Dynamics of Ultrafast Magnetization in Magnetic Thin Films and Heterostructures

这个重点研究小组项目包括两名教员和几名工业合作者，他们将研究超快、自旋相关的过程，这些过程反映了铁磁薄膜中的非平衡磁化动力学和皮秒及以下的异质结构。一个核心问题涉及磁化反转的最终“速度极限”，这将通过使用全光、超短脉冲激光技术来实验解决。不同于传统的利用脉冲磁场来研究存储介质中磁化开关的方法，本研究的物理重点是有序磁性介质中自旋的选择性光激发，从而以非热的方式利用光来调制交换相互作用和相关的电子关联。除了研究横向均匀磁性多层膜和交换偏置双层和多层膜中的光学激活磁电过程外，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，在高密度平面阵列中，单个亚微米磁性粒子通过偶极(或可能是交换偏置)力耦合。传统过渡金属(Co，NiFe)薄膜是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定的过渡金属氧化物，最著名的是半金属铁磁CrO2。这项研究让学生和博士后参与与当前技术直接相关的尖端基础研究。该培训为学生在学术界、工业界或政府的各种职业做好准备。%典型的计算机最慢的部件是磁性硬盘驱动器。虽然从薄膜盘介质存储和检索数据涉及几个步骤，但将信息编码成磁性排列的原子的过程正在达到其实际速度的极限。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，并在短短几万亿分之一秒内实现原子组的磁排列反转--这一速度大约是当今磁盘驱动器中这一过程的一百倍。全光技术使研究小组能够研究这种快速磁切换所涉及的基本相互作用，它可能会在未来导致极快的数据存储设备。一种特定的方法集中于将激光脉冲对准由两个磁耦合的薄膜磁性薄膜组成的夹层，其集体相互作用决定了双层的整体磁性，这在抵抗外部施加的磁场方面是有效的。通过选择性地吸收只有几个原子层厚的界面上的激光辐射，两种材料之间的磁耦合突然中断，释放出其中一层(自由的铁磁体)，使其迅速被从外部施加的相反方向的静态磁场逆转。虽然这一概念有朝一日可能会被用于快速数据存储，但该团队将主要用它来研究以前所未有的速度“翻转超小指南针”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，但同时反转数千个原子的磁矩的集体过程在基本层面上还没有被很好地理解。这项研究让学生和博士后参与与当前技术直接相关的尖端基础研究。该培训为学生在学术界、行业或政府的各种职业做好准备

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