The Crystal Structure,Magnetic Properties And Magnetocaloric Effect In RE-Co/Ni-Ga Materials

The magnetic refrigeration（MR）based on magnetocaloric effect（MCE）is considered to be a promising technology due to its high-efficiency,energy-saving,environmental-friendly,stability and reliability.The development and application of MCE materials are the core of magnetic refrigeration technology.Exploring magnetic refrigeration materials with excellent magnetocaloric properties will contribute to promoting the development of magnetic refrigeration technology.In recent years,liquid He,H₂,N₂ and O₂ have played an increasingly important role in aerospace,medical and health,industrial production,military defense and other fields.For better storage and transportation,gas liquefaction is particularly important.It has been shown that gas liquefaction can be achieved by magnetic refrigeration technology.Therefore,the purpose of this thesis is to explore low-temperature magnetocaloric materials suitable for gas liquefaction.We chose rare earth-cobalt/nickel-gallium（RE-Co/Ni-Ga）series to study.By tuning the stoichiometric ratio of each element or changing the types of rare earth elements,the phase transition temperatures can be continuously tuned in a wide temperature range and the cryogenic refrigeration materials with excellent MCE performance can be obtained.In this paper,a series of RE-Co/Ni-Ga compounds were fabricated successfully by arc-melting combined with an appropriate annealing process,and the crystal structure,magnetic properties,magnetic phase transition and magnetocaloric properties of these compounds were systematically investigated.In addition,as an extension,RENi Al₂ and RECo Si₂ series were also synthesized and their MCE properties were studied.The main conclusions are as follows:The Dy₂Co₂Ga,Ho₂Co₂Ga,Er₂Co₂Ga and Tm₂Co₂Ga compounds crystallizing in the orthorhombic W₂B₂Co-type structure were found to undergo a second-order magnetic phase transition from ferromagnetic（FM）to paramagnetic（PM）at around55.0,38.5,25.5 and 11.6 K,respectively.The effective magnetic moment(（?）_eff)values are 10.48,10.51,9.41,and 7.38（?）_B for RE is Dy,Ho,Er and Tm,respectively,which are close to the corresponding theoretical RE³⁺values,indicating that the magnetic contribution mainly origin from rare earth elements.For the field change（（35）H）of 0-5 T,the-（35）S_M^max values for Dy₂Co₂Ga,Ho₂Co₂Ga,Er₂Co₂Ga and Tm₂Co₂Ga are 6.2,11.7,9.6 and 11.3 J kg^-1 K^-1,respectively.This series show an excellent magnetocaloric properties in the low temperature range.Particularly,Ho₂Co₂Ga reaches its maximum-（35）S_M value at around 24 K,which is close to the H₂liquefaction temperature（20.4 K）,making it of great application value for cryogenic refrigeration.The magnetic phase transitions for RE₆Co₂Ga（RE=Ho,Dy and Gd）compounds（crystallizing in orthorhombic Ho₆Co₂Ga-type structure）could be tuned from antiferromagnetic（AFM）-paramagnetic（PM）to FM-PM phase transition by tuning the type of rare earth elements.Using the Banerjee criterion and the field dependence of MCE,the nature of phase transitions for Ho₆Co₂Ga and Dy₆Co₂Ga are proved to be first-order,while Gd₆Co₂Ga belongs to the second-order nature phase transition.In addition,Gd₆Co₂Ga shows three consecutive FM-PM phase transitions at around 55,78,and 129 K,respectively,which widens its working temperature range and makes its refrigeration capacity much higher than that of the other two compounds.For（35）H of 0-5 T,the-（35）S_M of Ho₆Co₂Ga and Gd₆Co₂Ga reach the maximum values of 10.1 and 9.1 J kg^-1 K^-1 at around 26 K and 75 K,respectively,which are close to the H₂（20.4 K）and N₂（77 K）liquefaction temperature range.This excellent magnetocaloric performance for RE₆Co₂Ga system indicating its good application prospect.The magnetization data of RE₂Ni_1.5Ga_2.5 compound（Ca In₂-type structure）indicates that there is almost no thermal and magnetic hysteresis.When RE is Dy,Ho,Er and Tm,the Curie temperatures（T_C）are 16.1,10.6,4.1,and 2 K,respectively.The Curie temperatures show a linear relationship with the de Gennes factor,indicating that the RE-RE exchange interactions play a dominant role in magnetism.All the compounds undergo a FM-PM second-order nature magnetic phase transitions at around their respective T_C.It is worth noting that when RE is Dy,the phase transition temperature is close to the H₂ liquefaction temperature,whereas for RE of Er and Tm,the phase transition temperatures are close to the He liquefaction temperature（4.2 K）.For the（35）H of 0-5 T,the-（35）S_M^max values of Dy₂Ni_1.5Ga_2.5,Ho₂Ni_1.5Ga_2.5,Er₂Ni_1.5Ga_2.5 and Tm₂Ni_1.5Ga_2.5 are 10.01,20.12,18.07 and 18.22 J kg^-1 K^-1,respectively.Excellent MCE performance for RE₂Ni_1.5Ga_2.5 system makes it quite competitive in low-temperature magnetic refrigeration candidate materials.For RENi Ga₂,RENi Al₂ and RECo Si₂ compounds in RETX₂ system,after tuning the types of elements,the structure type and the order of magnetic phase transition have changed,which provides some clues for the future alloy design.Rietveld refinement results proved that RENi Ga₂ and RENi Al₂ crystallize in orthorhombic Mg Cu Al₂ structure,while RECo Si₂crystallizes in Ce Ni Si₂-type structure.Dy Ni Ga₂,Ho Ni Ga₂,Er Ni Ga₂,Ho Ni Al₂ and Er Ni Al₂ undergo FM-PM second-order magnetic phase transitions at around 46.0,26.0,11.0,7.5 and 5.0 K,respectively,whereas for Er Co Si₂ and Tm Co Si₂,the first-order metamagnetic phase transition from AFM to PM occurred at around 6.9 and 5.4 K,respectively.For（35）H of 0-5 T,the maximum values of-（35）S_M for Dy Ni Ga₂,Ho Ni Ga₂,Er Ni Ga₂,Ho Ni Al₂,Er Ni Al₂,Er Co Si₂ and Tm Co Si₂ are determined to be 6.16,10.37,11.42,14.0,21.2,7.1 and 8.7 J kg^-1 K^-1,respectively.Their working temperature range is continuously tunable between the He liquefaction and H₂ liquefaction temperature range,making them have a broad application prospect.