Dynamics of Ultrafast Magnetization in Magnetic Thin Films and Heterostructures

这个重点研究小组项目涉及两名教员和几名工业合作者，他们将研究在皮秒及以下时间尺度上反映铁磁薄膜和异质结构中非平衡磁化动力学的超快、自旋相关过程。一个核心问题涉及到磁化逆转的最终“速度限制”，这将通过采用全光、超短脉冲激光技术在实验中得到解决。与传统方法利用脉冲磁场研究存储介质中的磁化开关不同，本研究的物理学重点是研究有序磁介质中自旋的选择性光激发，从而通过光以非热的方式调制交换相互作用和相关的电子相关。除了研究横向均匀磁性多层和交换偏置双层和多层中的光激活磁电子过程外，该项目还包括研究高密度平面阵列中集体微磁效应的动力学，其中单个亚微米磁性粒子通过偶极（或可能的交换偏置）力耦合。传统过渡金属（Co, NiFe）薄膜构成了项目工作的起始材料基础，但研究的一个重要组成部分强调选择过渡金属氧化物，最值得注意的是半金属铁磁体CrO2。该研究涉及与当前技术直接相关的前沿基础研究的学生和博士后。该培训为学生在学术界、工业界或政府的各种职业生涯做好准备。典型计算机中最慢的部分是磁性硬盘驱动器。虽然从薄膜磁盘介质中存储和检索数据需要几个步骤，但将信息编码为磁排列原子的过程正在达到其实际速度极限。在这个项目中，我们的目标是使用超短激光脉冲来影响磁盘材料的磁性，并在短短万亿分之一秒内实现原子团的磁性排列逆转——大约比今天磁盘驱动器的速度快100倍。全光技术使该团队能够研究这种快速磁开关中涉及的基本相互作用，并可能在未来导致极快的数据存储设备。一种具体的方法侧重于将激光脉冲瞄准两个磁耦合薄膜的夹层，其集体相互作用决定了双层的整体磁性能，从而有效地抵抗外部施加的磁场。通过在只有几个原子层厚的界面上选择性地吸收激光辐射，两种材料之间的磁耦合被突然中断，释放出其中一层（“自由”铁磁体），通过从外部施加的相反方向的静态磁场迅速逆转。虽然这个概念有一天可以用于快速数据存储，但该团队将主要使用它来研究以前所未有的速度“翻转超小型指南针”的基本过程。许多物理学家已经研究了单个原子磁矩的反转，但是同时翻转数千个原子的磁矩的集体过程在基本层面上还没有得到很好的理解。该研究涉及与当前技术直接相关的前沿基础研究的学生和博士后。该培训为学生在学术界、工业界或政府的各种职业生涯做好准备

This Focused Research Group project involves two faculty members and several industrial collaborators who will study ultrafast, spin dependent processes that reflect nonequilibrium magnetization dynamics in ferromagnetic thin films and heterostructures on a picosecond time scale and below. A core question relates to the ultimate "speed limits" of magnetization reversal, which will be approached experimentally by employing all-optical, ultrashort pulse laser techniques. Unlike conventional approaches, which use pulsed magnetic fields to study magnetization switching in storage media, the physics in this research focuses on selective optical excitations of spins within the ordered magnetic medium, so as to modulate the exchange interaction and related electronic correlations by light in an nonthermal manner. In addition to studying optically activated magnetoelectronic processes in laterally uniform magnetic multilayers and exchange biased bi- and multilayers, the project includes the study the dynamics of collective micromagnetic effects in high density planar arrays where the individual submicron magnetic particles are coupled via dipolar (or possibly exchange bias) forces. Thin films of conventional transition metals (Co, NiFe) form the starting materials base for the project work, but a significant component of the research emphasizes selected transition metal oxides, most notably the half metallic ferromagnet CrO2. The research involves students and postdocs in cutting-edge fundamental research that has immediate relevance to current technology. The training prepares student for a variety of careers in academe, industry or government.%%%The slowest part of a typical computer is the magnetic hard drive. While there are several steps involved in storing and retrieving data from the thin film disk medium, the process of encoding information into magnetically aligned atoms is reaching its practical limits of speed. In this project work we aim to use ultrashort laser pulses to influence the disk material's magnetic properties and to achieve the reversing the magnetic alignment of groups of atoms in as little as a few trillionth of a second-approximately a hundred times faster than the speed of the process in today's disk drives. The all-optical technique allows the team to investigate the fundamental interactions involved in such fast magnetic switching, and it may lead to extremely fast data storage devices in the future. One specific approach focuses on aiming the laser pulses at a sandwich of two magnetically coupled thin film magnetic films, whose collective interaction determines the overall magnetic properties of the bilayer which is efficient in resisting an externally applied magnetic field. By selectively absorbing the laser radiation at the interface, only a few atomic layers thick, the magnetic coupling between the two materials is abruptly interrupted, freeing one of the layers (the 'free' ferromagnet) to be rapidly reversed by an oppositely-directed static magnetic field, applied from the outside. While the concept could some day be used in fast data storage, the team will be using it mostly to study the basic processes of "flipping ultrasmall compass needles" at unprecedented speeds. Many physicists have studied the reversal of a single atom's magnetic moment, but the collective process of flipping the moments of many thousands of atoms at once is not well understood at a fundamental level. The research involves students and postdocs in cutting-edge fundamental research that has immediate relevance to current technology. The training prepares student for a variety of careers in academe, industry or government