PROPRIEDADES MAGNÉTICAS E EFEITO MAGNETOCALÓRICO DOS SISTEMAS RMAl3 (R = La, Ce, Pr E Nd; M = Ag E Cu) E R2AgCuAl6 (R = Pr E Nd)

Abstract

ABSTRACT: This study investigated the structural, magnetic, and magnetocaloric properties of intermetallic compounds in the RMAl3 (R = La, Ce, Pr, and Nd; M = Ag and Cu) and R2CuAgAl6 (R = Pr and Nd) systems. The samples were synthesized in an arc furnace under an argon atmosphere, subjected to heat treatment for 7 days at 700 °C, and structurally characterized via X-ray diffraction (XRD) and Rietveld refinement. Specific heat (Cp) measurements as a function of temperature and magnetic field revealed that CeAgAl3, PrAgAl3, and NdCuAl3 order magnetically at 4.8 K, 5.3 K, and 2.21 K, respectively, whereas NdAgAl3, PrCuAl3, and Nd2AgCuAl6 showed no detectable magnetic ordering above 1.8 K. Temperature-normalized Cp data indicate a Schottky anomaly in all analyzed samples, with the associated peak being most intense in the NdAgAl3, PrCuAl3, NdCuAl3, and Nd2AgCuAl6 compounds. The 4f-electron contribution to Cp in the CeAgAl3, PrAgAl3, PrCuAl3, NdAgAl3, and NdCuAl3 compounds was determined by subtracting the contributions of the non-magnetic analogs LaAgAl3 or LaCuAl3. This enabled the derivation of magnetic entropy (S4f) curves. These curves allowed for the estimation of the number of microstates contributing to the system's S4f within the 1.8–100 K temperature range. Parameters quantifying the Magnetocaloric Effect (MCE)—namely magnetic entropy change (-ΔS), adiabatic temperature change (ΔTad), and Relative Cooling Power (RCP)—were determined from the entropy (S) curves. For a magnetic field change (ΔH) of 5 T, the CeAgAl3 compound exhibited maximum values ​​of -ΔS (-ΔSmax) and ΔTad (ΔTad,max), as well as an RCP of approximately 10.32 J/kg·K, 3.15 K, and 56.87 J/kg, respectively. PrAgAl3 exhibits lower values ​​of -ΔSmax (~4.68 J/kg·K) and ΔTad,max (~2.61 K) but maintains a relatively high RCP (~62.49 J/kg). In this case, a significant portion of the contribution to the MCE is associated with crystal field effects, as evidenced by the results obtained for the non-magnetic compound PrCuAl3 within the measured range (-ΔSmax = 2.71 J/kg·K, ΔTad,max = 1.14 K, and RCP = 41.69 J/kg). On the other hand, NdCuAl3 demonstrates the best magnetocaloric performance (-ΔSmax = 17.08 J/kg·K, ΔTad,max = 3.80 K, and RCP = 159.66 J/kg). The results obtained for these materials are comparable to—and in the case of CeAgAl3, even superior to—intermetallics reported in the literature, indicating their potential for application in MCE-based magnetic refrigeration (MR) systems at cryogenic temperatures.