Investigation Of The Crystal Structural,Magnetic Properties And Magnetocaloric Effect In Rare-earth Based Double Perovskite Oxides

Magnetic Cooling(MC)technology is considered to be a kind of refrigeration with very extensive application prospects in the future.MC is the most straightforward use of the magnetocaloric effect(MCE)and has a high theoretic conversion efficiency and environment friendliness compared to traditional gas compression refrigeration.Therefore,the development of materials with large MCE is vital to promote the application of the MC.In this thesis,two series of rare earth(RE)based double perovskite oxides RE₂FeAlO₆(RE=Gd,Dy,Ho)and RE₂CrMnO₆(RE=Ho and Er)were fabricated successfully by sol-gel route,and their crystal structure,magnetic properties and MCE have investigated systematically.Moreover,the Gd₂₀Ho₂₀Tm₂₀Cu₂₀Ni₂₀ amorphous alloy was also fabricated by arc-melting together with the melt-spun method,and systematically checked the microstructure,magnetocaloric performance and refrigerant capacity.The obtained results are as follows:RE₂FeAlO₆(RE=Gd,Dy,Ho)double perovskite oxides have prove to be orthorhombic crystal structure,Pbnm space group,and RE₂FeAlO₆(RE=Gd,Dy,Ho)oxides belong to B-site disorder double perovskite structure.The consequences of magnetic measurement indicated that RE₂FeAlO₆(RE=Gd,Dy,Ho)oxides undergo a secondary magnetic transition from paramagnetic to ferromagnetic phase.The Curie temperature of Gd2FeAlO6 and Ho2FeAlO6 are both lower than 2 K,and the Curie temperature of Dy2FeAlO6 is?7.8 K.In addition,the calculated effective magnetic moments of RE₂FeAlO₆(RE=Gd,Dy,Ho)oxides are close to the theoretaical magnetic moments.Under the magnetic field change of 0-5 T,the maximum values of magnetic entropy change(-?SMmax)for Gd2FeAlO6,Dy2FeAlO6,Ho2FeAlO6 reach 18.5,8.5,and 7.1 J/kg-K,the values of Temperature averaged Entropy Change(i.e.,TEC(3 K))are 14.5?8.4?6.9 J/kg·K,respectively.In addition,the corresponding values of relative cooling power(RCP)are 187.5,212.5,183.5 J/kg and the values of refrigerant capacity(RC)are 136.1,163.5,144.5 J/kg for Gd2FeAlO6,Dy2FeAlO6,Ho2FeAlO6,respectively.The RE₂FeAlO₆ has value of potential application for cryogenic MC.The polycrystalline RE₂CrMnO₆(RE=Ho and Er)double perovskite oxides are also cystallized in orthorhombic structure belonging to the Pbnm space group.Both oxides are of B-site disordered double perovskite structure.The RE₂CrMnO₆(RE=Ho and Er)oxides undergo the magnetic phase transition process from paramagnetic to ferromagnetic phase.The Curie temperatures are?6.1 K and?5.2 K for Ho₂CrMnO₆ and Er₂CrMnO₆,respectively.The calculated effective magnetic moments of RE₂CrMnO₆(RE=Ho and Er)oxides are close to the theoretaical ones.Under the magnetic field change of 0-5 T,the values of maximum magnetic entropy change(?SMmax)and Temperature averaged Entropy Change(i.e.,TEC(3 K))for Ho₂CrMnO₆,Er₂CrMnO₆ oxides are 8.8,8.7 J/kg·K and 10.3,9.9 J/kg·K,and the corresponding RCP(RC)are 193.7(146.0),150.7(112.3)J/kg,respectively.The Gd₂₀Ho₂₀Tm₂₀Cu₂₀Ni₂₀ alloy is with full amorphous structures and all consistent elements are uniformly distributed.A paramagnetic-ferromagnetic phase transition with no hysteresis occurred around?32 K,indicating that the Gd₂₀Ho₂₀Tm₂₀Cu₂₀Ni₂₀ amorphous alloy has reversible MCE.Additionally,the magnetic transition of the Gd₂₀Ho₂₀Tm₂₀Cu₂₀Ni₂₀ amorphous alloy is more sensitive to the magnetic field and exhibits a table-like MCE.Under AH of 0-5 T,the calculated full width at half maximum(?TFWHM),magnetic entropy change maximum(|?SM|max),temperature averaged entropy change(i.e.,TEC(10 K)),RCP,RC reach 60.8 K,10.6 J/kg·K,10.5 J/kg·K,644.6 J/kg,527.9 J/kg,respectively,indicating that Gd₂₀Ho₂₀Tm₂₀Cu₂₀Ni₂₀ amorphous alloy is also considerable candidate for MC.