For ThCr2Si2 structure (space group I4/mmm) sample, e.g. CeMn2Ge2, Mn atoms occupy the 4d site - site position (0, 0.5, 0.25). It was said that this sample has a ferromagnetic spiral structure (see attached figure) on the Mn site.
http://www.sciencedirect.com/science/article/pii/0925838894013780
I try to using the conical magnetic structure (provided by FullProf, jbt=5) to simulate this structure. Some parameters for this model are explained as:
-> P1,P2,P3,P4 and P5 correspond to x,y,z,B,occ of the atom
-> P6 is the magnetic moment of the atom (in Bohr magnetons)
-> P7 is the half-angle cone of the atom (degrees)
-> P8 is the magnetic phase of the atom (in fractions of 2 pi)
-> P9 of the first atom correspond to Phi (degrees)
-> P10 of the first atom correspond to Theta (degrees)
(Phi,Theta) are the spherical angles(degrees) of the cone axis
Generally, the half-angle is 90°, a flat spiral structure occurs (magnetic moments rotate in the ab plane). If the half-angle is smaller than 90°, a ferromagnetic spiral structure can be obtained.
I tried to build the ferromagnetic spiral structure on the Mn sublattice in CeMn2Ge2 using this model. However the results just gave a flat spiral no matter the half-angle value. What is the problem here?
Hi Wang, my question is just that I cannot get/simulate a ferromagnetic (conical) spiral magnetic structure model in I4/mmm.
I am really new in this field, but if you want generate an effect you are describing with the attached images Y. Wang is from my point of view right. it is in maximum 4mm but not 4/m mm .
The FullProf manual tells for Jbt=5 "No symmetry operation can be introduced: all the magnetic atoms within a primitive unit cell must be given (constraints have to be introduced through the codes of the parameters)" and "We recommend to use the triclinic space group of symbol L -1 (where L = P, A, B, C, F, I, R) in order to have a full set of reflections with the proper multiplicity when the true magnetic symmetry is not known."
Generally, when you want to describe a magnetic ordering, you should switch from the usual space groups to the black and white (Shubnikov) groups. The real symmetry of your material will be described as a black and white subgroup of the grey group I4/mmm in case of the commensurate magnetic structure and you will have to go to the higher dimensions if the magnetic structure is incommensurate. FullProf simplifies the task by separating the diffraction pattern into structure reflections and magnetic satellites, but you have to allow the propagation vectors to be unconstrained with the symmetry of the high-temperature phase.
Hi Alexander, can you give some more detailed explanation about "you have to allow the propagation vectors to be unconstrained with the symmetry of the high-temperature phase" ? In this sample, the high temperature phase is still I4/mmm but with a canted magnetic structure.
Thank you every one. I have solved this problem. In my previous model, SYMM was wrong. It should be
SYMM x,y,z
SYMM u,v,w 0.00
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