[Wien] Magnetic moments converging in a different direction to the one they are defined

Vojtech Chlan chlan at mbox.troja.mff.cuni.cz
Wed Apr 17 15:59:39 CEST 2019


Dear Charlie,

I join Martin's recommendation to try LDA+U. By looking at the 
non-integer total magnetic moment per unit cell 14.88 muB, I guess you 
obtained (semi)metallic state, which might or might not be what you 
want. Pure GGA sometimes leads to metallic solution for iron oxides 
(even for those they are good insulators). I don't know how pronounced 
the semi-metallic character should be for greigite, probably much more 
than for magnetite, but even then the value of 14.88 muB looks quite far 
from 16.00 muB. (For magnetite you should reach this integer value even 
without LDA+U.)

Anyway, the LDA+U approach will also help you stabilizing the desired 
spin structure quite efficiently. I would be surprised if the moments 
flipped when you have some U (a few eV) applied to 3d states of Fe. In 
fact, you can use LDA+U just to get close to desired spin structure and 
then try to remove it ....

And additional note: when reducing the symmetry of magnetite be aware of 
the fact that it is a mixed-valence compound (and I guess greigite might 
be similar case) and has a tendency to localize the minority electron 
within the octahedral sublattice. When you split the eight octahedral Fe 
into four sorts they can readily differentiate to Fe2+ and Fe3+. (Even 
in the cubic phase.) Which again may or may not what you want to have in 
your model. (With +U you will definitely get the Fe2+ / Fe3+ scenario, 
unless you keep all octahedral Fe equivalent.)

Regards,
Vojtech


On 17-Apr-19 12:00, Penny, Charles wrote:
> Dear all,
>
> Thanks for your prompt and helpful replies.
>
> I have run runfsm initially before switching to runsp, but no luck. It 
> immediately converges back to the ferrimagnetic solution.
>
> This raises two further thoughts/questions in my mind.
>
> 1) I am assuming that runfsm is NOT an acceptable replacement for 
> runsp with regards to projecting the total energy onto the Heisenberg 
> model. In any case I obtain a large moment in the interstitial region 
> which likely invalidates such a move.
>
> 2) I find it interesting that this process works without problem for 
> Fe3O4, but seems presently rather futile with Fe3S4. Simply swapping O 
> for S in the lattice (not quite, of course) appears to create such 
> instability in alternative spin configurations. Is anyone aware of 
> anything I could look into which might be able to explain this, or is 
> it just a case of 'bad luck' for this material and move on if I can't 
> get any improvement?
>
> Re: Magnetite
>
> An interesting material without a doubt. Luckily I'm not investigating 
> the low temperature phase, otherwise I might have more of a headache!
>
> Regards,
>
> Charlie
> ------------------------------------------------------------------------
> *From:* Wien <wien-bounces at zeus.theochem.tuwien.ac.at> on behalf of 
> Stefaan Cottenier <Stefaan.Cottenier at UGent.be>
> *Sent:* 17 April 2019 08:50
> *To:* A Mailing list for WIEN2k users
> *Subject:* Re: [Wien] Magnetic moments converging in a different 
> direction to the one they are defined
> Dear Martin,
>
> You trigger long-forgotten memories... ;-)
>
> Amazing that you remember that talk. That was the only way in which 
> these results were ever communicated, there was no paper. The reason 
> for this was that we did observe very clear and smooth energy 
> dependences upon continuous rotation of the Fe-moments in different 
> ways and for different configurations, yet the overall picture was a 
> messy paradox (= looking at one individual result would lead to one 
> conclusion, looking at another individual result would lead to an 
> opposite conclusion).
>
> There has always been the feeling that once we should revisit this, 
> but it never got realized so far. Perhaps, if you feel like...
>
> Best regards,
> Stefaan
>
>
> > -----Original Message-----
> > From: Wien <wien-bounces at zeus.theochem.tuwien.ac.at> On Behalf Of
> > pieper
> > Sent: Tuesday, April 16, 2019 12:51 PM
> > To: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at>
> > Subject: Re: [Wien] Magnetic moments converging in a different 
> direction to
> > the one they are defined
> >
> > 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 at 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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