[Wien] AFM type II

[pieper]

I guess you are talking about the conventional classification of AFM 
ordering used in neutron diffraction. Consider AFM as a standing wave of 
spin orientations. Different Types of AFM are then distinguished by the 
wave vector of the standing wave in the crystal lattice.

AFM-I is the AFM order with the shortest possible wavelength, that is 
moments on nearest neighbor planes are antiparallel. In an fcc structure 
the nearest neighbors of an atom at (0,0,0) are on the two planes 
'above' and 'below' along the space diagonal of the cube at positions 
(1/2,1/2,0) and so on. The wave vector of the standing wave describing 
spin orientation is perpendicular to these planes of parallel spins. The 
length of the wave vector q in reciprocal space is such that going the 
distance d to the next plane with parallel moments (half the space 
diagonal) in that direction result in an identical situation in the wave 
function cos(qx), that is q*d=2*pi.

For AFM-II moments on planes with next nearest neighbors are 
antiparallel. And so on. The longer the wavelength of the standing wave 
(or the shorter q in reciprocal space) the more unit cells in the 
crystal lattice you will need to represent the AFM structure (depending 
on the distance between lattice planes in your structure without AFM 
order).

Good luck with figuring out the directions and lengths of wave vectors 
in your structure yourself,

Martin Pieper


---
Dr. Martin Pieper
Karl-Franzens University
Institute of Physics
Universitätsplatz 5
A-8010 Graz
Austria
Tel.: +43-(0)316-380-8564


Am 2020-01-26 21:01, schrieb djamel slamnia:
> THANKS AGAIN SIR
> 
> I NEED TO KNOW WHAT IS THE AFM ORDERS FOR P3M1 (156) ??? TYPE II OR
> III
> 
> THANKS IN ADVANCE
> 
>  Le dimanche 26 janvier 2020 à 20:51:41 UTC+1, Gavin Abo
> <gsabo at crimson.ua.edu> a écrit :
> 
>> THE DEFINITION FOR THE COMPOUND   A=B = 3.74 A   ALPHA = BETA = 90
>> GAMMA = 120
>> for AFM type I : i creat  superstructure  x super cell target
>> lattice H  :  x =1,   y = 1 ,  Z =2  then x sgroup, program define
>> automatically the space group the same of my original space group
>> 156 without warrning
> 
> As you have described above (for Z=2), your attempt at creating a
> supercell has failed as "x sgroup" collapsed the supercell structure
> back to the non-supercell structure.
> 
> As mentioned on the FAQ page for supercell construction, you need to
> displace an atom, change an atom, or use a special label:
> 
> http://susi.theochem.tuwien.ac.at/reg_user/faq/supercells.html
> 
> In order to keep the supercell without "x sgroup" reducing it back to
> the original structure, refer to previous posts in the mailing list
> archive about breaking the symmetry.  A few of the many posts about
> that as examples are at the three links below:
>  
> https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg18380.html
> https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg01866.html
> https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg15517.html
> 
> 
>> but when try to do it for type II : x super cell  P
>> :  x =1,   y = 1 ,  Z =1 x sgroup could not define the space group
> 
> In section "3.12 Setting up a new case" on page 29 in the WIEN2k 19.1
> usersguide [
> http://susi.theochem.tuwien.ac.at/reg_user/textbooks/usersguide.pdf ],
> there is the statement:
> 
> "Alternatively with the new StructGen you can specify the spacegroup
> and only the inequivalent positions. The equivalent ones will be
> generated automatically."
> 
> This means spacegroups in WIEN2k are defined according to the
> inequivalent positions and not by the equivalent positions.
> 
> For the case above (Z=1), it is likely that "x supercell" found some
> equivalent positions in the original structure and automatically added
> special labels to them changing them into inequivalent positions.
> Thus, a supercell structure was successfully created.  If you want the
> supercell structure to reduce back to the original structure, you
> would likely just need to remove all or some of the special labels in
> StructGen before running "x sgroup".
> 
> For understanding the inequivalent and equivalent positions with
> WIEN2k spacegroups, the example in the post at the following link
> might helpful:
> 
> http://zeus.theochem.tuwien.ac.at/pipermail/wien/2013-January/018171.html
> 
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