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Magnetic deflagration and detonation in crystals of nanomagnets
Umeå University, Faculty of Science and Technology, Department of Physics.
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 [en]
Nanomagnets, magnetic deflagration, front instability, Zeeman energy, magnetic instability, magnetic detonation, weak detonation
National Category
Physical Sciences
Research subject
Physics Of Matter
Identifiers
URN: urn:nbn:se:umu:diva-124445ISBN: 978-91-7601-534-6OAI: oai:DiVA.org:umu-124445DiVA: diva2:952143
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
List of papers
1. Anisotropic properties of spin avalanches in crystals of nanomagnets
Open this publication in new window or tab >>Anisotropic properties of spin avalanches in crystals of nanomagnets
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2013 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 87, no 1, 014409Article in journal (Refereed) Published
Abstract [en]

Anisotropy effects for spin avalanches in crystals of nanomagnets are studied theoretically with the external magnetic field applied at an arbitrary angle to the easy axis. Starting with the Hamiltonian for a single nanomagnet in the crystal, two essential quantities characterizing spin avalanches are calculated: the activation and Zeeman energies. The calculation is performed numerically for a wide range of angles and analytical formulas are derived within the limit of small angles. The anisotropic properties of a single nanomagnet lead to anisotropic behavior of the magnetic deflagration speed. Modifications of the magnetic deflagration speed are investigated for different angles between the external magnetic field and the easy axis of the crystals. Anisotropic properties of magnetic detonation are also studied, which concern, first of all, the temperature behind the leading shock and the characteristic time of spin switching in the detonation.

Place, publisher, year, edition, pages
American Physical Society, 2013
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-63800 (URN)10.1103/PhysRevB.87.014409 (DOI)000313157100003 ()
Funder
Swedish Research Council
Available from: 2013-01-09 Created: 2013-01-07 Last updated: 2016-08-12Bibliographically approved
2. Multidimensional instability and dynamics of spin-avalanches in crystals of nanomagnets
Open this publication in new window or tab >>Multidimensional instability and dynamics of spin-avalanches in crystals of nanomagnets
2014 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, no 21, 217206Article in journal (Refereed) Published
Abstract [en]

We obtain a fundamental instability of the magnetization-switching fronts in superparamagnetic and ferromagnetic materials such as crystals of nanomagnets, ferromagnetic nanowires, and systems of quantum dots with large spin. We develop the instability theory for both linear and nonlinear stages. By using numerical simulations we investigate the instability properties focusing on spin avalanches in crystals of nanomagnets. The instability distorts spontaneously the fronts and leads to a complex multidimensional front dynamics. We show that the instability has a universal physical nature, with a deep relationship to a wide variety of physical systems, such as the Darrieus-Landau instability of deflagration fronts in combustion, inertial confinement fusion, and thermonuclear supernovae, and the instability of doping fronts in organic semiconductors.

Place, publisher, year, edition, pages
American Physical Society, 2014
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-96684 (URN)10.1103/PhysRevLett.113.217206 (DOI)000345745800012 ()25479521 (PubMedID)
Funder
Swedish Research Council
Available from: 2014-11-26 Created: 2014-11-26 Last updated: 2016-08-12Bibliographically approved
3. Magnetic detonation structure in crystals of nanomagnets controlled by thermal conduction and volume viscosity
Open this publication in new window or tab >>Magnetic detonation structure in crystals of nanomagnets controlled by thermal conduction and volume viscosity
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 9, 094428Article in journal (Refereed) Published
Abstract [en]

Experimentally detected ultrafast spin avalanches spreading in crystals of molecular (nano) magnets [Decelle et al., Phys. Rev. Lett. 102, 027203 (2009)] have recently been explained in terms of magnetic detonation [Modestov et al., Phys. Rev. Lett. 107, 207208 (2011)]. Here magnetic detonation structure is investigated by taking into account transport processes of the crystals such as thermal conduction and volume viscosity. The transport processes result in smooth profiles of the most important thermodynamical crystal parameters, temperature, density, and pressure, all over the magnetic detonation front, including the leading shock, which is one of the key regions of magnetic detonation. In the case of zero volume viscosity, thermal conduction leads to an isothermal discontinuity instead of the shock, for which temperature is continuous while density and pressure experience jump. It is also demonstrated that the thickness of the magnetic detonation front may be controlled by applying the transverse-magnetic field, which is important for possible experimental observations of magnetic detonation.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-102360 (URN)10.1103/PhysRevB.91.094428 (DOI)000351875300002 ()
Available from: 2015-06-23 Created: 2015-04-23 Last updated: 2016-08-12Bibliographically approved
4. Counterpart of the Darrieus-Landau instability at a magnetic deflagration front
Open this publication in new window or tab >>Counterpart of the Darrieus-Landau instability at a magnetic deflagration front
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2016 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 13, 134418Article in journal (Refereed) Published
Abstract [en]

The magnetic instability at the front of the spin avalanche in a crystal of molecular magnets is considered. This phenomenon reveals similar features with the Darrieus-Landau instability, inherent to classical combustion flame fronts. The instability growth rate and the cutoff wavelength are investigated with respect to the strength of the external magnetic field, both analytically in the limit of an infinitely thin front and numerically for finite-width fronts. The presence of quantum tunneling resonances is shown to increase the growth rate significantly, which may lead to a possible transition from deflagration to detonation regimes. Different orientations of the crystal easy axis are shown to exhibit opposite stability properties. In addition, we suggest experimental conditions that could evidence the instability and its influence on the magnetic deflagration velocity.

Keyword
Molecular magnets, Flames, Mn-12-acetate, Stability, Fusion
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-119332 (URN)10.1103/PhysRevB.93.134418 (DOI)000373974800005 ()
Funder
Swedish Research Council
Available from: 2016-04-15 Created: 2016-04-15 Last updated: 2016-08-12Bibliographically approved
5. Multilevel model for magnetic deflagration in nanomagnet crystals
Open this publication in new window or tab >>Multilevel model for magnetic deflagration in nanomagnet crystals
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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
Nanomagnets, Zeeman energy, spin avalanches, magnetic deflagration
National Category
Physical Sciences
Research subject
Physics Of Matter
Identifiers
urn:nbn:se:umu:diva-124442 (URN)
Funder
Swedish Research Council
Available from: 2016-08-11 Created: 2016-08-11 Last updated: 2016-08-12Bibliographically approved

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