Ab initio Studies on Exchange Interactions in Metals and Correlated Oxides (dissertation)
ADAM JAKOBSSON
Abstract:
A report is given on the implementation of a frozen magnon technique in the FLAPW code FLEUR, showing that the procedure reduced computational costs compared with a previously implemented method, while delivering accurate results. Furthermore a generalisation was made of the well known one lattice formula that connects the total energy differences of spin-spirals to the adiabatic magnon excitation energies to multiple lattice systems. The implemented method was applied to B2 structured FeCo to investigate the changes of the Curie temperature with a tetragonal distortion and it was found that the FeCo films grown on Rh have characteristics that makes it a good candidate for HAMR storage media. The method was also applied to a selection of spin-gapless semiconductors in order to investigate their magnetic properties. It was found that all of the studied materials have a Curie temperature well above room temperature and excellent agreement with experimental results was obtained for the material Mn2CoAl that has been synthesised. Finally, the implemented frozen magnon method was adjusted to include constraining fields to restrict the directions of the magnetic moments. It was shown that this procedure significantly improved the agreement between the dispersion for all considered spin-spiral configuration and introduced significant adjustments of the exchange parameters. However for other materials, such as bcc-Fe, such corrections yield worse results and, motivated by the need for a consistent method, we considered a self consistent spin-spiral based method to extract the exchange parameters from the inverse transverse static magnetic susceptibility. Preliminary results show that the newly implemented method gives results close to the corrected frozen magnon method for FeCo and results close to the uncorrected frozen magnon method for bcc Fe and thus provides a consistent improvement over the two previously used methods.
Contents
1 Introduction .................................................................................................. 9
2 Ab initio Calculations ................................................................................ 11
2.1 Density Functional Theory ............................................................ 11
2.2 Spin-Density Functional Theory ................................................... 14
2.3 Energy Functionals ........................................................................ 14
2.4 The Kohn-Sham Equations in a Periodic Potential ...................... 16
2.5 The Eigenvalue Problem ................................................................ 17
2.6 The LAPW and APW+lo Methods ............................................... 18
2.7 Non-Collinear Magnetism ............................................................. 20
2.8 Spin-Spirals and the Generalised Bloch Theorem ....................... 21
3 Heisenberg Model Calculations ................................................................ 23
3.1 Exchange Parameters from Total Energy Calculations ............... 23
3.2 Magnons and Spin-Spiral Total Energies ..................................... 24
3.3 The Magnetic Force Theorem ....................................................... 26
3.4 Exchange Parameters from Susceptibility Calculations .............. 27
4 Key Results ................................................................................................ 29
4.1 Test case bcc Fe .............................................................................. 29
4.2 Transition-Metal Monoxides ......................................................... 30
4.3 Magnetic Properties of Distorted B2 structured FeCo ................ 35
4.4 Magnetic Properties of Inverse Heusler Materials ....................... 37
4.5 Constraining Fields, the MFT and Susceptibility Calculations . . 39
5 Perspectives and Outlook .......................................................................... 45
6 Acknowledgments ..................................................................................... 46
7 Sammanfattning på svenska ...................................................................... 47
8 References .................................................................................................. 50
To download the dissertation click on the link below:
https://uu.diva-portal.org/smash/get/diva2:794092/FULLTEXT03.pdf
Abstract:
A report is given on the implementation of a frozen magnon technique in the FLAPW code FLEUR, showing that the procedure reduced computational costs compared with a previously implemented method, while delivering accurate results. Furthermore a generalisation was made of the well known one lattice formula that connects the total energy differences of spin-spirals to the adiabatic magnon excitation energies to multiple lattice systems. The implemented method was applied to B2 structured FeCo to investigate the changes of the Curie temperature with a tetragonal distortion and it was found that the FeCo films grown on Rh have characteristics that makes it a good candidate for HAMR storage media. The method was also applied to a selection of spin-gapless semiconductors in order to investigate their magnetic properties. It was found that all of the studied materials have a Curie temperature well above room temperature and excellent agreement with experimental results was obtained for the material Mn2CoAl that has been synthesised. Finally, the implemented frozen magnon method was adjusted to include constraining fields to restrict the directions of the magnetic moments. It was shown that this procedure significantly improved the agreement between the dispersion for all considered spin-spiral configuration and introduced significant adjustments of the exchange parameters. However for other materials, such as bcc-Fe, such corrections yield worse results and, motivated by the need for a consistent method, we considered a self consistent spin-spiral based method to extract the exchange parameters from the inverse transverse static magnetic susceptibility. Preliminary results show that the newly implemented method gives results close to the corrected frozen magnon method for FeCo and results close to the uncorrected frozen magnon method for bcc Fe and thus provides a consistent improvement over the two previously used methods.
Contents
1 Introduction .................................................................................................. 9
2 Ab initio Calculations ................................................................................ 11
2.1 Density Functional Theory ............................................................ 11
2.2 Spin-Density Functional Theory ................................................... 14
2.3 Energy Functionals ........................................................................ 14
2.4 The Kohn-Sham Equations in a Periodic Potential ...................... 16
2.5 The Eigenvalue Problem ................................................................ 17
2.6 The LAPW and APW+lo Methods ............................................... 18
2.7 Non-Collinear Magnetism ............................................................. 20
2.8 Spin-Spirals and the Generalised Bloch Theorem ....................... 21
3 Heisenberg Model Calculations ................................................................ 23
3.1 Exchange Parameters from Total Energy Calculations ............... 23
3.2 Magnons and Spin-Spiral Total Energies ..................................... 24
3.3 The Magnetic Force Theorem ....................................................... 26
3.4 Exchange Parameters from Susceptibility Calculations .............. 27
4 Key Results ................................................................................................ 29
4.1 Test case bcc Fe .............................................................................. 29
4.2 Transition-Metal Monoxides ......................................................... 30
4.3 Magnetic Properties of Distorted B2 structured FeCo ................ 35
4.4 Magnetic Properties of Inverse Heusler Materials ....................... 37
4.5 Constraining Fields, the MFT and Susceptibility Calculations . . 39
5 Perspectives and Outlook .......................................................................... 45
6 Acknowledgments ..................................................................................... 46
7 Sammanfattning på svenska ...................................................................... 47
8 References .................................................................................................. 50
To download the dissertation click on the link below:
https://uu.diva-portal.org/smash/get/diva2:794092/FULLTEXT03.pdf
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