2014: Synchrotron radiation for magnetism

Physicists from Lomonosov MSU in an international team of scientists developed a novel experimental technique based on synchrotron radiation diffraction, which allows determining the sign of Dzyaloshinskii–Moriya interaction and opens up new possibilities for a studying magnetic and magnetoelectric materials.

Magnetism - the spontaneous alignment of atomic moments in a material - is driven by quantum-mechanical ‘exchange’ interactions. The interactions between atomic magnetic moments may be not direct, but mediated by the intervening matter. For instance, the oxygen-mediated antisymmetric superexchange -- Dzyaloshinskii�Moriya (DM) interaction (Dzyaloshinsky, Sov. Phys. JETP 5, 1259 (1957); J. Phys. Chem. Solids 4, 241 (1958); T. Moriya, Phys. Rev. Lett. 4, 228 (1960); Phys. Rev. 120, 91 (1960)) results in weak ferromagnetism in oxides. It can be expressed in terms of a DM vector D and the vector product of spins IDM~D[SixSj]. Moriya realized that the vector�s direction and magnitude are closely related to the local symmetry of the system. Currently the DM interaction is the key element in the physics of multiferroics, hence a great deal of effort is devoted to its theoretical and experimental studies.

In the paper published in Nature Physics. (V 10.�N.�3.�Pp 202-206 (2014)) an international team of scientists proposes a novel experimental technique based on interference between two x-ray scattering processes (one acts as a reference wave), which is combined with a second unusual approach of turning the atomic antiferromagnetic motif with a external magnetic field. Novel technique is applied to determine the phase of the magnetic x-ray scattering signal, and the sign of the DM interaction in FeBO3. FeBO3 is essentially antiferromagnetic, with sublattices almost antiparallel and a small local twist leading to weak ferromagnetism. Symmetry alone cannot say whether this local twist will be left-handed or right-handed. The absolute sign of the local twist can be found both experimentally and theoretically using techniques described in the paper.

In order to observe interference between the two scattering processes, XMaS (ESRF, Grenoble, France) measurements were carried out at the 003 reflection that is �forbidden� for the vastly stronger charge scattering processes, and has comparable amplitudes of resonant and magnetic scattering signals near the Fe absorption edge maximizing the effects of interference. The sign and amplitude of the magnetic scattering signal depends on the spin direction, which can be rotated with a magnetic field. We thus can control the amplitude and phase of both the magnetic scattering and resonant reference wave.

Three types of measurements are presented. The first shows a remarkable effect � an apparent jump in the energy of the resonant scattering peak as the magnetic motif is rotated by 180 degrees, as a result of constructive (destructive) interference to the low (high) energy side of the resonance. The opposite jump was observed when the phase of the resonant scattering was reversed by changing the sample rotation angle. The direction of the jump gives the phase of the magnetic scattering. The second measurement shows the intensity, measured as a continuous rotation of the field angle, for the low and high energy side of the resonance. For the final measurement, the sample azimuthal angle was varied continuously, with a fixed photon energy and field applied in two opposite directions. In all cases, the phase of the magnetic scattering is in agreement with the calculations. We find that the magnetic structure twist in the is in the same direction as that of the oxygen atoms. So, a new interference technique in which measurements are carried out with precise control of the amplitude and phase of a reference wave gives an unambiguous result for the sign of the DM interaction. Our experimental results are validated by state-of the-art ab initio calculations. Together, our experimental and theoretical approaches are expected to open up new possibilities for exploring, modeling and exploiting novel magnetic and magnetoelectric materials.

These results are published in the paper: V. E. Dmitrienko, E. N. Ovchinnikova, S. P. Collins, G. Nisbet, G. Beutier, Y. O. Kvashnin, V. V. Mazurenko, A. I. Lichtenstein & M. I. Katsnelson, "Measuring the Dzyaloshinskii�Moriya interaction in a weak ferromagnet", Nature Physics 10(3), 202-206 (2014).