Cs2NaAl1−xCrxF6: A family of compounds presenting magnetocaloric effect

Abstract

In this paper we explore the magnetocaloric effect (MCE) of chromium-doped elpasolite Cs 2 NaAl 1−x Cr x F 6 (x = 0.01 and 0.62) single crystals. Magnetization and heat capacity data show the magnetocaloric potentials to be comparable to those of garnets, perovskites, and other fluorides, producing magnetic entropy changes of 0.5 J/kg K (x = 0.01) and 11 J/kg K (x = 0.62), and corresponding adiabatic temperature changes of 4 and 8 K, respectively. These values are for a magnetic field change of 50 kOe at a temperature around 3 K. A clear Schottky anomaly below 10 K, which becomes more apparent when an external magnetic field is applied, was observed and related to the splitting of the Cr 3+ energy levels. These results hint at a new family of materials with potential wide use in cryorefrigeration. Magnetic refrigerators are promising devices based on the magnetocaloric effect (MCE), with applications including hydrogen liquefiers, high-speed computers, and superconducting quantum interference device (SQUID) cooling. Thus, the quest for new materials which exhibit the MCE and promise technological improvement has attracted much attention in recent years [1][2][3][4]. Two important thermodynamic quantities characterize the MCE: the temperature change in an adiabatic process ( T ad ) and the entropy change in an isothermal process ( S T ) upon magnetic field variation. The latter is strictly related to the efficiency of a thermomagnetic cycle. The MCE is usually indirectly measured by using specific heat and magnetization data [1].Compounds based in paramagnetic salts were used to break the 1 K barrier for the first time in magnetic refrigerators [2]. However, despite their many applications over the intervening years, the low thermal conductivity of these salts is detrimental in adiabatic demagnetization applications [5], leading to a search for new materials with MCEs at lower temperatures. Among such materials is gadolinium gallium garnet and magnetic nanocomposites based on iron-substituted gadolinium gallium garnets [6][7][8][9]. Although some of these compounds have S T values comparable to the Gd-standard material, of about 3.4 J/kg K for a field variation of H = 10 kOe close to room temperature [10], there are other systems that have a much larger entropy change, S T ≈ 30 J/kg K under H = 50 kOe, at 5 K, such as Gd 3 Ga 5−x Fe x O 12 [6]. Other successful examples consist of perovskite-type oxides [11], metal-organic frameworks containing gadolinium [12], and molecular nanomagnets, such as Mn 32 [13] and Fe 14 [14]. However, very few studies have so far reported on caloric effects in fluoride systems [15][16][17], which is the aim of this paper.From the point of view of optical and structural properties, Cs 2 NaAl 1−x Cr x F 6 elpasolite single crystals, which crystallize in the R3m space group, have been thoroughly investigated [18][19][20][21][22][23][24][25][26][27]. The choice of the Cr 3+ ion as a doping impurity is justified by the fact that its 3d unfilled shell produces electronic transitions that i...