Review of the influence of copper and chromium substitution on crystal structure, magnetic properties and magnetocaloric effect of GdFe_2−x(Cu, Cr)_x (x = 0, 0.1, 0.15 and 0.2) intermetallic compounds

Highlights

• GdFe₂-_x (Cu, Cr)_x (x = 0, 0.1, 0.15 and 0.2) samples prepared by arc melting subsequent annealing.

• The Rietveld analysis of XRD patterns proves that compounds crystallize in the cubic laves phase.

• Magnetocaloric effect at low field of GdFe_2-x(Cu, Cr)_x was studied for the first time.

• ΔS_M, RCP and TEC are calculated.

Abstract

The iron-based compounds GdFe_2−x(Cu,Cr)_x (x = 0, 0.1, 0.15 and 0.2) were synthesized successfully by means of arc-melting and annealing at 800 °C for one week. The structural, magnetic and magnetocaloric properties of these intermetallic compounds were investigated systematically in detail using X-ray powder diffraction (XRD) analysis, Scanning electron microscopy (SEM) equipped with Energy dispersive X-ray spectroscopy (EDS), and magnetic measurements. The Rietveld analysis of X-ray diffraction patterns proves that GdFe_2−x(Cu,Cr)_x (x = 0, 0.1, 0.15 and 0.2) compounds crystallize in the cubic laves phase MgCu₂-type structure with the $Fd\bar{3}m$ space group. Gadolinium and iron atoms statistically occupy the 8a and 16d sites respectively. Moreover, copper and chromium atoms substitute the iron atoms at site 8a in the MgCu₂-type structure. The temperature dependence of magnetization studied in a broad temperature range reveals that all compounds exhibit a ferromagnetic-paramagnetic transition at Curie temperature (T_C). We find that the substitution of iron by copper and chromium leads to a decrease in the magnetic ordering transition temperature. The Arrott plots of all our samples show the occurrence of a second-order phase transition. Besides, the performance of magnetocaloric effect (MCE) for GdFe_2−x(Cu,Cr)_x (x = 0, 0.1, 0.15 and 0.2) compounds was evaluated by the magnetic entropy change (ΔS_M) and the related Relative Cooling Power (RCP). In the vicinity of T_C, the ΔS_M reached a maximum value of 0.79 J/kg.K, 1.2 J/kg.K, 1.4 J/kg.K and 2.5 J/kg.K, while the RCP was found to be 13.2 J/kg, 21.6 J/kg, 31.7 J/kg and 44.3 J/kg, respectively for GdFe_2−xCu_x (x = 0, 0.1, 0.15 and 0.2) compounds. Furthermore, the GdFe_2−xCr_x (x = 0.1, 0.15 and 2) compounds show ΔS_M values of about 0.9 J/kg.K, 1.3 J/kg.K and 1.8 J/kg.K, and RCP values of 14.8 J/kg, 25.4 J/kg and 36.8 J/kg respectively, under a field change of 1.56 T. Through these results, the GdFe_2−x(Cu,Cr)_x (x = 0, 0.1, 0.15 and 0.2) compounds can be an attractive materials for use in magnetic refrigeration and heat pumping technology above room temperature.

Introduction

Over recent years, human consciousness about energy and environment has been highly emphasized. The toxic gases associated with global-warming, greenhouse effects and ozone-depleting gases, such as chlorofluorocarbons (CFCs), hydrofluoric carbons (HFCs) and ammonia (NH₃) [1,2] have posed a threat to human life, which limits the conventional gas-compression (CGC) refrigerators. Thus, from an environmental conservation perspective, research on future refrigeration technologies is increasingly oriented toward other techniques, such as magnetic refrigeration (MR), based on the magnetocaloric effect (MCE) [3,4]. The principle of this effect is based on the reversible heating and cooling of a magnetic material after the application of a magnetic field variation [5,6]. In addition, magnetic refrigeration is becoming a promising technology to replace the conventional cooling technique, because it is much more compactly built due to the fact of taking solid substances as working materials. The key advantage of magnetic refrigeration is that it has ecological, economic and environmental benefits [[7], [8], [9], [10]]. What is more, this technology doesn't cause atmospheric pollution or noise and it offers significantly more reliability and energy-efficient than the conventional refrigeration [11].

In this context, the intermetallic materials combining rare-earth elements and transition metals with large magnetocaloric properties have attracted a considered attention owing to its potential application in the magnetic refrigeration (MR) technology [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]].

Most of rare-earth transition-metals intermetallic compounds exhibit a second order magnetic transition (SOMT) [11,14,16,19,20]. Intermetallics that possess a SOMT generally exhibit negligible loss by thermal and magnetic hysteresis, consequently they could be a potential candidate for magnetic refrigeration.

In addition, many promising materials such as Gd₅Si₂Ge₂ [25,26] Mn₅Ge_3−xSi_x [27], and MnNiGa [28], have been broadly studied for their high magnetocaloric effect and their application as magnetic refrigerants.

Among numerous rare earth - transition metals-based intermetallic compounds, Laves-phases RM₂ (R stands for rare earth elements, and M stands for transition metals) compounds have been massively studied [29,30]. These phases crystallize in three structure types: MgCu₂ (C15) type structure, MgZn₂ (C14) type structure, and MgNi₂ type structure (C36) [31]. The RV₂ (R = Ti, Nb, Hf, and Ta) compounds present high melting point, good oxidation resistance, and high strength at elevated temperatures which make them available for high-temperature structural applications [[32], [33], [34]] and hydrogen-storage [35]. In addition, the rare earth Laves phases RCo₂ (R = Dy, Ho, Er) [[36], [37], [38], [39]], TbCo₂ [40], GdCo₂ [41], and SmNi₂ [42] with cubic MgCu₂-type structure (C15) exhibit attractive and useful properties such as significant anisotropic magnetostriction, large magnetovolume effect, and large magnetocaloric effect around the Curie temperature.

In the literature, GdFe₂ compound, which crystallizes in the cubic Laves-phase with MgCu₂ type structure, has been the subject of many investigations in the past few years [[43], [44], [45]]. These compounds are of considerable scientific and technological interests because of their excellent magnetic, electronic, elastic and thermal properties, due to the coexistence of complementary features of itinerant (3d) and localized (4f) electrons in these compounds.

In our previous papers [46,47], we have prepared a series of Gd-Fe-Cu and Gd-Fe-Cr alloys to determine the solid state phase equilibria in both ternary Gd-Fe-Cu and Gd-Fe-Cr systems at 800 °C, through the use of SEM-EDS and XRD techniques. The solid solutions GdFe_2−xCu_x and GdFe_2−xCr_x were identified to exist in these isothermal sections, respectively, which extended from about x = 0 to x = 0.2. This outcome opens the way in this experimental work, for investigating the effect of copper and chromium substitution for iron on the structural, magnetic, and magnetocaloric properties of the GdFe_2−x(Cu,Cr)_x (x = 0, 0.1, 0.15 and 0.2 compounds.