All-Optical Switching in Co-Heavy Rare Earth alloys

Abstract

Ultrafast all-optical switching (AOS) offers a route to field-free, energy-efficient magnetic writing. Single-shot helicity-independent AOS (HI-AOS) has long been associated with Gd-based ferrimagnets. This thesis investigates whether it can also occur in ultrathin, Gd-free Co–rare-earth alloys containing Ho, Dy, Tb, or Er.

By tuning alloy composition near magnetic compensation, this work demonstrates deterministic single-pulse HI-AOS in CoHo and CoDy. CoTb switches only partially, owing to anisotropy dispersion and similar demagnetization times of its sublattices, while CoEr does not switch at room temperature, where magnetic compensation is absent. Time-resolved measurements and micromagnetic simulations show that reversal in CoHo and CoDy develops over microseconds through domain-wall motion and coalescence. The laser pulse first demagnetizes Co; the more slowly demagnetizing rare-earth sublattice then helps establish the direction of reversal.

The thesis also shows that reversal speed can be tuned across three orders of magnitude in Co/Gd synthetic ferrimagnets. Changing the Gd thickness or inserting a Pt spacer alters interfacial spin-current transfer and shifts switching dynamics from picoseconds toward nanoseconds. Together, these findings connect fast and slower AOS pathways and provide guidance for designing optically controlled spintronic devices.

PhD thesis supervision

• Michel Hehn (supervisor), Institut Jean Lamour, University of Lorraine
• Salinporn Kittiwattanakul (co-supervisor), Chulalongkorn University

Examination jury members

• Laurent Ranno (reviewer), Institut Néel, Université Grenoble Alpes
• Christine Boeglin (reviewer), IPCMS, CNRS
• Nicolas Bergeard (examiner), IPCMS, CNRS
• Stéphane Mangin (examiner), Institut Jean Lamour, University of Lorraine
• Emmanuelle Jal (invited member), LCPMR, CNRS