The Perovskite Ho（Fe/Mn）O₃Single Crystal Growth And The Magnetocaloric Effect

In this paper, RMO₃（R is rare earth elements, M is the transition metal） system withthe perovskite structure is the object for the study. Crystal growth, magnetic andthermodynamic effects as a starting point, the good quality HoFeO₃singlecrystal growth conditions was system studied, a series of characterization is carriedout, the HoFeO₃single crystal is successful prepared. While a thorough andmeticulous research in the large magnetocaloric effect of HoFeO₃single crystal atlow temperatures is carried out, along with the change in the magnetic phasetransition. In addition, magnetic properties, magnetocaloric effect （MCE） andthe strain at low temperature of D_y1-xHo_xMnO₃system are depth researched.The main research work is as follows:1. Single crystal growth process of HoFeO₃is explored by the opticalfloating zone. The growth conditions which are suitable for the singlecrystal growth of this material, such as temperature, pressure, rotation,atomosphere, and gas flow. By XRD analysis, we determine the crystalstructure of the HoFeO₃single crystals, and confirm that the samples havewell single phase. By the EMPA （Electron microprobe analysis） foras-grown HoFeO₃single crystal, we find tiny cracks in the sample.Combined with the relationship of the crystal growth temperaturegradient, the growth rate, as well as single crystal diameter, we give thereasonable rational analysis and interpretation of phenomenon. And wealso determine the composition of the samples by EDS （Energy dispersiveX-ray spectroscopy） analysis, find there is no composition segregationwhich further confirms the rationality of our view. Meanwhile, we clearlydirect the crystal axis of HoFeO₃single crystal by Laue pattern,and confirm the accuracy of the orientation by Ramanscattering, provides a good sample for the next experiment. 2. We discuss the magnetic properties, thermal properties andmagnetocaloric effect of HoFeO₃single crystal according to a varietyof physical property measurements. By analyzing the magnetic propertiesof the sample, we identify the spin reorientation temperature zone ofHoFeO₃single crystal. In the sample, Due to the presence of magneticanisotropy and spin-orbit coupling, the relationship between thecoercive force and temperature is complex; the change is different alongdifferent axis. According to the Landau’s second order phase transitiontheory, there would be a change in the specific heat at the spinreorientation temperature zone. But as the value of K2（the four times termcoefficient of free energy relating with the temperature and material） isvery small for this sample, we cannot clearly observe the abnormal fromthe curve of hear capacity dependents on temperature directly at the spinreorientation temperature zone. So, it is difficult to calculate the magneticentropy change from the data of the specific heat, and we calculate themagnetic entropy change by Maxwell equation from the isothermal M-Hcurves. since the sample has a rich magnetic phase transitions, such as themagnetic odering of Ho3+, antiferromagnetic interaction between Ho3+and Fe3+, magnetic field-induced change magnetic transition, we findthere exists two maximum of the magnetic entropy change, called twinpeaks phenomenon. Compare to the other materials such as manganite,Heusler alloy, the HoFeO₃single crystal owns the large magnetic entropychange about19.2J/kg K and high value of refrigeration capacity （RC）about220J/kg, indicating it can be used as a very suitable for candidatemagnetic refrigerant materials for low temperature applications.3. We deeply study the magnetic, magnetocaloric effect and the strain at lowtemperature of D_y1-xHo_xMnO₃（x=0-1） polycrystalline samples. Theresults show that with the increasing of the doping level, the magnetism of the samples increasing, which is responding to the magnetism of the rareearth. Meanwhile, with the decreasing of temperature, there existsa rich of magnetic structures change. From the Arrott figure, we can seethat the magnetic phase transition, including a phase transition and secondorder phase transition. Calculating the magnetic entropy change, we findthat the peak temperature of the magnetic entropy change, the magneticordering temperature of the rare earth ions and the temperature of rareearth ions and Mn3+interaction are inconsistent. At the same time, wefind that with increasing the doping concentration, the magnetic entropychange of the sample increases, but the peak temperature without anyoffsetting. This shows that the doping level only change thevalue of the peak without changing the peak temperature. ForHoMnO3polycrystalline, with the increasing of the magnetic fieldvariation, the peak temperature of the magnetic entropy change offsets tothe high temperature. This shows that there is a change in themagnetic phase transition. So we think that the magnetic entropy changeis not just due to the magnetic ordering of rare earth ions and the changein magnetic phase transition. There should be more complex mechanism.We carry out the strain measurement to explore the complex mechanismand find an intrinsic structural phase transition at low temperatures,resulting a rich magnetic phase transition and the change of ionicinteractions between R3+and Mn3+.It is regards as the nature of theproduction of large MCE. The samples show a huge RC （refrigerationcapacity） values （312J/kg） and magnetic entropy change （12.5J/kg K） atlow temperatures indicates that the samples in this system can be usedas a promising magnetic refrigeration materials.