Dynamics of Ultrafast Magnetization in Magnetic Thin Films and Heterostructures

Brown University, Providence RI

Investigators

Abstract

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