Jean-Loïs BELLO: Magnetism, current-induced switchings and domain-wall propagation in single and exchange coupled Gd based ferrimagnet thin films

Abstract:
The discoveries ensuing the emergence of spintronics have revolutionized technology, especially in communication and data storage. Spintronics is a field of research that combines the study of an intrinsic property of the electron, the spin to which is bound an intrinsic magnetic moment and the charge of the electron. In a world where the energy consumption of internet traffic and data storage doubles every four years, Spintronics faces new challenges. To meet the need to increase the density, the operation speed and to lower the power consumption of memory devices, using spin-currents to manipulate the magnetization is very promising and has led to new ideas of possible devices in which such currents are used to propagate magnetic domain-walls. Materials with perpendicular to film plane magnetization have been the subject of intense research and they are crucial for the development of magnetic memories such as hard disk drive or MRAM. Among those, amorphous rare-earth transition-metal GdFeCo thin films are well-known to exhibit many interesting magnetic properties. Perpendicular magnetization can be stabilized with weak anisotropies and a total moment compensation can be achieved depending on the concentration and the temperature. In addition to magnetic fields, its magnetic moment has been shown to be switched either by single femtosecond laser pulse or spin-orbit torque. Since this material appears to be promising for future spintronics applications, we focused on the study of the magnetic properties of multilayers that include this material. We first report the dependence of magnetic properties in GdFe90Co10)100−x ferrimagnetic alloys single layers on the composition and on the nature of the interfaces. 5nm thick GdFe90Co10)100−x layers were integrated in different multilayers, all grown by magnetron sputtering. For x between 22% and 33%, we show that the composition at which the magnetic compensation occurs changes depending on the layers under and over the TE-RM layer. We studied different interfaces M/GdFeCo/N with M or N = Cu, Pt, Ta, Ir. Then, for GdFeCo layers having out of plane magnetization, more complex structures have been grown associating two GdFeCo layers. Especially, we grew GdFeCo/Ir/GdFeCo with varying Ir thickness. We show that it is possible to have an antiferromagnetic coupling. Those synthetic antiferromagnets are proven to improve the velocity of propagation of magnetic domain-wall. In our work, we propose to combine the properties of such a structure with the ones of GdFeCo. After micro-fabrication of devices, we have first investigated the magnetization reversal induced by spin-torques in our synthetic antiferromagnetic systems and the layers that compose them. If standard switching that requires an external magnetic field has been observed, we also have found that switching occurs in the absence of magnetic field. After further measurements, especially with magnetic domain imaging with a Kerr microscope, we have shown that the field induced by the current injection cannot be neglected, leads to a peculiar behavior of the magnetization in our devices and strongly affects spin-torque switching. Finally, we looked into the domain-wall propagation and our preliminary results show promising features at the condition to eliminate the currentinduced fields. This study has given us first hints and possibilities to be explored as a function of the numerous interactions and parameters that can be tuned especially in GdFeCo-based synthetic antiferromagnets.

Keywords:
Thin films deposition, Ferrimagnets, Synthetic antiferromagnets, Spin-Torques, Domain-wall, Spintronics.

Composition of the jury:
> Reporters:
- Mrs Stefania PIZZINI, Institut Néel, France
- M. Gilles GAUDIN, SPINTEC, France
> Examiners:
- M. Andrew KENT, New York University, USA
- M. Roman MORGUNOV, Institute of Problems of Chemical Physics, Russia
> Direction of thesis:
- M. Michel HEHN, Thesis director, Institut Jean Lamour, France
- M. Stéphane MANGIN, Thesis co-director, Institut Jean Lamour, France