Laser-Induced Femtosecond Magnetic Recording

Here is a summary of the 2008 thesis ‘Laser-Induced Femtosecond Magnetic Recording‘ by Stanciu, Claudiu Daniel from Radboud University Nijmegen, The Netherlands:

A hard disk drive (HDD) stores information in the form of small magnetic areas magnetized in opposite directions, like tiny bar magnets pointing north or south. Depending on the orientation of these bits the information can be later retrieved as ‘ones’ and ‘zeros’. The higher the information density on a hard disk, the smaller the magnetic bits must be. The higher the writing speed of information on a hard disk, the faster the manipulation (i.e. reversal) of the tiny bar magnets must become. Thus smaller and faster represents the driving force for the advancement of the hard disk drive technology in particular, and of the magnetic storage technology in general. This thesis focuses on the speed of manipulating the magnetization in metallic magnets relevant for data storage.

The conventional way to reverse the magnetization is by applying a magnetic field pulse either parallel or perpendicular to the magnetization of a bit, as described in Chapter 1 of this thesis. Today, a HDD stores a magnetic bit in ~1 nanosecond. The speed of this process is proportional to the magnetic field strength. This would imply that the reorientation time could be as short as desired, provided that suficiently high fields would be available. However, it was recently predicted that there is a natural limit of magnetization switching on the picosecond time scale, beyond which magnetization reversal becomes chaotic. Thus, finding new approaches to reverse magnetization in a reproducible way on a time scale shorter than picoseconds is a fundamental challenge with important consequences for magnetic data storage technology.

In this thesis two different ways of reversing the magnetization faster than picosecond were experimentally demonstrated, breaking what was previously called the speed limit of magnetic recording. In addition, this thesis demonstrates the feasibility of an effect that was previously believed fundamentally impossible in metals: controlled magnetization reversal with circularly polarized light.

The experiments were performed on a rare-earth transition-metal (RE-TM) amorphous alloy, i.e. GdFeCo. The magnetic and magneto-optical properties of the RE-TM metallic magnets are detailed in Chapter 2. The main experimental techniques used to investigate and control the magnetization dynamics on the femtosecond timescales are presented in Chapter 3. The results are presented in Chapter 4 and Chapter 5.

Chapter 4 shows that the angular momentum compensation point, that can be present in some ferrimagnetic materials such as GdFeCo, is accompanied by a highspeed and strongly damped spin dynamics. Next, by ultrafast heating the ferrimagnet across its compensation points, under an applied magnetic field, a subpicosecond magnetization reversal is experimentally demonstrated. This shows for the first time the feasibility of subpicosecond magnetization reversal, in a time-resolved experiment.

In Chapter 5, the effect of the angular momentum of the photons on spins in metallic systems was investigated. Previously, this effect was thought to be of no relevance. A full 180 degrees reorientation of a magnet by femtosecond laser pulses was considered to be even less possible based on the physics as known today. However, it is demonstrated here that the magnetization of the metallic amorphous alloy GdFeCo can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. With this thesis a new avenue in the field of ultrafast magnetization reversal has been opened. Since the experiments presented here are the first to demonstrate the feasibility of an effect that was previously believed fundamentally impossible, the microscopic mechanism responsible for this effect is still a matter of debate. As described above, many experiments have to be performed in order to completely understand the opto-magnetic switching mechanism.

Finally, it must be noted that although the all-optical magnetization reversal by circularly polarized light is presented from the point of view of applications in the data storage, it is obvious that this new effect has the potential to strongly affect also other fields of research and technology.