Magnetic Structure And Negative Thermal Expansion In RE₂Fe₁₇-based Compounds(RE:rare Earth)

Rare earth magnetic alloys have attracted wide attention due to their unique magnetic properties,such as permanent magnetism,magnetostriction,magneto-volume,and magneto-caloric effects,etc.The knowledge of the magnetic structure benefits the deep understanding of the magnetic properties.As one of candidates for the rare earth permanent magnet,RE2Fe17-based compounds possess unique magnetic structures and spin-lattice coupling.This dissertation focused the negative thermal expansion behaviors of RE2Fe17-based compounds.A series of RE2Fe17-based compounds are designed and prepared with different chemical compositions.The crystal structure,magnetic structure and thermal expansion were investigated systematically.The complex spin-lattice coupling was explored in detail,and the relationship between spin-lattice coupling and thermal expansion was built quantificationally.The desired zero thermal expansion behaviors were discovered in the series of RE2Fe17-based compounds.The magnetic field as an effective route was developed for controlling the NTE behaviors of magnetic materials,which lays not only the foundation for the NTE design and understanding but also the related magnetic properties.First,by optimizing the Fe-Co composition,a series of RE2(Fe,Co)17 intermetallic compounds were obtained with zero thermal expansion(ZTE)behaviors over wide temperature range.Among them,the ZTE temperature window reaches 3 to 461 K in Ho2Fe16Co composition,almost twice broader than the known super invar alloy.The comprehensive study of in-situ neutron diffraction,low-temperature Mossbauer spectra and first-principles calculations revealed that the 3d bonding state transition of Fe-sublattice favors extra lattice stress upon magnetic ordering.Co content entangles in the dramatic enhancement of internal molecular field,which can be manipulated to control the magnetic ordering temperature range and thermal expansion behavior.Based on this knowledge,the external magnetic field as effective route was developed to tailor the thermal expansion of RE2(Fe,Co)17 compounds.A series of Ho2(Fe,Al)17 compounds were designed and synthesized with increasing Al substitutions for Fe.X-ray powder diffraction showed that the crystal structure was different at Al concentrations.It possessed a hexagonal symmetry at low concentration,a rhombohedral one at high concentration,and the two symmetries coexist over the intermediate concentration.The magnetization of the Ho2Fe11Al6 compound showed an abnormal behavior at low temperature,demonstrating a ZTE behavior.The investigation of variable temperature neutron powder diffraction revealed that the total magnetic moment of Ho and Fe sublattices had similar value but opposite direction at low temperature.Thus,the total magnetic moment of Ho2Fe11Al6 compound is close to zero from 5 to 80 K,demonstrating the zero magnetization.A zero thermal expansion behavior was also ontained over this temperature range.As the temperature increases,the discrepancy of the rate of the total magnetic moment of Ho and Fe sublattices with respect to temperature illuminated the peak of magnetization around 220 K.The atomic ratio of Lu to Fe was adjusted in the non-stoichiometric Lu2-x?xFe17 compound by introducing vacancy.Room temperature synchrotron X-ray powder diffraction determined that the extra Fe atoms can occupy the sites of Lu vacancies.The magnetic measurements showed that the magnetic structure of Lu2-x?xFe17 compounds changed significantly as the concentration of iron atoms increased.Variable temperature neutron powder diffraction clearly determined the novel spiral magnetic structure,in which the magnetic moments of Fe atoms lies in the basal plane parallelly while turn spiral between adjacent layers along the c direction.The neutron powder diffraction refinements showed that the magnetic structure changes induced by the higher Fe concentration affected slightly the negative thermal expansion behavior of the Lu2-x?xFe17 compound.The negative thermal expansion were intimately involved in the over the changes of the degree to magnetic order magnetic ordering temperature range.Finally,by introducing the same non-magnetic Y atoms into the non-stoichiometric Lu1.7Fe17 compounds,the magnetic structure and thermal expansion behaviors were studied for(Lu,Y)1.7Fe17 compound.The magnetic measurements showed that the original spiral magnetic structure of Lu1.7Fe17 weakened and disappeared gradually with increasing Y substitutions.The negative thermal expansion is also weakened simultaneously.Variable temperature neutron powder diffraction and synchrotron X-ray powder diffraction revealed that the appearance of spiral magnetic structure is highly correlated to the negative thermal expansion along the c-axis.The critical lattice parameter c and temperature were determined.The magnetic phase diagram was depicted unambiguously as a function of rare earth composition for(Lu,Y)1.7Fe17 compounds.