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Interacting Magnetic Nanosystems: An Experimental Study Of Superspin Glasses
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 74
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1505
Keywords [en]
spin glass, SQUID magnetometry, maghemite, magnetism, nanoparticles
National Category
Engineering and Technology Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-319717ISBN: 978-91-554-9893-1 (print)OAI: oai:DiVA.org:uu-319717DiVA, id: diva2:1087448
Public defence
2017-06-02, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2017-05-10 Created: 2017-04-07 Last updated: 2017-05-16
List of papers
1. Ageing dynamics of a superspin glass
Open this publication in new window or tab >>Ageing dynamics of a superspin glass
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2014 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 108, no 1, p. 17004-Article in journal (Refereed) Published
Abstract [en]

Magnetization dynamics of a model superspin glass system consisting of nearly monodispersed close-packed maghemite particles of diameter 8 nm is investigated. The observed non-equilibrium features of the dynamics are qualitatively similar to those of atomic spin glass systems. The intrinsic relaxation function, as observed in zero-field-cooled magnetization relaxation experiments, depends on the time the sample has been kept at constant temperature (ageing). Accompanying low-field experiments show that the archetypal spin glass characteristics -ageing, memory and rejuvenation- are reproduced in this dense system of dipolar-dipolar interacting superspins.  

National Category
Materials Engineering Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-238588 (URN)10.1209/0295-5075/108/17004 (DOI)000343970300018 ()
Available from: 2014-12-16 Created: 2014-12-14 Last updated: 2017-12-05Bibliographically approved
2. Size-dependent surface effects in maghemite nanoparticles and its impact on interparticle interactions in dense assemblies
Open this publication in new window or tab >>Size-dependent surface effects in maghemite nanoparticles and its impact on interparticle interactions in dense assemblies
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2015 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 26, no 47, article id 475703Article in journal (Refereed) Published
Abstract [en]

The question of the dominant interparticle magnetic interaction type in random closely packed assemblies of different diameter (6.2-11.5 nm) bare maghemite nanoparticles (NPs) is addressed. Single-particle magnetic properties such as particle anisotropy and exchange bias field are first of all studied in dilute (reference) systems of these same NPs, where interparticle interactions are neglible. Substantial surface spin disorder is revealed in all particles except the smallest, viz. for diameters d = 8-11.5 nm but not for d = 6.2-6.3 nm. X-ray diffraction analysis points to a crystallographic origin of this effect. The study of closely packed assemblies of the d >= 8 nm particles observes collective (superspin) freezing that clearly appears to be governed by interparticle dipole interactions. However, the dense assemblies of the smallest particles exhibit freezing temperatures that are higher than expected from a simple (dipole) extrapolation of the corresponding temperatures found in the d >= 8 nm assemblies. It is suggested that the nature of the dominant interparticle interaction in these smaller particle assemblies is superexchange, whereby the lack of significant surface spin disorder allows this mechanism to become important at the level of interacting superspins.

Keywords
nanoparticles, magnetism, dipolar, superexchange
National Category
Nano Technology
Identifiers
urn:nbn:se:uu:diva-272272 (URN)10.1088/0957-4484/26/47/475703 (DOI)000366209100010 ()
Funder
Swedish Research CouncilGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology
Available from: 2016-01-18 Created: 2016-01-13 Last updated: 2017-11-30Bibliographically approved
3. Particle size-dependent superspin glass behavior in random compacts of monodisperse maghemite nanoparticles
Open this publication in new window or tab >>Particle size-dependent superspin glass behavior in random compacts of monodisperse maghemite nanoparticles
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2016 (English)In: MATERIALS RESEARCH EXPRESS, ISSN 2053-1591, Vol. 3, no 4, article id 045015Article in journal (Refereed) Published
Abstract [en]

Dense random assemblies made from highly monodisperse gamma-Fe2O3 nanoparticles with sizes ranging from 6.2 to 11.5 nm have been investigated by DC and AC magnetometry. It is found that all assemblies undergo superspin glass phase transitions. The superspin glass phase transition temperature is strongly dependent on the particle size and the nature of the interparticle interaction. However the transition from superparamagnet to superspin glass, as evidenced by the shape of the ac-susceptibility curves and the dynamic critical exponents associated with the transition, is similar in all systems.

Keywords
magnetic nanoparticles, maghemite, superspin glass, phase transition
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-299615 (URN)10.1088/2053-1591/3/4/045015 (DOI)000377811500016 ()
Funder
Swedish Research CouncilGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology
Available from: 2016-07-25 Created: 2016-07-25 Last updated: 2017-04-07Bibliographically approved
4. Effects of the individual particle relaxation time on superspin glass dynamics
Open this publication in new window or tab >>Effects of the individual particle relaxation time on superspin glass dynamics
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2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 5, article id 054407Article in journal (Refereed) Published
Abstract [en]

The low temperature dynamic magnetic properties of two dense magnetic nanoparticle assemblies with similar superspin glass transition temperatures T-g similar to 140 K are compared. The two samples are made from batches of 6 and 8 nm monodisperse gamma-Fe2O3 nanoparticles, respectively. The properties of the individual particles are extracted from measurements on reference samples where the particles have been covered with a thick silica coating. The blocking temperatures of these dilute assemblies are found at 12.5 K for the 6 nm particles and at 35 K for the 8 nm particles, which implies different anisotropy energy barriers of the individual particles and vastly different temperature evolution of their relaxation times. The results of the measurements on the concentrated particle assemblies suggest a strong influence of the particle energy barrier on the details of the aging dynamics, memory behavior, and apparent superspin dimensionality of the particles.

National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-279564 (URN)10.1103/PhysRevB.93.054407 (DOI)000369726000001 ()
Funder
Swedish Research Council
Available from: 2016-03-02 Created: 2016-03-02 Last updated: 2017-04-07Bibliographically approved
5. Magnetic properties of nanoparticle compacts with controlled broadening of the particle size distribution
Open this publication in new window or tab >>Magnetic properties of nanoparticle compacts with controlled broadening of the particle size distribution
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 18, article id 184431Article in journal (Refereed) Published
Abstract [en]

Binary random compacts with different proportions of small (volume V) and large (volume 2V) essentially bare maghemite nanoparticles are used to investigate the effect of controllably broadening the particle size distribution on the magnetic properties of magnetic nanoparticle assemblies with strong dipolar interaction. A series of eight random mixtures of highly uniform 9.0- and 11.5-nm-diameter maghemite particles prepared by thermal decomposition is studied. In spite of the severely broadened size distributions in the mixed samples, well-defined superspin glass transition temperatures are observed across the series, their values increasing linearly with the weight fraction of large particles.

National Category
Engineering and Technology Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-319714 (URN)10.1103/PhysRevB.95.184431 (DOI)000405203000010 ()
Funder
Swedish Research Council
Note

Title in List of papers in Thesis: Magnetic properties of nanoparticles compacts with controlled broadening of the particle size distribution

Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2017-11-29Bibliographically approved
6. Demagnetization effects in dense nanoparticle assemblies
Open this publication in new window or tab >>Demagnetization effects in dense nanoparticle assemblies
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2016 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 109, no 15, article id 152404Article in journal (Refereed) Published
Abstract [en]

We highlight the relevance of demagnetizing-field corrections in the characterization of dense magnetic nanoparticle assemblies. By an analysis that employs in-plane and out-of-plane magnetometry on cylindrical assemblies, we demonstrate the suitability of a simple analytical formula-based correction method. This allows us to identify artifacts of the demagnetizing field in temperature-dependent susceptibility curves (e.g., shoulder peaks in curves from a disordered assembly of essentially bare magnetic nanoparticles). The same analysis approach is shown to be a straightforward procedure for determining the magnetic nanoparticle packing fraction in dense, disordered assemblies.

National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-308922 (URN)10.1063/1.4964517 (DOI)000386534800031 ()
Funder
Swedish Research Council
Available from: 2016-12-01 Created: 2016-12-01 Last updated: 2017-11-29Bibliographically approved
7. Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles
Open this publication in new window or tab >>Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles
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2017 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 29, no 19, p. 8258-8268Article in journal (Refereed) Published
Abstract [en]

Remanence magnetization plots (e.g., Henkel or delta M plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that delta M plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the delta M plots of gamma-Fe2O3 nanoparticles isolated by thick SiO2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mossbauer measurements, that has a pronounced effect on the delta M plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the delta M plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and delta M plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles.

Place, publisher, year, edition, pages
Washington, D.C., USA: American Chemical Society (ACS), 2017
National Category
Engineering and Technology Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-319716 (URN)10.1021/acs.chemmater.7b02522 (DOI)000412965800027 ()
Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2018-02-01Bibliographically approved

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