Modeling of Ultrafast Magnetization Dynamics at High temperatures

由于对当前读/写方案施加的基本限制，当今用于诸如硬盘驱动器和磁随机存取存储器的更小和更快的信息存储设备的技术进步遇到了不可避免的挑战。迎接这一挑战的最令人兴奋的想法之一是利用超快激光或超短脉冲电流来操纵磁性状态。事实上，实验上已经发现，通过这些方法可以在不到一皮秒(PS)的时间内控制磁态。不可避免的是，人们进入了一个新的科学和技术领域，涉及高温下的超快动力学，而不是现在的磁技术，它在室温下慢1000倍(纳秒)。在这个研究方案中，我们将重点放在对物理过程的详细了解上，确定控制磁化状态的关键因素，并开发能够以期望的精度模拟磁化动力学的定量建模工具。预计研究成果可广泛应用于解释和预测实验观察到的新的磁化动力学现象，更重要的是为基于超快和高温磁化动力学的未来磁技术的应用提供必要的计算工具，如热辅助磁记录技术。其他拟议的活动包括与以前建立的行业进行强有力的合作，通过广泛的指导和行业实验室访问来培训研究生。一门新的自旋电子学课程已经开始，重点是自旋电子学物理和器件的最新进展，并将于2014年秋季学期开课。该科学计划旨在开发一个有效的方程，可以广泛用于在广泛的温度范围和超快时间尺度下的磁化动力学的定量建模。目前，对室温纳秒磁化动力学的可靠而强大的模拟工具是基于Landau-Lifshitz(LL)方程，该方程被认为在接近或高于居里温度时失效。从理论和模拟的角度来看，需要一种更好的模拟工具来代替LL方程，以便能够定量地描述高温和超快时间尺度下的磁化动力学。基于对快速弛豫微观起源的初步研究，量子动力学方法将被用来建立一个自洽的动力学方程。在所提出的磁化强度矢量的动力学方程得到验证后，将对磁性多层膜和合金中的特定元素的动力学进行广泛的数值模拟。与器件相关的两个特殊的动力学过程，激光诱导退磁和热辅助磁写，将被广泛研究和优化。

Technological advances today for smaller and faster information storage devices such as hard disk drives and magnetic random access memory have encountered unavoidable challenges due to fundamental limitations placed on the current read/write schemes. One of the most exciting ideas to meet this challenge is to utilize an ultrafast laser or an ultra-short pulsed electric current to manipulate magnetic states. Indeed, it has been experimentally discovered that it is possible to control the magnetic states in less than one picosecond (ps) by these methods. Inevitably, one enters a new area of science and technology involving ultrafast dynamics at high temperatures, as oppose to the present-day magnetic technology which is 1000 times slower (nanoseconds) at room temperatures. In this research proposal, one focuses on the detailed understanding of the physical processes, determines key factors that control the magnetic states and develops quantitative modeling tools which can simulate magnetization dynamics with desired accuracy. It is anticipated that the research outcome can be broadly applied to explain and predict novel magnetization dynamic phenomena observed experimentally, and more importantly, to provide essential computational tools for application of future magnetic technologies based on ultrafast and high temperature magnetization dynamics such as heat-assisted magnetic recording technology. The other proposed activities include strong collaboration with industry previously established, training of graduate students via extensive mentoring and industry laboratories visiting. A new spintronics course with the emphasis on the recent progresses in spintronics physics and devices has been initiated and will be taught in the Fall semester of 2014.This scientific program aims at developing an effective equation that can be broadly used for quantitatively modeling of magnetization dynamics at a wide range of temperatures and at ultrafast time scales. At present, the reliable and powerful simulation tool for room-temperature nanosecond magnetization dynamics is based on the Landau-Lifshitz (LL) equation, which is deemed to fail at the temperature close or above the Curie temperature. What is needed, from the view point of theory and simulation, is a better simulation tool to replace the LL equation so that the magnetization dynamics at high temperature and at ultrafast timescales can be quantitatively addressed. Built on the preliminary study of the microscopic origins of fast relaxations, the quantum kinetic approach will be used to establish a self-consistent dynamic equation. After the proposed dynamic equation for the magnetization vector is validated, extensive numerical simulations will be carried out for the element-specific dynamics in magnetic multilayers and alloys. Two particular device-relevant dynamics processes, laser induced demagnetization and heat-assisted magnetic writing, will be extensively studied and optimized.