Magnetic structures and reorientation transitions in noncentrosymmetric uniaxial antiferromagnets

Abstract

A phenomenological theory of magnetic states in noncentrosymmetric tetragonal antiferromagnets is developed, which has to include homogeneous and inhomogeneous terms (Lifshitz-invariants) derived from Dzyaloshinskii-Moriya couplings. Magnetic properties of this class of antiferromagnets with low crystal symmetry are discussed in relation to its first known members, the recently detected compounds Ba2CuGe2O7 and K2V3O8. Crystallographic symmetry and magnetic ordering in these systems allow the simultaneous occurrence of chiral inhomogeneous magnetic structures and weak ferromagnetism. New types of incommensurate magnetic structures are possible, namely, chiral helices with rotation of staggered magnetization and oscillations of the total magnetization. Fieldinduced reorientation transitions into modulated states have been studied and corresponding phase diagrams are constructed. Structures of magnetic defects (domain-walls and vortices) are discussed. In particular, vortices, i.e. localized non-singular line defects, are stabilized by the inhomogeneous Dzyaloshinskii-Moriya interactions in uniaxial noncentrosymmetric antiferromagnets.

The quantum nature of skyrmions and half-skyrmions in Cu2OSeO3

“…This is similar in spirit to MnSi 37 , but in Cu 2 OSeO 3 the canting must lead to a weakly antiferromagnetic modification of the primary ferrimagnetic order parameter that is related to the long-range ordering of the S t ¼ 1 tetrahedron spins. This weak antiferromagnetism in Cu 2 OSeO 3 is the dual counterpart of the weak ferromagnetism that is present in chiral acentric bipartite antiferromagnets 38 . The presence of this antiferromagnetic superstructure implies that in neutron (or resonant x-ray) diffraction experiments there be an additional set of corresponding magnetic Bragg peaks.…”

Skyrmions in square-lattice antiferromagnets

“…Finally, we also consider an external magnetic field normal to the thin film. Previous work by Bogdanov et al [26] considered the same system at zero temperature and in the continuum limit. These authors identified three phases: an antiferromagnetic phase, a spin-flop phase, and a phase where inhomogeneous structures persist.…”

“…For these reasons, skyrmions have been investigated in many different antiferromagnetic systems, ranging from doped bulk materials [21], Bose-Einstein condensates [22], and various triangular lattice antiferromagnets [23,24] to nanodisks [25]. Isolated AFM skyrmions [26,27], as well as moving skyrmions in AFMs, have been considered theoretically [20,28,29].…”

Phase diagram of the Dzyaloshinskii-Moriya helimagnet Ba2CuGe2O7in canted magnetic fields

“…Surprisingly, unlike the isostructural commensurate weak ferromagnet Ba 2 CoGe 2 O 7 [14][15][16], Ba 2 CuGe 2 O 7 shows no signs of spontaneous ferroelectricity [15,16] in zero field. Moreover, Ba 2 CuGe 2 O 7 is one of the rare materials where spiral spin textures and weak ferromagnetism are predicted to coexist [17]. It was suggested by theory that Ba 2 CuGe 2 O 7 could support stable skyrmion phases [17] 1, D-85747 Garching, Germany.…”

“…Moreover, Ba 2 CuGe 2 O 7 is one of the rare materials where spiral spin textures and weak ferromagnetism are predicted to coexist [17]. It was suggested by theory that Ba 2 CuGe 2 O 7 could support stable skyrmion phases [17] 1, D-85747 Garching, Germany. Sebastian.muehlbauer@frm2.tum.de similar to those of the B20 compounds, however such structures have not been observed so far.…”