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Register a new userLattice pinning of magnetic domains in the helimagnet Ba_2CuGe_2O_7
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The layered magnetic compound Ba_2CuGe_2O_7 exhibits spiral antiferromagnetic order thanks to a Dzyaloshinskii-Moriya (DM) anisotropy that is allowed by crystal symmetry. Here we theoretically examine some finer issues such as the experimentally observed lattice pinning of the propagation vector of helical magnetic domains. We find that DM anisotropy alone would lead to incorrect pinning, but agreement with experiment is restored upon including an additional exchange anisotropy that is also consistent with symmetry. The present results shed light on the so-called bisection rule which has been abstracted from experiment in presence of an in-plane magnetic field.
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Anisotropic low-temperature properties of the cubic spinel helimagnet ZnCr2Se4 in the single-domain spin-spiral state are investigated by a combination of neutron scattering, thermal conductivity, ultrasound velocity, and dilatometry measurements. In an applied magnetic field, neutron spectroscopy shows a complex and nonmonotonic evolution of the spin-wave spectrum across the quantum-critical point that separates the spin-spiral phase from the field-polarized ferromagnetic phase at high fields. A tiny spin gap of the pseudo-Goldstone magnon mode, observed at wave vectors that are structurally equivalent but orthogonal to the propagation vector of the spin helix, vanishes at this quantum critical point, restoring the cubic symmetry in the magnetic subsystem. The anisotropy imposed by the spin helix has only a minor influence on the lattice structure and sound velocity but has a much stronger effect on the heat conductivities measured parallel and perpendicular to the magnetic propagation vector. The thermal transport is anisotropic at T < 2 K, highly sensitive to an external magnetic field, and likely results directly from magnonic heat conduction. We also report long-time thermal relaxation phenomena, revealed by capacitive dilatometry, which are due to magnetic domain motion related to the destruction of the single-domain magnetic state, initially stabilized in the sample by the application and removal of magnetic field. Our results can be generalized to a broad class of helimagnetic materials in which a discrete lattice symmetry is spontaneously broken by the magnetic order.
Magnetic and dielectric properties with varying magnitude and direction of magnetic field H have been investigated for a triangular lattice helimagnet MnI2. The in-plane electric polarization P emerges in the proper screw magnetic ground state below 3.5 K, showing the rearrangement of six possible multiferroic domains as controlled by the in-plane H. With every 60-degree rotation of H around the [001]-axis, discontinuous 120-degree flop of P-vector is observed as a result of the flop of magnetic modulation vector q. With increasing the in-plane H above 3 T, however, the stable q-direction changes from q||<1-10> to q||<110>, leading to a change of P-flop patterns under rotating H. At the critical field region (~3 T), due to the phase competition and resultant enhanced q-flexibility, P-vector smoothly rotates clockwise twice while H-vector rotates counter-clockwise once.
Ferroelectric properties in magnetic fields of varying magnitude and direction have been investigated for a triangular-lattice helimagnet CuFe1-xGaxO2 (x=0.035). The magnetoelectric phase diagrams were deduced for magnetic fields along [001], [110], and [1-10] direction, and the in-plane magnetic field was found to induce the rearrangement of six possible multiferroic domains. Upon every 60-degree rotation of in-plane magnetic field around the c-axis, unique 120-degree flop of electric polarization occurs as a result of the switch of helical magnetic q-vector. The chirality of spin helix is always conserved upon the q-flop. The possible origin is discussed in the light of the stable structure of multiferroic domain wall.
In the light of recent results obtained for the prototype helimagnet MnSi we examine the possible magnetic structures of compounds of the same family, consistent with the crystal symmetries when the magnetic propagation vector is parallel to the [001] axis. The analysis of a published muon spin rotation spectrum recorded in MnGe [Phys. Rev. B 93, 174405 (2016)] shows no deviation from the canonical helimagnetic structure, unlike in MnSi. This qualitative difference calls for further theoretical works on chiral magnets.
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