New magnetic CIF dictionary for your approval

[James Hester]

Dear COMCIFS,

The Commission for Magnetic Structures, headed by Branton Campbell, has put
in an enormous effort over the past 3 years to create and harmonise a set
of CIF tags for describing magnetic structures.  The resulting magCIF
dictionary is attached to this email, together with an overview document
and set of examples.

The discussion period for approval of this dictionary is 3 weeks. The
magnetic community are planning an important workshop in early December at
which this dictionary will be officially launched, with software support,
so timely consideration is important.

A browseable summary version of the dictionary is viewable at
http://comcifs.github.io/magCIF.dic.html
You may view the recent history of dictionary development at the
dictionary's github page:
https://github.com/COMCIFS/magnetic_dic/blob/master/magCIF.dic

James.

-- 
T +61 (02) 9717 9907
F +61 (02) 9717 3145
M +61 (04) 0249 4148
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***** Purpose
The magCIF dictionary provides CIF infrastructure for the presentation of both commensurate and incommensurate magnetic structures exhibiting long-range three-dimensional magnetic order.  Tools for treating short-range order could be added in the future, but have not yet been discussed in earnest.  Incommensurate magnetic structures are to be presented using magnetic superspace groups in a manner analogous to incommensurate non-magnetic structures.  Commensurate magnetic structures can alternatively employ (1) a Belov-Neronova-Smirnova (BNS) setting, (2) an Opechowski-Guccione (OG) setting, (3) an arbitrary non-standard setting, or (4) an incommensurate (wave) description in some appropriate magnetic-superspace-group setting.

***** Helpful definitions
For convenience, we define the "derived non-magnetic structure" associated with a magnetic structure to be the structure obtained by simply zeroing all of its magnetic moments.  The symmetry group of the derived non-magnetic structure is obtained by removing the primes from each of time-reversed operators of the magnetic symmetry group of the magnetic structure.  For a supercell or wave description of a magnetic structure, the symmetry group of the derived non-magnetic structure will be a non-magnetic space group or superspace group, respectively.

The "magnetic cell" (conventional or primitive) is a unit cell defined by a basis of the translation subgroup (conventional or primtive) of the magnetic space group of the magnetic structure.

The "derived non-magnetic cell" (conventional or primitive) is a unit cell defined by a basis of the translation subgroup (conventional or primitive) of the space group of the derived non-magnetic structure.

A BNS setting employs a magnetic cell while an OG setting employs a derived non-magnetic cell.  For type 1-3 MSGs the primitive BNS-setting and OG-setting cells have the same volume; but for type-4 MSGs the primitive OG-setting cell has half the volume of the primitive BNS-setting cell.  Non-standard settings are also possible.

***** BNS-supercell description
(a) _cell_length_* for the parameters of the BNS unit cell
(b) _space_group_symop_magn_operation loop to contain one representative MSG operator corresponding to each element of the point group of the derived non-magnetic structure.
(c) _space_group_symop_magn_centering loop for the magnetic centering and anti-centering translations of the MSG. Alternatively, all MSG operations can be included in the _space_group_symop_magn_operation loop.
(d) _atom_site loop for the properties of all atoms in the BNS assymetric unit of the MSG.  At minimum, one should include the labels, scattering types, positions, and occupancies of the magnetic atoms in this list.
(e) _atom_site_moment loop for the magnetic moments of all magnetic atoms in the assymetric unit of the MSG, which can alternatively be included in the _atom_site loop.
(f) _space_group_magn.transform_BNS_Pp_abc and _space_group_magn.transform_OG_Pp_abc for transformations to reference settings.  Optionally, the _space_group.magn_transforms loop allows one to present the same information along with transforms to other settings of interest.

***** OG(k)-supercell description
(a) _cell_length_* for the parameters of the OG unit cell
(b) _space_group_symop_magn_operation loop to contain one representative MSG operator corresponding to each	element of the point group of the derived non-magnetic structure.
(c) _space_group_symop_magn_OG_centering loop for the centering translations of the derived non-magnetic space group.
(d) _space_group.magn_OG_wavevector_kxkykz for the OG magnetic propagation vector, which associates a time-reversal component to each of the translations of the lattice of the derived non-magnetic structure.	It's value is zero for type 1-3 MSGs and non-zero for type-4 MSGs.
(e) _atom_site loop for the properties of all atoms in the OG assymetric unit of the MSG.  At minimum, 	one should include the labels, scattering types, positions, and occupancies of the magnetic atoms in this list.
(f) _atom_site_moment loop for the magnetic moments of all magnetic atoms in the OG assymetric unit of the MSG, which can alternatively be included in the _atom_site loop.
(g) _space_group.magn_transform_BNS_Pp_abc and _space_group.magn_transform_OG_Pp_abc for transformations to reference settings.  Optionally, the _space_group.magn_transforms loop allows one to present the same information along with transforms to other settings of interest.

***** Wave description of an incommensurate magnetic structure
(a) _cell_length_* for the parameters of the magnetic unit cell (primitive, conventional, or non-standard)
(b) _space_group_symop_magn_ssg_operation loop to contain one representative MSSG operator corresponding to each element of the point group of the derived non-magnetic structure.
(c) _space_group_symop_magn_ssg_centering loop for the magnetic centering and anti-centering translations of the MSSG.	Alternatively, all MSSG operations can be included in the _space_group_symop_magn_ssg_operation loop.
(d) _atom_site loop for the non-magnetic properties of all atoms in the assymetric unit of the BMSG.  At minimum, one should include the labels, scattering types, positions, and occupancies of the magnetic atoms in this list.
(e) _atom_site_moment loop for the magnetic moments of all magnetic atoms in the assymetric unit of the BMSG, which can alternatively be included in the _atom_site loop.
(f) _cell_wave_vector loop for the fundamental wave vectors that define the superspace.
(g) _atom_site_Fourier_wave_vector loop for the wave vectors of the magnetic and non-magnetic modulations of the structure.
(h) _atom_site_moment_Fourier loop for the magnetic Fourier amplitudes of each wave vector component and each magnetic atom of the assymetric unit of the basic cell.
(g) _space_group_magn_ssg_transforms loop for transformations to reference settings of interest.

This is the general case, where the magnetic modulations may be truly incommensurate, where we may treat commensurate and incommensurate magnetic modulations differently, and where the magnetic modulations may be combined with incommensurate non-magnetic modulations.  Once again, this description presents the complex Fourier amplitude vectors of each of the magnetic plane waves that apply to each of the atoms of the assymetric unit of the basic unit cell.  Use a BNS setting for the basic magnetic space group (BMSG), which will be of type-4 if one or more of the commensurate magnetic modulations has a non-zero propagation vector.

***** Wave description of a commensurate magnetic structure
Same as for wave description of an incommensurate magnetic structure except for the following restrictions: (1) each contributing magnetic wave with non-zero wave-vector is treated as though it were incommensurate, and (2) no non-magnetic modulations are permitted.  These restrictions ensure that the BMSG will be of type 1-3 and have the same primitive cell as the unmodulated derived non-magnetic structure.

The relevant MSSG operators will relate the wave parameters of the atoms in the assymetric unit to those of all other atoms in the basic cell.  If desired, one can present a trivial MSSG and provide a complete listing of the magnetic wave parameters of every atom in the primitive cell of the BMSG, though it is generally helpful to employ any magnetic superspace symmetries that are actually present in the structure.  Employ a BNS-type description of the basic magnetic cell in such a description.

***** Transformations to other commensurate settings
For any setting of any commensurate magnetic-structure description, basis transformations to the Litvin-OG setting (_space_group_magn.transform_OG_Pp_abc) and the ISO-MAG BNS setting (_space_group_magn.transform_BNS_Pp_abc) should be provided, even if the transformation presented is the identity tranformation.  This is important to the convenient comparison of magnetic structures presented in different settings.  The _space_group_magn.transform loop can contain this same information, along with transformations to other convenient settings, such as a literature reference, a database reference, or another data-block within the same file.

***** Multiple magnetic data blocks
One can add additional data blocks to any CIF for the purpose of presenting alternative settings and/or descriptions of the same magnetic structure.  Blocks for reference settings can exclude the atom loop (i.e. contain only symmetry operators) if desired.  When multiple data blocks are used in this way, each block should present the basis transformations to each of the other data blocks.

***** Parent (non-magnetic) structure
The designation of a parent structure is common but optional for a magnetic-structure description, and is facilitated by the _parent_space_group category, which conveys information about the symmetry group of the parent designated structure and the basis transformation that relates it to the magnetic structure.  This category is not intended to facilitate a complete description of the parent structure.

The non-magnetic parent structure is not generally unique -- there may be a whole sequence of increasingly idealized non-magnetic structures, any one of which could be selected as the parent.  But the non-magnetic parent structure selected must have a group-subgroup relationship with the child magnetic structure.

The parent structure may not have the same symmetry group or cell size as the derived non-magnetic structure.  But the primitive cell of the parent structure will always be smaller than or equal to the primitive cell of the derived non-magnetic structure.  When smaller, we'll call the parent a "small-volume" parent; when equal, we'll call it an equi-volume parent.

When magnetic and non-magnetic atoms and atom properties are presented jointly within the magnetic data block, one can either (1) include both the magnetic and non-magnetic atom properties in the _atom_site loop, or (2) present the magnetic moments in a separate _atom_site_moment loop.  Both structures will unavoidably share the same symmetry constraints -- one cannot impose different symmetry groups on the magnetic and non-magnetic atom properties within the same data block.

As an alternative to or in addition to the_parent_space_group category, one can present the parent structure in a separate data block, and then relate the parent and child space-group settings via an appropriate inter-data-block basis transformation.  This approach allows one to describe the magnetic structure and some higher-symmetry approximation to the non-magnetic-parent structures with different symmetry groups.

A future CIF dictionary for representational-analysis (symmetry-mode) parameters will make it easy to impose different effective symmetries on various subsystems (e.g. magnetic, displacive, occupational, ADP) of a single crystal structure within the same datablock.

For commensurate magnetic structures with type 1-3 MSGs, the BNS-supercell and OG(k)-supercell descriptions are equally convenient in most respects since the both employ the same cell.  For type-4 MSGs, the desire to describe the magnetic structure using the cell of an equi-volume parent structure may outweigh the complications that arise from a primitive OG(k) cell that is half the size of the magnetic primitive cell.  When both working with a type-4 MSG and a small-volume parent structure, the primitive OG(k) cell is neither the primitive parent cell nor the primitive magnetic cell, and is therefore less likely to be helpful than a BNS cell.
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