纳米尺寸自旋阀中电流驱动的自旋波发射

提出了在纳米赝自旋阀中的电流感应自旋传输矩的磁动力学描述,成功地解释了在磁纳米多层结构中的电流驱动微波发射和电流感应磁化翻转现象。自旋流极化由在电导匹配时的自旋流和化学势连续性边界条件决定。自旋矩的纵向和横向分量在自旋阀的电流驱动微波发射和电流感应磁化翻转现象中扮演了不同的角色:纵向自旋矩分量决定了电流感应磁化翻转(CIMS)效应,而横向自旋矩是自旋波发射(SWE)效应所不可缺少的。根据这一理论,由LLG方程自然得到自旋波发射的双模,分别为横向自旋矩引发的X和Y方向的振动,并引起磁多层电阻以频率2w或w(进动频率)随时间变化。磁场和自旋流共同决定了自旋波发射的频率和功率,这一理论预言了某种特殊的磁多层结构,如磁层相互垂直的结构,将具有大得多的微波发射效率,这一结论已经被实验所证实。     

纳米磁多层结构中电流驱动自旋波发射

回顾了纳米磁多层结构中电流感应自旋矩传输和电流驱动磁化矢量进动引起的自旋波发射等新量子效应的研究现状及发展。基于该效应的纳米磁多层微波振荡器件具有结构简单、无须外加磁场、容易集成等特点,在现代通信领域具有广阔的应用前景,备受国内外研究者的关注。介绍了自旋波发射效应的理论处理方法和实验研究进展,讨论了自旋波发射器件的工作原理和铁磁膜的磁化方向、外磁场方向、大小及驱动电流对器件性能的影响。目前研制的器件的效率较低、振荡功率小,采用新的垂直磁化结构有助于解决上述问题。     

纳米磁多层结构中电流驱动自旋波发射

回顾了纳米磁多层结构中电流感应自旋矩传输和电流驱动磁化矢量进动引起的自旋波发射等新量子效应的研究现状及发展。基于该效应的纳米磁多层微波振荡器件具有结构简单、无须外加磁场、容易集成等特点,在现代通信领域具有广阔的应用前景,备受国内外研究者的关注。介绍了自旋波发射效应的理论处理方法和实验研究进展,讨论了自旋波发射器件的工作原理和铁磁膜的磁化方向、外磁场方向、大小及驱动电流对器件性能的影响。目前研制的器件的效率较低、振荡功率小,采用新的垂直磁化结构有助于解决上述问题。     

对称磁多层结构纳米自旋传输矩微波振荡器

提出一种新型的电流驱动对称磁多层结构纳米自旋传输矩微波振荡器结构。由两个铁磁膜组成,具有相同厚度,中间被一薄的非磁层隔开。纳米磁多层柱的上、下都有金属层作为电极。一个恒定不变的直流电流垂直通过该磁多层结构时,产生自旋极化和自旋传输矩,并施加自旋矩于每一磁层。当电流超过临界值时,引起两个磁膜的磁化矢量交替翻转方向,并导致磁多层电阻的周期性变化,从而产生稳定的微波振荡。振荡频率与电流呈线性关系,变化范围1～100GHz。微波功率也可以用电流调节,范围在1μW～1mW。     

自旋场效应晶体管的电学特性研究(英文)

用非平衡格林函数方法研究一种自旋场效应晶体管的电子输运特性。结果表明,不考虑自旋散射的作用,当漏极电压比较小时该器件能达到很高的磁阻比率。对该器件在考虑自旋散射和不考虑自旋散射下的输出电流进行对比,发现在铁磁平行(反平行)的条件下,考虑自旋散射时的输出电流要比不考虑自旋散射时的输出电流小(大)。研究结果揭示了该器件的物理机制,为该器件的优化设计提供了理论指导。     

NiFe/Ag/NiFe纳米结自旋晶体管研究

利用直流磁控溅射工艺和掩膜技术研制出新型NiFe/Ag/NiFe全金属自旋晶体管试样。该薄膜磁阻系数ΔR/R≥9%,全金属自旋晶体管试样集电极电流变化MC>587%(常温),且其性能随基极Ag层厚度减小而增强。全金属自旋晶体管具有电流放大、存贮及逻辑电路等功能,,有望替代现有的晶体管而应用于半导体大规模集成电路。     

自旋阀中的极化输运与相关自旋新材料及结构研究

电子既是电荷的载体又是自旋的载体。电子作为电荷的载体,使二十世纪成为了微电子学的天下。而随着1988年巨磁电阻(GMR)效应发现以来,通过操纵电子的另一量子属性——自旋,使新一代的电子器件又多了一维控制手段。电子自旋的研究涵盖了金属磁性多层膜、磁性氧化物、磁性半导体等众多体系,探寻这些体系中自旋输运的基本原理是研究的重点。目前,基于传统自旋阀中极化输运及自旋电子学的发展,对新材料和新结构的研究尚不成熟,还有众多科学问题亟待解决,诸如:如何在室温下获得更大的巨磁电阻变化率、提高器件的稳定性及灵敏度、自旋阀中交换偏置场产生的物理根源、实现自旋同半导体完美结合的材料、结构及方法等。因此,基于国内外自旋电子学研究的重点,首先围绕最基本的自旋阀纳米多层膜结构,开展了自旋阀多层膜制备、设计、结构优化、自旋阀交换偏置核心结构物理机制探索等研究;其次,提出了三种异质结新结构,并以大自旋极化率Fe3O4磁性半金属为核心材料,开展了自旋阀、新异质结研究;最后,在理论与材料研究的基础上,对自旋器件进行了设计与实验研究,获得了一些有益的结果:(1)理论方面,基于自旋电子器件进一步发展对新结构、新材料发展的需求,提出了磁性半导体/半导体、磁性半导体/磁性半导体、自旋滤波材料/自旋滤波材料的新自旋异质结模型。理论分析发现,利用磁性半导体/半导体异质结,在负偏压的作用下可实现自旋电子的极化输运,而利用磁性半导体/磁性半导体、自旋滤波材料/自旋滤波材料异质结可实现趋于100%的磁电阻变化率。另外通过计算,对可实现的磁阻效应及对材料的要求进行了详细研究,为新材料的应用奠定了一定的理论基础。(2)虽然基于自旋阀核心结构的自旋电子器件研究已开展了多年,但如何进一步提高自旋电子器件的磁电阻效应、灵敏度、工作范围、工作稳定性和解决这些问题的物理机制,仍是自旋电子学中的一个热点。因而,首先基于Mott二流体模型发现自旋阀巨磁电阻受磁性材料、非磁性材料、自旋极化率、自旋扩散长度、厚度、尺寸、电阻率等影响明显,因而可通过改善制备工艺条件及各层的材料、厚度改善自旋阀的性能,探寻提高巨磁电阻变化率、灵敏度等的有效途径。其次,以理论分析为指导,实验上首先制备Ta/NiFe/Cu/NiFe/FeMn传统自旋阀多层膜,研究了自由层、隔离层、钉扎层、反铁磁层厚度对巨磁电阻效应的影响,找到了最佳的制备工艺;其次,研究了缓冲层材料对自旋阀灵敏度、巨磁电阻效应的影响。发现由于缓冲层元素表面自由能的影响导致了自旋阀灵敏度的改变,指出选择适当表面自由能的缓冲层,可有效改善自由层薄膜的性能,为提高器件的灵敏度提供了有效的途径;最后,基于室温磁场下制备自旋阀交换偏置场较小、工作范围较窄的问题,通过对传统结构的改进,提出了新型双交换偏置场自旋阀模型,为增大器件工作稳定性、人为调制器件工作范围,提供了有效手段。(3)交换偏置在自旋电子器件中具有核心地位,但到目前为止,其产生的物理根源、影响其大小的因素仍是未解决的难题。因而,基于自旋阀的核心结构——铁磁/反铁磁交换偏置效应,研究了NiFe/FeMn双层膜钉扎层、被钉扎层厚度、材料微结构、底钉扎、顶钉扎结构等对交换偏置的影响,分析了交换偏置产生的物理根源;研究了制备磁场大小对钉扎场大小的影响,发现了利用大磁场可实现提高交换偏置的新方法,并利用52kA/m(650Oe)的大磁场在1~2nm的NiFe钉扎层中实现了接近48kA/m(600Oe)的交换偏置场。(4)基于自旋阀测试,研究了初始测试磁场平行与反平行于交换偏置场方向,测试电流的大小对交换偏置场的影响。并用大脉冲电流,在初始测试磁场反平行于交换偏置场方向的样品中,首次实现电流矩在电流沿膜面流动自旋阀结构中对钉扎场的翻转,为铁磁/反铁磁双层膜体系产生交换偏置的机理提供了新的研究途径,并对自旋阀的应用提出了新的挑战。(5)为探寻高自旋极化率的新材料,开展了半金属磁性材料Fe3O4薄膜制备工艺的研究。通过改变溅射功率、退火温度、缓冲层、磁场沉积等,在200W溅射功率、300℃的退火温度、24kA/m(300Oe)沉积磁场的最佳条件下获得了高晶粒织构、成分单一的Fe3O4薄膜,并通过对氧气氛的调节,实现了无缓冲层高性能Fe3O4薄膜的制备。(6)利用所制备的Fe3O4薄膜,进行了基于Fe3O4自旋阀的制备,发现Fe3O4薄膜同其它金属材料间电阻率的失配,是造成巨磁电阻效应低的原因;另外,基于理论提出的磁性材料/半导体异质结,制备了Fe3O4/n-Si纳米结,初步实现了磁性材料到半导体的自旋注入与输运。     

线性自旋阀磁头的材料性能及结构参数优化

优化材料性能参数和自旋阀巨磁阻磁头的结构参数是微磁器件实用化的关键.本文基于建立的物理计算模型,首先利用传输线方法和微磁学理论对铁磁自由层中信号场和磁化强度分布进行了计算,优化了材料性能参数;其次讨论了磁头结构尺寸和磁参数对信号场和饱和磁感应强度的影响;最后将自旋阀巨磁阻磁头传感单元作为有源磁电阻放入前端放大电路,对输出电压。二次谐波及线性区间进行模拟,发现在缝宽g1 g2≤0.7,线性偏电流10~50 mA范围内,自旋阀磁头相对记录介质有线性信号输出。     

自旋电子学研究的最新进展

自旋极化电子的高效注入、自旋霍尔效应和自旋流的产生与探测都是目前自旋电子学中热门研究专题,世界一些著名学术刊物屡见报道。对这些重要内容的理论和实验的最新研究成果进行了介绍。通过自旋极化电子高效注入方法和材料的研究,人们期望研制出新一代自旋电子器件,进而实现应用电子自旋传输、记录和存储信息的目标。近期实验给出,自旋极化电子从铁磁金属注入半导体和金属都获得较高的极化率。各种注入方法中,自旋流直接注入法目前备受关注,因为自旋霍尔效应为自旋流的产生与探测提供了新的途径,即自旋霍尔效应可以产生自旋流,但因无霍尔电压故不容易测量;而逆自旋霍尔效应又将自旋流转化为电流,使得难以测量的自旋流又可以直接用电学方法测量。     

纳米尺度自旋转移矩器件的制备工艺

提出一种改进的自旋转移矩器件的制备工艺:在电子束曝光形成纳米级图形之后,依次采用离子束刻蚀、带胶绝缘层淀积再正胶剥离的图形转移方法,成功制备了纳米柱状赝自旋阀结构磁性多层膜CoFe/Cu/CoFe/Ta,器件的横向尺寸为140nm×70nm。对该结构进行了电磁学性质的测试:在变化范围为-500～+500Oe(1A/m=4π×10-3 Oe)的外加磁场下,观测到巨磁阻效应;在零外加磁场下,施加垂直于膜平面的电流时,观测到电流诱导的磁化翻转效应,其临界电流密度为108 A/cm2量级。该方法具有工艺步骤少、易于实现的特点,在自旋转移矩器件等纳米级器件的制备中具有广泛的应用前景。     

Magnetoresistive Sensoren

Depending on the angle between magnetization and current density in a thin permalloy film, the anisotropic magnetoresistive effect is utilized for high performance sensors. Both the parameters of the sputtering process and the sensor layout have to be optimized. Aiming a magnetic field resolution of some nT in a frequencey range from 0–1 kHz, the results of Wheatston-bridge circuits are discussed.     

Giant magnetoresistance materials for read heads

The sensitivity of magnetoresistive read heads can be increased by using layered magnetic materials showing the giant magnetoresistance effect, instead of a single magnetic film showing the anisotropic magnetoresistance effect. For this purpose, exchange-biased spin-valve layered structures are very suitable. For well-chosen compositions and nanometer-scale layer thicknesses these materials combine a fair giant magnetoresistance effect with a very small field interval in which the resistance change takes place. In this paper we give an overview of aspects which determine the functioning of materials of this class in read heads, including their preparation, magnetotransport properties and the magnetic interactions which determine the magnetization reversal process.     

Magnetic Tunnel Junctions for Spintronic Memories and Beyond

In this paper, recent developments in magnetic tunnel junctions (MTJs) are reported with their potential impacts on integrated circuits. MTJs consist of two metal ferromagnets separated by a thin insulator and exhibit two resistances, low (R_p) or high (R_ap) depending on the relative direction of ferromagnet magnetizations, parallel (P) or antiparallel (AP), respectively. Tunnel magnetoresistance (TMR) ratios, defined as (R_ap $R_p)/R_p as high as 361%, have been obtained in MTJs with Co₄₀Fe₄₀B₂₀ fixed and free layers made by sputtering with an industry-standard exchange-bias structure and post deposition annealing at Ta = 400 degC. The corresponding output voltage swing DeltaV is over 500 mV, which is five times greater than that of the conventional amorphous Al-O-barrier MTJs. The highest TMR ratio obtained so far is 500% in a pseudospin-valve MTJ annealed at Ta = 475 degC, showing a high potential of the current material system. In addition to this high-output voltage swing, current-induced magnetization switching (CIMS) takes place at the critical current densities (J_CO) on the order of 10⁶ A/cm² in these MgO-barrier MTJs. Furthermore, high antiferromagnetic coupling between the two CoFeB layers in a synthetic ferrimagnetic free layer has been shown to result in a high thermal-stability factor with a reduced J_CO compared to single free-layer MTJs. The high TMR ratio enabled by the MgO-barrier MTJs, together with the demonstration of CIMS at a low J_CO, allows development of not only scalable magnetoresistive random-access memory with feature sizes below 90 nm but also new memory-in-logic CMOS circuits that can overcome a number of bottlenecks in the current integrated-circuit architecture     

Structure and soft magnetic properties of Fe-Co-Ni-based multicomponent thin films

Soft magnetic thin films with suitable uniaxial anisotropy and high saturation magnetization are required for high frequency applications, since the ferromagnetic resonance frequency (f_FMR) is proportional to the multiplication of the saturation magnetization and anisotropy magnetic field. In this study, multicomponents of Fe-Co-Ni-based soft magnetic thin films were deposited on the Si substrate by radio frequency (RF) magnetron sputtering with various Ar/N₂ ratios at room temperature. The composition, crystal structure, surface morphology, and magnetic domain were analyzed. Without nitrogen doping, the domain of the magnetic thin film was arranged randomly. The effect of N₂ content in the thin film on the magnetic properties was evaluated and further discussed. Magnetic properties, including saturation magnetization (M_s) and coercivity (H_c), were determined. The saturation magnetization of the undoped magnetic thin film was around 1.3 T. However, when the nitrogen was added, the magnitude of the anisotropy field could reach 30 Oe, while the saturation magnetization was around 1 T. It is expected that the derived magnetic thin film is a promising candidate for potential usage in high frequency inductors.     

Possibility of exchange switching of the ferromagnetic-antiferromagnetic junctions

The current flow is investigated in the structures consisting of a ferromagnetic metal (FM) an antiferromagnetic (AFM) conductor and a nonmagnetic metal that closes the electric circuit. In such a structure, the FM layer is characterized by a relatively strong anisotropy and fixed lattice spins and serves as the injector of spins into the AFM layer. In the presence of a relatively strong external magnetic field, the resulting magnetization is induced in the AFM layer and the magnitude of the magnetization can be controlled. It is demonstrated that the polarized current from the FM layer generates a torque and allows the exchange switching of the magnetization vector. The distinctive feature of the effect lies in the fact that the switching threshold with respect to current can be decreased by several orders of magnitude due to the dependence of the magnetization on the external field     

An automated stand for express-diagnostics of magnetoresistive structures

The design of an automated measuring stand for express-diagnostics of magnetoresistive structures are presented. This computer-controlled stand allows measurements of the magnetoresistances of film structures with high accuracy in a low alternating magnetic field. The stand has been tested on single- and multiple-layer magnetoresistive structures.     

Fundamental characteristics of a magnetoresistive sensor using Y1Ba2Cu3O7-x ceramicsuperconducting films

The fundamental characteristics of a magnetic sensor fabricated from a Y₁Ba₂Cu₃O_7-x ceramic superconducting film are investigated. The operation principle of the sensor is based on the magnetoresistive properties of the material. The Y₁Ba₂Cu₃O_7-x ceramic film was prepared by the spray pyrolysis method. An element of the magnetic sensor consists of the film patterned to a meander shape. the sensitivity is discussed in connection with the noise measurement of the element. A magnetic field resolution of 2×10^-6 G/(Hz) ^1/2 at 100 Hz was obtained in the 0.1-100 G range. The sensitivity is much higher than that of a conventional semiconductor magnetic sensor. With an AC modulation provided by a magnetic field, phase detection of the magnetic field can be performed using a lock-in amplifier. An improvement in the low-frequency (<1 Hz) magnetic field resolution of about one order of magnitude was obtained using this method     

Switching the direction of circumferential magnetization of square epitaxial Fe (001) microstructures by spin-polarized current

The change in the direction of the circumferential lattice magnetization by using the spin-polarized current is found by magnetic force microscopy to be possible for square epitaxial Fe (001) microstructures in the basic state. Such a change of the magnetization occurs when applying the magnetic saturation field to the structure with the subsequent switching off of the field accompanied by passing the current with the density above a certain threshold on the order 10¹² A/m² through the structure. In the basic state of the structure, the change of the magnetization depends not only on the current density but also on the spin polarization and the region of the current flow. The effect of the spin-polarized current on the circumferential magnetization of the structure, which is related to the injection of spins by the current, is discussed.