{"id":891,"date":"2020-06-14T22:27:27","date_gmt":"2020-06-14T20:27:27","guid":{"rendered":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/?page_id=891"},"modified":"2020-06-22T12:57:22","modified_gmt":"2020-06-22T10:57:22","slug":"dissertations","status":"publish","type":"page","link":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/?page_id=891","title":{"rendered":"Dissertations"},"content":{"rendered":"\n<h3 class=\"wp-block-heading\">Noise, chirality, and chaos in the dynamics of magnons and magnetic solitons<\/h3>\n\n\n\n<p><strong>Habilitation \u00e0 diriger des recherches<br><\/strong><em>Facult\u00e9 des Sciences \u2013 Universit\u00e9 Paris-Sud<\/em><\/p>\n\n\n\n<p>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.<\/p>\n\n\n\n<p><em>Ce m\u00e9moire pr\u00e9sente un r\u00e9sum\u00e9 des activit\u00e9s de recherche th\u00e9orique en magn\u00e9tisme que j\u2019ai men\u00e9es depuis mon recrutement au CNRS en 2004. Il traite la th\u00e9orie stochastique des nano-oscillateurs \u00e0 transfert de spin, les oscillateurs \u00e0 vortex dans les nanocontacts (y compris leurs phases chaotiques), la canalisation et la propagation non r\u00e9ciproque des ondes de spin dans les syst\u00e8mes chiraux, les processus thermiquement activ\u00e9s des parois de domaine et la dynamique des skyrmions. Je conclus en d\u00e9taillant quelques perspectives issues de ces travaux, notamment le traitement de l\u2019information par le chaos, les ph\u00e9nom\u00e8nes de r\u00e9troaction dans le micro-magn\u00e9tisme et les processus stochastiques dans les aimants chiraux.<\/em><\/p>\n\n\n\n<p>This dissertation was defended on 20 June 2018 in Orsay before the examination panel comprising: <\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Hans-Benjamin Braun (<em>Rapporteur<\/em>) \u2013 University College Dublin<\/li><li>Ursula Ebels (<em>Rapporteur<\/em>) \u2013 SPINTEC, Grenoble<\/li><li>Eric Fullerton (Examiner) \u2013 University of California at San Diego<\/li><li>Claudio Serpico (Examiner) \u2013 Universit\u00e0 di Napoli &#8220;Federico II&#8221;<\/li><li>Andr\u00e9 Thiaville (President) \u2013 Laboratoire de Physique des Solides, Orsay<\/li><li>Jan Vogel (<em>Rapporteur<\/em>) \u2013 Institut N\u00e9el, Grenoble<\/li><li>Claude Chappert \u2013 C2N, Orsay<\/li><li>Thibaut Devolder \u2013 C2N, Orsay<\/li><\/ul>\n\n\n\n<p>[<a href=\"http:\/\/hebergement.universite-paris-saclay.fr\/joovon\/pdf\/HDR_UPSud_Kim,JV.pdf\" target=\"_blank\" rel=\"noreferrer noopener\">PDF<\/a>]<\/p>\n\n\n\n<div style=\"height:100px\" aria-hidden=\"true\" class=\"wp-block-spacer\"><\/div>\n\n\n\n<h3 class=\"wp-block-heading\">Microscopic disorder, finite temperatures and spin waves in domain-wall driven exchange bias<\/h3>\n\n\n\n<p><strong>Doctor of Philosophy<br><\/strong><em>University of Western Australia<\/em><\/p>\n\n\n\n<p>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.<\/p>\n\n\n\n<p>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.<\/p>\n\n\n\n<p>This dissertation was submitted in August 2002 and examined by an external panel comprising:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Jean-Fran\u00e7ois Bobo \u2013 Laboratoire de Physique de la Mati\u00e8re Condens\u00e9e, Toulouse<\/li><li>Bernard Di\u00e9ny \u2013 SPINTEC, Grenoble<\/li><li>Roy Chantrell \u2013 University of York<\/li><\/ul>\n\n\n\n<p>[<a rel=\"noreferrer noopener\" href=\"http:\/\/hebergement.universite-paris-saclay.fr\/joovon\/pdf\/PhD_UWA_Kim,JV.pdf\" target=\"_blank\">PDF<\/a>]<\/p>\n<div class=\"post-meta\">Last modified 22\/06\/20<\/div>","protected":false},"excerpt":{"rendered":"<p>Noise, chirality, and chaos in the dynamics of magnons and magnetic solitons Habilitation \u00e0 diriger des recherchesFacult\u00e9 des Sciences \u2013 Universit\u00e9 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, [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":168,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-891","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=\/wp\/v2\/pages\/891","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=891"}],"version-history":[{"count":15,"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=\/wp\/v2\/pages\/891\/revisions"}],"predecessor-version":[{"id":994,"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=\/wp\/v2\/pages\/891\/revisions\/994"}],"up":[{"embeddable":true,"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=\/wp\/v2\/pages\/168"}],"wp:attachment":[{"href":"https:\/\/hebergement.universite-paris-saclay.fr\/joovon\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=891"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}