Large refrigerant capacity induced by table-like magnetocaloric effect in amorphous Er0.2Gd0.2Ho0.2Co0.2Cu0.2 ribbons

ABSTRACT The microstructure, magnetism, and magnetocaloric properties in melt-spun Er0.2Gd0.2Ho0.2Co0.2Cu0.2 ribbons were reported. The ribbons are fully amorphousized and all the constituent elements are distributed uniformly. The large table-like magnetocaloric effect (MCE) from 25 to 75 K has been observed, resulting in a large value of refrigerant capacity (RC). With the magnetic field change (Δµ0H) of 0–5 T, the values of maximum magnetic entropy change reaches 11.1 J/kg K, and the corresponding value of RC are as large as 806 J/kg, make the amorphous Er0.2Gd0.2Ho0.2Co0.2Cu0.2 ribbons extremely attractive for cryogenic magnetic refrigeration. GRAPHICAL ABSTRACT IMPACT STATEMENT Table-like magnetocaloric effect (MCE) was observed in amorphous Er0.2Gd0.2Ho0.2Co0.2Cu0.2 ribbon, the MCE parameters are comparable or obviously larger than most of reported materials, making it attractive for magnetic refrigeration.

During last three decades, the MCE, referring to the change of entropy or temperature induced by the change of applied magnetic field, has attracted much special attention due to their potential use for magnetic refrigeration (MR) as well as for a better understanding the related properties of the corresponding magnetic solids [1][2][3][4][5]. The MR based on MCE is expected to replace the traditional gas compression refrigerant due to its more environmental friendly and higher conversion efficiency. Up to the present, the MR is still in its early development stage, only limit for the laboratory work. Therefore, most of the researchers in this community are searching for new magnetic solids with promising magnetocaloric parameters. Moreover, it is well accepted that the Ericsson cycle [5][6][7] is the best choice for cryogenic MR technology which consists two isothermal and two isomagnetic field processes. For the ideal Ericsson cycle, the refrigeration performance should be the same in the whole working temperature range, i.e. the tablelike MCE. This performance can be indirectly identified by the parameter of refrigerant capacity (RC). Therefore, searching or exploring proper magnetic solids that exhibit large values of RC and table-like MCE is a key issue for active applications. For this purpose, series of magnetic alloys and compounds have been systematically studied recently with respect to their magnetic and magnetocaloric properties [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. And the table-like MCE has been found in some of the magnetic materials that undergo multiple successive magnetic transitions [15][16][17][18][19][20].
Among the MCE materials, the magnetic solids in the amorphous state have also attracted some special attention due to their special physical and chemical properties [25][26][27][28][29], such as high chemical stability, very soft magnetic properties, excellent mechanical properties, superior thermal conductivity, high electrical resistivity, etc. Due to the absence of long-range ordering of amorphous materials, the magnetic transition is rather broad for the amorphous alloys, therefore, the peak values of − S M are usually smaller than that of their corresponding crystallized forms, whereas, the peaks in the temperature dependence of − S M curves would be getting broad which may result in larger RC. Very recently, some heavy rare-earth (HRE)-based intermetallic compounds are found to exhibit large/giant MCE [8][9][10][11][12], however, only limit to around their own T C . It also has been theoretically shown that rod-or wire-shaped magnetic refrigerant materials are more suitable for actual cooling devices than their spherical counterparts [30,31], thus, the MCE in these type magnetic materials have been experimentally investigated recently, and some of them could outstanding candidate for active MR [26,[32][33][34]. Moreover, to our knowledge, no systematically research related to the triple HRE-based amorphous alloys has been reported. To develop new magnetic solids that are suitable for ideal cryogenic MR Ericsson cycle, in this letter, the triple HRE-based amorphous ribbon has been developed and its microstructure, magnetism, and magnetocaloric properties are presented. A large tablelike cryogenic MCE in a wide temperature range from 25 to 75 K as well as large RC has been realized in the Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 amorphous ribbons. Our study demonstrates that this kind of material appears to be an ideal candidate for active cryogenic MR.
The alloy ingot of Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 was fabricated from high-purity Er, Gd, Ho, Co, and Cu metals by the arc-melting method under argon gas. The alloy ingot was flipped and re-melted for five times and the weighted loss was ∼ 0.28 wt. % for the overall melting processes. Then, the ribbons with a typical size of 12-20 cm in length, 1.5-2.5 mm in width, and 25-35 μm in thickness were produced by melt-spun technology under argon gas at a surface linear speed of ∼ 36 m/s from the alloy ingot of Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 . The structure was ascertained by room temperature X-ray diffraction (XRD) using a PANalyical X'pert Pro diffractometer with Cu K α radiation. The thermal analysis was carried out in a Netzsch STA 4091 differential scanning calorimeter (DSC) with a heating rate of 0.33 K/s under argon gas flow. The transmission electron microscope (TEM) images and energy-dispersive X-ray (EDX) results were obtained by a Tecnai G2 F20 S-TWIN (FEI) high-resolution electron microscope. A small tetragonal pellet sample was used for the magnetization (M) measurements which were conducted by a superconducting quantum interference device (Quantum Design, SQUID-VSM) magnetometer. The oscillating mode is selected to minimize the demagnetization field when the field is decreasing to zero, and the temperature dependence of magnetization are collected with the speed of 3 K per minutes.
The XRD pattern for the melt-spun Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 ribbon is presented in Figure 1. No visible sharp crystalline peaks can be indexed for the ribbon and the observed broad diffraction halo with its maximum at 2θ ∼ 36°is expected for the amorphous Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 ribbon. To obtain more information of the amorphous nature, the DSC for Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 ribbon is also performed and the trace is presented in the inset of Figure 1. An endothermic reaction peak at the amorphous transition temperature can be found in the DSC traces which are similar to previous reported amorphous materials [27][28][29]. The temperature of T m (melting point), T g (glassy transition temperature), T x (first crystallization temperature), and T l (liquidus temperature) as indicated in the figure by arrows are 919, 488, 553, and 961 K for Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 , respectively. Accordingly, the values of undercooled liquid region T x ( = T x −T g ) and the reduced glass transition temperature T rg = (T g /T l ) [21,23] which are taken as the figure of merits to evaluate the glass forming ability (GFA) are evaluated to be 68 K and 0.51, respectively. The high-resolution TEM (HRTEM) has been used to further confirm the amorphous nature of Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 ribbons. The HRTEM image and the selected area electron diffraction (SAED) pattern [as presented in Figure 2(a,b), respectively] also indicate full amorphous structure for Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 ribbon, which are in good agreement with the XRD and DSC results. The element amorphous ribbon undergoes a rather broad magnetic transition from paramagnetic to ferromagnetic at the Curie temperature (T C ∼ 49 K) which is a typical behaviour for magnetic solids in amorphous state due to the absence of long-range ordering. The linearity in the curve of 1/χ vs. T above 140 K in Figure 3 indicates that 1/χ follows the Curie-Weiss law: 1/χ = (T−θ p )/C (θ p is the paramagnetic Curie temperature and C is the Curie constant). The values of effective magnetic moment ((μ eff = (3k/μ 0 N) × C ≈ 2.83 √ C)) and θ p are evaluated to be 7.36 µ B /f. u. and 62.3 K, respectively. The M(T) curves Besides the S M , the RC is another crucial figure of merits to justify the potential suitability of magnetic solids as a magnetic refrigerant which indirectly quantifies the amount of heat transfer from the cold to the hot reservoirs in one ideal MR cycle. In practice, three different criteria have been applied to estimate the values of the RC [4,35]: (1) from the product of S max M and δT FWHM  In summary, the structure, magnetism, and magnetocaloric properties of melt-spun multi-HRE-based  (289), 649 (245), and 549 (198) J/kg, which are obviously higher than most of potential MR materials with similar working temperature regime. These promising MCE parameters indicate the present Er 0.2 Gd 0.2 Ho 0.2 Co 0.2 Cu 0.2 amorphous ribbon is an excellent candidate material for active cryogenic MR.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The present work was supported by the National Natural Science Foundation of China [grant number 51671048], [grant number 11374081].