A Study of Anisotropy in Antiferromagnets

自旋电子学或自旋电子学是应用物理学的一个新兴领域，它研究电子的固有自旋及其相关的磁矩。迄今为止最成功的自旋电子器件是基于巨磁电阻效应的，并已在硬盘驱动器的读头传感器中使用了20多年。直到最近，大多数研究都是基于铁磁性材料。2014年，有人宣布在金属反铁磁（AF）材料中发生了显著的自旋电子现象。这些现象是一种意想不到的自旋霍尔效应，以及使用自旋极化电流脉冲操纵AF轴方向的能力。后一种现象至关重要，因为它已被证明AF材料的弛豫速度比典型的铁磁器件快1000倍。因此，原则上，基于AF的存储或开关设备将能够比传统的磁随机存取存储器（MRAM）设备快许多倍，MRAM设备本身的开关速度比互补金属氧化物半导体（CMOS）设备快。这样的系统将需要更低的功率，重要的是，任何产生的方向都不会受到正常退磁场效应的影响，而正常退磁场效应限制了基于铁磁体的设备的性能。由于这些潜在的主要优势和涉及操纵AF材料的新物理，世界范围内对AF材料的研究水平得到了迅速发展。因此，这是一个新兴的努力领域，由于科学和技术原因，它正迅速成为主导。对于潜在的存储和开关应用，AF材料的各向异性将是一个关键参数，其起源尚不清楚。我们的目标是通过ab-initio建模和实验测量的综合方案来解决这个问题。前者将通过密度泛函理论（DFT）建模进行，该模型不需要对晶体结构等进行假设。这对于广泛使用的系统IrMn(x)很重要，其中非化学计量（x>3）组成的AF各向异性增加。实验方案将侧重于测量各向异性。测量这一特性的技术是由我们的团队开发的，基于所谓的交换偏置效应。

Spintronics or spin electronics is an emerging field of applied physics which studies the intrinsic spin of an electron and its associated magnetic moment. The most successful spintronic device to date is based on the Giant Magneto Resistance effect and has been used in the read head sensor of hard disk drives for over 20 years. Until recently, most studies were based on ferromagnetic materials. In 2014 it was announced that significant spintronic phenomena occurred in metallic antiferromagnetic (AF) materials. These phenomena are an unexpected spin-Hall effect and, critically, the ability to manipulate the orientation of the AF axes using a spin polarised current pulse. This latter phenomenon is of critical importance as it has been shown that an AF material can relax 1000 times faster than a typical ferromagnetic device. Hence, in principle, an AF based storage or switching device would be capable of being many times faster than a conventional Magnetic Random Access Memory (MRAM) device which itself switches faster than a Complementary Metal-Oxide Semiconductor (CMOS) device. Such system would require significantly lower power and, importantly, any resulting orientation would not be subject to the normal demagnetising field effect that limits the performance of a device based on a ferromagnet. Because of these potential major advantages and the new physics involved in manipulating an AF material the level of worldwide research in AF materials has burgeoned. Hence this is a new emerging field of endeavour which is rapidly becoming dominant for both scientific and technological reasons. For both potential storage and switching applications the anisotropy of AF materials will be a critical parameter whose origin(s) are not yet understood.We aim to address this via an integrated programme of ab-initio modelling and experimental measurements. The former will be undertaken via Density Functional Theory (DFT) modelling which requires no assumptions regarding, for example, crystal structure. This is important for the widely used system IrMn(x) where the AF anisotropy increases for the non-stoichiometric (x>3) composition. The experimental programme will focus on the measurement of the anisotropy. The technique to measure this property was developed by our group and is based on what is known as the exchange bias effect.