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- Activity Report 2018 -

Masuda and Hiroi Groups

Magnetic State Selected by Magnetic Dipole Interaction in Kagome Antiferromagnet NaBa2Mn3F11

Fig. 1. (a) The tail-chase 120° structure on the kagome lattice. (b) Neutron diffraction profile for NaBa2Mn3F11 measured at 0.25 K. The solid square and curves show experimental data and simulation, respectively. Arrows indicate magnetic Bragg peaks.

Long-range magnetic dipole-dipole (MDD) interaction is ubiquitous in nature. In most bulk magnets, the MDD interaction is not necessarily a primary interaction because its energy scale is much smaller than an exchange interaction. The MDD interaction, however, plays a key role in geometrically frustrated magnets, where the geometry causes macroscopic degeneracy of the magnetic state. For instance, the origin of an exotic monopole state in pyrochlore oxides is explained by the MDD interaction [1]. The frustrated geometry and the MDD interaction are, thus, a good combination for search of a new magnetic state.

In a classical Heisenberg kagome antiferromagnet, the ground state is known to be 120° structure with an infinite degeneracy. A small perturbation lifts the degeneracy and selects a unique structure, e.g., an all-in all-out structure in case of Dzyaloshinskii-Moriya interaction [2]. The MDD is theoretically predicted to select a tail-chase structure (Fig. 1(a)) [3]. Such a structure, however, has not been experimentally identified yet to date. NaBa2Mn3F11 having a unique kagome-triangular lattice was synthesized by Hiroi group in ISSP [4]. It exhibits an antiferromagnetic transition at TN = 2 K. The estimated Curie-Weiss temperature of -32 K is smaller than those of many kagome magnets, indicating that the exchange spin interaction is small. The MDD interaction may be important for the determination of the magnetic state.

Recently we carried out neutron powder diffraction experiment to identify the magnetic structure of NaBa2Mn3F11[5]. We observed magnetic Bragg peaks at 0.25 K as shown in Fig. 1(b). They were indexed by a commensurate magnetic propagation vector k0 = (0,0,0), and two incommensurate vectors k1 = (0.3209,0.3209,0) and k2 = (0.3338,0.3338,0). The magnetic structure analysis by combination of Rietveld refinement and the representation analysis reveals that the tail-chase 120° structure modulated by the incommensurate vectors gives a satisfactory agreement with the experiment. Furthermore, we calculated the phase diagram with the parameters of the nearest- and second-neighbor exchange interaction, and MDD interaction. We identified that the MDD interaction solves the infinite degeneracy and selects the tail-chaise 120° structure in NaBa2Mn3F11. For the future perspective, the study of magnetic dynamics is beneficial for the search for exotic sates such as zero-energy mode induced by the MDD interaction.


References

  • [1] J. S. Gardner, M. J. P. Gingras, and J. E. Greedan, Rev. Mod. Phys. 82, 53 (2010).
  • [2] M. Elhajal, B. Canals, and C. Lacroix, Phys. Rev. B 66, 014422 (2002).
  • [3] M. Maksymenko, V. R. Chandra, and R. Moessner, Phys. Rev. B 91, 184407 (2015).
  • [4] H. Ishikawa, T. Okubo, Y. Okamoto, and Z. Hiroi, J. Phys. Soc. Jpn. 83, 043703 (2014).
  • [5] S. Hayashida H. Ishikawa, Y. Okamoto, T. Okubo, Z. Hiroi, M. Avdeev, P. Manuel, M. Hagihala, M. Soda, and T. Masuda, Phys. Rev. B 97, 054411 (2018).

Authers

  • S. Hayashida, H. Ishikawa, Y. Okamotoa, T. Okubob, Z. Hiroi, M. Avdeevc, P. Manueld, M. Hagihala, M. Soda, and T. Masuda
  • aNagoya University
  • bDepartment of Physics, The University of Tokyo
  • cAustralian Nuclear Science and Technology Organization
  • dISIS Pulsed Neutron and Muon Source, Rutherford Appleton Laboratory