Study On The Irradiation-induced Defects And Magnetic Properties In MgO Single Crystals

Ferromagnetism of materials generally originates from partially filled shells of d or f electrons.Defects induced room temperature ferromagnetism was,however,observed in undoped HfO2 films in 2004.The ferromagnetism of such systems is termed d0 ferromagnetism,which challenges our conventional understanding of the origin of magnetism in insulators.In most previous studies the d0 ferromagnetism was attributed to point defects,but it is still in intense debate on the nature of the defects,i.e.,anions or cations.In this thesis,MgO single crystal was routinely employed to study the d0 ferromagnetism.The point defects in MgO were generated by y-ray or neutron irradiation.The relationship between the point defect configuration and magnetic properties in irradiated MgO was then scrutinized in detail.The d0 ferromagnetism in MgO was rationalized by point-defect theory.These results can be valuable for developing new materials with doLferromagnetism.The main results are summarized as follows:(1)Based on the Huang's scattering theory,the X-ray diffuse scattering induced by different defects near various reciprocal-lattice points was explored.The isointensity profiles around distrinct reciprocal-lattice points,which were caused by cubic and double-force point defects in MgO,were calculated by MATLAB software.The calculations showed that there existed a significant difference in the isointensity profiles of the diffuse scattering for the cubic and double-force point defects near the same reciprocal-lattice points.The point defect configurations were identified by a comparison of the experimental reciprocal space mappings(RSMs)with the calculated diffuse scattering intensity curves.(2)Commercial(100)MgO single crystals were irradiated with different doses by 60Co y-ray radiation source.The X-ray diffuse scattering measurements were carried out at Shanghai Synchrotron Radiation Facility(SSRF)and Beijing Synchrotron Radiation Facility(BSRF)diffuse scattering station.The rocking and ?-2? curves for(200)and(311)reflections were achieved at different temperatures.The results showed that the ?-ray irradiation resulted in lattice distortions and a certain number of point defects were introduced in irradiated(100)MgO single crystals.The measured reciprocal space mappings(RSMs)of the irradiated MgO single crystals matched very well with the calculated diffuse scattering intensity profiles for double-force point defects.Frenkel defects were introduced to(100)MgO single crystals by y-ray irradiation.The UV-Vis absorption spectra verified the y-ray irradiation produced anion defects in(100)MgO single crystals.Combined UV-Vis absorption spectra and diffused scattering measurements,showed that the anion Frenkel defects were indeed generated in(100)MgO single crystals by y-ray irradiation,but their concentration was low.M-H curves demonstrated that the irradiated(100)MgO was still diamagnetic at room temperature,but transformed to paramagnetic at 5K.The paramagnetism might be due to the y-ray irradiation-induced anion defects.Since the anion defect concentration was too low to reach the threshold,the d0 ferromagnetism could not be experimentally observed in the y-ray irradiated(100)MgO single crystals.(3)(100),(110)and(111)-oriented MgO single crystals were neutron-irradiated with different doses ranging from 1.0×1016 to 1.0×1020 cm-2.The ?-2? scans and rocking curves strongly indicated that neutron irradiation caused lattice distortions and introduced a number of point defects in MgO single crystals.As revealed by RSMs,double-force defects in MgO single crystals were produced upon neutron irradiation.The UV-Vis absorption spectral measurements showed that O vacancies(F centers and F+ centers)were present in neutron-irradiated MgO single crystals.Meanwhile,the O vacancies could aggregate in irradiated MgO at higher doses of 1.0×1019 and 1.0×1020cm-2.The magnetic properties of the irradiated samples were experimentally measured,The results showed that the irradiated MgO was ferromagnetic at low temperature.The low temperature d0 ferromagnetism in the neutron-irradiated MgO single crystals was attributed to be stemming from the anion defects.The ferromagnetism in MgO had nothing to do with the crystal orientation and could be elucidated by the F+ center exchange mechanism.