Spin-wave directional anisotropies in antiferromagnetic Ba3NbFe3Si2O14

Abstract

Ba3NbFe3Si2O14 (langasite) is structurally and magnetically single domain chiral with the magnetic helicity induced through competing symmetric exchange interactions. Using neutron scattering, we show that the spin-waves in antiferromagnetic langasite display directional anisotropy. On applying a time reversal symmetry breaking magnetic field along the c-axis, the spin wave energies differ when the sign is reversed for either the momentum transfer ± Q or applied magnetic field ± µ0H. When the field is applied within the crystallographic ab-plane, the spin wave dispersion is directionally isotropic and symmetric in ± µ0H. However, a directional anisotropy is observed in the spin wave intensity. We discuss this directional anisotropy in the dispersion in langasite in terms of a field induced precession of the dynamic unit cell staggered magnetization resulting from a broken twofold symmetry. Directional anisotropy, or often referred to as non reciprocal responses, can occur in antiferromagnetic phases in the absence of the Dzyaloshinskii-Moriya interaction or other effects resulting from spin-orbit coupling.

SOS: symmetry-operational similarity

“…1(b) [16]. A similar magneto-chiral spin wave effect was theoretically calculated, and also experimentally observed in monoaxial chiral Ba3NbFe3Si2O14 (Fe langasite) when the chiral axis, magnetic field, and spin wave propagation direction are all aligned [17][18]. Another example is nonreciprocal spin wave propagation in helical (screw-type) spins in magnetic fields along the helical spin axis or in conical spins, which corresponds to Fig.…”

Absolute crystal and magnetic chiralities in the langasite compound Ba3NbFe3Si2O14 determined by polarized neutron and x-ray scattering

“…Recently, electromagnons have been observed using THz spectroscopy [4] in a pure enantiomer crystal of the langasite compound Ba 3 NbFe 3 Si 2 O 14 . This compound possesses a chiral crystal structure (space group P321) and develops a spiral magnetic ordering below T N = 27 K. Inelastic neutron scattering experiments have revealed directional anisotropy in the spin-wave intensity upon applying a time-reversal symmetry breaking magnetic field along the c axis [5].…”

Neutron diffraction in MnSb2O6 : Magnetic and structural domains in a helicoidal polar magnet with coupled chiralities

“…As is the case for many compounds having a cycloidal magnetic structure, MnSb 2 O 6 is predicted to hold an electric polarization through the inverse DM interaction [9] or spincurrent induced [8] mechanisms which predicts an electric polarization P given by P ∝ r i j × (S i × S j ), (15) which couples to the magnetic polarity P m in the phenomenological free energy through a term ∝ λP m • P. Therefore the electric polarization P lies parallel or antiparallel to to the magnetic polarity P m depending on the sign of the coupling constant λ [17]. Using our definitions in Eqs.…”

“…Additional interest can be found in materials having a crystallographic chirality that may naturally stabilize a noncentrosymmetric magnetic structure. For example, iron based langasite (Ba 3 NbFe 3 Si 2 O 14 [11][12][13][14][15]) crystallizes in the chiral, trigonal space group P321, and the structural chirality is coupled to the chirality of its magnetic helix through symmetric Heisenberg exchanges. Recently, a mag-netic field induced long-wavelength spin spiral modulation has been discovered in this compound giving rise to an electric polarization [16].…”