Neutron diffraction study and theoretical analysis of the antiferromagnetic order and the diffuse scattering in the layered kagome system CaBaCo2Fe2O7

Abstract

The hexagonal swedenborgite, CaBaCo 2 Fe 2 O 7 , is a chiral frustrated antiferromagnet, in which magnetic ions form alternating Kagome and triangular layers. We observe a long range √ 3 × √ 3 antiferromagnetic order setting in below TN = 160 K by neutron diffraction on single crystals of CaBaCo 2 Fe 2 O 7 . Both magnetization and polarized neutron single crystal diffraction measurements show that close to TN spins lie predominantly in the ab-plane, while upon cooling the spin structure becomes increasingly canted due to Dzyaloshinskii-Moriya interactions. The ordered structure can be described and refined within the magnetic space group P 31m . Diffuse scattering between the magnetic peaks reveals that the spin order is partial. Monte Carlo simulations based on a Heisenberg model with two nearest-neighbor exchange interactions show a similar diffuse scattering and coexistence of the √ 3 × √ 3 order with disorder. The coexistence can be explained by the freedom to vary spins without affecting the long range order, which gives rise to ground-state degeneracy. Polarization analysis of the magnetic peaks indicates the presence of long-period cycloidal spin correlations resulting from the broken inversion symmetry of the lattice, in agreement with our symmetry analysis.

Magnetic particles and strings in iron langasite

“…There are other antiferromagnets, in which noncollinear spin orders result from frustrated exchange interactions and which can host unusual topological defects, such as manganese nitrides with the cubic inverse perovskite crystal structure showing a variety of non-collinear spin structures and a giant negative thermal expansion effect [57][58][59], Pb 2 MnO 4 with a noncentrosymmetric tetragonal crystal lattice and a rare 90 • spin ordering [60], swedenborgites with alternating triangular and Kagome spin lattices, which similar to Felangasite are both frustrated and non-centrosymmetric [39,61], and the conducting non-collinear antiferromag-nets, MnGe 3 and Mn 3 Sn, showing large anomalous Hall and Nernst effects and allowing for electric control of magnetic states [26][27][28][29][30][31][32][33]62]. The unusual topological defects discussed in this paper can be a new avenue of research in antiferromagnetic spintronics.…”

“…The 120 • order of the classical spins S 1 , S 2 , S 3 in triangles with S 1 + S 2 + S 3 = 0 can be described by two orthogonal unit vectors, V 1 and V 2 [38,39],…”

“…LIs for collinear magnets are expressed in terms of components of the magnetization (or Néel) vector and their derivatives. For antiferromagnets with a 120 • spin structure described by a more complex order parameter, such as swedenborgaties and langasites, LIs can be written in terms of the two vectors, V 1 and V 2 , and their derivatives [38,39]. Finding such LIs is greatly simplified by the use of one-dimensional complex representations of 3 z .…”

Field-induced double spin spiral in a frustrated chiral magnet

“…Next, we discuss the origin of this unprecedented magnetic texture. This can be done based on the parametrization introduced in [24] (see also [25]), which uses the fact that for the 120 • spin order within the Fe triangles S 1 + S 2 + S 3 = 0 and S 2 1 = S 2 2 = S 2 3 :…”

Unconventional states and topological defects in Fe-langasite

“…Larger number of variables needed to describe an ordered state gives rise to a greater diversity and complexity of topological defects, a prominent example being the A-phase of superfluid 3 He. The order parameter describing non-collinear antiferromagnetic orders in the swedenborgite, CaBaCo2Fe2O7, and Fe-langasite, Ba3TaFe3Si24O14, is an SO(3) matrix [1,2]. The iron langasite spin lattice is built of triangles formed by antiferromagnetically coupled Fe 3+ -ions (S = 5/2).…”