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0Magnetocaloric effect of Gd-based microwires from binary to quaternary system
Y. F. Wang, F. X. Qin, Y. H. Wang, H. Wang, R. Das, M. H. Phan, and H. X. Peng
Citation: AIP Advances 7, 056422 (2017); doi: 10.1063/1.4975356 View online: http://dx.doi.org/10.1063/1.4975356 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics
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Magnetocaloric effect of Gd-based microwires from binary to quaternary system
Y.F.Wang,1 F. X. Qin,1a Y. H. Wang,1 H.Wang,1 R. Das,2 M. H. Phan,2 and H. X. Peng1
1 Institute for Composites Science Innovation (InCSI), School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
2 Department of Physics, University of South Florida, Tampa, Florida 33620, USA
(Presented 3 November 2016; received 23 September 2016; accepted 15 November 2016; published online 30 January 2017)
We have studied the magnetocaloric effect (MCE) of Gd-based amorphous microwires from binary to quaternary system. We find that with increase of components from binary GdNi to ternary GdNiCo, there is a significant increase in magnetic entropy change (AS m) from 1.43 to 2.73 J ■ kg 1 ■ K 1 and an increase of temperature interval from 90K to 115K; further comparison between the quaternary GdNiCoDy and ternary GdNiCo shows a continuing increase of temperature interval while retaining the similar ASm. Such an improvement of MCE can be ascribed to the enhancement of amorphicity with increasing number of components, which leads to the improved magnetic softness and homogeneity. The increase of the Curie temperature with increasing number of components also indicates the enhanced Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic interactions caused by the addition of alloying elements as comparing binary, ternary and quaternary system or by optimized composition in terms of such as Ni/Co ratio in a typical ternary system of GdNiCo. These results have demonstrated that appropriately designed Gd-based microwires are very useful for active magnetic refrigeration in the liquid nitrogen temperature regime. © 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4975356]
Modern society relies very much on readily available refrigeration. Current refrigeration technology for cooling applications is mainly based on vapor compression, which has already reached an upper limit of cooling efficiency and is harmful to our living environment due to the usage of hazardous gas such as chlorofluorocarbons (CFCs) and hydro-chlorofluorocarbons (HCFCs).1 To meet the requirement of environmental friendliness and energy saving, magnetic refrigeration (MR) based on the magnetocaloric effect (MCE),2 emerges as the most promising alternative to conventional refrigerators, through a long time of continuous and in-depth research.3 The MCE is defined as the adiabatic temperature change (ATad) of a magnetic material in response to a varying magnetic field (like magnetization and demagnetization), the essence of which is the isothermal magnetic entropy change (ASm) due to the aligning or disturbing of the magnetic dipoles of atoms when a field is applied or removed. So ATad and ASm are often considered to be two important parameters for characterizing MCEs in magnetic materials. In view of its practical application, however, the refrigerant capacity (RC) is a more reasonable figure to characterize the cooling efficiency of magnetic materials, which is defined as the integration of -ASm(T) from the temperature of cold sink to hot sink in an ideal refrigeration cycle.4 Therefore, a desired magnetocaloric material should possess a large magnetic entropy change (ASm) over a wide temperature range, resulting in the large RC.5 On account of the giant magnetocaloric effect (GMCE) discovered in Gd5(SixGe1-x)4(0<xw < 0.5),6
2158-3226/2017/7(5)/056422/6 7,056422-1 ~ "nil if ) "fir I 1 ■
Corresponding author: faxiangqin@zju.edu.cn
namely the alloy with a significantly large MCE, much effort has been devoted to flrst-order magnetic transition(FOMT) materials. Many families of alloys have proved to exhibit large ASm, such as MnAsi-xSbx,7 MnFePi-xAsx8 and NiMnGa Heusler alloys,9 but there are still some challenging issues that restrict the implementation of these FOMT materials; the temperature change is restricted to a relatively narrow temperature range, and the large thermal and field hysteresis losses in the temperature range of interest are detrimental to the active magnetic refrigeration.10 By contrast, studies11-14 have shown that the materials with a second-order magnetic transition (SOMT) have smaller MCEs but span over broader temperature ranges, thus leading to the larger RC values and negligible hysteresis losses. As a consequence, in single phase materials, it is difficult to attain large ASm and large RC at the same time, which means a wide and high platform in the diagram of ASm vs. T and is necessary to Ericsson cycle.
To overcome the difficulty, metallic glasses seem to be a good solution, because of a series of progress achieved in property studies of rare-earth based materials.15-17 Since the heavy rare-earth based amorphous materials have a large magnetic moment, which determine them a large magnetic entropy change, and a tunable ordering temperature, these materials are excellent magnetic refrigerants, as compared to Fe based, Co based and Pd based amorphous materials. Among the amorphous materials, Gd-based metallic glasses18-20 appear a great potential in magnetic refrigeration. The peak value of -ASm for Gd53Al24Co20Zr3 bulk metallic glass (BMG) reaches 9.40Jkg-1K-1, which is comparable with that of pure Gd as known with a high cost, in the work.21 Notably the RC (590Jkg-1) of the BMG is much larger than that of the FOMT material, Gd5Si2Ge2, due to a much wide temperature range of the large ASm. The previous work of Qin et al.22 has shown the larger values of -ASm and RC in Gd53Al24Co20Zr3 microwire samples relative to their bulk counterparts. Current efforts are to improve the MCE and increase the Curie temperature of Gd alloy microwires.
In this paper, a series of Gd-based microwires, including binary, ternary and quaternary components, were fabricated and their magnetocaloric properties were systematically characterized. The enhancements of both the MCE and Curie temperature achieved in these microwires make them promising candidates for AMR applications.
Gd based microwires of composition Gd71Ni29, Gd55Ni10Co35, Gd55Ni15Co30, Gd55Ni20Co25 and Gd53Ni24Co20Dy3 were prepared using a melt-extraction method. The specific production process is described below: The ingot, i.e. precursor alloy with the same composition of the microwires finally obtained, was manufactured from raw materials Gd (99.9%), Ni (99.9%), Co (99.9%) and Dy (99.9%) in argon gas atmosphere by arc-melting. The melt extraction process was performed using a cooper wheel with of 160 mm and 60° knife edge, with a linear velocity of the wheel rim fixed at 30 m min-1 and a feed rate of the molten material of 90^m s-1. The morphology of microwires was observed by a field emission scanning electron microscope (SEM Utral 55) at 5kV. The amorphous structure of the microwires was examined by the X-ray diffraction characterization using SHIMADZU XRD-6000 with Cu Ka radiation. The magnetic properties were measured utilizing a commercial Physical Property Measurement System (PPMS-9T) from Quantum Design in a temperature range of 10-300K and with a magnetic field up to 2T.
Figure 1 shows the X-ray diffraction (XRD) patterns of the Gd based amorphous microwires with the binary, ternary and quaternary components (Gd71Ni29, Gd55Ni20Co25, Gd53Ni24Co20Dy3 respectively). The profile indicates a high diffraction peak exists between 30° to 35° and a more broader and lower diffraction peak distributes around a larger angle, which is agreed with the previous work.5,21 By comparison, there is an appreciable peak in the Gd71Ni29 XRD pattern, however, the peak gets lower and indistinct and has a slight trend to the right with component increasing. There is a mixture of crystalline and amorphous phases in Gd71Ni29 and a fully amorphous state is formed in Gd55Ni20Co25 and Gd53Ni24Co20Dy3. This phenomenon can be demonstrated by the empirical rule,23 that multicomponent systems consisting of more than three components can enhance glass forming ability (GFA).The picture of the cross section shows the roundness of microwires is poor, suggesting that the fabrication parameters need to be optimized in order to receive better roundness.
The temperature dependence of magnetization was measured under a field of 0.2T between 50K and 300K, as shown in Fig. 2a. The ternary Gd55Ni20Co25 and quaternary Gd53Ni24Co20Dy3 have a similar behavior, with magnetization decreasing rapidly to zero when heating, indicating a change from ferromagnetism to paramagnetism. For the binary Gd71Ni29 a rapid decline of magnetization
FIG. 1. XRD patterns of the binary Gd71Ni29, the ternary Gd55Ni2oCo25 and quaternary Gd53Ni24Co2oDy3 amorphous microwires. The inset picture shows the cross section of the microwires.
occurs till Tc, after which the magnetization reduces much more slowly. At room temperature, the material is paramagnetic, which is consistent with that of ternary and quaternary samples. The picture illustrates that there is a single homogeneous magnetic phase in ternary and quaternary amorphous microwires and dual magnetic phases in binary amorphous microwires because of the coexistence of crystalline and amorphous phases. The Curie temperatures, Tc, which are defined as the temperature at the minimum of dM/dT, are 121K, 129K and 141K for binary, ternary and quaternary amorphous microwires (Fig. 2b). The Curie temperatures are much lower than that of the crystalline Gd (293K), which is resulted from the structural disordering and the addition of alloying elements.5 Figure 2c shows the M-T curves of ternary Gd55Ni10+5xCo35-5x (x=0,1,2) also under a 0.2T field in a temperature range of 50K-300K. The three amorphous alloys show a similar rapid decrease with increasing temperature, suggesting a homogeneous single magnetic phase. The Tc values obtained for the three alloys are 189K, 157K and 129K, respectively (Fig. 2d), indicating that the content of Ni and Co has a significant effect on Tc, more Co and less Ni resulting in a higher Tc. This is consistent with what has been reported in Ref. 24, which elucidates the phenomenon using a theoretical model based on
FIG. 2. Temperature (T) dependence of magnetization (M) taken at 0.2T (a) for Gd71 Ni29, Gd55Ni20Co25, Gd53Ni24Co20Dy3 and (c) for ternary Gd55Ni10+5xCo35-5x (x=0, 1, 2); dM/dT (b) for Gd71Ni29, Gd55Ni20Co25, Gd53Ni24Co20Dy3 and (d) for Gd55Ni10+5xCo35-5x (x=0, 1, 2).
FIG. 3. Isothermal magnetization curves at different temperatures for (a) Gd71Ni29, (b) Gd55NiioCo35, (c) Gd55Nii5Co30, (d) Gd55Ni20Co25 and (e) Gd53Ni24Co20Dy3.
hypothetical "GdCo" and real GdNi compound, reasoning that enhanced effective magnetic exchange interactions resulted from Ni sites replaced by Co lead to this phenomenon.
The MCE of the Gd-based amorphous microwires was evaluated by measuring the isothermal magnetization curves at different temperatures. Fig.3 (a-e) shows the results of the isothermal magnetization, measured with a field step of 2000e in a range of 0-20k0e and temperature intervals of
FIG. 4. Magnetic entropy changes -ASm under different magnetic field for (a) Gd71Ni29, (b) Gd55Ni^Co35, (c) Gd55Ni15Co30, (d) Gd55Ni20Co25 and (e) Gd53Ni24Co20Dy3.
TABLE I. Maximum entropy change, -ASm, peak temperature, Tp, Curie temperature,Tc, and RC, for the present samples.
Materials Tc(K) Tp(K) Ц0 AH(T) -ASm(J/kg •K) RC(J/kg)
Gd71Ni29 121 110 2 1.56 143.7
Gd55Ni10Co35 189 190 2 2.51 229.8
Gd55Ni15Co30 157 165 2 3.55 323.3
Gd55Ni20Co25 129 135 2 2.89 204.7
Gd53Ni24Co20Dy3 141 145 2 2.68 245.4
10K away from the Tc and 5K around the Tc. The plots show the magnetization reaches its saturation state rapidly at a low field, with little magnetic hysteresis loss. The behavior suggests that Gd based amorphous microwires have excellent soft magnetic properties, which is desired for AMR. Based on the isothermal magnetization curves (M-H), Arrott plots25 exhibit all positive slopes of H/M-M2 (not shown here), indicating that all Gd-based amorphous microwires possess a typical SOMT feature, which is desirable for the magnetic refrigeration over wide temperature span.
The isothermal entropy change(ASm) is calculated through the M-H curves by using Maxwell equation.26 The plots of -ASm - T are displayed in Fig. 4, under a magnetic field range from 0-2T for all the samples. As shown, all samples have broad and large peaks, which can result in a large RC. The peak value of -ASm is 1.56J/Kg+K, 2.51 J/Kg+K, 3.55J/Kg+K, 2.89J/Kg+K and 2.68 J/Kg+K for Gd71Ni29, Gd55Ni10Co35, Gd55Ni15Co30, Gd55Ni2oCo25 and Gd53Ni24Co2oDy3 at a 2T field, respectively. For a more intuitive comparison, -ASm and RC values of the present samples are summarized in Table I. The binary amorphous microwires have the minimum -ASm, while Gd55Ni15Co30 shows the highest -ASm value, demonstrating that composition have a significant impact on the MCE. By optimization of component and proportion of the composition, superior MCE can be attained in Gd-based amorphous microwires.
To evaluate the RC of the samples, the value of RC is defined as the integration of -ASm from the cold end to the hot end in an ideal refrigeration cycle. It should be noted that the temperatures at half maximum of the peak are usually used as the integration interval. Fig. 5 shows the comparison of RC-H plots of different microwires. The RCs of the samples increase with the increasing field. The largest RC value is obtained in ternary Gd55Ni15Co30 at a 2T magnetic field, approaching 325J/Kg, corresponding with the largest -ASm value.
The MCE performance of the microwires is strongly dependent on the composition and microstructure. There exists a relation between the composition and Curie temperature: increasing the atomic ratio of Co/Ni can yield a larger Curie temperature. This can be understood as an appropriate ratio of Ni and Co ions has enhanced Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic interactions and hence induced the maximum polarization. Another important factor is the
number of elements involved as more components can to some extent benefit the formation of amorphous structure, which is likely to lead to a broadened temperature span.5 This explains why we see the Gd55Nii5Co3o ranks NO.1 and the Gd53Ni24Co20Dy3 the second in terms of RC ranking. It is expected with optimization of the composition of the quaternary system, the RC can be significantly improved to overtake the ternary system in any case.
In summary, we have studied the MCE of a set of Gd-based microwires with different composition from binary to quaternary. It is found that the key indices of MCE performance such as Curie temperature, entropy change and cooling capacity are governed by the amorphicity of the microwires and composition through the RKKY magnetic interactions. The improvement of amorphous microwires engineering technique and exploration of optimized composition recipe are therefore anticipated to yield better MCE microwires for active refrigeration applications.
ACKNOWLEDGMENTS
FXQ would like to thank the financial support of NSFC No. 51671171 and No. 51501162, and 'National Youth Thousand Talent Program' of China and 'Hundred Talents Program' of Zhejiang University.
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