Demonstration of high-frequency oscillation in a Co-based Heusler alloy tunnel junction

关键词：半金属，Heusler合金，MRAM，Gilbert减振和交换偏置目标：1.建立一种可重复制造磁隧道结(MTJ)的工艺，该工艺由夹有外延CO2FeAl0.5Si0.5薄膜电极的氧化镁势垒组成。证明了室温下非常大的隧道磁阻(TMR)比。证明了基于小的Gilbert阻尼常数和大的自旋极化的高效电流感应磁化开关(CIMS)。首次利用CO2FeAl0.5Si0.5结纳米制造高频自旋振荡原型。方法：1.采用超高真空(UHV)溅射/分子束外延(MBE)生长器件，并在NIMS上结合最先进的扫描电子显微镜(STEM)界面原子分析进行纳米制造。基于上述反馈工艺，纳米级制备了高质量的Co2FeAl0.5Si0.5/MgO/CO2FeAl0.5Si0.5结。在NIMS[平面隧道电流分析和随温度变化的TMR测量]和YORK[振动样品磁强计(VSM)和磁光克尔效应(MOKE)]下进行了高灵敏度的磁化分析。预期结果：1.揭示CO2FeAl0.5Si0.5/MgO界面的原子结构和弹道自旋极化电子隧道性质之间的关联。改善了世界记录的TMR比(在RT时为386%，在9K时为832%)及其温度依赖关系，以遵循经验的磁化温度依赖关系。临界电流密度降至106A/cm24以下。Co2FeAl0.5Si0.5薄膜的阻尼常数估计。最近，为了实现未来的自旋随机存取存储器应用，人们对一种在室温下具有100%自旋极化的新型铁磁材料进行了深入的研究。我们将使用Co2FeAl0.5Si0.5 Heusler合金薄膜，它具有最高的自旋极化，在室温下产生了迄今为止最大的隧道磁阻。带有Heusler薄膜的磁性隧道结将通过超高真空溅射和分子束外延在NIMS外延生长，然后在York用最先进的电子显微镜和磁测量进行表征。通过改善薄膜与氧化镁隧道势垒的界面原子结构，可以实现更大的TMR比。还将进行高频测量，以确定它们的阻尼常数，这些常数将比传统的铁磁体小。这些结有望在~100%的自旋极化和非常小的阻尼常数方面具有显著的优势，以便在自旋存储器中实现快速和有效的开关。在本项目的最后，我们将首次尝试用Heusler合金薄膜制作高频振荡器的原型。

Keywords: half-metals, Heusler alloys, MRAM, Gilbert damping and exchange biasGoals: 1. Establishment of a reproducible process to fabricate a magnetic tunnel junction (MTJ), consisting of a MgO barrier sandwiched with epitaxial Co2FeAl0.5Si0.5 film electrodes.2. Demonstration of very large tunnelling magnetoresistance (TMR) ratio at room temperature (RT).3. Demonstration of efficient current-induced magnetisation switching (CIMS) based on a small Gilbert damping constant and large spin polarisation.4. Nanofabrication of a prototype of a high-frequency spin oscillation with the Co2FeAl0.5Si0.5 junction for the first time.Approach: 1. Device fabrication using ultrahigh vacuum (UHV) sputtering/molecular beam epitaxy (MBE) growth and nanofabrication at NIMS combined with interfacial atomic analysis by state-of-the-art scanning transmission electron microscopy (STEM) at York.2. Based on the above feedback process, nanopillar fabrication of high-quality Co2FeAl0.5Si0.5/MgO/Co2FeAl0.5Si0.5 junctions.3. Highly sensitive magnetisation analysis both at NIMS [current in plane tunnelling analysis and temperature-dependent TMR measuremsnts] and York [vibrating sample magnetometer (VSM) and magneto-optical Kerr effect (MOKE)].4. High-frequency operation of the CIMS in a Heusler-based nanopillar and electrical detection by coplanar waveguide.Expected outcome: 1. To reveal a correlation between atomic structures at the Co2FeAl0.5Si0.5/MgO interfaces and ballistic spin-polarised electron tunnelling properties.2. Improvement of the world-record TMR ratio (386% at RT and 832% at 9 K) and its temperature dependence to follow the empirical temperature dependence of magnetisation.3. Decrease of a critical current density below 106 A/cm24. Estimation of a damping constant of the Co2FeAl0.5Si0.5 film.An intensive search for a new ferromagnetic material with 100% spin polarisation at room temperature has been carried out recently for the realisation of a future spin random access memory application. We will employ Co2FeAl0.5Si0.5 Heusler alloy films, which hold the highest spin polarisation, resulting the largest tunnelling magnetoresistance at room temperature to date. Magnetic tunnel junctions with the Heusler films will be epitaxially grown at the NIMS by ultrahigh vacuum sputtering and molecular-beam epitaxy, and will then be characterised at York with the state-of-the-art electron microscopy and magnetometry. By improving the interfacial atomic structures of the films against a MgO tunnel barrier, larger TMR ratios will be demonstrated. High-frequency measurements will also be performed to define their damping constants, which are to be smaller than the conventional ferromagnets. These junctions are expected to take significant advantages in both ~100% spin polarisation and a very small damping constant for the realisation of fast and efficient switching in a spin memory. At the end of this project, we will attempt to fabricate a prototype of a high-frequency oscillator with Heusler alloy films for the first time.