Current-driven domain wall motion and magnetomemristance in FeRh-based nanostructures

本项目将研究ferh基纳米结构中反铁磁/铁磁畴壁（AF/FM）的电流驱动运动。这将阐明相变的基本物理原理，并基于我们之前在具有掺杂梯度的FeRh脱膜中温度/场控制DW运动的演示，探索（磁致）忆阻器器件的潜力。忆阻器是适用于超密集非易失性存储器的器件，也显示了人工突触的许多特征，为新型神经形态存储器和逻辑架构打开了大门，这些架构有望在未来几代ITC硬件中增强功能和低能耗运行。为了实现这一目标，我们首先需要知道需要什么样的掺杂材料和密度来控制AF-FM相变温度，在过渡中提供适当的滞后（以存储信息），以及AF和FM相之间最大可能的电阻率变化（用于读出），以及什么样的参数（材料和密度）可以描述理想的掺杂梯度。接下来，我们需要确定FeRh可以形成多小的磁性纳米结构，并保持合适的AF/FM过渡，以及允许最小纳米结构稳定的精确条件和要求。然后，有必要确定在FeRh相分离体系中移动分离铁磁和反铁磁区域的畴壁所需的电流密度。最后，我们将需要在具有合适掺杂梯度的纳米结构中证明电流驱动的畴壁运动下的忆阻作用。我们的项目将结合最先进的磁性材料生长，表征，这些新型DWs的直接成像，以及设备制造和测试，将我们从基础材料开发到完全可操作的磁忆阻器原型纳米结构。我们将从溅射沉积均匀和梯度掺杂的FeRh薄膜材料开始，并测量它们的磁性和磁输运性质，这将告诉我们实现最佳忆阻作用所需的掺杂材料和掺杂水平。然后，我们将从最佳掺杂的FeRh层制造纳米结构到10s纳米尺度，无论是作为单个纳米结构（用于显微镜）还是作为大规模纳米结构阵列（用于磁强计），这将揭示保留对忆阻作用有用的相变的最小尺寸。接下来，我们将在世界上首次开展电流驱动的横向高铁纳米线AF/FM畴壁运动实验，利用磁显微镜跟踪DW响应电流脉冲的运动，揭示自旋注入对DW运动的效率以及不同来源引起的DW钉住的程度和性质。然后，我们将从梯度掺杂层中绘制纳米柱，并研究由垂直电流驱动的DW运动，在原型忆阻器器件中，使用磁输运测量和器件内部结构的直接成像。关键的结果将是磁阻作为器件尺寸，温度和磁场的函数。我们将获得的结果不仅将通过揭示仍然知之甚少的FeRh相变本质的基本问题而导致高影响力的出版物和会议报告，而且还将揭示世界上第一个磁记忆电阻器的性能特征，在新型神经形态计算机体系结构领域开发潜在的有价值的知识。

This project will study current-driven motion of antiferromagnetic/ferromagnetic (AF/FM) domain walls (DWs) in FeRh-based nanostructures. This will both elucidate the fundamental physics of the phase transition and also explore the potential for (magneto-)memristor devices, based on our prior demonstration of temperature/field controlled DW motion in an FeRh epilayer with a doping gradient. Memristors are devices suitable for ultradense non-volatile memory and also show many of the characteristics of an artificial synapse, opening the door to novel neuromorphic memory and logic architectures that promise enhanced functionality and low energy operation in future generations of ITC hardware. To achieve this goal we first need to know what doping materials and densities are required to control the AF-FM phase transition temperature, provide appropriate hysteresis in the transition (to store information), and largest possible resistivity change (for readout) between the AF and FM phases, as well as what parameters (materials and densities) would describe an ideal doping gradient. Next, we will need to establish how small a magnetic nanostructure can be formed from FeRh and retain a suitable AF/FM transition, and the precise conditions and requirements that permit the smallest nanostructures to be stable. Then it will be necessary to establish the current densities needed to move the domain walls that separate ferromagnetic and antiferromagnetic regions in the phase-separated regime of FeRh. Last, we will need to demonstrate a memristive action under current-driven domain wall motion in a nanostructure with a suitable doping gradient.Our project will combine state-of-the-art magnetic materials growth, characterisation, direct imaging of these novel DWs, and device fabrication and test, taking us from basic materials development to a fully operational magneto-memristor prototype nanostructure. We will begin by sputter-depositing uniformly- and gradient-doped FeRh epilayer materials and measuring their magnetic and magnetotransport properties, which will tell us the dopant materials and doping levels needed to achieve optimal memristive action. We will then fabricate nanostructures down to the few 10s of nm scale from the optimally doped FeRh layers, either as individual nanostructures (for microscopy) or as large-scale arrays of nanostructures (for magnetometry), which will reveal the minimum size at which a phase transition that is useful for memristive action is retained. Next, we will carry out world-first experiments on current-driven AF/FM domain wall motion in lateral FeRh nanowires, using magnetic microscopy to track the motion of DWs in response to current pulses, revealing the efficiency of spin injection for DW motion and the degree and nature of DW pinning arising from different sources. We will then pattern nanopillars from gradient-doped layers and study DW motion driven by a vertical current in a prototype memristor device, using both magnetotransport measurements and direct imaging of the internal structure of the device. The key result will be the magneto-memristance as a function of device size, temperature, and magnetic field. The results we shall obtain will not only lead to high impact publications and conference presentations by shedding light on the still poorly understood fundamental problem of the nature of the phase transition in FeRh, but also reveal the performance characteristics of the world's first magneto-memristor, developing potentially valuable knowhow in the field of novel neuromorphic computer architectures.