Madrid Meeting

First meeting of the Commission on Magnetic Structures in Madrid

Report of the activities of the Commission of Magnetic Structures (CMS) at the 2011 IUCr meeting in Madrid

On Wednesday afternoon (24 August), the IUCr Executive Committee reviewed the commission's Terms of Reference and gave the following instructions: (1) A representative of the commission must attend the General Assembly in order to make a statement and to answer questions when the formation of the commission is put to a vote. (2) The IUCr has created an online crystallographic dictionary at www.iucr.org (Resources Tab -> Online Dictionary). We are invited to formulate and submit definitions related to magnetic structures for this dictionary. (3) All members of the commission must be listed in the IUCr's World Directory of Crystallographers at www.iucr.org (People Tab -> Directory), which is free. (4) The commission needs to create a formal website within the www.iucr.org domain. This is in addition to any external website that we might have. Brian McMahon in the Chester office will assist us in setting up and managing the content. (5) In order to coordinate the activities of the CMS and the Commission on Neutron Scattering (CNS), each commission should designate one member to serve as a consultant to the other. (6) It has been proposed that a new online volume of International Tables of Crystallography be devoted to magnetic space groups. Because the Commission on International Tables (CIT) has expressed strong interest in this proposal, the CMS was encouraged to discuss how it could best contribute to the preparation and review of such tables. The CIT chair suggested that Daniel Litvin (author of an online ebook of magnetic space group tables) might play a key role in its development.

At the General Assembly on Wednesday evening (24 August), Branton explained that magnetic structures have been primarily the responsibility of the Commission on Neutron Scattering, which has done a great deal to promote neutron scattering facilities, magnetic scattering research, and instruments and tools for determining magnetic structures. The Commission on Magnetic Structures was created in order to bring greater resources into this field in order to initiate a new focus on the development of standards for describing magnetic structures and their symmetries. The formation of the commission was then approved by the General Assembly. Manuel Perez-Mato, in addition to being a consultant to the commission, was later appointed by the Executive Committee to supervise the efforts of the commission.

At the open commission meeting on Friday (26 August), commission members and consultants in attendance included Vladimir Pomjakushin, Juan Rodriguez-Carvajal, Wieslawa Sikora, Daniel Litvin, Manuel Perez-Mato and Branton Campbell, with Taku Sato connected via Skype. Harold Stokes (BYU), Teresa Fernandez-Diaz (ILL) and Vladimir Hutanu (ESRF) were also present. Branton chaired the meeting in Laurent's absence, and Vladimir P. kept a record of discussion items.

(1) It was agreed that the decisions of the commission will be made by simple majority vote, with no less than 5 members participating. So long as the commission continues to function smoothly, we will not rely on formal rules of parliamentary procedures, but will hear and vote on proposals from individuals or subcommittees.

(2) It was agreed that all members of the commission should create listings in the IUCr's World Directory of Crystallographers as soon as possible (takes about 5 minutes to complete).

(3) In order to coordinate our activities with those of the Commission on Neutron Scattering (CNS), J. Rodriguez-Carvajal of our commission has been invited to serve as a consultant to the CNS, while Teresa Fernandez-Diaz from the CNS will be appointed to serve as a consultant to our commission. We will submit this appointment to the Executive Secretary of the IUCr for approval.

(4) It was agreed that the commission will form subcommittees to work on certain key endeavors. Subcommittees will complete assignments and make recommendations to the commission. The commission will consider these recommendations carefully in making subsequent decisions.

(5) A subcommittee was created to study the issue of a new IT volume on magnetic space groups. Danny and Branton volunteered to serve on this subcommittee. Other members who would like to participate should make their interest known. It may be helpful to appoint additional consultants to assist with this work.

(6) It was suggested that a subcommittee be created to consider a magnetic CIF dictionary. Wieslawa and Branton both expressed interest in serving on this committee. Other members who would like to participate should make their interest known. Gotzon Madariaga from Bilbao was suggested as a possible consultant. This subcommittee will initiate work after other more fundamental progress has been made.

(7) It was suggested that a subcommittee be created to consider a magnetic structure database. Wieslawa shared her extensive experience with such a database and described the challenges of creating and maintaining it. Taku and Alexander both expressed interest in the work of this committee. Other members who would like to participate should make their interest known. This subcommittee will initiate work after other more fundamental progress has been made.

(8) Massimo Nespolo and Mois Aroyo informed us that the Mathematical and Theoretical Crystallography commission proposed and were approved to conduct a 3-day satellite workshop prior to the 2012 ECM meeting in Bergen, Norway (4-6 August). They invited the Magnetic Structure commission to co-sponsor and to help organize/conduct the workshop. While this is an attractive opportunity, we felt that we first need to achieve some consensus on how to describe a magnetic structure and how to interconvert between alternative descriptions before we will have material to present at a workshop. In other words, a large workshop seems premature. A smaller conference session might be more appropriate. It's not yet clear who is able/willing to help organize the workshop, though Manuel offered to contribute some material on magnetic superspace-groups for incommensurate magnetic structures.

(9) Danny presented an introduction to his magnetic space-group tables, along with several examples and their interpretation. He also expressed the understanding that the IUCr will put his magnetic space-group tables online in their current form very soon, but that they may be modified in the future (e.g. changing axes labels in diagrams, etc.). Branton understood that the tables would eventually go through a review process (by our commission and by the CIT). Clarification on these points are needed. Manuel suggested that BNS settings be included beside the existing OG settings in these tables. Branton suggested that it would be helpful to have a computer program that can generate magnetic space-group diagrams in the style of Litvin, which would allow things like axes labels to be modified uniformly for all groups.

(10) The bulk of the commission's discussion centered on a minimal description of a magnetic structure. Historically, there are two schools of thought: (1) The "crystallographic" approach that employs magnetic Shubnikov symmetry groups, and (2) the "propagation vector" approach that treats amplitudes and phases of Fourier components of the magnetic structure. For the general case, if the amplitudes and phases of an infinite number of k-vector harmonics are specified, then the crystallographic and propagation-vector approaches are mathematically equivalent. Juan presented the theory and practice of the propagation vector approach, and Vladimir presented several examples.

(11) The (a) "representational-analysis" description of a magnetic structure involves basis functions of the irreps of the associated paramagnetic parent symmetry, or rather the amplitudes attributed to these modes. In contrast, the (b) "traditional" description specifies the x,y,z components of the moments or Fourier coefficients of individual atoms or Wyckoff orbits. If all secondary irreps that couple to the primary irrep(s) are included, the two bases are mathematically equivalent and are related by a linear transformation. A representational description is best presented as a set of symmetry-mode vectors (i.e. irrep basis functions) that are defined in terms of the traditional basis.

(12) Either the crystallographic or propagation-vector approaches can be presented using either the representational-analysis or traditional bases. We thus have 2x2=4 different cases to treat. The relationship between the traditional propagation-vector description and the traditional crystallographic description is perhaps the most fundamental issue for us to address in any standard or paper that we prepare as a commission. The relationship between the traditional and representational bases needs standards but is conceptually less difficult to treat.

(13) Manuel pointed out that the development of magnetic ordering also lowers the symmetry of the parent nuclear structure, even if the actual lattice distortion is too small to be experimentally accessible. Practitioners often work in the "undistorted lattice approximation", effectively describing the magnetic propagation vectors in terms of the undistorted parent cell. But it is often helpful or necessary to recognize that the lattice has been distorted. In general, the lattice distortion must be accounted for when translating a propagation-vector description into a crystallographic description. Manuel recommends that we distinguish between propagation-vector descriptions of magnetic structures that employ a "distorted lattice" versus an "undistorted-lattice-approximation".

(14) Several possible structures were suggested as working examples:

• a) single k = (0,0,0): A-type LaMnO₃ - Branton
• b) single k = (1/2, 0, 1/2): Ho₂BaNiO₅ - Juan
• c) single k = (beta,0,0): TmMnO3 (Pnma setting) - Vladimir
• d) multi k from different stars, k= (0,1/2,0)+(delta,1/2,1/2): Ho₃Cu₄Sn₄ - Wieslawa
• e) multi k from same star, k=(1/2,1/2,0)+(1/2,-1/2,0): Sr₃(CuNi₂)(PO₃)₄ - Vladimir

Commission members agreed to collaborate in efforts to prepare multiple mutually-consistent descriptions of these examples using propagation-vector and crystallographic approaches (both traditional and representational bases) may lead us to simpler cases.

(15) We hope to meet formally as a commission at least once a year. An international neutron-scattering meeting was suggested as a possible venue. And Laurent has suggested that he may be able to obtain some funding to support a meeting in Grenoble next year