Double-kphase of the Dzyaloshinskii-Moriya helimagnet Ba2CuGe2O

Abstract

Neutron diffraction is used to re-investigate the magnetic phase diagram of the noncentrosymmetric tetragonal antiferromagnet Ba2CuGe2O7. A novel incommensurate double-k magnetic phase is detected near the commensurate-incommensurate phase transition. This phase is stable only for field closely aligned with the 4-fold symmetry axis. The results emphasize the inadequacy of existing theoretical models for this unique material, and points to additional terms in the Hamiltonian or lattice effects. Dzyaloshinskii-Moriya (DM) antisymmetric exchange interactions [1,2] and their role in incommensurate magnetic structures are once again at the forefront of condensed matter research. One reason for the renewed interest is the discovery of novel topological skyrmion phases in the DM helimagnet MnSi and related compounds [3,4]. The other is the role that DM interactions play in certain scenarios of the multiferroic effect [5,6]. Arguably, no material has attracted more attention simultaneously in both these contexts than the tetragonal DM helimagnet Ba 2 CuGe 2 O 7 . This compound is one of the very few systems where DM helimagnetism can coexist with weak-ferromagnetic behavior [7]. Moreover, theory suggested that it can support stable skyrmions, similar to those in MnSi [7]. Ba 2 CuGe 2 O 7 has an unusual soliton lattice structure in zero field [8], and shows a very interesting behavior in applied fields [9][10][11]. While the material itself does not demonstrate spontaneous electric polarization, the isostructural and magnetically commensurate Ba 2 CoGe 2 O 7 [12, 13] is, in fact, an unconventional multiferroic material [14].The most spectacular feature of Ba 2 CuGe 2 O 7 is the commensurate-incommensurate (CI) transition that occurs in this material in applied magnetic fields [9,11]. Despite the uniqueness and fundamental importance of this compound, and years of systematic studies, its exact nature remains unresolved. Theory predicts a continuous transition with a divergent periodicity, that maps onto the classic Frenkel-Kontorova model [15]. In obvious contradiction, experiments indicate a discontinuous reorientation of spins with a poorly defined "intermediate state". The latter has been largely dismissed as a result of defect pinning [9,11], phase separation associated with a spin-flop-like discontinuous transition [16] or a poor alignment of the applied field relative to the unique 4-fold axis of the tetragonal structure [17]. In the present study we show that none of these explanations apply. In fact, the "intermediate state" is a novel double-k incommensurate phase that exists only for magnetic fields almost perfectly aligned along the 4-fold axis. Being very unlike anything predicted for this material, the double-k structure undermines all existing theoretical descriptions of Ba 2 CuGe 2 O 7 .The crystal and magnetic structures of Ba 2 CuGe 2 O 7 are reviewed in detail in Ref. 11. The magnetism is due to Cu 2+ ions that form a square lattice, cell-centered in the (a, b) plane of the tetragonal non-centric P 42 1 ...

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

“…For magnetic field H along the c-axis, the almost cycloidal spin structure of Ba 2 CuGe 2 O 7 distorts to a soliton lattice with divergent periodicity until an incommensurate/commensurate (I/C) transition is observed at ≈ 2.4 T [20,21]. However, we could already show that this model of Ba 2 CuGe 2 O 7 is still fairly incomplete [18]: Close to the I/C transition, an intermediate phase of unknown origin has been attributed to a distinct phase with a peculiar AF-cone 2k-structure [18]. The AF-cone phase was found for a narrow range of magnetic fields aligned closely to the crystallographic c-axis.…”

“…In contrast, recent neutron diffraction experiments showed the existence of an unexpected AF-cone phase [18]. In addition, it turned out that the spin structure of Ba 2 CuGe 2 O 7 sensitively depends on the orientation of the magnetic field and undergoes several phase transitions [13,18]. This paper provides a detailed follow-up study of the recently discovered magnetic structures of Ba 2 CuGe 2 O 7 [18].…”

“…The AF-cone phase was found for a narrow range of magnetic fields aligned closely to the crystallographic c-axis. In contrast, for a large misalignment of H, a crossover to a complexely distorted nonsinusoidal phase was speculated [18].…”

“…In addition, it turned out that the spin structure of Ba 2 CuGe 2 O 7 sensitively depends on the orientation of the magnetic field and undergoes several phase transitions [13,18]. This paper provides a detailed follow-up study of the recently discovered magnetic structures of Ba 2 CuGe 2 O 7 [18]. In particular, we systematically investigate their evolution as function of the orientation of the magnetic field.…”

“…Due to the equivalency of the (1,±1,0) directions, two equally Cu (1) Cu (2) Cu (3) D z (1,2) D z (2,3) D y (2,3) D y (1,2) (a) (c) The angle enclosed by the (1,1,0) direction and the propagation vector q is denoted φ, similar to the notation used in Refs. [18,19]. The angle enclosed by the c-axis and the magnetic field is denoted α.…”