Study On Magnetocaloric Effect Of Kagome Lattice Compound RE₃BWO₉(RE=Gd?Dy?Ho)

Since the 1990s,the advantages of low-temperature magnetic refrigeration materials,such as pollution-free,zero-emission,green,energy-saving and environmental protection which have attracted extensive attention from scientific researchers.Through the doping operation of magnetic and non-magnetic elements,the magnetic moment of the material is adjusted at the microscopic level.The material used shows weak hysteresis,small thermal hysteresis,large magnetic entropy,and outstanding cooling power.Due to their large spin and orbital angular momentum,heavy rare earth elements often exhibit large magnetocaloric properties.Therefore,in this article,we have conducted related magnetic and magnetocaloric studies on the tungstate RE₃BWO₉(RE=Gd,Dy,Ho)with kagome magnetic structure.The main contents are as follows:1.The research background of magnetic refrigeration,the principle of refrigeration in the view of thermodynamics and the description of measurement methods of magnetocaloric effect have been introduced briefly.The crystal structure and magnetic ground state of Kagome lattice compounds RE₃BWO₉(RE=Gd,Dy,Ho)have been simply summarized.2.The polycrystalline powder RE₃BWO₉(RE=Gd,Dy,Ho)and the sample of 20%Mo atoms doped parent phase Gd₃BWO₉were prepared by solid-state reaction.The structure of the prepared sample was measured and characterized by powder XRD which showed that the sample crystallinity and purity are very good and no impurities can be detected.3.Characterize the dependence of the temperature and magnetic field on the magnetic properties of the compound RE₃BWO₉(RE=Gd,Dy,Ho).The experimental results show that there is a weak antiferromagnetic interaction between the magnetic Gd³⁺ions in the compound Gd₃BWO₉and the Mo-doped sample Gd₃BW_0.8Mo_0.2O₉.There is no long-range magnetic order transition occured when the temperature is as low as 2 K,indicating that the ground state of the compound may be a paramagnetic state;the low-temperature magnetization curves of isomorphic compounds Dy₃BWO₉ and Ho₃BWO₉ show a deviation of the Curie-Weiss fitting around 8K,indicating that the two compounds have a short-range antiferromagnetic spin correlation at low temperatures.4.The magnetic entropy data of the compound RE₃BWO₉(RE=Gd,Dy,Ho)under differen magnetic fields calculated based on Maxwell's equation.The maximum magnetic entropy change and adiabatic transition temperature of the paramagnetic salt compound Gd₃BWO₉ at low field 3T are 36.75J·Kg^-1·K^-1 and 5.56K respectively.The maximum magnetic entropy change at 7T is 54.80 J·Kg^-1·K^-1,which is better than many known low-temperature Gd-based magnetic refrigeration compounds.The compound Gd₃BWO₉has no large magnetic hysteresis and thermal hysteresis,indicating that the material has potential application prospects in the field of low-temperature magnetic refrigeration.When the applied magnetic field changes from 0 T to 7 T,the maximum magnetic entropy changes of the compounds Dy₃BWO₉and Ho₃BWO₉are 28.50 J?kg^-1?K^-1at 4 K,29.76 J?kg^-1?K^-1at 6K respectively.With the increase of the external magnetic field,the magnetic entropy peaks of the compounds Dy₃BWO₉and Ho₃BWO₉gradually shifted to the high temperature region and the peak widths gradually became wider.Therefore,the compounds Dy₃BWO₉and Ho₃BWO₉can be applied to a larger temperature range in the refrigeration low temperature range.