Structure,Magnetism And Magnetocaloric Effects Of GdFeTeO₆ And GdFeTiO₅

Magnetic refrigeration technology is an environment-friendly refrigeration technology based on magnetocaloric effect of magnetic materials.In recent years,Gd-based transition metal oxides have attracted extensive attention due to their huge magnetocaloric effect.In this paper,Gd based materials GdFeTeO₆ and GdFeTiO₅ have been synthesized.The structure,magnetic properties and magnetocaloric effect of these compounds have been studied.The main contents are as follows:1.The development background of magnetic refrigeration technology,basic research methods of magnetocaloric effect,magnetic properties of rare earth transition metal materials and research status are summarized.High temperature solid state reaction method,superconducting quantum interference magnetometer?SQUID?and pulsed high magnetic field electron spin resonance?ESR?devices are briefly introduced..2.Polycrystalline powders of Gd based GdFeTeO₆ and GdFeTiO₅ have been prepared by solid-state method.X-ray diffraction data indicated that both samples were single phase.The crystal structures of GdFeTeO₆ and GdFeTiO₅ were obtained by Retiveld structure refinement.The magnetic properties of samples were measured by magnetic measurement system and electron spin resonance?ESR?device with pulsed high magnetic field.3.Magnetic measurement results show that GdFeTeO₆ undergoes paramagnetic to ferromagnetic phase transition when the temperature is near about 3 K,and the magnetic interaction is very weak.No long range magnetic ordering was observed in GdFeTiO₅among the experimental measurement temperature range.The fitting result of reciprocal magnetic susceptibility shows that the Curie-Weiss temperature of GdFeTeO₆ and GdFeTiO₅ is 3 K and-5.6 K,respectively.The effective magnetic moment is 9.27?_B/f.u.and 8.3?_B/f.u.,respectively.Brillouin fitting result shows that there is weak magnetic exchange interaction in GdFeTiO₅.ESR data show that ground state of GdFeTeO₆ is ferromagnetic state,while GdFeTiO₅ is a non-pure paramagnetic substance.4.The study of magnetocaloric effect shows that the maximum magnetic entropy change of GdFeTeO₆ and GdFeTiO₅ is 38.5 J?kg^-1?K^-1and 33.6 J?kg^-1?K^-1,respectively,with a magnetic field change of 0-7 T.For a field range of 0-3 T,the maximum magnetic entropy change of GdFeTeO₆ can reach 23.6 J?kg^-1?K^-1,while for a field change of 0-5 T,the maximum magnetic entropy of GdFeTiO₅ can reach 27.7 J?kg^-1?K^-1,which significantly higher than other magnetocaloric materials in the same series under the same magnetic field range.The first-principles calculation of GdFeTeO₆ shows that the huge magnetocaloric effect is due to the competing ferromagnetic and antiferromagnetic interactions.In addition,the hysteresis of GdFeTeO₆ and GdFeTiO₅ can be nearly neglected.Thus,GdFeTeO₆ and GdFeTiO₅ can be considered as low-temperature magnetic refrigeration materials.