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Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1505 Interacting Magnetic Nanosystems An Experimental Study Of Superspin Glasses MIKAEL SVANTE ANDERSSON ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-554-9893-1 UPPSALA urn:nbn:se:uu:diva-319717 2017 Dissertation presented at Uppsala University to be publicly examined in Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Friday, 2 June 2017 at 13:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Dino Fiorani (Istituto di Struttura della Materia, Italy). Abstract Andersson, M. S. 2017. Interacting Magnetic Nanosystems. An Experimental Study Of Superspin Glasses. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1505. 74 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9893-1. This thesis presents experimental results on strongly interacting γ-Fe2O3 magnetic nanoparticles and their collective properties. The main findings are that very dense randomly packed (≈60%) γ-Fe2O3 nanoparticles form a replica of a spin glass. The magnetic properties of the nanoparticle system are in most regards the same as those of an atomic spin glass. The system is therefore proposed as a model superspin glass. In superspin glasses the interacting building blocks that form the collective state are single domain nanoparticles, superspins with a magnetic moment of about 10000 μB, which can be compared to the atomic magnetic moment in spin glasses of approximately 1 μB. It was found that the relaxation time of the individual nanoparticles impacts the collective properties and governs the superspin dimensionality. Several dense compacts, each prepared with nanoparticles of a specific size, with diameters 6, 8, 9 and 11.5 nm, were studied. All the studied compacts were found to form a superspin glass state. Non- interacting reference samples, consisting of the same particles but coated with a silica shell, were synthesized to determine the single particle magnetic properties. It was also found that the effects of the nanoparticle size distribution, which lead to a variation of the magnetic properties, can be mitigated by having strong enough interparticle interactions. The majority of the work was carried out using SQUID magnetometry. Keywords: spin glass, SQUID magnetometry, maghemite, magnetism, nanoparticles Mikael Svante Andersson, Department of Engineering Sciences, Solid State Physics, Box 534, Uppsala University, SE-751 21 Uppsala, Sweden. © Mikael Svante Andersson 2017 ISSN 1651-6214 ISBN 978-91-554-9893-1 urn:nbn:se:uu:diva-319717 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-319717) Dedicated to the ones that came before and gave us what we have today. Allt har jag sett, både fröjd och misär, det har tillsammans fört mig hit Nu står jag här, och jag är den jag är och jag vill inte ändra en bit Allt jag har sett, Sånger ur Sten 1994 Big Fish MIKAEL SVANTE ANDERSSON born in Uppsala in 1988 earned his Master’s Degree in Materials Engineering from Uppsala University in spring 2013. The same spring he joined the di- vision of Solid State Physics at Uppsala University. His re- search is focused on magnetic materials and especially inter- acting magnetic nanoparticles. PREFACE This thesis is based on my research on strongly interacting magnetic nanoparticle systems during 2013-2017. This re- search was supervised by Dr. Roland Mathieu, Prof. Per Nord- blad and Prof. Peter Svedlindh at the department of Engineer- ing Sciences, Division of Solid State Physics at Uppsala Uni- versity in Sweden and Dr. Jose A. De Toro at the department of Physics at University Castilla-La Mancha in Spain. The the- sis is based on work published in peer-review journals and some unpublished work. Published work which do not con- tribute to the discussion of interacting magnetic nanosystems is only mentioned as “Other publications” in the list of pub- lications. In Sweden there are two common ways to write a thesis; a monograph or a comprehensive summary with the publications attached, this thesis is written as a comprehensive summary. The thesis is divided into the following chapters: In- troduction, in which an short introduction to the field of mag- netism, with a focus on spin glasses, is given. The introduction also covers the relevant parts of the field of supermagnetism to set the stage for the rest of the thesis. Experiments, in which the measurement protocols and equipment are described as well as the synthesis of the samples used in the studies cov- ered in this thesis. Results and discussion, in which the main results are presented and discussed. Concluding remarks and outlook, in which the results are summarized and concluding remarks are made, as well as a short discussion of the future in interacting nanosystems. Summary in Swedish includes a short summary of the thesis written in Swedish. The papers which this thesis is based upon are attached at the end (the pa- pers are omitted in the digital version). vii CONTENTS Preface vii Contents ix List of Publications xi 1 Introduction 1 1.1 Magnetism .............................. 1 1.2 Spin glasses ............................. 3 1.3 Superspin glasses .......................... 5 2 Experiments 7 2.1 Sample Fabrication ......................... 7 2.2 Magnetic Measurements ...................... 7 2.3 Magnetic measurement protocols ................. 8 2.3.1 DC magnetization as a function of temperature . 8 2.3.2 DC-Memory ....................... 8 2.3.3 Low field isothermal remanent magnetization as a function of temperature ................. 9 2.3.4 Relaxation measurements ................ 10 2.3.5 Magnetization as a function of applied magnetic field 12 2.3.6 δM(H) ........................... 12 2.3.7 Magnetic AC-susceptibility as a function of temper- ature ............................ 13 3 Results and discussion 15 3.1 Non-interacting magnetic nanoparticles ............. 16 3.1.1 Surface spin disorder and its impact on single parti- cle magnetic properties ................. 16 3.1.2 Intraparticle interactions and their impact on δM .. 20 3.2 A dense ensemble of magnetic nanoparticles with spin glass like properties ............................ 23 3.3 Size dependence of particle interactions and superspin glass properties .............................. 28 3.3.1 Interparticle interactions ................ 28 3.3.2 Size dependence of superspin glass properties .... 30 3.4 Superspin glass properties in a system consisting of mixed particles ............................... 33 3.5 Demagnetization effects for high concentrations of magnetic nanoparticles ............................ 36 3.6 Effects of the individual particle relaxation time on the super- spin glass state ........................... 42 4 Concluding remarks and outlook 47 Contents 4.1 Concluding remarks ........................ 47 4.2 Outlook ............................... 47 5 Summary in Swedish 49 Acknowledgments 53 References 55 LIST OF PUBLICATIONS Publication discussed in this thesis I Ageing dynamics of a superspin glass M. S. Andersson, J. A. De Toro, S. S. Lee, R. Mathieu and P. Nordblad EPL (Europhysics Letters) 108, 17004 (2014) II Size-dependent surface effects in maghemite nanopar- ticles and its impact on interparticle interactions in dense assemblies M. S. Andersson, R. Mathieu, S. S. Lee, P. S. Normile, G. Singh, P. Nordblad and J. A. De Toro Nanotechnology 26, 475703 (2015) III Particle size-dependent superspin glass behavior in random compacts of monodisperse maghemite nanoparticles M. S. Andersson, R. Mathieu, P. S. Normile, S. S. Lee, G. Singh, P. Nordblad and J. A. De Toro Materials Research Express 3, 045015 (2016) IV Effects of the individual particle relaxation time on superspin glass dynamics M. S. Andersson, J. A. De Toro, S. S. Lee, P. S. Normile, G. Singh, P. Nordblad and R. Mathieu Physical Review B 93, 054407 (2016) V Magnetic properties of nanoparticle compacts with controlled broadening of the particle size distributions M. S. Andersson, R. Mathieu, P. S. Normile, S. S. Lee, G. Singh, P. Nordblad and J. A. De Toro In manuscript xi VI Demagnetization effects in dense nanoparticle assem- blies P. S. Normile, M. S. Andersson, R. Mathieu, S. S. Lee, G. Singh and J. A. De Toro Applied Physics Letters 109, 152404 (2016) VII Spin disorder in nanoparticles: A cautionary tale on the use of Henkel and δM plots to assess interparticle interactions J. A. De Toro, M. Vasilakaki, S. S. Lee, M. S. Andersson, P. S. Normile, N. Yaacoub, P. Murray, P. Muñiz, D. Peddis, R. Mathieu, K. Liu, J. Geshev, K. N. Trohidou, and J. Nogués In manuscript Contributions to papers I Planned the experiment and performed the measure- ments. Analyzed the data and had an active roll in the discussion of the results. Wrote most of the paper. II Planned and performed some of the magnetic measure- ments. Analyzed the data for the magnetic measure- ments and had an active roll in the discussion of the re- sults. Wrote most of the paper. III Planned and performed the magnetic measurements. Analyzed the data for the magnetic measurements and had an active roll in the discussion of the results. Wrote the paper. IV Planned and performed the magnetic measurements. Analyzed the data for the magnetic measurements and had an active roll in the discussion of the results. Wrote the paper. V Performed the magnetic measurements. Analyzed the data for the magnetic measurements and had an active roll in the discussion of the results. Wrote the paper. VI Took part in the data analysis of the the magnetic mea- surements and had an active roll in the discussion of the results. Took part in the writing of the paper. VII Took part in the data analysis of the magnetic measure- ments and had an active roll in the discussion of the re- sults. Took part in the writing of the paper. xii List of Publications Reprints were made with permission from the respective publisher. Disclaimer This thesis is based on my licenciate thesis "PROPERTIES OF A MODEL SUPERSPIN GLASS SYSTEM: An experimental study of densely packed γ-Fe2O3 nanoparti- cles", which was written as a half-time report during my Ph.D studies.
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