ISSN 1027-4510, Journal of Surface Investigation. X-ray, Synchrotron and Neutron Techniques, 2008, Vol. 2, No. 4, pp. 560–563. © Pleiades Publishing, Ltd., 2008. Original Russian Text © A.A. Antonenko, E.N. Ovchinnikova, V.E. Dmitrienko, S.P. Collins, 2008, published in Poverkhnost’. Rentgenovskie, Sinkhrotronnye i Neitronnye Issledovaniya, No. 7, pp. 56–59. 560 INTRODUCTION Recently, resonant scattering of x-ray radiation has become a very popular method for studying physical properties of condensed media [1–3]. The method is selective, i.e., allows the study of scattering at only “resonant” atoms with absorption edges close to the energy of incident electromagnetic radiation. In some cases, purely resonant (“forbidden”) reflections can be observed near absorption edges, which are absent far from absorption edges due to symmetry properties of an object under study. Magnetic reflections were the first type of “forbidden” reflections studied experimen- tally [4, 5]. However, the appearance of purely resonant reflections is also possible in non-magnetic crystals due to the local non-cubic symmetry of the resonant atom position, which causes atomic factor anisotropy [6, 7]. Currently, purely resonant reflections were experimen- tally detected in a quite large number of magnetic and non-magnetic structures. In almost all studied cases, any contribution to the resonant part of the atomic fac- tor, magnetic or non-magnetic, is dominant. In [8], res- onant diffraction of synchrotron radiation was studied in the GdB 4 crystal, where both magnetic and non-mag- netic resonant reflections were observed. Magnetic and non-magnetic contributions to the tensorial atomic fac- tor were dominant at low temperatures and at room temperature, respectively. Interference of both contri- butions which were considered to be additive was also observed. The problem of the additivity of the contribu- tions to resonant scattering of x-ray radiation, which were caused by several anisotropic factors, was dis- cussed in [9–11]; it was shown that, apart from additive contributions, the resonant atomic factor can contain an additional contribution called the combined one [10]. This name arose by analogy with the case of combined interaction in Mössbauer spectroscopy. In the case of combined interaction during Mössbauer radiation dif- fraction, i.e., the simultaneous presence of magnetic and quadrupole hyperfine splittings, the appearance of forbidden reflections is possible, which would be absent in the presence of each separately [12–13]. Nev- ertheless, the presence of the combined contribution to resonant reflections in resonant diffraction of synchro- tron radiation has not yet been detected. This can be caused by various factors, among which are the preva- lence of the non-magnetic contribution for resonant scattering at the absorption K-edge and, on the contrary, the magnetic contribution at the L-edge, as well as rather broad resonances (~1–10 eV). In this paper, we discuss the problem of the appear- ance of combined effects in purely resonant reflections when any of anisotropic factors is dominant. The effect of the combined contribution to resonant scattering on the azimuthal dependence of purely resonant reflec- tions experimentally measured in [14] was considered by the example of the HoFe 2 crystal. TENSORIAL ATOMIC FACTOR IN THE PRESENCE OF SEVERAL ANISOTROPIC FACTORS In the dipole–dipole approximation, the tensorial atomic factor is phenomenologically described by the second-rank symmetric tensor whose components res- onantly depend on the incident radiation energy. This tensor can be divided into the isotropic, symmetric, and antisymmetric parts. The presence of the anisotropic Resonant Scattering of X-Ray Radiation in Magnetic Crystals with Non-Cubic Local Anisotropy A. A. Antonenko a , E. N. Ovchinnikova a , V. E. Dmitrienko b , and S. P. Collins c a Moscow State University, Moscow, 119992 Russia e-mail: ovtchin@mail.ru b Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 117333 Russia c Diamond Light Source, Chilton, Didcot, Oxfordshire OX11 0DE, United Kingdom Received June 26, 2007 Abstract—Features of resonant diffraction of synchrotron radiation in magnetic crystals in which the local symmetry of resonant atom positions is lower than cubic are considered. It was shown that the simultaneous presence of two anisotropic factors can cause asymmetry of the azimuthal dependence of purely resonant reflec- tions. The observed azimuthal dependence of the 002 reflection in the HoFe 2 crystal was numerically simulated. DOI: 10.1134/S1027451008040113