Study On Phase Equilibrium Of Re-Fe-Rh(Re=Nd,Ho) System And Magnetic Properties Of Related Compounds

Compared with the traditional gas compression refrigeration,magnetic refrigeration technology has attracted much attention because of its advantages of environmental protection,high efficiency and energy saving,and has a wide application prospect.Rare earth transition compounds have been an important research system in the field of magnetic materials because of their rich magnetic structures and magnetic phase transitions.Phase diagram is the foundation of material science research,in this paper,we chose the rare earth transition ternary system RE-Fe-Rh(RE=Nd,Ho)as the research object,experimentally studied the phase equilibrium relationship of RE-Fe-Rh at 873 K and constructed the isothermal cross section diagram of RE-Fe-Rh system at this temperature.In addition,the magnetic and magnetocaloric effects of some RE-Rh binary compounds,as well as the crystal structure,cell parameters and phase transition temperature of the ternary compound Fe_2-xRh_xHo were also studied.The results are as follows:(1)The phase relations of Nd-Fe-Rh and Ho-Fe-Rh ternary systems at 873 K were studied by XRD and SEM/EDS techniques,and the solid solubility of the third component in binary compounds and solid solution phases was determined.On the basis of the phase equilibrium relationship,the isothermal cross-section diagrams of Nd-Fe-Rh and Ho-Fe-Rh ternary systems at 873 K were constructed.The isothermal section of Nd-Fe-Rh is composed of 13 single-phase regions,23 two-phase regions and 11 three-phase regions,the solid solubility of Fe in Nd₃Rh,Nd₇Rh₃ and Nd Rh₂ is about 4 at%,3 at%and 17 at%,respectively.The Ho-Fe-Rh isothermal section consists of 15 single-phase regions,28 two-phase regions and 14 three-phase regions,the solid solubility of Fe in Ho₃Rh and Ho Rh₂ is about 5.5 at%and 19.5 at%,respectively,the solid solubility of Rh in Fe₃Ho and Fe₂Ho is 3 at%and 9.6at%,respectively.(2)The magnetic and magnetocaloric properties of binary RE₇Rh₃(RE=Nd,Ho)polycrystalline samples with hexagonal Th₇Fe₃ structure were measured by means of a comprehensive physical property measurement system(PPMS).Both Nd₇Rh₃ and Ho₇Rh₃compounds undergo continuous magnetic phase transitions at low temperatures,PM-AFM phase transitions below T_N and AFM-FM phase transitions near T_t,respectively.In addition,another magnetic transition of about 7.2 K was observed in Ho₇Rh₃,which may be related to spin gravity orientation.The effective magnetic moments of RE₇Rh₃(RE=Nd,Ho)are determined to be 3.39?_B/Nd³⁺and 10.2?_B/Ho³⁺,and the corresponding paramagnetic Curie temperatures are 38.6 K and-10 K,respectively.The magnetic entropy of Nd₇Rh₃ and Ho₇Rh₃ changes to 6.40 J/kg K and 3.06 J/kg K at 0-5 T magnetic field.These continuous magnetic entropy peaks overlap to a certain extent,resulting in a larger working temperature span and significant MCE.The corresponding RC values of Nd₇Rh₃ and Ho₇Rh₃ are 164J/kg and 85 J/kg.(3)The magnetic properties and magnetocaloric effects of Er₃Ni₂-type Nd₃Rh₂ and Y₃Rh₂-type RE₃Rh₂(RE=Nd,Ho,Er)have been studied experimentally.For Nd₃Rh₂,two continuous second-order magnetic transitions at low temperatures were observed,T_t=13 K and T_N=17 K,respectively,namely,the FM-AFM transition and the AFM-PM transition.For RE₃Rh₂(RE=Ho,Er),there is an FM-PM second-order magnetic transition at T_C=23 K and8 K,respectively.The effective magnetic moments of RE₃Rh₂(RE=Ho,Er)were determined to be 10.30?_B/Ho³⁺and 10.62?_B/Er³⁺,and the paramagnetic Curie temperatures were 13 K and 4.5 K,respectively,indicating that the interaction of Fm was the main factor in RE₃Rh₂(RE=Ho,Er)compounds.The magnetic entropy changes of RE₃Rh₂(RE=Nd,Ho and Er)were calculated to be 3.78 J/kg K,14.01 J/kg K and 20.01 J/kg K.Under the magnetic field of 0-5 T,the RC values of RE₃Rh₂(RE=Nd,Ho and Er)are 68 J/kg,382 J/kg and 267 J/kg,respectively.(4)The compounds RERh(RE=Nd,Ho)also showed two different crystal structural characteristics,namely Tl I structure and Cscl structure.The compound Nd Rh has a first-order magnetic phase transition of AFM-PM at 11 K,while the compound Ho Rh has a second-order magnetic phase transition of PM-FM at T_C=10.3 K.The effective magnetic moments of RERh(RE=Nd,Ho)were determined to be 3.63?_B/Nd³⁺and 10.46?_B/Ho³⁺,which were close to the theoretical values.The corresponding paramagnetic Curie temperatures of RERh(RE=Nd,Ho)were-5.6 K and-6.9 K,respectively.The negative paramagnetic Curie temperatures confirms that antiferromagnetism plays a dominant role in the two compounds.Under the magnetic field of 0-5 T,the maximum magnetic entropy changes of RERh(RE=Nd,Ho)are 1.8 J/kg K and 9.9 J/kg K,respectively.(5)The linear solid solution compounds Fe_2-xRh_xHo(x=0,0.09,0.18,0.27)were characterized in this paper,and the effect of Rh on the structure and magnetic properties of Fe₂Ho was studied.It is found that the maximum solid solubility of Rh in Fe₂Ho is about 9atomic percentage.With the increase of Rh content,the cell constant A and cell volume V showed a linear trend.Using DSC,it was found that Curie temperature of a series of compounds decreased with the increase of Rh content.At the same time,the hysteresis loops at room temperature were measured by VSM,and all of them showed ferromagnetic state without magnetic hysteresis,the saturation magnetization decreased with the increase of Rh substitution.