Structure Characterization And Raman Spectroscopy Research Of β-MnO₂ Single Crystal Film Grown By MBE

The growth technology, structure characterization, and Raman research forβ-MnO2 films have been one of the active and abundant subject embranchment in the fields of magnetic materials and perovskite-like structures, since new phenomena were found in transition metal oxides and spintronics was attached great importance. Molecular beam epitaxy (MBE) technology is applied to grow theβ-MnO2 film controlled by Reflection high energy electron diffraction (RHEED) in situ. The MBE is supplied with an oxygenic plasma source and a high-quality crystal Manganese source, where high vacuum stably remains 10-10 mbar before growth. Single crystal MgO (100) is selected as a substrate to insure a satisfied mismatch and a preferred orientation needed in subsequent studies. It makes convenient to investigate the Raman spectra that the MgO is not activated for the first-order Raman vibration. The RHEED patterns show obvious stripes during the growth process, which prove a layer by layer growth mode of the single crystalβ-MnO2 film. Single peak ofβ-phase MnO2 is shown with the peak of the substrate in the XRD pattern, which indicates a satisfied single crystal of theβ-MnO2 film. ESCALAB250-XPS measurement is adopted to collect X-ray photoelectron spectroscopy (XPS) data which suggest a good elements match, satisfied ionic valent states, and little gas impurity attached at the surface. The lattice ofβ-MnO2 belongs to non-symmorphic space group as D4 14h- P 42 /mnm and is anti-ferromagnetic tetragonal phase. It transforms to helical structure below Neel temperature (92K). For lattice structure, a MnO6 octahedron is formed with a strong correlation between Mn atoms at center and surrounding 6 O atoms. The corresponded main axis points in (100) and crosses at (0,0,2c ). The Mn atom has covalent bonds to O atoms, and 3d states of Mn atoms have orbital hybridization with 2p states of O atoms (p-d hybridization) in the MnO6 octahedron. These effects lead to a dissociative eg state and a localized t2g state, which result in a large numbers of new electronic phenomena of transition metals. On the other hand, the difference among the exchange effects between the magnetic 3d electrons at center and ones at corners causes the helical structure below the Neel temperature. A confocal (Renishaw invia) Raman spectroscopy with two types of linear polarized incidences at room temperature and around Neel temperature shows a redshift of Eg mode and change of A1g modes. According to normal Raman analysis, a new view as"vibration mode projection"is introduced to research the interaction between the magnetic branch of polarized incidence and vibration modes in helical structure. It suggests an important influence of special magnetic structure on linear polarized incidence. The main innovations are as follows:A rutileβ-MnO2 film was grown on MgO (001) substrate using Plasma assistant molecular beam epitaxy monitored (MBE) by reflection high-energy electron diffraction (RHEED).The MBE is one of most important techniques to grow various semiconductive films at present, which performs the growth in atomic and subatomic layers controlling the compositions and thickness of films by adjusting the evaporative conditions in situ. The rutileβ-MnO2 film is grown on MgO(100) substrate by the MBE equipment manufactured by German Omicron Company. The layer by layer growth mode and little defects inside the sample are indicated the RHEED pattern in situ. The XRD pattern shows the rutile structure oriented by the MgO(001) substrate. The XPS data show a satisfied element match and valent states, which support the theoretical calculation at the neighborhood of Fermi surface.The Local density approximation (LDA) plus Dynamical mean-field theory (DMFT) was developed to compute the electron spectrum ofβ-MnO2 and pointed out the contributions of d electrons.Theβ-MnO2 behaves magnetic anisotropy below Neel temperature (TN). However, because some localized magnetic anisotropy occurs, it is necessary to determine the orientation of spins in corresponding calculations. The first-principle calculation in solid states of electrons is an important theoretical method to predict and investigate the properties for various solid materials. The theories represented by local density approximation (LDA) on electronic structure calculations cannot be used to study the correlations between electrons satisfactorily. Therefore, the LDA approach is not suitable to be adopted in the first-principle calculation. The theories represented by LDA+DMFT (dynamic mean-field theory) are developed well, which major in the first-principle calculations taking account of both the orbital hybridization and anisotropy of crystal structure. However, plane wave functions introduced by LDA+DMFT approach cannot determine the orientation of spins. The Wannier function has been introduced instead of plane wave function in LDA+DMFT to determine the orientation of spins including the factors of plane wave. Considering the accuracy and time for the calculation, a crystal model has been established in a pitch of 7c/2. The results at neighborhood of Fermi surface shows the contributions of the eg and t2g states for electronic density of states (DOS) and spectrum of energy. The computed curves satisfied the XPS data well, and the fact is explained that the XPS shoulder of main peak below the Fermi surface is not reproduced. It is expected to be proved in experiments in the future that a BIS result above the Fermi surface is predicted in theory.Some special changes with temperature were observed in Raman spectra ofβ-MnO2. A new view as vibration mode projection (VMP) was introduced to study the influence of magnetic structure on Raman spectra.The polarized Raman spectra have been taken with the confocal Raman microscope near Neel temperature with a 514.5nm linear polarized incidence. A red shift occurs in Eg mode and the intensities of A1g modes change at different temperature. In conditions keeping the point of incidence and temperature invariant, the spectra show difference with different orientation of polarized incidence. The character of Raman spectra is determined by vibration modes of electrons in unit cells. The intensities and frequencies are corresponded to the peaks of phonon spectrum. Essentially, changes of electronic polarization lead to the Raman effects. As a kind of electromagnetic wave, light transmits with coupling of electronic part and magnetic part. The magnetic part can be influenced by magnetic structure (electronic spins) of the sample. The interaction only occurs in special orientations. The reasons why the interactions between magnetic part of incidence and magnetic structure of sample are as follows: 1) electronic and magnetic influences on the materials are not separated when the incidence is circle polarized or non-polarized, even on materials with special magnetic structure; 2) the sample has not obvious magnetic anisotropy. In this work, the electronic structure changed little at low temperature. Therefore, the main reason of Raman spectra lies in the electronic anisotropy. The Raman spectra are influenced by the interactions between magnetic part of linear polarized incidence and oriented spins.