[Wien] Magnetic moments converging in a different direction to the one they are defined
pieper
pieper at ifp.tuwien.ac.at
Tue Apr 16 12:51:11 CEST 2019
Fe3O4 being an old but unsatisfied love of mine a few additional
comments:
Determining exchange constants by spin reversal only makes sense if the
changes in electronic structure are small (see e.g. P. Novak et. al
PHYSICAL REVIEW B 71, 184433, 2005).
This (usually) works best in insulators, it is a delicate problem in
metals, and much more so in Fe3O4 with its Vervey transition. Here a
very intricate coupling between electronic and structural degrees of
freedom is at work. I seem to recall that the low temperature phase is a
comlicated mess (see e.g. Novak et al, PRB 61, 1256, 2000 and references
therein). So complicated that, as far as I remember, in the early
2000nds S.Cottenier, R. Laskowski, J. Rusz, M. Rots and P. Novak gave a
talk on a Wien2k Workshop calculating exchange interactions in magnetite
using the non-collinear magnetism version NCM-Wien2k. Unfortunately I
don't have time to search for literature on that one, but you probably
don't want to get into NCM anyway.
However, I don't think you can avoid DFT+U or +EECE - at least not for
Fe3O4. I am sure you can find a lot of literature on DFT+U and +EECE of
Fe3O4, among others by Novak, Madsen, ... This may introduce an
additional parameter in your comparisons of your structures. If you are
using an older version of Wien2k, upgrade! Wien26_16 had a bug with
DFT+U (see
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg15590.html).
Best regards,
Martin Pieper
---
Dr. Martin Pieper
Karl-Franzens University
Institute of Physics
Universitätsplatz 5
A-8010 Graz
Austria
Tel.: +43-(0)316-380-8564
Am 2019-04-16 10:29, schrieb Penny, Charles:
> Dear all,
>
> I am running spin-polarised calculations on a range of iron-spinel
> structures (namely, magnetite (Fe3O4), maghemite (gamma-Fe2O3) and
> greigite (Fe3S4)) with the objective of calculating magnetic exchange
> energies in these minerals. This requires calculating total energies
> of lot of different spin configurations. This process has worked well
> for magnetite and maghemite, but I have encountered a problem with
> greigite.
>
> When I run a calculation on a spin configuration of greigite that
> isn’t the ferrimagnetic ground state (e.g. a ferromagnetic
> configuration) the calculation converges to the ferrimagnetic
> solution, with the sublattice moments pointing in opposing directions.
>
>
> In the examples below, I have used a low-symmetry unit cell with
> eight unique iron atoms which allows me to calculate the required
> number of spin configurations for estimating J_ij. Atoms 1-4
> correspond to A site iron atoms in the spinel structure, atoms 5-8
> correspond to B site iron atoms and atoms 9-16 are sulphur atoms. In a
> ferrimagnetic system the A and B sites have opposing moments and
> sulphur atoms are non-magneitc.
>
> When I define a ferrimagnetic spin configuration, the calculation
> proceeds as expected, with the final moments looking like;
>
> rkmax_8_k_500.scf::MMINT: MAGNETIC MOMENT IN INTERSTITIAL =
> -0.05116
>
> rkmax_8_k_500.scf::MMI001: MAGNETIC MOMENT IN SPHERE 1 =
> 2.47349
>
> rkmax_8_k_500.scf::MMI002: MAGNETIC MOMENT IN SPHERE 2 =
> 2.47348
>
> rkmax_8_k_500.scf::MMI003: MAGNETIC MOMENT IN SPHERE 3 =
> 2.47348
>
> rkmax_8_k_500.scf::MMI004: MAGNETIC MOMENT IN SPHERE 4 =
> 2.47348
>
> rkmax_8_k_500.scf::MMI005: MAGNETIC MOMENT IN SPHERE 5 =
> -3.01699
>
> rkmax_8_k_500.scf::MMI006: MAGNETIC MOMENT IN SPHERE 6 =
> -3.01699
>
> rkmax_8_k_500.scf::MMI007: MAGNETIC MOMENT IN SPHERE 7 =
> -3.01699
>
> rkmax_8_k_500.scf::MMI008: MAGNETIC MOMENT IN SPHERE 8 =
> -3.01699
>
> rkmax_8_k_500.scf::MMI009: MAGNETIC MOMENT IN SPHERE 9 =
> -0.03675
>
> rkmax_8_k_500.scf::MMI010: MAGNETIC MOMENT IN SPHERE 10 =
> -0.03675
>
> rkmax_8_k_500.scf::MMI011: MAGNETIC MOMENT IN SPHERE 11 =
> -0.03675
>
> rkmax_8_k_500.scf::MMI012: MAGNETIC MOMENT IN SPHERE 12 =
> -0.03675
>
> rkmax_8_k_500.scf::MMI013: MAGNETIC MOMENT IN SPHERE 13 =
> -0.03675
>
> rkmax_8_k_500.scf::MMI014: MAGNETIC MOMENT IN SPHERE 14 =
> -0.03675
>
> rkmax_8_k_500.scf::MMI015: MAGNETIC MOMENT IN SPHERE 15 =
> -0.03675
>
> rkmax_8_k_500.scf::MMI016: MAGNETIC MOMENT IN SPHERE 16 =
> -0.03675
>
> rkmax_8_k_500.scf::MMTOT: SPIN MAGNETIC MOMENT IN CELL =
> -14.88108
>
> Final energy;
>
> rkmax_8_k_500.scf::ENE : ********** TOTAL ENERGY IN Ry =
> -43322.30312592
>
> However, when I define a ferromagnetic spin configuration the system
> converges to a ferrimagnetic solution with final moments;
>
> k_500_rkmax_8.scf::MMINT: MAGNETIC MOMENT IN INTERSTITIAL =
> 0.05118
>
> k_500_rkmax_8.scf::MMI001: MAGNETIC MOMENT IN SPHERE 1 =
> -2.47348
>
> k_500_rkmax_8.scf::MMI002: MAGNETIC MOMENT IN SPHERE 2 =
> -2.47347
>
> k_500_rkmax_8.scf::MMI003: MAGNETIC MOMENT IN SPHERE 3 =
> -2.47346
>
> k_500_rkmax_8.scf::MMI004: MAGNETIC MOMENT IN SPHERE 4 =
> -2.47346
>
> k_500_rkmax_8.scf::MMI005: MAGNETIC MOMENT IN SPHERE 5 =
> 3.01697
>
> k_500_rkmax_8.scf::MMI006: MAGNETIC MOMENT IN SPHERE 6 =
> 3.01697
>
> k_500_rkmax_8.scf::MMI007: MAGNETIC MOMENT IN SPHERE 7 =
> 3.01697
>
> k_500_rkmax_8.scf::MMI008: MAGNETIC MOMENT IN SPHERE 8 =
> 3.01697
>
> k_500_rkmax_8.scf::MMI009: MAGNETIC MOMENT IN SPHERE 9 =
> 0.03675
>
> k_500_rkmax_8.scf::MMI010: MAGNETIC MOMENT IN SPHERE 10 =
> 0.03675
>
> k_500_rkmax_8.scf::MMI011: MAGNETIC MOMENT IN SPHERE 11 =
> 0.03675
>
> k_500_rkmax_8.scf::MMI012: MAGNETIC MOMENT IN SPHERE 12 =
> 0.03675
>
> k_500_rkmax_8.scf::MMI013: MAGNETIC MOMENT IN SPHERE 13 =
> 0.03675
>
> k_500_rkmax_8.scf::MMI014: MAGNETIC MOMENT IN SPHERE 14 =
> 0.03675
>
> k_500_rkmax_8.scf::MMI015: MAGNETIC MOMENT IN SPHERE 15 =
> 0.03675
>
> k_500_rkmax_8.scf::MMI016: MAGNETIC MOMENT IN SPHERE 16 =
> 0.03675
>
> k_500_rkmax_8.scf::MMTOT: SPIN MAGNETIC MOMENT IN CELL =
> 14.88103
>
> Final energy is the same as in the ferrimagnetic case;
>
> k_500_rkmax_8.scf::ENE : ********** TOTAL ENERGY IN Ry =
> -43322.30312578
>
> Charge distance looks like it converges in both cases. Note that
> whilst the two calculations have the same saved name, they are in
> completely different CASE files. To outline my procedure, I initially
> call,
>
> instgen -ask
>
> And define the moments as ‘u u u u u u u u n n n n n n n n’ for a
> ferromagnetic calculation and ‘u u u u d d d d n n n n n n n n’
> for a ferrimagnetic calculation.
>
> Then,
>
> init -b -sp -numk 500 -rkmax 8.00
>
> runsp -ec 0.0001 -cc 0.0001 -fc 1.0 -p -i 200
>
> I am at a loss as to what is going on and can’t find anything in
> the mailing list to explain this. Any help on this matter would be
> greatly appreciated.
>
> Regards,
>
> Charlie
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