Gd, resulting in a large single-spin magnetic moment [17,18,19,20,21,22,23]. The movement speed of Gd atoms is faster, and it is difficult to form an ordered crystalline structure, but it is more inclined to form a disordered amorphous structure. Pecharsky and Gschneidner [24] found that the T_C of Gd₅Si₂Ge₂ alloy is 274 K, which has a large magnetocaloric effect. The maximum magnetic entropy −ΔS_m^max is 18.5 J/kg K under the change of a 0~5 T magnetic field.

Compared with thin ribbons and blocks, microfibers prepared by the rotated-dipping process [25,26,27,28] have the advantages of a high degree of amorphization and small diameter. The T_C of Gd₅₅Co₁₅Al₂₄Si₁Fe₅ metallic glass reaches 126 K, which is about 25% higher than that of the original amorphous alloy. In the Gd₂₅RE₂₅Co₂₅Al₂₅RE = Tb, Dy, Ho system, the quaternary high entropy amorphous alloy system shows excellent magnetocaloric effect, and the T_C can be adjusted with the change in rare-earth elements, whereas, on account of the high cooling rate during the fabrication process of the metallic microfibers, the large residual stress is generated inside, which has a certain effect on their magnetic properties. Annealing treatment can effectively reduce the residual stress and improve the magnetic domain structure of the metallic microfibers, thereby improving the magnetic properties. In general, the annealing processes commonly used for metallic microfibers include current annealing, magnetic field annealing, laser annealing, vacuum annealing, and so on [29,30]. Gao J.E. et al. [31] induce nanocrystals into the Fe-based amorphous alloys by vacuum annealing, which enhances the soft magnetic properties, with M_s and H_c values of 1.79 T and 11 A/m, respectively.

The research by Zhang R. C. et al. [32] showed that, after annealing at 1373 K for 20 min, the interaction between the nanocrystals and the amorphous phase inside LaFe_11.2Si_1.8 metallic microfibers formed a magnetic coupling, which increased the range of the secondary magnetic transition temperature; the −ΔS_m^max was 6.2 J/kg·K, and the RC was improved. A.F. Manchon-Gord [15] studied the amorphous structure relaxation of Fe₇₀Zr₃₀ amorphous alloy and found that the magnetization derivative curve narrows with annealing treatment. However, the magnetocaloric effect is more manifested in the temperature change that occurs when the magnetic field is magnetized due to the change in the magnetic domain structure. In addition, after heat treatment, the dependence of the domain structure and magnetocaloric effect can be improved by inducing the precipitation of nanocrystals. Luo Q. et al. [33] found that aging treatment will lead to nanocrystallization inside the alloy Gd₅₁Al₂₄Co₂₀Zr₄Nb₁. After aging at 923 K for 5 h, the internal structure of the alloy changed from amorphous to polycrystalline, and its magnetic properties decreased markedly.

In summary, the purpose of this paper is to analyze the influence of nanocluster structure on magnetic and magnetocaloric behavior in Gd-based metallic microfibers. Therefore, in order to further regulate the behavior of the nanoclusters, the precipitation of nanoclusters was induced by vacuum annealing. By analyzing the structure, morphology, and atomic order of the metallic microfibers, the effect of the metallic microfiber annealing temperature on the magnetic properties and the magnetocaloric behavior of the nanoclusters is obtained, which provides theoretical guidance and technical support for practical engineering applications.

2. Materials and Methods

We adopted Gd, Al, Co, and Ni with a purity of 99.9% as raw materials and determined the proportion according to the nominal composition Gd₅₇Al₁₉Co₁₉Ni₅ [34] (expressed as Gd-based metallic microfibers in the article). The non-consumable magnetron tungsten vacuum arc melting furnace was utilized for melting while filled with high-purity Ar gas for protection. After repeated electromagnetic stirring and smelting, copper mold suction casting was carried out to obtain a master alloy preform with a diameter of 8 mm. Gd-based metallic microfibers with a diameter of 35–40 μm and a length of more than 600 mm were prepared with rotated-dipping equipment with a copper roller speed of 1700 r/min and a feed ratio of 15 μm/s. The thermophysical parameters of the metallic microfibers were obtained with differential scanning calorimetry (type: DSC131 Evo, Setaram, Lyon, France) at a heating rate of 20 °C/min, providing a reference for the vacuum annealing process. The microfibers were vacuum-sealed with a vacuum rotary sealing device (type: MRVS-1002, Balab, Wuhan, China), and samples were annealed in a box-type resistance furnace, with the setting of annealing temperatures at 300 °C, 340 °C, and 380 °C for 10 min and air cooled. Figure 1 shows the heat-treatment process for the Gd-based metallic microfibers and the samples’ appearance.

The phase composition of the metallic microfibers was analyzed with a Rigaku D/MAX-2500/PC rotating anode X-ray diffractometer (XRD). The surface morphology of the metallic microfibers was characterized with secondary electron imaging by using an FEI QUANTA 650 FEG scanning electron microscope (SEM) with an accelerating voltage of 20 kV, and the elemental distribution of metallic microfibers was analyzed by energy dispersive spectrometry (EDS). The patterns of the metallic microfibers accordingly proceeded with a JEOL JEM2010 transmission electron microscope (TEM). In addition, the magnetic properties of the microfibers were measured with a SQUID-VSM-type magnetic property measurement system (MPMS).

Generally, magnetic entropy change −ΔS_m, RC, and RCP are used to evaluate the magnetocaloric performance of magnetic materials. And −ΔS_m can be calculated by bringing the data from the isothermal magnetization curve into Maxwell’s equation [35]:

$$\Delta S_m(T,H)=S(T,H)-S(T,0)={\displaystyle {\int }_0^{H_{\max }}{(\frac{\partial M}{\partial T})}_H}dH$$

(1)

where S—magnetic entropy; H_max—maximum external magnetic field strength; T—temperature.

The temperature interval corresponding to half of the maximum magnetic entropy change −ΔS_m^max value of the metallic microfiber is defined as the working temperature range. The integral of the area of the magnetic entropy change curve in the half maximum width temperature range is defined as the refrigeration capacity, RC. The product of the half peak width δT_FWHM and −ΔS_m is defined as the relative cooling power, RCP. The refrigerating efficiency of magnetocaloric materials is measured according to the values of RC and RCP [36], which can be expressed as:

$$RC={\displaystyle {\int }_{T_1}^{T_2}-\Delta S_m^{\max }(T)dT}$$

(2)

$$RCP=-\Delta S_m^{\max }\times \delta T_{FWHM}=-\Delta S_m^{\max }$$

(3)

3. Results and Discussion

3.1. Structural Analysis of Gd-Based Metallic Microfibers

Figure 2 shows the XRD diffraction patterns of the Gd-based metallic microfibers before and after annealing and the DSC curves. From Figure 2a, the diffuse scattering peak appeared around 2θ = 35°, with no sharp crystal diffraction peaks as a whole, which shows that the microstructure of the metallic microfibers is still in the amorphous state after annealing. A short time of vacuum annealing at the glass transition temperature T_g does not change the surface into a crystal structure. [33] As seen in Figure 2b, the metallic microfibers have two exothermic peaks, there is secondary crystallization during the heating process, and the first crystallization releases more heat than the secondary crystallization. From Figure 2c, it can be seen that an exothermic phenomenon occurs, which is due to the change in the material from the melting state to the solidification state.

As listed in

Table 1. Statistics for thermophysical parameters of Gd-based metallic microfibers.

Parameter	Tg (°C)	Tx1 (°C)	Tx2 (°C)	ΔT (°C)	−∆H (J·g−1)
As-Prepared	303	346	487	44	30.5

, the thermophysical parameters of the metallic microfibers are obtained from the DSC curve, where T_g is the glass transition temperature, T_x1 is the initial crystallization temperature, T_x2 is the secondary crystallization temperature, −ΔH is the mixing enthalpy, and ΔT is the glass transition region. It can be seen from the table that the metallic microfibers have high thermal stability. Furthermore, accordingly, T_g, T_x1, and T_x2 are 303 °C, 346 °C, and 384 °C, respectively. The annealing temperatures are determined to be 300 °C, 340 °C, and 380 °C.

Figure 3 indicates the SEM morphology and EDS analysis of the metallic microfibers before and after the annealing process. From Figure 3a,c,e,g, the surface of the metallic microfibers is smooth, uniform, and continuous. The EDS spectra of the metallic microfibers shown in Figure 3b,d,f,h show that the chemical compositions have not changed after annealing. And the positions of the energy spectrum peaks of each element have not deviated, indicating that the annealing process of the appropriate temperature has not altered the chemical composition of the metallic microfibers.

Figure 4 illustrates the TEM analysis of the Gd-based metallic microfibers before and after the annealing process. Figure 4a,b show that the microstructure of the metallic microfibers is uniform in the as-prepared and annealed states at 380 °C, with no nanocrystalline regions. Figure 4c,d show that the SAED patterns of the as-prepared and 380 °C annealed metallic microfibers are both amorphous halos, indicating that the metallic microfibers remain in the amorphous state after annealing.

The autocorrelation function (ACF) is modified to quantitatively characterize the order-degree change of the metallic microfibers both before and after annealing. It is possible to express the domain structure order degree (DSO) of the metallic microfibers as follows:

$$\psi ={\displaystyle \frac{\varsigma }{\kappa }}\times 100\%$$

(4)

where $\varsigma$—the number of ordered areas; $\kappa$—the total number of divided areas; and $\kappa =64$.

The HRTEM, FFT, IFFT, and ACF pictures of the metallic microfibers before and after the annealing process are shown in Figure 4e,f. The HRTEM result displays the obvious bright and dark stripes in the metallic microfibers’ local area after annealing, along with the appearance of atomic clusters. On the other hand, the IFFT atomic conformation diagram illustrates the random arrangement of atoms within the metallic microfibers, which is caused by the extreme cooling rate during formation, resulting in the internal stress of the fibers being non-uniformly distributed along the axial direction during condensation. According to the ACF ordered statistics, the order degrees of the as-prepared and 380 °C annealed microfibers are 4.69% and 20.31%, respectively. In conclusion, after annealing at 380 °C, atomic clusters formed due to structural relaxation inside the metallic microfibers, and an orderly micro-domain of atomic arrangement was formed in a certain area, which can affect the magnetic properties, especially in the process of magnetization, hindering the movement of the internal domain wall and the rotation of the magnetic moment of the metallic microfibers, thereby affecting the synergy of the magnetic moment formed under higher external magnetic field conditions. As mentioned above, it provides the structural basis for the change in the magnetic properties in the following research.

3.2. Magnetic Property Analysis of Gd-Based Metallic Microfibers

Figure 5 exhibits the general magnetization curves of the metallic microfibers at 20 K before and after annealing with the applied magnetic field of 0~5 T, from which the M_s of metallic microfibers decreases with the rising of the annealing temperature. At the annealing temperature of 380 °C, the M_s is the smallest, and the value is 155.98 emu/g.

Figure 6 shows the hysteresis loop of the metallic microfibers at 20 K before and after the annealing process and presents that the residual magnetization M_r, coercive force H_c, and magnetic permeability μ_m of the metallic microfibers increase with the rise in the annealing temperature, and when the annealing temperature is 380 °C, its maximum values are 17.83 emu/g, 125.12 Oe, and 0.58, respectively. At the annealing temperatures of 300 °C and 340 °C, the hysteresis loops are similar to that of the as-prepared state, showing a “Z shape”, and the area of the hysteresis loop indicates small hysteresis loss, while at the annealing temperature of 380 °C, the hysteresis loss increases.

Combining Figure 5 and Figure 6, it can be found that the softly magnetic properties of the microfibers decreased after annealing. This is due to the atomic clusters appearing and forming ordered micro-domains inside the microfibers after the annealing processing, in which there are atomic magnetic moments in different directions from the external magnetic field. Since the spin magnetic moments of the electron are different, the magnetic moments of the atoms are not equal, so the opposite magnetic moments can incompletely offset, resulting in a significant decrease in saturation magnetization.

Figure 7 presents the M-T curves of the metallic microfibers before and after the annealing processing. It can be seen that the T_C of the metallic microfibers in the as-prepared state and annealed at 300 °C and 380 °C is 109.3 K, 108.7 K, and 84.4 K, respectively. They all have a rapid transition from ferromagnetism (FM) to paramagnetism (PM) around the T_C. In the paramagnetic region, the magnetization of the microfibers disappears and does not vary with the change in the applied magnetic field. In the ferromagnetic region, the heating and cooling M-T curves of the as-prepared metallic microfibers and those annealed at 300 °C overlap, respectively. However, the M-T curves of the metallic microfibers annealed at 380 °C in the temperature range from 20 K to 60 K do not overlap, indicating that magnetothermal hysteresis occurs, which is related to the appearance of a large number of atomic cluster micro-domains inside. The atomic magnetic moment rotation process is affected by the cluster pinning effect, reflecting the magnetocrystalline anisotropy characteristics, so it has strong hysteresis characteristics.

Figure 8 displays the isothermal magnetization curves of the metallic microfibers before and after the annealing process. It can be seen that the changing trend of the metallic microfibers is consistent and has a higher magnetic susceptibility in the temperature range below the T_C. The magnetization increases with the rise in the external magnetic field and can quickly reach a saturation state, showing obvious ferromagnetic properties. Around the T_C, the metallic microfibers carry out a rapid transition from FM to PM. The magnetic susceptibility of the metallic microfibers is small and completely transitions to the paramagnetic state above T_C. Particularly, the metallic microfibers need to be in a relatively high external magnetic field to reach the magnetic saturation state at the annealing temperature of 380 °C, which is caused by the inconsistency in the direction of easy magnetization inside the micro-domains of the atomic clusters formed after annealing and the direction of the magnetic field.

Figure 9 presents the Arrott curves of the Gd-based metallic microfibers before and after annealing. All the curves are approximately aligned in a parallel manner with the positive slopes and without obvious inflection points, showing the typical characteristics of secondary magnetic phase change materials. The results of the Arrott curves [37,38] of the Gd-based metallic microfibers are consistent with the related research results. No change occurs in the type of phase transition in the metallic microfibers, which is still the FM-PM secondary transition, indicating that the metallic microfibers before and after annealing can be used as a magnetic refrigeration working material.

Figure 10 shows the magnetic entropy change curves of the Gd-based metallic microfibers before and after the annealing process, from which the −ΔS_m value of the microfibers increases firstly and then decreases with the rise in the external temperature, the value of −ΔS_m^max appears near T_C, and the curves show symmetry with T_C as the axis. This is a typical second-order magnetic phase transition. At the external magnetic field of 5 T, the −ΔS_m^max of the as-prepared metallic microfibers is 11.57 J/kg·K. When the annealing temperature increases, the −ΔS_m^max change tends to decrease, the value of which is 7.20 J/kg·K at the annealing temperature of 380 °C. This is closely related to the change in the atomic arrangement structure. After the annealing process, the degree of the internal order of the microfibers increases, and the arrangement of the magnetic moments in the micro-regions becomes orderly, which reduces the disorder of the internal magnetic moments. This leads to a decrease in the magnetic entropy of the metallic microfibers.

The RC and RCP of the Gd-based metallic microfibers before and after the annealing process are plotted in Figure 11. When the external magnetic field increases, the RC and RCP of the microfibers increase approximately linearly. At the external magnetic field of H_ex = 5 T, the as-prepared metallic microfibers reach a maximum cooling efficiency, and the RC and RCP are 834.14 J/kg and 1138.16 J/kg, respectively. Compared with the RC and RCP values [39] mentioned in previous studies, the cooling capacity has been improved. The RC and RCP of the metallic microfibers after annealing at 300 °C are 667.8 J/kg and 894.6 J/kg, and they are 459.4 J/kg and 588.7 J/kg, respectively, after annealing at 380 °C.

To sum up, the magnetic refrigeration capacity of the Gd-based metallic microfibers before and after the vacuum annealing process is comprehensively evaluated according to magnetocaloric performance indicators such as −ΔS_m^max, RC, and RCP, and it can be found that the cooling capacity of the annealed metallic microfibers decreased compared with that of the as-prepared microfibers.

Therefore, the appropriate annealing temperature can improve the magnetic properties of the amorphous alloys. However, the magnetocaloric properties of the amorphous alloys will be reduced due to the change in the internal magnetic domain distribution at high temperatures [31,32], which provides a theoretical basis for the application of Gd-based metallic microfibers in magnetic refrigeration devices.

As listed in

Table 2. Statistics for magnetocaloric parameters of Gd-based alloys.

Compositions	−ΔSmmax (J/kg·K)	RC (J/kg)	RCP (J/kg)	Refs.
Gd57Al19Co19Ni5	11.57	1138.16	834.14	This work
Gd70Co10Al20	6.36	538	-	[4]
Gd36Tb20Co20Al24	12.36	417	-	[8]
Gd3SbO7	3.54	-	-	[12]
Gd70Co30	5.13	-	277	[17]
Gd70Co20Fe10	2.78	-	353	[17]
Gd71Ni29	-	804.3	-	[19]
Gd73.5Si13B13.5	6.4	790	885	[21]
Gd34Ni33Al33	11.06	-	-	[22]
Gd60Al25(NiCo)15	6.31	-	890	[23]
Gd55Co15Al24Si1Fe5	7.26	857	-	[33]
Gd60Al20Co20	10.12	698.28	936.19	[34]

, the −ΔS_m^max and RC values of the Gd₅₇Al₁₉Co₁₉Ni₅ metallic microfibers are better compared with those alloys. The different transition temperatures of the magnetocaloric materials are applied to different working environment requirements and fields. The T_C value of the Gd-based metallic microfibers is about 109 K. The Gd-based metallic microfibers have excellent magnetocaloric properties. The development and performance optimization of Gd-based metallic microfibers can greatly accelerate the engineering application process of magnetic refrigeration technology in various fields. Its application fields include medicine and health care, civil life, precision instruments, and aerospace.

3.3. Mechanism Analysis of Annealing Gd-Based Metallic Microfibers

The mechanism of the effect of the annealing temperature on the magnetic properties of the Gd-based metallic microfibers can be analyzed from the two aspects of magnetic moment arrangement and magnetic domain structure change to further explore the influence of the interaction between the atoms and the evolution of the microstructure on the magnetic properties. For ferromagnetic materials, without an external magnetic field, the vector sum of their internal intrinsic magnetic moments is zero, showing non-magnetism, while under the condition of an external magnetic field, the intrinsic magnetic moment of the material changes and exhibits magnetism. Therefore, the study of the magnetic change in the Gd-based metallic microfibers needs to start with the atomic magnetic moment. The effect of annealing on the magnetic properties of the Gd-based metallic microfibers is explained in Figure 12.

After the vacuum annealing process, the residual internal stress of the metallic microfibers caused by rapid cooling is greatly eliminated, as shown in Figure 12a. Meanwhile, it can be seen from the HRTEM image that, after annealing, there are atomically arranged micro-domains inside where the atomic magnetic moments are aligned. As shown in Figure 12b, the magnetic moment of the microfiber surface is reversed when the magnetic moment rotates from low energy to high energy with the increase in the magnetic field induction intensity. From Figure 12c, it can be seen that vacuum annealing increases the number of ordered atomic micro-regions without crystallization occurring. In addition, the direction of the magnetic moment remains consistent along the direction of the magnetic field. Under the action of an external magnetic field, the domain wall is displaced, as shown in Figure 12d, and the energy consumed is much greater than that at the angle of 90°, so the M_s value of the metallic microfibers decreases. In addition, after the microfiber is magnetized, a circular induced current (eddy current) will be generated, and the eddy current will excite a magnetic field opposite to the external magnetic field, thereby reducing the M_s of the material. After the annealing process of the microfibers, the internal atomic spin interaction is enhanced, the interatomic bonding force is also enhanced, and under the condition of an external magnetic field, the magnetoelastic performance of the microfibers increases, which eventually increases the magnetization work of the metallic microfibers and reduces the M_s. The internal ordering of the annealed microfibers is improved in a certain region, and the ordered arrangement of the magnetic moments reduces the disorder of the internal magnetic moments, thus lowering the −ΔS_m value.

4. Conclusions

In summarizing, the structure, general magnetic properties, and magnetocaloric properties of Gd-based metallic microfibers before and after annealing processing were systematically compared and analyzed, and the conclusions are as follows:

• Gd-based metallic microfibers have a favorable thermal stability and glass-forming ability. The local atoms in the microfibers are arranged in an orderly manner and the ordered micro-regions are formed after annealing @ 380 °C.
• The M_r, H_c, and μ_m of the Gd-based metallic microfibers increase with the rising of the annealing temperature, which reached 17.83 emu/g, 125.12 Oe, and 0.58, respectively at the annealing temperature of 380 °C. The −ΔS_m^max, RC, and RCP for the microfibers annealed at 380 °C are 7.20 J/kg K, 459.4 J/kg, and 588.7 J/kg, respectively, at the H_ex of 5 T.
• After annealing @ 380 °C, the local atoms in the metallic microfibers are rearranged to generate ordered micro-regions of atomic clusters, which will hinder the movement of the domain wall and the rotation of the magnetic moment during the magnetization process, thereby affecting the synergy of the magnetic moment under the external magnetic field. Simultaneously, the magnetic entropy value of the microfibers decreases with the formation of the locally ordered micro-regions, which ultimately affects the magnetocaloric properties.