Magnetization And Photoluminescence Of Mn- And Fe- Based Optical Materials In High Magnetic Fields

The 3d-transition metal?such as Mn,Fe,Co?compounds often show magnetization due to the unpaired 3d-electrons,and optical absorption/luminescence in the visible light region resulting from their underfilled 3d-electronic orbitals with the energy gap of few eVs.These properties can be taken advantage by material scientists to fabricate multifunctional devices for the application of data storage,spin-electronics,and sensing.Low temperature and high magnetic fields are appropriate conditions for the study of magnetic and optical properties of the 3d transition metal compounds.At the temperatures down to liquid helium point,thermal fluctuation induced electron-phonon interaction would be suppressed,and magnetic ordering becomes a predominant factor to influence the optical transitions.In an applied magnetic field,ferromagnetic ordering of the original antiferromagnetic or paramagnetic states would seriously modulate the optical transition selection rules,and the related correlations between magnetic states and optical properties could be revealed.In this thesis,photoluminescence of Mn and Fe based compounds were studied at various temperatures and magnetic fields,and novel results were obtained.Contents of the thesis include:?.Temperature and magnetic field dependent photoluminescence of?CH3NH3?2MnCl4single crystalTemperature and magnetic field dependent photoluminescences?PL?of a layered perovskite single crystal?CH₃NH₃?₂MnCl₄ were investigated.Firstly,the mechanism of PL for the single crystal?CH₃NH₃?₂MnCl₄ is revealed by the temperature dependent PL spectra and PL lifetime.The excitation was accomplished by ion absorption via optical transition from the Mn?⁶A₁?to Mn?⁴T₁?states,and the excited energy can be transferred to the adjacent Mn ions due to the strong magnetic interactions.The Mn ions in a defect state will give rise to PL through radiative process.Secondly,it is found that the PL intensity could be modulated by an external magnetic field with axial anisotropic behavior.An enhancement of PL intensity was observed along the in-plane crystal direction.Finally,an in-plane antiferromagnetic ordering at low temperature was observed by the magnetic susceptibility and ESR measurements.The above results indicate that antiferromagnetic ordering would enhance the Mn-related PL intensities.?.Temperature and magnetic field dependent energy transfer of CsPbCl₃:Mn²⁺nanocrystalsThe PL spectra of CsPbCl₃:Mn²⁺nanocrystals were measured at different temperature and magnetic field.Firstly,from the temperature-dependent PL spectra,the energy transfer process is suppressed at cooling from 300 K to 60 K with decreasing thermal fluctuation.At further cooling,the energy transfer process recovers.The variation of PL peak width,peak intensity and peak position with temperature indicated that the thermal-induced electron-phonon coupling facilitate the energy transfer process in the high temperature region.Secondly,from the temperature dependent magnetic susceptibility and ESR measurements,it is found that in the low temperature region?<60 K?,antiferromagnetic Mn-Mn ion pairs are formed in the nanocrystals,which break the local symmetry,relieve the optical transition selection rules,and enhance the transfer process.Finally,the energy transfer process is suppressed by ferromagnetic ordering in high magnetic fields,which further verify the low temperature enhancement of energy transfer by antiferromagnetic couplings.?.Temperature dependent up-conversion PL in Gd₃Ga_5-xFe_xO₁₂:Yb³⁺/Er³⁺nanocrystalsUp-conversion PL?UCPL?of Fe³⁺-doped Gd₃Ga₅O₁₂:Yb³⁺/Er³⁺nanocrystals were investigated at the temperature down to liquid helium point.Firstly,XRD and magnetization of the nanocrystals with different Fe doping concentrations show that Fe ions replace Ga sites in the lattice.Secondly,temperature dependent UCPL intensity of the nanocrystals with different Fe concentrations were studied.The results show that energy transfer from Fe ions to Er ions occurs and effectively enhances the up-conversion PL intensity at low temperatures.This result indicates the Gd₃Ga_5-xFe_xO₁₂:Yb³⁺/Er³⁺nanocrystals is applicable for temperature sensing from room temperature to cryogenic temperatures.