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Microscopic aspects of magnetic lattice demagnetizing factors
KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
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2017 (English)In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 1, no 4, article id 044406Article in journal (Refereed) Published
Abstract [en]

The demagnetizing factor N is of both conceptual interest and practical importance. Considering localized magnetic moments on a lattice, we show that for nonellipsoidal samples, N depends on the spin dimensionality (Ising, XY, or Heisenberg) and orientation, as well as the sample shape and susceptibility. The generality of this result is demonstrated by means of a recursive analytic calculation as well as detailed Monte Carlo simulations of realistic model spin Hamiltonians. As an important check and application, we also make an accurate experimental determination of N for a representative collective paramagnet (i.e., the Dy2Ti2O7 spin ice compound) and show that the temperature dependence of the experimentally determined N agrees closely with our theoretical calculations. Our conclusion is that the well-established practice of approximating the true sample shape with "corresponding ellipsoids" for systems with long-range interactions will in many cases overlook important effects stemming from the microscopic aspects of the system under consideration.

Place, publisher, year, edition, pages
2017. Vol. 1, no 4, article id 044406
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-220295DOI: 10.1103/PhysRevMaterials.1.044406ISI: 000416582000002OAI: oai:DiVA.org:kth-220295DiVA, id: diva2:1169094
Note

QC 20171222

Available from: 2017-12-22 Created: 2017-12-22 Last updated: 2018-05-02Bibliographically approved
In thesis
1. Spin ice and demagnetising theory
Open this publication in new window or tab >>Spin ice and demagnetising theory
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Frustration, or the inability to simultaneously minimise all local interactions is, a phenomenon occurring in a broad number of physical systems. We will in this thesis focus on a class of frustrated ferromagnetic materials called spin ices and how both numerical and experimental techniques can be used to understand their properties. Spin ices show a number of peculiar properties such as low temperature residual entropy and magnetic monopole excitations. 

Considering a dipolar Hamiltonian model with exchange interactions we verify a qualitative and previously established agreement with experimental data of the quantity χT/C , where χ is the magnetic susceptibility, T  the temperature and C the Curie parameter. We find a quantitative agreement by identifying that further near-neighbour interactions are sensitive probes of χT/C and the neutron structure factor, in particular its zone boundary scattering and relative peak intensities. 

In systems passing from being governed by ferromagnetic interactions into potentially ordered anti-ferromagnets at low temperature we define special temperatures in close relation with real gases. These temperatures enable a new classication of "inverting" magnets of which spin ice is a member. 

Due to rich complex long-range interactions in spin ice and a high sensitivity of the quantity χT/C, we identify demagnetising corrections to be crucial in extracting the correct physics. Apart from previously reported results we find the demagnetising factor to be clearly temperature and lattice structure dependent and not just shape dependent. The large moment of the Dy ions in Dy2Ti2O7  thus implies that an incorrect demagnetising treatment can shift the important features in χT/C  outside of the relevant temperature range considered. Employing our refined demagnetising theory we obtain good agreement with experiments down to sub-kelvin temperatures. 

The magnetic ions in spin ice enable neutron scattering as an excellent tool to study spin ice. A massively parallel computer code is developed in order to obtain high resolution neutron scattering factors in Fourier space. These high resolution charts are in good agreement with carefully verified experimental data down to 350 mK.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2018. p. 61
Series
TRITA-SCI-FOU ; 2018:19
Keywords
Geometrical frustration, spin ice, Monte Carlo simulations, demagnetising theory, special temperatures, Ewald summation, neutron scattering, structure factors, loop algorithms, parallel tempering, frustrated ferromagnets, equilibration issues, long-range interactions, exchange interactions.
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-227037 (URN)978-91-7729-733-8 (ISBN)
Public defence
2018-05-31, FB42, AlbaNova, Roslagstullsbacken 21C, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2013-03968Stiftelsen Olle Engkvist Byggmästare, 2014/807
Note

QC 20180502

Available from: 2018-05-02 Created: 2018-05-02 Last updated: 2018-05-02Bibliographically approved

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Twengström, MikaelHenelius, Patrik

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