【作者】 于晓龙；

【导师】 李树玮；

【作者基本信息】 中山大学 ， 凝聚态物理， 2008， 博士

【摘要】 自从过渡金属氧化中发现大量新的物理现象和自旋电子学被高度重视以来,β-MnO2薄膜材料的生长、结构表征和拉曼光谱研究在磁性材料和类钙钛矿结构材料中是研究内涵丰富并且具备科研前景的研究方向之一。我们外延生长的β-MnO2薄膜中采用原位RHEED(反射高能电子衍射)实时监控的MBE(分子束外延)生长,配有氧plasma(等离子体)发射源和生长必需的高纯度Mn单质金属源,预生长前真空腔气压稳定在高真空10-10mbar,衬底选择单晶MgO(100)以保证良好的晶格匹配和研究必需的晶格择优取向,同时因为MgO衬底没有一阶拉曼激活的振动模式也很好的避免拉曼测试中不必要的干扰。生长过程中RHEED图样为明暗交替变化的清晰条纹,证明外延薄膜为层状生长的高质量的单晶相薄膜。XRD(X射线衍射)谱图中只有清晰的衬底峰和MnO2的β相(200)峰,证明了样品的单晶相。XPS(X射线光电子能谱)谱图给出令人满意的Mn4+、O2-离子价态和元素配比,极少量的表面吸附气体和杂质。β-MnO2属于非symmorphic空间群D 414h- P 42 /mnm,具有四方相反铁磁结构,在奈尔温度以下相变为磁螺旋结构。晶体结构方面,中心位置附近的Mn原子和周围6个O原子由强关联作用形成MnO6八面体,八面体中轴线沿(110)方向穿过c/2高度处。MnO6八面体的Mn、O原子以共价键结合,Mn的3d轨道和O的2p轨道杂化而在MnO6八面体中形成游离的eg和局域的t2g态,这就引起了过渡金属氧化物各种新的电学现象的产生。另一方面,Mn的3d自旋电子具有磁性,顶点和中心Mn原子间交换作用的差别导致了低温磁螺旋结构的形成。我们采用不同偏振方向的线偏振入射光的共焦拉曼光谱仪( Renishaw inVia)在室温和奈尔温度附近进行测试,发现了β-MnO2的Eg模式峰值的红移和A1g模式峰强的变化。不同于常规的拉曼分析,我们提出“振动模式投影”的方法来解释线偏振光的磁分支和磁螺旋结构的振动模式之间的相互作用,认为具有特殊磁性结构的样品对线偏振入射光拉曼光谱有重要的影响。本文创新点如下:■采用装配射频Plasma的分子束外延(MBE)设备在MgO(001)衬底上成功生长了β-MnO2薄膜。分子束外延生长是目前生长各种半导体薄膜的重要方法之一。生长过程中可以通过精确控制各个蒸发源的蒸发温度、蒸发时间等参数,并结合各种原位监控手段,实现对外延薄膜的厚度、成分的控制,实现亚单原子层精度的生长。我们采用德国Omicron公司制造的MBE设备在MgO衬底上生长β-MnO2薄膜,通过原位实时的反射高能电子衍射(RHEED)条纹状图样可知生长模式为层状生长,内部缺陷较少。X射线衍射(XRD)的结果表明我们的样品为金红石结构,晶向(001)来自衬底的择优取向。X射线光电子能谱( XPS)测试显示了令人满意的元素配比和各元素价态,同时为费米面附近电子能谱的理论计算提供了实验支持。■改进了密度泛函近似(LDA)+静态平均场( DMFT)理论模型,计算β-MnO2的电子能谱,指明Mn的d电子贡献。β-MnO2材料在低于奈尔温度显现各向异性磁螺旋结构,但是在室温情况也会有局域的局部磁螺旋结构形成,这就需要在相关的计算中引入晶格电子自旋方向的给定。基于第一性原理的固体电子结构计算方法,是我们对各种固态材料的物性进行预测和解释的重要理论方法。以局域密度近似(LDA)为代表的电子结构计算方法,不能很好地处理电子之间的关联效应,因此很难应用于强关联材料的第一性原理计算。而以LDA+DMFT为代表的,针对强关联材料的第一性原理计算新方法发展非常迅速,可以兼顾轨道的杂化、晶格结构的各向异性和电子的强关联相互作用。LDA+DMFT方法延用的波函数是平面波,不能提供自旋方向的确定,我们引入万涅尔(Wannier)函数为基态波函数,本质上保留了平面波的基本特点同时也考虑了自旋的定向。计算中以c轴方向上7层单胞作为主体晶体模型,兼顾计算时间和精度,就费米面附近的计算结果分析了eg和t2g电子态对电子态密度和电子能谱的贡献,并对照样品的XPS实验结果解释了费米面以下附近电子能谱主峰的肩部没有完美模拟的原因,同时理论上预测了费米面以上BIS的实验结果,期待能够进一步验证。■在奈尔温度附近研究材料的拉曼光谱,发现了β-MnO2光谱随温度而有特定趋势的变化;就其特殊的磁螺旋结构,作者提出“振动截面投影”(Vibration Mode Pojection)的概念解释了拉曼散射中,样品的磁性结构对光谱的影响。我们采用装配514.5nm的线偏振激光的共焦拉曼仪器在奈尔温度附近对样品进行拉曼光谱测量,发现随温度降低Eg模式有明显的红移同时A1g模式峰强变化显著,同时保持测量点和温度不变的情况下改变线偏振方向,光谱也有所不同。拉曼光谱的特征主要来自于原胞中带电原子的振动模式,其峰强和频率对应声子谱中各个振动峰,本质上拉曼光谱项来自电子极化率的变化。作者认为光是电磁波,由电学波部分和磁学波部分耦合在一起而传播的,磁(电子自旋)结构变化同样会影响拉曼光谱,本质上来自入射光的磁学部分和样品的磁性结构,且其相互作用有特定的方向性。对于之前很多拉曼测试中,这些特点没有体现的原因有二:一、大多采用圆偏振入射光或者非偏振入射光,材料对于电和磁的响应没有被分离开来,即使材料有特殊的磁性结构也很难被定性研究;二、材料没有明显的磁结构各向异性。而对于本文中的测试条件,样品在低温的电学结构是没有变化或者变化很小的,所以导致光谱变化的原因来自电子自旋的各向异性,是有了显著方向性的电子自旋与线偏振光中的磁学部分相互作用影响了光谱的特点。更多还原

【Abstract】 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.更多还原