Magnetism And Magnetocaloric Effects In One-Dimensional Zigzag Chain SrR₂O₄（R=Gd,Ho,Dy）

Magnetic refrigeration is a new refrigeration technology based on the magnetocaloric effect of materials.Low temperature magnetic refrigeration technology can be widely used in military defense,aerospace and basic physical properties measurements and other fields.Therefore,it is great significance to study low-temperature magnetic refrigerant materials.Rare earth-based materials often show large magnetocaloric effects due to their large spin or orbital angular momentum,which makes them receive widespread attention.In this paper,the magnetism and magnetocaloric effects of one-dimensional zigzag chain compounds SrR₂O₄（R=Gd,Ho,Dy）have been studied.The main contents are as follows:1.The principle of magnetic refrigeration,the basis for selecting materials and the research progresses of magnetocaloric effect materials have been introduced briefly.A bacKground of one-dimensional zigzag chain compounds SrR₂O₄（R=Gd,Ho,Dy）has been presented.The sample preparation and magnetic measurement methods have been introduced briefly.Especially,the detailed measurement principle and flow chart of electron spin resonance in pulsed high magnetic field（ESR）have been presented.2.Polycrystalline powders of SrR₂O₄（R=Gd,Ho,Dy）were prepared by solid state method.XRD was used to characterize the phase and structure of the samples.The experimental data displayed that all of the samples have good crystalinity and were in single phase.3.The magnetization of sample was measured by superconducting quantum interference magnetometer（SQUID）.The susceptibility data show that SrGd₂O₄ exhibits antiferromagnetic ordering near T=2.83 K,and SrHo₂O₄ exhibits antiferromagnetic short-range ordering at about 4 K.Nevertheless,no magnetic phase transition was observed in SrDy₂O₄.The fitting results of reciprocal susceptibility curves of SrHo₂O₄ and SrDy₂O₄show that there exists spin-spin correlation.There is weak ferromagnetism in SrHo₂O₄induced by an external magnetic field,and SrDy₂O₄ shows an indistinct 1/3 plateau in the magnetization process.ESR measurements further show that there is a magnetic field-induced order to paramagnetic-like transition;there exists spin-spin correlation in SrGd₂O₄ above the phase transition temperature.4.The magnetocaloric effects of SrR₂O₄（R=Gd,Ho,Dy）polycrystalline powders have been studied.It is shown that all polycrystalline powders display large magnetocaloric effects.When the magnetic field changes to 7 T,the maximum magnetic entropy changes of SrGd₂O₄,SrHo₂O₄ and SrDy₂O₄ are 27 J·kg^-1·K^-1,18.3 J·kg^-1·K^-1 and 17.7 J·kg^-1·K^-1,respectively.Among them,SrGd₂O₄ shows large magnetic entropy variation and almost no thermal and field hysteresis,showing that it is suitable for magnetic refrigeration near liquid helium temperature.The–?S_M（T）curve of SrHo₂O₄ shows a large peak width,which could make the magnetocaloric effects less affected by temperature,for which it has a good stability in refrigeration process.SrHo₂O₄ polycrystalline powder is expected to be a candidate material for low temperature magnetic refrigeration with a wide refrigeration temperature range.For SrDy₂O₄,the temperature of the position of maximum magnetic entropy change is low but the magnetic entropy change is still large at 2 K,which is suitable for low temperature magnetic refrigeration below liquid helium temperature.