PROPRIEDADES MAGNÉTICAS E MAGNETOCALÓRICAS DO SISTEMA RNiGe3 (R = Pr, Nd, Eu, Dy, Ho e Er).

Abstract

Structural, magnetic and magnetocaloric properties of the RNiGe3 (R = Pr, Nd, Eu, Dy, Ho and Er) compounds were studied. The samples were synthesized by arc melting technique under argon atmosphere. X-ray diffraction (XRD) measurements and analysis by using the Rietveld method confirm the desired crystallographic phase for all studied samples. Magnetization data shows antiferromagnetic ordering for RNiGe3 compounds, with Néel temperature (TN) around 21 K, 12.5 K, 14 K, 21 K, 9 K and 6 K for R = Pr, Nd, Eu, Dy, Ho and Er, respectively. For T<TN, the compounds NdNiGe3 and EuNiGe3 present an additional transition around 6.1 K and 5 K, respectively, while PrNiGe3 presents two additional transitions around 18.5 and 15.5 K. External magnetic field application induces metamagnetic phase transitions from antiferromagnetic to ferromagnetic state. However, the observed values of the magnetic moments in high magnetic field (H= 60 and 90 kOe) and low temperatures (T = 2 K) are reduced compared to the respective free ions. The metamagnetic critical magnetic field (HC) occurs around 16 kOe, 17.3 kOe, 26.5 kOe, 10.5 kOe and 9.5 kOe, for R = Pr, Nd, Dy, Ho and Er, respectively, and around 20 and 28 kOe for R=Eu. Magnetocaloric effect (MCE) of studies compounds was evaluated by means of the isothermal magnetic entropy change (-ΔSM), in which a broad peak in -ΔSM curve associated with the magnetic transition was observed. In addition, negative MCE was observed for all samples (in ΔH < HC) due to antiferromagnetic ordering. Among the studies compounds, the highest value of -ΔSM was observed for HoNiGe3. The maximum values of -ΔSM (−𝛥𝑆𝑀𝑚𝑎𝑥) and relative cooling power (RCP) observed for RNiGe3 series were respectively, 6.1 J/kg.K and 72.9 J/kg, 4.2 J/kg.K and 68.2 J/kg, 7.5 J/kg.K and 113.6 J/kg, 5.0 J/kg.K and 106 J/kg, 16.7 J/kg.K and 308.5 J/kg, and 11.3 J/kg.K and 214.7 J/kg, for R = Pr, Nd, Eu, Dy, Ho and Er, in ΔH = 50 kOe. These values are comparable to those reported in the literature for other magnetocaloric compounds, in the same temperature range. The obtained results indicate that the RNiGe3 compounds presented interesting magnetocaloric properties for application in magnetic refrigeration at cryogenic temperatures.