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Ferromagnetic/Antiferromagnetic Exchange Bias Nanostructures for Ultimate Spintronic Devices Kamil Akmaldinov

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Ferromagnetic/Antiferromagnetic Exchange Bias Nanostructures for Ultimate Spintronic Devices Kamil Akmaldinov Ferromagnetic/antiferromagnetic exchange bias nanostructures for ultimate spintronic devices Kamil Akmaldinov To cite this version: Kamil Akmaldinov. Ferromagnetic/antiferromagnetic exchange bias nanostructures for ultimate spin- tronic devices. Condensed Matter [cond-mat]. Université Grenoble Alpes, 2015. English. NNT : 2015GREAY009. tel-01161786 HAL Id: tel-01161786 https://tel.archives-ouvertes.fr/tel-01161786 Submitted on 9 Jun 2015 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. THÈSE Pour obtenir le grade de DOCTEUR DE L’UNIVERSITÉ DE GRENOBLE Spécialité : Physique Arrêté ministériel : 7 août 2006 Présentée par Kamil AKMALDINOV Thèse dirigée par Vincent BALTZ codirigée par Bernard DIENY préparée au sein du Laboratoire SPINTEC et de la société Crocus Technology dans l'École Doctorale de Physique de Grenoble Ferromagnetic/antiferromagnetic exchange bias nanostructures for ultimate spintronic devices Thèse soutenue publiquement le 6 février 2015 , devant le jury composé de : M. Manuel BIBES Directeur de recherche du CNRS, Rapporteur M. Robert STAMPS Professeur de l’Université de Glasgow, Rapporteur M. Olivier BOURGEOIS Directeur de recherche du CNRS, Membre M. Luc LECHEVALLIER Maître de conférences de l’Université de Cergy-Pontoise, Membre Mme Clarisse DUCRUET Ingénieur R&D chez Crocus Technology, Membre M. Vincent BALTZ Chargé de recherche du CNRS, Membre Université Joseph Fourier / Université Pierre Mendès France / Université Stendhal / Université de Savoie / Grenoble INP Contents INTRODUCTION ........................................................................................................... 5 Chapter 1. Introduction to ferromagnetic / antiferromagnetic exchange bias ....... 7 1.1. Exchange bias phenomenology ................................................................................... 7 1.1.1. Discovery .............................................................................................................. 7 1.1.2. Setting exchange bias ........................................................................................... 9 1.2. Theoretical models .................................................................................................... 11 1.2.1. Meikeljohn and Bean intuitive picture ............................................................... 11 1.2.2. Néel/Mauri domain wall model ......................................................................... 13 1.2.3. Magnetic frustrations: Malozemoff’s random field and Takano models .......... 16 1.2.4. Todays’ macroscopic picture: granular model plus spin-glass like phases ........ 19 1.3. Quantifying the amount of spin-glass like phases ..................................................... 22 Chapter 2. Applications of exchange bias and issues to be solved ....................... 29 2.1. Introduction of key concepts ..................................................................................... 29 2.1.1. Giant magnetoresistance ................................................................................... 29 2.1.2. Tunnel magnetoresistance ................................................................................. 31 2.1.3. Spin-valve ........................................................................................................... 33 2.2. Magnetoresistive read heads for hard disk drives .................................................... 34 2.3. Memories ................................................................................................................... 37 2.3.1. Various magnetic random access memories (MRAM) approaches ................... 38 2.3.2. Focus on thermally-assisted MRAM (TA-MRAM) .............................................. 43 2.4. Issues to be dealt with in the present thesis ............................................................. 47 1 Chapter 3. Minimizing the amount of spin-glass like phases ............................... 49 3.1. The role of Mn in the formation of spin-glasses ....................................................... 49 3.1.1. Direct imaging of Mn diffusion using atom-probe tomography [1] .................. 49 3.1.2. Further influence of neighbouring getters ......................................................... 58 3.2. Barriers to the diffusion of Mn .................................................................................. 61 3.2.1. Dual barriers [2] .................................................................................................. 61 3.2.2. Attempt with more complex barriers ................................................................ 69 Chapter 4. Insights of spin-glass like phases for applied spintronics .................... 71 4.1. Mixing antiferromagnets to tune TA-MRAM interfacial spin-glasses [3] .................. 71 4.2. Amount of spin-glasses over thin films and bit-cell dispersion in TA-MRAM [4] ...... 77 CONCLUSION AND PERSPECTIVES ............................................................................... 85 Bibliographical references .......................................................................................... 89 Glossary – acronyms and abbreviations ..................................................................... 103 Appendices ........................................................................................................ 105 Appendix 1: using single barrier to the diffusion of Mn………………………………………………105 Appendix 2: tuning the interface vs volume contributions via ion irradiations…………..108 Appendix 3: comparing blocking temperature distributions and dichroism results..….111 2 3 4 INTRODUCTION The research presented here is focused on ferromagnetic/antiferromagnetic (F/AF) exchange bias (EB) nanostructures for ultimate spintronics devices and more specifically for the improvement of thermally-assisted random access memories (TA-MRAM) now developed by the CROCUS technology company. It was conducted within the SPINTEC laboratory (spintronics and technology of compounds), joint research unit between French institutions: University of Grenoble Alpes, CNRS, CEA (INAC) in the frame of a joint research and development program between the CROCUS technology company and the SPINTEC laboratory. Such a program financially supported this PhD thesis under a CIFRE grant (Convention Industrielle de Formation par la REcherche ). Basically, a MRAM comprises a magnetic tunnel junction, an exchange biased reference layer and a storage layer. The specificity of a TA-MRAM is that the storage layer is also exchange biased for improved data retention. The bit writing scheme of every memory cell involves additional heating to temporarily unblock the exchange bias coupling between the F and AF stacks of the storage layer. To ensure stable device functioning and to compete with other memory types the memory cell to memory cell parameters dispersion such as the exchange bias loop shift and the write power should be as low as possible. It is well-known, that EB can be strongly affected by diverse external conditions - from the deposition rates and layers thicknesses to annealing temperatures and fields. It becomes even more critical when talking about nano-dimensional structures, where the shape and the size of a bit also matters. No doubts, all these parameters should be taken into account to control the variability and in the literature there are numerous studies focused on shape, growth and fabrication reasons for cells dispersions. Apart from that, it is widely recognised that EB is strongly dependent on the F-AF interfacial quality and in particular on the amount of highly unstable regions - spin-glass like phases. It was supposed few years ago that randomly spread spin-glass like phases at the F/AF interface or within the bulk of the AF layer significantly contribute to the distributions of EB properties in devices after processing. This PhD thesis aimed at factually addressing this point. The manuscript is therefore structured as follows: Chapter 1 introduces the exchange bias phenomenology and state-of-art. It covers early theoretical suggestions considering ideal systems and to more complicated systems accounting for interfacial spin-glass like phases. It continues with the description of nowadays macroscopic picture based on both experimental findings and theoretical view. 5 The latter assumes a granular model coupled to F/AF spin-glass like phases and will be used throughout. Finally, and given the description of today’s macroscopic view, this chapter ends with a paragraph about the recent way proposed to quantify spin-glass like phases via bimodal blocking temperature distributions. Chapter 2 first introduces to the reader some of the basic principles in use in spintronics devices such as giant- and tunnel magnetoresistance and spin transfer torque. In a second step devices employing the EB phenomenon are described. In particular, the chapter ends with a focus on TA-MRAM devices and sets the issues to be addressed in the frame of this PhD thesis: i) better understanding and finding ways to tune the
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