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Multilevel model for magnetic deflagration in nanomagnet crystals
Umeå University, Faculty of Science and Technology, Department of Physics.
Nordita, KTH Royal Institute of Technology and Stockholm University.
Umeå University, Faculty of Science and Technology, Department of Physics.ORCID iD: 0000-0003-3096-1972
Department of Applied Physics, Chalmers University of Technology.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We extends the existing theoretical model for determining the characteristic features of magnetic deflagration in nanomagnet crystals. The Zeeman energy, the deflagration velocity, and other parameters are computed taking into account all spin energy levels of the molecular magnet.  We also consider the effect of a strong transverse magnetic field, and show that the latter significantly modifies the spin-state structure, leading to an uncertainty concerning the activation energy of the spin flipping. We show that taking into account all energy levels reduces the final temperature as well as the Zeeman energy released, affecting the velocity of propagation of the spin-flipping front. The results obtained for the front velocity are in very good agreement with experimental data for a crystal of Mn12-acetate in a longitudinal magnetic field.

Keyword [en]
Nanomagnets, Zeeman energy, spin avalanches, magnetic deflagration
National Category
Physical Sciences
Research subject
Physics Of Matter
Identifiers
URN: urn:nbn:se:umu:diva-124442OAI: oai:DiVA.org:umu-124442DiVA: diva2:952033
Funder
Swedish Research Council
Available from: 2016-08-11 Created: 2016-08-11 Last updated: 2016-08-12Bibliographically approved
In thesis
1. Magnetic deflagration and detonation in crystals of nanomagnets
Open this publication in new window or tab >>Magnetic deflagration and detonation in crystals of nanomagnets
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis we cover the dynamics of the macro magnetic transformations (spin avalanches) in crystals of molecular nanomagnets, also known as magnetic deflagration and detonation.

Taking a single-molecule Hamiltonian, we calculate the dependence of Zeeman energy and the activation energy as a function of an external magnetic field at different angles relative to the easy axis of the crystal. Using quantum mechanical calculations, we show that the energy levels of the molecule exhibit complex behavior in presence of a transverse component of the magnetic field. For an arbitrarily aligned magnetic field, the energy levels do not arrange in a simple "double-well" manner. We extend existing theoretical models by generalizing the Zeeman energy for a wide range of magnetic fields and its different orientations.

We obtain a new type of front instability in magnetization-switching media. Due to the dipole-dipole interaction between the molecules magnetic instability results to the front banding and change in the front propagation velocity. The magnetic instability has a universal physical nature similar to the Darrieus-Landau instability. The instability growth rate and the cutoff length are calculated for the spin avalanches in the crystals of nanomagnets.

Finally, we investigate the internal structure of the magnetic detonation front. We calculate the continuous shock profile using the transport processes of the crystal such as thermal conduction and volume viscosity. Such an approach can be applied to any weak shock wave in solids. Zero volume viscosity leads to an isothermal jump, i.e., the temperature changes continuously while the pressure and the density experience discontinuity. The analysis has shown that the volume viscosity plays a major role in the formation of the detonation front.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2016. 40 p.
Keyword
Nanomagnets, magnetic deflagration, front instability, Zeeman energy, magnetic instability, magnetic detonation, weak detonation
National Category
Physical Sciences
Research subject
Physics Of Matter
Identifiers
urn:nbn:se:umu:diva-124445 (URN)978-91-7601-534-6 (ISBN)
Public defence
2016-09-05, MC413, MIT-huset, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2016-08-15 Created: 2016-08-11 Last updated: 2016-08-15Bibliographically approved

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