Noise, chirality, and chaos in the dynamics of magnons and magnetic solitons

Habilitation à diriger des recherches
Faculté des Sciences – Université Paris-Sud

This dissertation provides an overview of the research activities I have undertaken in theoretical magnetism since taking up my CNRS position in 2004. It covers work on the stochastic theory of spin-torque nano-oscillators, nanocontact vortex oscillators including chaotic phases, channelling and nonreciprocal spin wave propagation in chiral systems, thermally-driven domain wall processes, and skyrmion dynamics. I conclude with some perspectives that follow from this research, namely chaos-based information processing, time-delay phenomena in micromagnetism, and stochastic processes in chiral magnets.

Ce mémoire présente un résumé des activités de recherche théorique en magnétisme que j’ai menées depuis mon recrutement au CNRS en 2004. Il traite la théorie stochastique des nano-oscillateurs à transfert de spin, les oscillateurs à vortex dans les nanocontacts (y compris leurs phases chaotiques), la canalisation et la propagation non réciproque des ondes de spin dans les systèmes chiraux, les processus thermiquement activés des parois de domaine et la dynamique des skyrmions. Je conclus en détaillant quelques perspectives issues de ces travaux, notamment le traitement de l’information par le chaos, les phénomènes de rétroaction dans le micro-magnétisme et les processus stochastiques dans les aimants chiraux.

This dissertation was defended on 20 June 2018 in Orsay before the examination panel comprising:

  • Hans-Benjamin Braun (Rapporteur) – University College Dublin
  • Ursula Ebels (Rapporteur) – SPINTEC, Grenoble
  • Eric Fullerton (Examiner) – University of California at San Diego
  • Claudio Serpico (Examiner) – Università di Napoli “Federico II”
  • André Thiaville (President) – Laboratoire de Physique des Solides, Orsay
  • Jan Vogel (Rapporteur) – Institut Néel, Grenoble
  • Claude Chappert – C2N, Orsay
  • Thibaut Devolder – C2N, Orsay


Microscopic disorder, finite temperatures and spin waves in domain-wall driven exchange bias

Doctor of Philosophy
University of Western Australia

Exchange bias is an interface effect that results from the exchange interaction between a ferromagnet in contact with an antiferromagnet layer. The existence of bias depends on the magnetic order in the antiferromagnet and the effect is commonly characterized by a hysteresis loop displacement along the field axis and enhancements to the coercivity in the ferromagnet. These features are sensitive to the details of the physical and chemical structure of the ferromagnet/antiferromagnet interface. Although exchange bias was discovered over forty years ago, there still remains a host of unanswered questions and contradictions between experimental observations and theoretical predictions. The problem poses many challenges to test our current understanding of interfacial magnetism and frustrated spin systems.

A theoretical study of exchange bias in bilayer systems, based on partial domain wall formation near the interface in the antiferromagnet, is presented in this dissertation. A continuum theory developed for mixed interfaces demonstrates a link between micro- scopic spin order and phenomenological bilinear and biquadratic terms coupling the two magnetic layers. Particular attention is given to the role of impurities, such as interface roughness in the form of geometrical imperfections and magnetic defects within the film layers. Results from numerical calculations show that dramatic modifications to the hysteresis can occur with the presence of such impurities, and some effort is made at providing clues to help identify defect types in experiment. Periodic imperfections at the interface are shown to modify the angular dependence of the bias and the behaviour can be understood in terms of changes in the natural orientation of the ferromagnet magnetization. Local spatial variations in the magnetic constants result in domain wall pinning effects and are shown to give rise to coercivity enhancement. The effects of finite temperatures are studied with a local mean-field theory and the results demonstrate the importance of thermally-driven wall pinning processes in the antiferromagnet. Suggestions for alternative means of characterizing antiferromagnetic order are made in light of studies of the magnetic heat capacity. Finally, the behaviour of long-wavelength spin excitations in the bilayer is examined. Changes to the ferromagnet spin wave spectra due to the interlayer coupling are studied in detail and some estimates are given for frequency shifts and linewidth variations resulting from interfacial inhomogeneities.

This dissertation was submitted in August 2002 and examined by an external panel comprising:

  • Jean-François Bobo – Laboratoire de Physique de la Matière Condensée, Toulouse
  • Bernard Diény – SPINTEC, Grenoble
  • Roy Chantrell – University of York