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First Principles Theory of the hcp-fcc Phase Transition in Cobalt
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
KTH, Centra, SeRC - Swedish e-Science Research Centre.ORCID-id: 0000-0001-6509-5782
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.ORCID-id: 0000-0003-4341-5663
Vise andre og tillknytning
2017 (engelsk)Inngår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, artikkel-id 3778Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Identifying the forces that drive a phase transition is always challenging. The hcp-fcc phase transition that occurs in cobalt at similar to 700 K has not yet been fully understood, although early theoretical studies have suggested that magnetism plays a main role in the stabilization of the fcc phase at high temperatures. Here, we perform a first principles study of the free energies of these two phases, which we break into contributions arising from the vibration of the lattice, electronic and magnetic systems and volume expansion. Our analysis of the energy of the phases shows that magnetic effects alone cannot drive the fcc-hcp transition in Co and that the largest contribution to the stabilization of the fcc phase comes from the vibration of the ionic lattice. By including all the contributions to the free energy considered here we obtain a theoretical transition temperature of 825 K.

sted, utgiver, år, opplag, sider
NATURE PUBLISHING GROUP , 2017. Vol. 7, artikkel-id 3778
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-211395DOI: 10.1038/s41598-017-03877-5ISI: 000403650300021PubMedID: 28630476Scopus ID: 2-s2.0-85021051213OAI: oai:DiVA.org:kth-211395DiVA, id: diva2:1130121
Merknad

QC 20170808

Tilgjengelig fra: 2017-08-08 Laget: 2017-08-08 Sist oppdatert: 2017-11-10bibliografisk kontrollert
Inngår i avhandling
1. A theoretical study of magnetism and its extension to finite temperatures in random alloys
Åpne denne publikasjonen i ny fane eller vindu >>A theoretical study of magnetism and its extension to finite temperatures in random alloys
2017 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

This work presents new theoretical developments of atomistic spin simulations of magnetic materials at finite temperatures. Special focus is put on the description of longitudinal magnetic fluctuations and the application in random transition metal alloys. A new computational scheme is proposed for mapping total energies from electronic structure calculations to an extended atomistic spin model. The proposed model has some new appealing features from previous models. To be more specific, the proposed model successfully eliminates the reference state dependency of the mapping that previous models have suffered from. Moreover, the proposed model includes longitudinal magnetic fluctuations that gives an improved description of the magnetic properties over a larger temperature interval. The proposed model strives to find the right compromise between accuracy and computational feasibility and it is applied not only to the elemental systems Fe, Co and Ni, but also to a number of binary transition metal alloys such as Permalloy (Fe$_{20\%}$Ni$_{80\%}$) and Fe-Co systems.

Electronic structure calculations of Gilbert damping and the closely related magnetodynamic properties, the saturation magnetization and exchange stiffness, have been conducted for a number of different magnetic systems including Permalloy with additional doping of $4d$ or $5d$ transition metal impurities and the full Heusler alloy Co$_2$FeAl. Regarding the Permalloy based systems, a systematic study of the magnetodynamic properties was performed and compared with existing experimental data. In general we found good agreement and manage to explain the main trends regarding the Gilbert damping across the series with a simple model that captures the most important material properties to the damping, namely the spin orbit coupling and density of states at the Fermi level. In Co$_2$FeAl, we calculated the Gilbert damping in different existing crystal structures and compare those with new experimental data and found good agreement between them.

Magnon properties of random alloys, like Permalloy, are studied using two complementory methods, the adiabatic magnon spectra valid at zero temperature and from finite temperature atomistic spin dynamics through the dynamical structure factor. The influence of chemical disorder and temperature effects on the magnon properties are investigated that hopefully could motivate new experimental studies of these materials.

sted, utgiver, år, opplag, sider
Stockholm: KTH Royal Institute of Technology, 2017. s. 82
Serie
TRITA-FYS, ISSN 0280-316X
HSV kategori
Forskningsprogram
Fysik
Identifikatorer
urn:nbn:se:kth:diva-213460 (URN)978-91-7729-502-0 (ISBN)
Disputas
2017-09-29, Sal B, Electrum 229, Kistagången 16, Kista, 10:00 (engelsk)
Opponent
Veileder
Forskningsfinansiär
Swedish Research Council, 2015-04608Swedish Research Council, 2016-05980Swedish Energy Agency, P40147-1
Merknad

QC 20170904

Tilgjengelig fra: 2017-09-04 Laget: 2017-08-31 Sist oppdatert: 2017-09-04bibliografisk kontrollert

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