Bilbao Crystallographic Server MAXMAGN Help |

Extended tutorial: download

MAXMAGN is an interactive program whose main purpose is to provide the possible maximal magnetic symmetries that are compatible with a given parent space group Gp of a paramagnetic phase and a (single) commensurate propagation vector **k** (in conventional basis). This permits to know the possible maximal symmetries that a commensurate magnetic structure can have, starting from the knowledge of Gp and the **k** vector detected in the magnetic diffraction pattern. Only 1**k** magnetic orderings are considered. Optionally, the crystal structure of the paramagnetic phase can be introduced. If so, the magnetic orderings corresponding to each of the possible configurations of maximal magnetic symmetry can be obtained. These magnetic models are given in a cif-type form (mcif file), using Shubnikov magnetic space groups in BNS setting, with explicit indication of the symmetry constraints on the magnetic moments, and the parameters that can be refined in each case.

Most of the determined magnetic structures have a single primary propagation vector (1**k** structures) and display magnetic orderings that have a maximal symmetry compatible with its propagation vector **k** (i.e., no magnetic ordering can exist with this propagation vector and with a higher magnetic space group containing the one being observed). MAXMAGN therefore provides the set of magnetic orderings that are most probable for a given propagation vector. For simple vectors this set is very limited, not surpassing 10 in most cases, and being very often 4 or 6. In the exceptional case that these symmetries are too restrictive to explain the observations, MAXMAGN also allows a controlled descent to lower symmetries.

The permitted maximal magnetic symmetries are determined by the program as *subgroups* of the grey magnetic group Gp1', which represents the magnetic symmetry of the paramagnetic phase. Each of these subgroups should fulfill the condition that it is consistent with the propagation vector (**k**-vector) observed, and that there is no intermediate magnetic group between it and the paramagnetic space group Gp1' (i.e. supergroup of one and subgroup of the other), which is also consistent with the same **k**-vector. We shall call these subgroups "**k**-maximal" subgroups (not to be confused with *klassengleiche* subgroups), and they represent the set of possible maximal symmetries that a magnetic ordering with this **k**-vector can have.

MAXMAGN is very efficient for simple **k**-vectors that imply small multiplications of the magnetic primitive cell with respect to that of the paramagnetic structure. **k**-vectors with multiplication factors 0, 2, 3, 4 are the most frequent ones, and can be analyzed with this program using reasonable computing times. On the other hand, **k**-vectors implying large multiplication factors of the magnetic unit cell can be problematic, with the program exhausting the allowed computing time. In these cases, however, the use of a description in terms of Shubnikov magnetic space groups is normally not very useful, and a spin wave description using superspace magnetic groups, which is outside the scope of this program, is usually more appropriate. Thus, the highest multiplication factor accepted by MAXMAGN hash been set to 8, which is far enough for the purpose of the program.

The table of subgroups also includes a link to the listing of the systematic absences of magnetic diffraction for each subgroup. This is obtained with MAGNEXT, so they are relevant for unpolarized neutron magnetic diffraction. Also, the button "alternatives (domain-related)" allows the user to choose and use an alternative subgroup belonging to the same conjugacy class (see below). This option permits to obtain all domain-related configurations of a magnetic ordering with one of these

Finally, if a particular crystal structure for the paramagnetic phase has been introduced, the alternative models of the magnetic structure complying with each of the listed

The subgroups are defined by means of a matrix and a vector, which describe the transformation from the setting of the parent space group Gp (parent setting) to the BNS standard setting of the subgroup. This matrix is the one MAXMAGN uses, if the subgroup is chosen, for describing the magnetic structure and its symmetry in a standard setting. This setting transformation can be changed to another equally valid through the button "Alternatives". The listing of alternative transformations is not necessarily complete and the menu "Check your transformation" allows the introduction of a user-designed transformation, that can be used as an alternative. MAXMAGN checks if the introduced transformation is valid for defining one of the subgroups in the list. This option is only necessary, if the magnetic ordering is to be described in a specific standard setting.

Every possible choice of subgroup and corresponding transformation appearing in this option is shown with a "Choose" button. Clicking on this button, the program goes back to the listing of

The description of the magnetic structure can be changed to a standard setting for its magnetic symmetry by clicking the appropriate button at the end of the output page. Alternatively, the user can introduce the description of a setting of its own choice (to be valid the chosen unit cell must be a supercell of the magnetic primitive cell).

Specific values (assumed to be given in Bohr magnetons) for the free parameters of the magnetic moments can be introduced in this page, and obtain with them a cif-like file of the resulting model for the magnetic structure to be downloaded. Clicking on the button "Export data to mcif file" a link to download the cif-like file is shown, as well as a non-editable textbox showing the content of the file. Also, for 3D visualization of magnetic structures with Jmol, a button to submit the cif-like file to MVISUALIZE is provided. This cif-like file (with extension .mcif ) follows the format created by the developers of ISOCIF and ISODISTORT and uses magnetic space group symmetry in a systematic and consistent form. These mcif files can be used for a refinement (as a starting model), for graphical purposes, for representation analysis, for generating a full listing of atomic positions and spins within a unit cell (to do ab-initio calculations, for instance), or for any other kind of study. The "mcif" format is supported at present by the programs JANA2006, ISODISTORT, ISOCIF, VESTA, MVISUALIZE and StrConvert. This latter is also a tool in the Bilbao Crystallographic Server, which allows to manipulate, edit and visualize this kind of files.

Finally, the option "Go to a subgroup" can be used to descend to lower subgroups (see below).

Bilbao
Crystallographic Server http://www.cryst.ehu.es |
For comments, please mail to cryst@wm.lc.ehu.es |