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Growth, Magnetic and Spin Polarized Vacuum Tunneling Properties of Ultrathin Ferromagnetic Films.

机译：超薄铁磁薄膜的生长，磁极化和自旋极化真空隧穿特性。

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摘要

Vacuum tunneling in field emission from sharp W tips coated with ultrathin ferromagnetic films attracts interest because it could potentially generate bright, coherent spin polarized electron beams. Investigations of spin polarized field emission (SPFE) also may contribute to a better understanding of the spin polarized tunneling process in spin polarized scanning tunneling microscopy. SPFE has been achieved from ultrathin film Fe- and Co-coated W(001) and W(111) tips in their spontaneously magnetized state at 300K. In each case, a transverse spin polarization component is detected. A modest magnitude of spin polarization up to 35% was obtained. For (001) tips, the azimuthal orientation of polarization showed a strong preference for alignment with the transverse low-index crystallographic directions, i.e. the ⟨110⟩ for Co and ⟨100⟩ for Fe. In contrast, the polarization direction for W(111) tips exhibited only a weak preferential alignment with the set of ⟨1¯10⟩ and ⟨112¯⟩ tip directions due to the competing influence of magnetocrystalline anisotropy and tip morphology on tip magnetization. Superparamagnetic fluctuations of the tip magnetization are evident in the polarization direction of emitted electrons when film coatings are very thin. This superparamgentic behavior effectively imposes a limit to spin polarized vacuum tunneling applications in the very thin film limit. This limit can be overcome by cooling very thin film coated tips below 300K or by increasing the volume of the emitting domain at the tip apex through increasing the film thickness. At marginally larger thickness, long-term stability of the polarization magnitude and direction is observed at 300K. A method for changing the stable spin polarization direction is also presented that exploits spontaneous flipping of the tip magnetization at elevated temperature. A new instrument for measuring the spin-resolved field emission energy distribution (SPFEED) was also developed. Its design, simulation, control and operation are presented as well as initial SPFEED results for Fe/W(001) tips.;In order to aid our understanding of the polarized emission from Fe/W(001) tips, the growth and magnetic properties of ultrathin Fe films on a W(001) single crystal surface were also investigated. These investigations were carried out using low energy electron microscopy (LEEM) and spin polarized LEEM (SPLEEM). The three key growth regimes were observed --- rough growth of highly-strained pseudomorphic (ps) films at room temperature, kinetically-limited layer-by-layer growth of ps films at intermediate temperature and Stranski-Krastanov (SK) growth of three-dimensional (3D) crystallites on top of a thermodynamically stable 2 monolayer (ML) thick wetting layer at higher temperature. In the SK mode, a metastable 4 ps ML Fe film forms before the nucleation of 3D Fe islands and consequent dewetting of the top two unstable layers. Regardless of growth temperature, ferromagnetic order is detected in 3 ML and 4 ML films but not in 2 ML films at 300K. We observe in-plane magnetization with easy axis along the substrate ⟨110⟩ and ⟨100⟩ directions in 3 ML films and along the ⟨100⟩ directions in 4 ML and thicker films. Large magnetic domains are observed in metastable 4 ML films grown at intermediate temperature, which break up into smaller domains due to the spontaneous formation of dislocations when thickness is increased. This trend is slightly delayed when strain is partially relieved by roughness during growth of Fe films at room temperature. Depending on the size and shape of an isolated Fe island that forms in the SK growth regime, the magnetic domain structure can be vortex, quasisingle domain with flux closure ends or single domain.
机译：从涂覆有超薄铁磁膜的尖锐W尖端场发射中的真空隧穿引起了人们的兴趣，因为它可能会产生明亮的，相干的自旋极化电子束。自旋极化场发射（SPFE）的研究也可能有助于更好地了解自旋极化扫描隧道显微镜中的自旋极化隧道过程。 SPFE是通过在300K时自发磁化状态的超薄膜Fe-和Co-coated W（001）和W（111）吸头实现的。在每种情况下，都检测到横向自旋极化分量。获得了高达35％的适度自旋极化强度。对于（001）尖端，极化的方位取向显示出强烈希望与横向低折射率晶体学方向对齐，即Co的〈110〈和Fe的〈100〉。相反，由于磁晶各向异性和尖端形态对尖端磁化的竞争影响，W（111）尖端的极化方向仅与〈1′10¯和〈112′〈尖端方向表现出弱的优先排列。当薄膜涂层非常薄时，尖端磁化强度的超顺磁波动在发射电子的极化方向上很明显。这种超超自然行为在非常薄的薄膜极限内有效地限制了旋转极化真空隧穿应用的极限。通过将低于300K的非常薄的薄膜涂层尖端冷却或通过增加膜厚来增加尖端顶点处的发射区域的体积，可以克服这一限制。在稍大的厚度下，在300K时观察到极化强度和方向的长期稳定性。还提出了一种用于改变稳定自旋极化方向的方法，该方法利用了在高温下尖端磁化的自发翻转。还开发了一种用于测量自旋分辨场发射能量分布（SPFEED）的新仪器。介绍了其设计，仿真，控制和操作以及Fe / W（001）电极头的初始SPFEED结果。；为了帮助我们理解Fe / W（001）电极头的极化发射，生长和磁性能还研究了W（001）单晶表面上的超薄铁膜的制备。这些研究是使用低能电子显微镜（LEEM）和自旋极化LEEM（SPLEEM）进行的。观察到三个关键的生长机制-室温下高应变的假晶（ps）膜的粗生长，中温下ps膜的动力学受限的逐层生长以及三种的Stranski-Krastanov（SK）生长高温下热力学稳定的2单层（ML）厚润湿层顶部的三维（3D）微晶。在SK模式下，在3D Fe岛形核并随后对顶部的两个不稳定层进行去湿之前，形成了亚稳态的4 ps ML Fe铁膜。无论生长温度如何，在300 K的3 ML和4 ML膜中均检测到铁磁顺序，而在2 ML膜中则检测不到。我们观察到在3 ML薄膜中沿基板〈110〉和〈100〉方向以及在4 ML及更厚膜中沿〈100〉方向具有易轴的面内磁化。在中等温度下生长的亚稳态4 ML薄膜中观察到较大的磁畴，由于厚度增加时自发形成位错，该磁畴分裂成较小的磁畴。当在室温下生长Fe膜时，通过粗糙度部分缓解应变时，这种趋势会稍微延迟。取决于在SK生长过程中形成的孤立的Fe岛的大小和形状，磁畴结构可以是涡流，具有磁通封闭端的quasingle磁畴或单个磁畴。

著录项

  • 作者

    Niu, Yuran.;

  • 作者单位

    Hong Kong University of Science and Technology (Hong Kong).;

  • 授予单位 Hong Kong University of Science and Technology (Hong Kong).;
  • 学科 Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 160 p.
  • 总页数 160
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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