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Collective properties of magnetic mesospins
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.ORCID iD: 0000-0001-5752-1980
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mesoscopic spin systems consist of an ensemble of lithographically patterned nanomagnetic elements - mesospins. The interactions between the mesospins, can be designed at will by altering their lateral arrangement, enabling the study of collective magnetic order in a wide range of systems.          

  The spin dimensionality of a mesospin is controlled by its shape and form. Thin elongated elements are Ising-like, with only two possible magnetization directions. Disc shaped elements can be single domain and behave XY-like, with a magnetization direction free to rotate in the plane of the disc. Larger disc sizes result in magnetic vortices. Tuning the material parameters of the elements enables mesospin dynamics at and below room temperature. Combining all of the above, the magnetic state of a lattice is then defined by the mesospins lateral arrangement, their spin dimensionality, and the temperature.          

  In this Thesis we investigate the magnetic order and dynamic properties in a series of different configurations, where the nano-magnetic elements are in the vortex state, Ising-like mesospins or of mixed mesospin dimensionality. Chains of Ising-mesospins were investigated and shown to be successfully described by the Ising model. A lossless transition between the magnetic vortex state and the collinear state, was found in square arrays of magnetic discs. In a more complicated interaction regime, square artificial spin ice, the dynamical range of the Ising-like mesospins in the lattice was probed, in terms of magnetization relaxation studies.          

  Utilizing the configurational freedom in mesoscopic spin systems, together with the possibility to alter the spin dimensionality of the elements, it is possible to create a lattice with no naturally occurring analogue. In such a lattice, where XY mesospins were added to square artificial spin ice, it was found that the degeneracy of the square ice model was restored. Furthermore, using a reciprocal space analysis tool, the magnetic spin structure factor, the system was shown to possess the characteristic features of a Coulomb spin liquid with strong local correlations and absence of long range order. Increasing the interaction between the elements, results in an emergent magnetic order on a large length-scale.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. , p. 89
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1619
Keywords [en]
artificial spin ice, magnetic nano-structures, mesoscopic spin systems, mesospins, mixed spin dimensionalities, interaction modifier, engineering of energy landscape, emergence
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-338722ISBN: 978-91-513-0205-8 (print)OAI: oai:DiVA.org:uu-338722DiVA, id: diva2:1173262
Public defence
2018-03-02, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2018-02-06 Created: 2018-01-12 Last updated: 2018-03-08
List of papers
1. Ising like behaviour of mesoscopic magnetic chains
Open this publication in new window or tab >>Ising like behaviour of mesoscopic magnetic chains
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-338704 (URN)
Available from: 2018-01-11 Created: 2018-01-11 Last updated: 2018-01-12
2. Hysteresis-free switching between vortex and collinear magnetic states
Open this publication in new window or tab >>Hysteresis-free switching between vortex and collinear magnetic states
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2014 (English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 16, p. 053002-Article in journal (Refereed) Published
Abstract [en]

We demonstrate a lossless switching between vortex and collinear magnetic states in circular FePd disks arranged in a square lattice. Above a bifurcation temperature (Te) we show that thermal fluctuations are enough to facilitate flipping between the two distinctly different magnetic states. We find that the temperature dependence of the vortex annihilation and nucleation fields can be described by a simple power law relating them to the saturation magnetization.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-224245 (URN)10.1088/1367-2630/16/5/053002 (DOI)000335383800002 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2018-01-12Bibliographically approved
3. Thermally induced magnetic relaxation in square artificial spin ice
Open this publication in new window or tab >>Thermally induced magnetic relaxation in square artificial spin ice
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2016 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 37097Article in journal (Refereed) Published
Abstract [en]

The properties of natural and artificial assemblies of interacting elements, ranging from Quarks to Galaxies, are at the heart of Physics. The collective response and dynamics of such assemblies are dictated by the intrinsic dynamical properties of the building blocks, the nature of their interactions and topological constraints. Here we report on the relaxation dynamics of the magnetization of artificial assemblies of mesoscopic spins. In our model nano-magnetic system - square artificial spin ice - we are able to control the geometrical arrangement and interaction strength between the magnetically interacting building blocks by means of nano-lithography. Using time resolved magnetometry we show that the relaxation process can be described using the Kohlrausch law and that the extracted temperature dependent relaxation times of the assemblies follow the Vogel-Fulcher law. The results provide insight into the relaxation dynamics of mesoscopic nano-magnetic model systems, with adjustable energy and time scales, and demonstrates that these can serve as an ideal playground for the studies of collective dynamics and relaxations.

National Category
Physical Sciences Materials Engineering
Identifiers
urn:nbn:se:uu:diva-315927 (URN)10.1038/srep37097 (DOI)000392061500001 ()27883013 (PubMedID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologyThe Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Available from: 2017-02-22 Created: 2017-02-22 Last updated: 2018-01-12Bibliographically approved
4. Interaction modiers in articial spin ices
Open this publication in new window or tab >>Interaction modiers in articial spin ices
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics Nano Technology
Identifiers
urn:nbn:se:uu:diva-328864 (URN)
Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-01-12

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