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Publications (10 of 77) Show all publications
von Malottki, S., Dupe, B., Bessarab, P. F., Delin, A. & Heinze, S. (2019). Enhanced skyrmion stability due to exchange frustration (vol 7, 12299, 2017). Scientific Reports, 9, Article ID 8158.
Open this publication in new window or tab >>Enhanced skyrmion stability due to exchange frustration (vol 7, 12299, 2017)
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2019 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, article id 8158Article in journal (Refereed) Published
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-252969 (URN)10.1038/s41598-019-44360-7 (DOI)000469219900001 ()31133684 (PubMedID)2-s2.0-85067078379 (Scopus ID)
Note

Erratum

20190812

Available from: 2019-08-12 Created: 2019-08-12 Last updated: 2019-08-12Bibliographically approved
Huttmann, F., Rothenbach, N., Kraus, S., Ollefs, K., Arruda, L. M., Bernien, M., . . . Wende, H. (2019). Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator. Journal of Physical Chemistry Letters, 10(5), 911-917
Open this publication in new window or tab >>Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator
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2019 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 10, no 5, p. 911-917Article in journal (Refereed) Published
Abstract [en]

We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M-5,M-4 edge XMCD, orbital and spin magnetic moments of (-0.1 +/- 0.3)mu(B) and (+7.0 +/- 0.6)mu(B), respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M-5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2019
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-250564 (URN)10.1021/acs.jpclett.8b03711 (DOI)000461271700003 ()30717591 (PubMedID)2-s2.0-85061921184 (Scopus ID)
Note

QC 20190430

Available from: 2019-04-30 Created: 2019-04-30 Last updated: 2019-04-30Bibliographically approved
von Malottki, S., Bessarab, P. F., Haldar, S., Delin, A. & Heinze, S. (2019). Skyrmion lifetime in ultrathin films. Physical Review B, 99(6), Article ID 060409.
Open this publication in new window or tab >>Skyrmion lifetime in ultrathin films
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 6, article id 060409Article in journal (Refereed) Published
Abstract [en]

We show that thermal stability of skyrmions due to entropic effects can be strongly affected by external control parameters such as magnetic field and interface composition. The lifetimes of isolated skyrmions in atomic Pd/Fe bilayers on Ir(111) and on Rh(111) are calculated in the framework of harmonic transition state theory based on an atomistic spin model parametrized from density functional theory. Depending on the system the attempt frequency for skyrmion collapse can change by up to nine orders of magnitude with the strength of the applied magnetic field. We demonstrate that this effect is due to a drastic change of entropy with skyrmion radius which opens a route toward stabilizing sub-10-nm skyrmions at room temperature.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-246256 (URN)10.1103/PhysRevB.99.060409 (DOI)000459933200003 ()2-s2.0-85062498277 (Scopus ID)
Note

QC 20190401

Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-04Bibliographically approved
Fan, X., Elgammal, K., Smith, A. D., Östling, M., Delin, A., Lemme, M. C. & Niklaus, F. (2018). Humidity and CO2 gas sensing properties of double-layer graphene. Carbon, 127, 576-587
Open this publication in new window or tab >>Humidity and CO2 gas sensing properties of double-layer graphene
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2018 (English)In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 127, p. 576-587Article in journal, Editorial material (Refereed) Published
Abstract [en]

Graphene has interesting gas sensing properties with strong responses of the graphene resistance when exposed to gases. However, the resistance response of double-layer graphene when exposed to humidity and gasses has not yet been characterized and understood. In this paper we study the resistance response of double-layer graphene when exposed to humidity and CO2, respectively. The measured response and recovery times of the graphene resistance to humidity are on the order of several hundred milliseconds. For relative humidity levels of less than ~ 3% RH, the resistance of double-layer graphene is not significantly influenced by the humidity variation. We use such a low humidity atmosphere to investigate the resistance response of double-layer graphene that is exposed to pure CO2 gas, showing a consistent response and recovery behaviour. The resistance of the double-layer graphene decreases linearly with increase of the concentration of pure CO2 gas. Density functional theory simulations indicate that double-layer graphene has a weaker gas response compared to single-layer graphene, which is in agreement with our experimental data. Our investigations contribute to improved understanding of the humidity and CO2 gas sensing properties of double-layer graphene which is important for realizing viable graphene-based gas sensors in the future.

Place, publisher, year, edition, pages
Netherlands: Elsevier, 2018
Keywords
Graphene, humidity, gas sensing, CO2
National Category
Other Engineering and Technologies not elsewhere specified
Research subject
Electrical Engineering; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-218275 (URN)10.1016/j.carbon.2017.11.038 (DOI)000417484000065 ()2-s2.0-85034837689 (Scopus ID)
Projects
M&MWaveGraphGEMS
Funder
EU, European Research Council, 277879Vinnova, 2016-01655Swedish Research Council, 2015-05112
Note

QC 20171127

Available from: 2017-11-25 Created: 2017-11-25 Last updated: 2019-08-20Bibliographically approved
Quellmalz, A., Smith, A. D., Elgammal, K., Fan, X., Delin, A., Östling, M., . . . Niklaus, F. (2018). Influence of Humidity on Contact Resistance in Graphene Devices. ACS Applied Materials and Interfaces, 10(48), 41738-41746
Open this publication in new window or tab >>Influence of Humidity on Contact Resistance in Graphene Devices
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2018 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 48, p. 41738-41746Article in journal (Refereed) Published
Abstract [en]

The electrical contact resistance at metal–graphene interfaces can significantly degrade the properties of graphene devices and is currently hindering the full exploitation of graphene’s potential. Therefore, the influence of environmental factors, such as humidity, on the metal–graphene contact resistance is of interest for all graphene devices that operate without hermetic packaging. We experimentally studied the influence of humidity on bottom-contacted chemical-vapor-deposited (CVD) graphene–gold contacts, by extracting the contact resistance from transmission line model (TLM) test structures. Our results indicate that the contact resistance is not significantly affected by changes in relative humidity (RH). This behavior is in contrast to the measured humidity sensitivity  of graphene’s sheet resistance. In addition, we employ density functional theory (DFT) simulations to support our experimental observations. Our DFT simulation results demonstrate that the electronic structure of the graphene sheet on top of silica is much more sensitive to adsorbed water molecules than the charge density at the interface between gold and graphene. Thus, we predict no degradation of device performance by alterations in contact resistance when such contacts are exposed to humidity. This knowledge underlines that bottom-contacting of graphene is a viable approach for a variety of graphene devices and the back end of the line integration on top of conventional integrated circuits.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
Keywords
graphene, bottom-contact, contact resistance, humidity sensitivity, integration, sheet resistance
National Category
Nano Technology Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-232554 (URN)10.1021/acsami.8b10033 (DOI)000452694100088 ()30387599 (PubMedID)2-s2.0-85057551886 (Scopus ID)
Funder
VINNOVA, 2016-01655 2017-05108Swedish Research Council, VR 2015-04608 VR 2016-05980Swedish Energy Agency, STEM P40147-1 STEM P40147-1EU, European Research Council, 277879 307311
Note

QC 20181207

Available from: 2018-07-25 Created: 2018-07-25 Last updated: 2019-01-08Bibliographically approved
Bessarab, P. F., Mueller, G. P., Lobanov, I. S., Rybakov, F. N., Kiselev, N. S., Jonsson, H., . . . Delin, A. (2018). Lifetime of racetrack skyrmions. Scientific Reports, 8, Article ID 3433.
Open this publication in new window or tab >>Lifetime of racetrack skyrmions
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2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 3433Article in journal (Refereed) Published
Abstract [en]

The skyrmion racetrack is a promising concept for future information technology. There, binary bits are carried by nanoscale spin swirls-skyrmions-driven along magnetic strips. Stability of the skyrmions is a critical issue for realising this technology. Here we demonstrate that the racetrack skyrmion lifetime can be calculated from first principles as a function of temperature, magnetic field and track width. Our method combines harmonic transition state theory extended to include Goldstone modes, with an atomistic spin Hamiltonian parametrized from density functional theory calculations. We demonstrate that two annihilation mechanisms contribute to the skyrmion stability: At low external magnetic field, escape through the track boundary prevails, but a crossover field exists, above which the collapse in the interior becomes dominant. Considering a Pd/Fe bilayer on an Ir(111) substrate as a well-established model system, the calculated skyrmion lifetime is found to be consistent with reported experimental measurements. Our simulations also show that the Arrhenius pre-exponential factor of escape depends only weakly on the external magnetic field, whereas the pre-exponential factor for collapse is strongly field dependent. Our results open the door for predictive simulations, free from empirical parameters, to aid the design of skyrmion-based information technology.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-224023 (URN)10.1038/s41598-018-21623-3 (DOI)000425590600057 ()29467438 (PubMedID)2-s2.0-85042361199 (Scopus ID)
Note

QC 20180323

Available from: 2018-03-23 Created: 2018-03-23 Last updated: 2018-03-23Bibliographically approved
Borlenghi, S., Boman, M. & Delin, A. (2018). Modeling reservoir computing with the discrete nonlinear Schrodinger equation. Physical review. E, 98(5), Article ID 052101.
Open this publication in new window or tab >>Modeling reservoir computing with the discrete nonlinear Schrodinger equation
2018 (English)In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 98, no 5, article id 052101Article in journal (Refereed) Published
Abstract [en]

We formulate, using the discrete nonlinear Schrodinger equation (DNLS), a general approach to encode and process information based on reservoir computing. Reservoir computing is a promising avenue for realizing neuromorphic computing devices. In such computing systems, training is performed only at the output level by adjusting the output from the reservoir with respect to a target signal. In our formulation, the reservoir can be an arbitrary physical system, driven out of thermal equilibrium by an external driving. The DNLS is a general oscillator model with broad application in physics, and we argue that our approach is completely general and does not depend on the physical realization of the reservoir. The driving, which encodes the object to be recognized, acts as a thermodynamic force, one for each node in the reservoir. Currents associated with these thermodynamic forces in turn encode the output signal from the reservoir. As an example, we consider numerically the problem of supervised learning for pattern recognition, using as a reservoir a network of nonlinear oscillators.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-239083 (URN)10.1103/PhysRevE.98.052101 (DOI)000448929900001 ()2-s2.0-85056391374 (Scopus ID)
Funder
Swedish Energy Agency, STEM P40147-1Swedish Research Council, VR 2016-05980Swedish Research Council, VR 2016-01961Swedish Research Council, VR 2015-04608
Note

QC 20181121

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2019-08-20Bibliographically approved
Borlenghi, S. & Delin, A. (2018). Stochastic Thermodynamics of Oscillators' Networks. Entropy, 20(12), Article ID 992.
Open this publication in new window or tab >>Stochastic Thermodynamics of Oscillators' Networks
2018 (English)In: Entropy, ISSN 1099-4300, E-ISSN 1099-4300, Vol. 20, no 12, article id 992Article in journal (Refereed) Published
Abstract [en]

We apply the stochastic thermodynamics formalism to describe the dynamics of systems of complex Langevin and Fokker-Planck equations. We provide in particular a simple and general recipe to calculate thermodynamical currents, dissipated and propagating heat for networks of nonlinear oscillators. By using the Hodge decomposition of thermodynamical forces and fluxes, we derive a formula for entropy production that generalises the notion of non-potential forces and makes transparent the breaking of detailed balance and of time reversal symmetry for states arbitrarily far from equilibrium. Our formalism is then applied to describe the off-equilibrium thermodynamics of a few examples, notably a continuum ferromagnet, a network of classical spin-oscillators and the Frenkel-Kontorova model of nano friction.

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
stochastic thermodynamics, heat transfer, oscillators' networks, entropy production, KKER H, 1977, PHYSICAL REVIEW A, V16, P2126 me Tania, 2010, PHYSICAL REVIEW E, V82, KKER H, 1979, PHYSICA A, V95, P311 aun OM, 1998, PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS, V306, P1 rlenghi Simone, 2016, PHYSICAL REVIEW E, V93, KKER H, 1975, ZEITSCHRIFT FUR PHYSIK B-CONDENSED MATTER, V21, P295 xty Denes, 2014, PHYSICS LETTERS B, V729, P108 sager L, 1931, PHYSICAL REVIEW, V38, P2265 rlenghi Simone, 2014, PHYSICAL REVIEW B, V89, lls R., 1980, Differential Analysis on Complex Manifolds, rlenghi Simone, 2014, PHYSICAL REVIEW LETTERS, V112, llgaard A., 2013, PHYSICAL REVIEW D, V88
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-241215 (URN)10.3390/e20120992 (DOI)000454282000100 ()2-s2.0-85058962757 (Scopus ID)
Note

QC 20190118

Available from: 2019-01-18 Created: 2019-01-18 Last updated: 2019-08-20Bibliographically approved
von Malottki, S., Dupe, B., Bessarab, P. F., Delin, A. & Heinze, S. (2017). Enhanced skyrmion stability due to exchange frustration. Scientific Reports, 7, Article ID 12299.
Open this publication in new window or tab >>Enhanced skyrmion stability due to exchange frustration
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2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 12299Article in journal (Refereed) Published
Abstract [en]

Skyrmions are localized, topologically non-trivial spin structures which have raised high hopes for future spintronic applications. A key issue is skyrmion stability with respect to annihilation into the ferromagnetic state. Energy barriers for this collapse have been calculated taking only nearest neighbor exchange interactions into account. Here, we demonstrate that exchange frustration can greatly enhance skyrmion stability. We focus on the prototypical film system Pd/Fe/Ir(111) and use an atomistic spin model parametrized from first-principles calculations. We show that energy barriers and critical fields of skyrmion collapse as well as skyrmion lifetimes are drastically enhanced due to frustrated exchange and that antiskyrmions are metastable. In contrast an effective nearest-neighbor exchange model can only account for equilibrium properties of skyrmions such as their magnetic field dependent profile or the zero temperature phase diagram. Our work shows that frustration of long range exchange interactions -a typical feature in itinerant electron magnets -is a route towards enhanced skyrmion stability even in systems with a ferromagnetic ground state.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-215818 (URN)10.1038/s41598-017-12525-x (DOI)000411677500003 ()28951587 (PubMedID)2-s2.0-85030030341 (Scopus ID)
Note

QC 20171017

Available from: 2017-10-17 Created: 2017-10-17 Last updated: 2017-10-17Bibliographically approved
Pan, F., Chico, J., Delin, A., Bergman, A. & Bergqvist, L. (2017). Extended spin model in atomistic simulations of alloys. Physical Review B, 95(18), Article ID 184432.
Open this publication in new window or tab >>Extended spin model in atomistic simulations of alloys
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 18, article id 184432Article in journal (Refereed) Published
Abstract [en]

An extended atomistic spin model allowing for studies of the finite-temperature magnetic properties of alloys is proposed. The model is obtained by extending the Heisenberg Hamiltonian via a parametrization from a first-principles basis, interpolating from both the low-temperature ferromagnetic and the high-temperature paramagnetic reference states. This allows us to treat magnetic systems with varying degree of itinerant character within the model. Satisfactory agreement with both previous theoretical studies and experiments are obtained in terms of Curie temperatures and paramagnetic properties. The proposed model is not restricted to elements but is also applied to binary alloys, such as the technologically important material permalloy, where significant differences in the finite magnetic properties of Fe and Ni magnetic moments are found. The proposed model strives to find the right compromise between accuracy and computational feasibility for accurate modeling, even for complex magnetic alloys and compounds.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-211618 (URN)10.1103/PhysRevB.95.184432 (DOI)000405203000011 ()2-s2.0-85024367724 (Scopus ID)
Funder
Swedish Research Council, VR 2015-04608Swedish Research Council, VR 2016-05980Swedish e‐Science Research CentereSSENCE - An eScience CollaborationEU, FP7, Seventh Framework Programme, 600382Swedish Energy Agency, STEM P40147-1
Note

QC 20170810

Available from: 2017-08-10 Created: 2017-08-10 Last updated: 2017-11-29Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7788-6127

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