本文選題：稀磁半導體 + 鐵磁序　； 參考：《中國科學院大學(中國科學院物理研究所)》2017年博士論文

【摘要】：稀磁半導體材料(DMS)是可以同時調(diào)控電荷自由度和自旋自由度的功能性材料,在材料科學中展現(xiàn)出了豐富卓越的物理性能,引起了人們廣泛的研究興趣。在經(jīng)典的Mn摻雜的III-V族稀磁半導體中,Mn對三價的Ga的替代會引入一個受主,同時也是磁矩的來源。Mn的這種雙重作用復雜了相應的理論解釋。最近發(fā)現(xiàn)的Mn摻雜的I-II-V半導體Li_(1+x)(Zn_(1-y)Mn_y)As和Mn摻雜的II-II-V semiconductor(Ba_(1-x)K_x)(Zn_(1-y)Mn_y)_2As_2是新型的稀磁半導體,在這些鐵磁體中空穴和自旋的注入是可以分別調(diào)控的。在稀磁半導體中,人們已經(jīng)在交換作用的控制中取得了一定的進展,然而對其在物理特性的微觀起源中的理解仍然有限。具有元素和軌道殼層分辨的同步輻射譜學技術(發(fā)射、吸收和二色性)有著非常高的能量分辨率,非常適合探測特定電子態(tài)的結構,而高能X射線衍射實驗能分辨出晶格的微小變化。將(Ba_(1-x)K_x)(Zn1-y Mny)_2As_2選為目標材料,研究了空穴摻雜下,導致其高溫鐵磁有序Tc的間接交換作用機制。除了X射線譜學技術,壓力也做為另一個熱力學維度引入,用來探究這些新型稀磁半導體中晶格、磁性和電子自由度之間的復雜相互關系。As的K邊磁圓二色譜(XMCD)信號強度隨著樣品磁化強化的變化而變化,同時強烈依賴于樣品Tc的大小。因此,As的K邊磁圓二色信號可以用來代表塊材樣品的磁化強度。結合X射線發(fā)射譜(XES)、吸收譜(XAS)和磁圓二色譜的測量,實驗結果表明空穴摻雜以減小Mn(3d)局域自旋電子態(tài)密度為代價,增強p-d軌道雜化作用強度(MnAs4四面體中電子的重排),從而增強Mn離子間的交換作用強度�？昭▋�(yōu)摻雜和空穴欠摻雜的樣品在壓力作用下,都表現(xiàn)出帶隙的減小,調(diào)控長程鐵磁序的空穴媒介載流子變的弱局域,從而抑制樣品的磁有序。這兩個作用效果都是由于壓力作用下As的4p態(tài)的強烈反應(能帶展寬)所引起的。實際上,陰離子p態(tài)和陽離子d態(tài)的電子結構隨壓力的演變是和MnAs4四面體中幾何構型的改變和層間As-As距離緊密相連的。這對理解結構和磁性之間的關系提供了重要的信息。使用金剛石壓砧,結合同步輻射X射線衍射實驗,研究了Li_(1+x)(Zn_(1-y)Mn_y)As壓力作用下的結構演變。在11.6GPa左右,探測到了壓力誘導的結構轉變,高壓下的新相被提出具有Pmca的空間群。結構轉變誘導的元胞體積坍縮大概在8%。相變前的結構隨壓力的演變關系對將來的Li(Zn,Mn)As中的鐵磁性研究具有重要的指導意義。通過金剛石壓砧結合具有非常高的能量分辨率的同步輻射穆斯堡爾譜(SMS),研究了1111型鐵基超導體SmFeAs O在良好靜水壓條件下的磁(Fe)相變。隨著壓力的增加,磁轉變溫度降低,最終在25GPa左右反鐵磁序消失。結合給出超導轉變溫度的電輸運實驗結果,得到了SmFeAsO的P-T相圖。這為理解高溫超導體中超導性與反鐵磁性之間的相互作用關系補充了重要的信息。
[Abstract]:Dilute magnetic semiconductor material (DMS) is a kind of functional material which can control the degree of freedom of charge and spin at the same time. It has shown a great deal of excellent physical properties in material science and aroused extensive research interest. The substitution of mn for trivalent Ga in the classical Mn-doped III-V family of diluted magnetic semiconductors will introduce a acceptor, which is also the source of magnetic moment. Recently, Mn-doped I-II-V semiconductor Li_(1 x)(Zn_(1-y)Mn_y)As and Mn-doped II-II-V semiconductor(Ba_(1-x)K_x)(Zn_(1-y)Mn_y)_2As_2 are new dilute magnetic semiconductors. The hole and spin injection in these ferromagnets can be controlled separately. In dilute magnetic semiconductors, some progress has been made in the control of exchange interactions. However, their understanding of the microcosmic origin of physical properties is still limited. Synchrotron radiation spectroscopy (emission, absorption and dichroism), which has elemental and orbital shell resolution, has a very high energy resolution and is well suited for detecting the structure of particular electronic states, The high energy X-ray diffraction can distinguish the small changes of lattice. The mechanism of indirect exchange interaction of Mny)_2As_2 with hole doping leading to ferromagnetic ordering Tc at high temperature is studied. In addition to X-ray spectroscopy, pressure was introduced as another thermodynamic dimension to explore the lattice in these new dilute magnetic semiconductors. The complex relationship between magnetic and electronic degrees of freedom. The intensity of the K-edge magnetic circular dichroism (XMCD) signal of as varies with the magnetization of the sample and strongly depends on the size of the sample Tc. Therefore, the K edge magnetic circular dichroism signal of as can be used to represent the magnetization of bulk samples. Combined with X-ray emission spectra, absorption spectra and magnetic circular dichroism, the experimental results show that hole doping is at the cost of decreasing the density of local spin electron states. The electron rearrangement in MnAs4 tetrahedron is enhanced to enhance the exchange intensity between mn ions. Under the pressure, both the sample with excellent hole doping and the sample with underdoped hole show the decrease of the band gap and the weak localization of the carrier of the hole which regulates the long range ferromagnetic order, thus restraining the magnetic order of the sample. Both of these effects are caused by the strong reaction (band broadening) of the 4p state of as under pressure. In fact, the evolution of the electronic structure of the anionic p-state and cationic d-state with the pressure is closely related to the change of the geometric configuration in the MnAs4 tetrahedron and the As-As distance between the layers. This provides important information for understanding the relationship between structure and magnetism. By using diamond anvil and synchrotron radiation X-ray diffraction experiment, the structure evolution under Li_(1 x)(Zn_(1-y)Mn_y)As pressure was studied. The pressure induced structural transition was detected around 11.6GPa, and the new phase at high pressure was proposed as a space group with Pmca. The cell volume collapse induced by structural transformation is about 8%. The relationship between the structure and the pressure before the phase transition is of great significance for the study of ferromagnetism in the future. By using diamond anvil and synchrotron radiation Mossbauer spectroscopy with very high energy resolution, the magnetic phase transition of 1111 iron based superconductor SmFeAs O under good hydrostatic pressure was studied. With the increase of pressure, the magnetic transition temperature decreases, and finally the antiferromagnetic order disappears around 25GPa. The P-T phase diagram of SmFeAsO is obtained by combining the experimental results of electrical transport at superconducting transition temperature. This provides important information for understanding the interaction between superconductivity and antiferromagnetism in high temperature superconductors.
【學位授予單位】：中國科學院大學(中國科學院物理研究所)
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TN304

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