摘要
自旋电子学是以自旋极化载流子的产生、注入、输运和检测作为研究对象的一门新兴的学科。作为下一代信息技术的发展方向之一,自旋电子学的产生有其深刻的历史原因。众所周知,电子具有电荷和自旋两种属性。传统的微电子学是以调控电子电荷来进行信息逻辑处理的,在此基础上的各种功能器件是现代信息技术的基础。作为电子的另外一种属性,自旋,主要在信息存储领域发挥作用。随着技术的发展,基于电荷调控的微电子器件尺度越来越小,已经达到数10nm的量级。小尺度下,微电子器件热损伤严重,稳定性变差,并且存在量子限域效应的根本性阻碍。寻找新型器件是当今电子学领域的一个重点。通过改变电子的自旋状态而实现信息逻辑处理比操纵电荷所需要的能量更少,并且速度更快。随着GMR等自旋器件的发现,以及一系列基于电子自旋的原型器件的提出,自旋电子学迅速成为近年来的一个研究热点。要实现基于电子自旋的新型器件,首先必须制备出具有室温铁磁性、高自旋极化率的材料。
磁性半导体是目前最有希望的自旋电子学材料之一。磁性半导体按照过渡族金属含量的多少,可分为稀磁半导体和浓磁半导体。所谓稀磁性半导体是指在现有的半导体母体材料中掺杂少量的过渡族金属元素,过渡族金属元素以替位掺杂的方式进入母体材料的晶格中,并通过局域的磁性离子与载流子自旋之间的耦合作用,在材料中产生宏观铁磁性及自旋极化的载流子。基于过渡族金属替位掺杂的传统半导体材料如GaAs、Si、Ge材料的稀磁半导体的居里温度普遍不高。研究最为成熟的(Ga,Mn)As磁性半导体目前报道的最高居里温度为200K,远低于微电子器件应用的温度。为了获得室温铁磁性的材料,研究人员开始转向氧化物稀磁半导体,如过渡族金属掺杂的ZnO、TiO2、SnO2、In2O3等。到目前为止,大部分研究集中在材料的铁磁性起源,而对自旋电子学材料的基本要求--高自旋极化载流子的研究还很少。另一方面,虽然很多研究小组都报道了具有室温铁磁性的氧化物磁性半导体,由于过渡族金属在半导体晶格中的溶解度不高,这使得替位掺杂量很难提高,从根本上限制了材料居里温度以及饱和磁化强度的提高,难以满足在自旋电子学器件中的应用。
为了克服以上问题,我们课题组从稀磁半导体转向浓磁半导体的研究。我们提出：在非热平衡条件下,通过交替沉积原子层厚度的半导体层和过渡族金属层的方法制备高过渡族金属含量的非晶浓磁半导体。过去几年的工作证明,我们独特的生长方式制备的浓磁半导体具有室温铁磁性,高饱和磁化强度,是有潜力的自旋电子学备选材料。但是,我们的浓磁半导体依然存在几个根本问题需要解决。首先,以前制备的浓磁半导体薄膜存在纳米尺度的不均匀,必须制备出成份均匀的样品来确定其铁磁性起源。其次,自旋电子学材料的本质要求--载流子的自旋极化一直没有给出定性或者定量的研究,对于浓磁半导体的应用这是一个必须跨越的障碍。
本论文中,作者制备了单相非晶氧化物浓磁半导体(In1-xCox)2O3-v和Zn0.32Co0.68O1-v。(In0.23Co0.73)2O3-v浓磁半导体的结构和成分表征表明样品铁磁性来源于非晶相。在输运测量中首次观察到了自旋极化载流子的直接证据--反常霍尔效应。对(In0.23Co0.77)2O3-v的研究表明,改变制备氧分压,可以调控变程跃迁过程中电子-空穴对的交换耦合作用,进而导致正-负磁电阻的变号。对于ZnO基浓磁半导体,我们得到了Zn0.32Co0.68O1-v浓磁半导体的自旋极化率高达67%,目前为止这是在氧化锌磁性半导体中最高的。通过控制界面势垒,在Zn0.32Co0.68O1-v/ZnO/Pb超导异质结中观察到了无反射隧穿现象。最后,对新型铁基超导体母体材料La0.25Pr0.75Co2P2连续自旋翻转的研究表明,等电子掺杂同样可以破坏磁性层的反铁磁耦合,这为铁基超导机理研究提供了一个重要提示。
论文主要研究内容分为三部分：
一、研究了(In1-xCox)2O3-v浓磁半导体的制备、氧分压调控的磁性和输运、电子-空穴交换作用导致的正-负磁电阻以及变程跃迁区的反常霍尔效应。
(1)在非热平衡条件下用交替溅射的方法生长了单相(In0.23Co0.73)2O3-v浓磁半导体薄膜。通过控制薄膜制备时的氧分压,其磁性和导电性质可以被精确调制。该系列的浓磁半导体都具有室温铁磁性,其饱和磁化强度,载流子浓度和电导率都随着制备氧分压的降低而增加,氧分压最小的样品具有最强的铁磁性。这为自旋注入提供了一种有效的调控手段。掠入射X射线衍射,软X射线吸收谱,高分辨TEM的测量结果表明样品的磁性来源于(In0.23Co0.73)2O3-v非晶相,氧分压的增加导致了结晶相的出现和铁磁性的减弱。
(2)通过控制氧分压,在相同成分的(In0.23Co0.77)2O3-v样品中实现了正-负磁电阻的转变。两种样品电输运都处于Efros变程跃迁区。通过自旋依赖的变程跃迁理论定量拟合,表明磁电阻符号的取决于变程跃迁过程中电子-空穴对的交换相互作用是铁磁耦合还是反铁磁耦合。这为电子-空穴对交换相互作用的大小和符号测量给出了一种全新的电学方法。
(3)在绝缘体导电机制的样品中,我们首次在实验上观察到了变程跃迁区反常霍尔信号随着温度变号的现象。通过改变制备氧分压,浓磁半导体(In0.23Co0.77)2O3-v的导电性质从金属导电到绝缘体导电连续变化,绝缘性的样品在低温下为变程跃迁导电性。反常霍尔效应的实验结果和理论分析表明变程跃迁区的反常霍尔系数与纵向电阻的标度关系,不能用现有的理论描述。我们提出,在不同温度下,发生变程跃迁的局域态电子的局域能级不同,其自旋轨道耦合强度也不同,因此不能作为常量处理。
二、基于平面型半导体/超导体异质结,研究了Zn0.32Co0.68O1-v浓磁半导体的自旋极化率,以及Zn0.32Co0.68O1-v/ZnO/Pb异质结中的无反射隧穿。
(1)对超导异质结的Andreev Reflection谱测量表明,浓磁半导体Zn0.32Co0.68O1-v的自旋极化率高达67%,这是目前报道的氧化锌磁性半导体中最高的。我们制备了平面结构的Zn0.32Co0.68O1-v/Pb超导异质结,并对微分电导谱进行了自旋极化的BTK理论拟合。实验结果表明非晶化以及高含量过渡族金属这两种方式可以有效提高磁性半导体的自旋极化率。另一方面,界面上大量磁性杂质和无序氧化物的存在对自旋极化电子产生了很强的非弹性散射,降低了自旋极化率的测量值。
(2)通过在Zn0.32Co0.68O1-v/Pb超导异质结界面上引入ZnO势垒层,我们在隧穿背景下观察到了零偏压电导的提高,这是第一次在磁性半导体／超导体系统中观察到无反射隧穿现象。对零偏压电导的温度和磁场依赖性的研究表明,磁性半导体中的电子相位相干长度和变程跃迁长度在数量级上是一致的。在超导带隙外,实验还观察到了微弱的带隙外电导峰,这起源于Zn0.32Co0.68O1-v磁性半导体中费米能级以下的局域态能级。
三、研究了铁基超导体的母体材料La0.25Pr0.75Co2P2的两次自旋翻转,并由磁性和磁电阻结果一致证实两次自旋翻转起源于反铁磁耦合的Pr磁层。
对单晶La0.25Pr0.75Co2P2样品的磁性和磁电阻测量一致表明：La替换Pr在c轴方向上诱导了两次连续的自旋翻转转变。磁化率-温度测量显示样品存在两个反铁磁转变,TN1=240K, TN2=11K,分别起源于Co的磁矩和Pr的磁矩。在外磁场作用下,磁电阻的两步急剧下降为自旋翻转提供了进一步的证据,表明这两次自旋翻转都起源于反铁磁耦合的Pr磁层。基于实验结果,我们提出了一个具有不同磁环境的磁结构模型来解释连续的自旋翻转转变。研究结果表明,等电子掺杂也可以破坏磁性层间反铁磁耦合,这为铁基超导新材料寻找和机理研究提供了一种有益提示。
Spintronics is a new scientific field that focuses on generation, injection, transport and detection of spin polarized carrier. As known to all, electron has two intrinsic properties, charge and spin. The traditional microelectronics, which mainly studies and utilizes the transport properties of electron as a charge carrier, has become the cornerstone of modern information technology, while the spins, occupy a central place in information storage. As development of microelectronics technology, the dimension of microelectronic device becomes smaller and smaller and had reached several tens nanometers limit. In the nanoscal limit, the microelectronics had met problems such as heat damage and thermal stability in devices. Furthermore, the quantum confinement effect is the insuperable obstacle that will result in electronic band theory failure finally. One of most important challenge to microelectronics is look for novel devices to overcome this obstacle. It will be less energy consumption and faster speed if we can control spin to do logic operation than that of charge. As discover of GMR device and many proposal of prototype spin-base devices, Spintronics has becomes one of hottest research field recently. To realize spin-base device, the foremost mission is to fabricate a novel material which has room temper ferromagnetic and high spin polarized carrier.
Magnetic semiconductor is one of most promising material for spintronics application. According to concentration of transition metal in semiconductor, there are two types of magnetic semiconductors:diluted magnetic semiconductor and concentrated magnetic semiconductor. The preparation method of a typical diluted ferromagnetic semiconductor was to dope transition element into the current used semiconductor system. It is expected that the transition elements may enter the crystal lattice by substituting some cation's positions, and through ferromagnetic coupling between localized transition metal ions and itinerant carriers, ferromagnetism and spin-polarized carrier may appear in semiconductors. The Curie temperature is low for transition metal doped tranditional semiconductor as GaAs, Si, Ge et al. Mn doped GaAs prepared by low temperature molecular beam epitaxy method is one of the most matured diluted ferromagnetic semiconductor syste such ms. The Curie temperature of (Ga,Mn)As is lower than200K, which does not meet the requirements of room temperature application. In order to obtain material that has room temperature ferromagnetism, researchers turn their focus into oxide semiconductor such as ZnO、 TiO2、SnO2、In2O3and so on. On the one hand, most researcher pay their attention to origin of ferromagnetic of magnetic semiconductor while there are very few report s on the most important property of spintronics materials, spin polarization. On the other hand, the low solubility of transition metal elements in semiconductor lattice has limited the further increases of the institutional doping level that results in low Curie temperature and saturated magnetic moment of diluted magnetic semiconductor.
In order to overcome mentioned problem, our group turn research into concentrated magnetic semiconductor. We proposal that grown of amorphous concentrated magnetic semiconductors by using the creative thin film growth method that alternatively deposit an atomic-thin layer of semiconductor and transition metal elements under thermal nonequilibrium condition. In our previous work, we had fabricated concentrated magnetic semiconductor with room temperature ferromagnetic and high saturated magnetic moment. However, there are still several basic problems in this material. The first one is that we need to fabricate uniform thin film to offer evidence for the ferromagnetic origin in this material. The second one is we hadn't yet observed any spin polarized carrier signal indirectly or directly in our previous research. These two problems are necessary mission to magnetic semiconductor devices application.
In this dissertation, the author had grown amorphous oxide concentrated magnetic semiconductor (In0.27Co0.73)2O3-v and Zn0.32Co0.68O1-v.We proved that ferromagnetic origin from amorphous phase of thin film. In transport measurements, we found the spin polarized carrier signal-anomalous Hall effect (AHE). More important, we reported spin polarization of67%in Zn0.32Co0.68O1-v, which is the highest spin polarization in oxide magnetic semiconductor as far as we know. In the final part of this dissertation, we found two successive spin-flip transitions in La0.25Pr0.75Co2P2。This result indicates that equal electron doping disturb antiferromagnetic coupling as well as electron and hole doping in iron-base superconductor.
There are three parts for this dissertation.
For the first part, we investigated fabrication, oxygen partial pressure tuned magnetism,transport and anomalous Hall Effect in concontrated magnetic semiconductor (Ino.27Coo.73)203-v.
(1) We prepared concentrated magnetic semiconductor (Ino.27Coo.73)203-v by alternating sputtering under nonequilibrium thermal substrate, which has high Cobalt concentration. The magnetism can be control through tuning of oxygen partial pressure in sputtering process. Room temperature ferromagnetic was observed in this series magnetic semiconductor thin film. The saturated moment, carrier density and conductivity increases as oxygen partial pressure in sputtering process decreasing. The graze incident X-ray diffraction results, soft X-ray absorption spectra and HRTEM results show that ferromagnetism originated from amorphous thin film while crystallization and weaker ferromagnetic occur as oxygen partial pressure increasing in sputtering process.
(2) The concentrated magnetic semiconductor (Ino.23Coo.77)203-v with higher Co concentration were grown under different oxgen pressure. All samples belong to variable range hopping regime(VRH) at low temperature. However, those samples show positive or negative magnetoresistance. The spin dependent VRH fitting indicates that the sign of magnetoresistance depends on exchange interaction of electron-hole during VRH process. We proposed a new method to measure exchange interaction item through transport measurement.
(3) The conducting properties of can be tuned by oxygen partial pressure in sputtering process. The transport regime transit from metallic to insulating as oxygen partial pressure increasing. The transport regime is Variable Range Hopping (VRH) for insulating samples at low temperature range. We reported firstly the experimental observation of sign change of anomalous Hall resistivity in variable range hopping. We also observed that the scaling relationship between the anomalous Hall resistivity and the longitudinal resistivity disagrees with the existed theoretical prediction. According to our experimental results, we proposed that the spin-orbital coupling of electrons in the VRH region changes as temperature varies, in which has been considered as a constant in previous theory. As a result, our experiment is of significant importance for a better understanding of AHE in the VRH regime.
The second part is about spin polarization of concentrated magnetic semiconductor Zn0.32Co0.68O1-v and reflectionless tunneling in disordered magnetic semiconductor/superconductor hybrid junction.
(1) We fabricated Zn0.32Co0.68O1-v/Pb superconducting hybrid junction. From Andreev Reflection spectroscopy, we got the spin polarization of Zn0.32Co0.68O1-v is as high as67%, which is the highest spin polarization in oxide magnetic semiconductor as far as we know. This indicates that amorphous phase and high Cobalt concentration can enhanced spin polarization effectively. On the other hand, spin polarization is reduced due to interface contamination of interface such as reduction of superconducting gap and inelastic scattering.
(2) We introduced ZnO barrier into the interface of Zn0.32Co0.68O1-v/Pb superconducting hybrid junction. The enhanced zero bias conductance peak (ZBCP) was observed at the background of tunneling. As temperature decreasing, the finite bias conductance peak (FBCP) occurs in differential conductance spectroscopy. The ZBCP and FBCP can be understood in the regime of reflectionless tunneling. Furthermore, we observed above-gap conductance peak which originates from localized states below Fermi level of impurity state in disordered Zn0.32Co0.68O1-v magnetic semiconductor.
At the third part, we studied magnetic and magnetore si stance properties of one possible parent material of iron-based superconductor, La0.25Pr0.75Co2P2. For magnetic field H//c, two antiferromagnetic (AFM) transitions were observed at Tn1=240K, Tn2=11K that originated from Co and Pr magnetic moments, respectively. Compared to PrCo2P2, magnetization, electrical transport measurements show two spin-flip transitions in La0.25Pr0.75Co2P2below the AFM transition temperature Tn2.These two spin-flip transitions originate from antiferromagnetically coupled Pr magnetic moments along the c axis. Furthermore, one small spin flop transition originated from antiferromagnetically coupled Co moments was observed in La0.25Pr0.75Co2P2above TN2. The two spin-flip transitions were explained in different magnetic environments based on magnetic structure of La0.25Pr0.75Co2P2. Our results indicate that equi-valence electron doping also disturbs the antiferromagnetical coupling of magnetic layers as well as electron or hole doping in iron-based superconductor. This is may shed light on mechanism of iron-based superconductor research.
引文
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