Evolution of two-dimensional antiferromagnetism with temperature and magnetic field in multiferroicBa2CoGe2O7

Abstract

We report on spherical neutron polarimetry and unpolarized neutron diffraction in zero magnetic field as well as flipping ratio and static magnetization measurements in high magnetic fields on the multiferroic square lattice antiferromagnet Ba 2 CoGe 2 O 7 . We found that in zero magnetic field the magnetic space group is Cm m2 with sublattice magnetization parallel to the [100] axis of this orthorhombic setting. The spin canting has been found to be smaller than 0.2 • in the ground state. This assignment is in agreement with the field-induced changes of the magnetic domain structure below 40 mT as resolved by spherical neutron polarimetry. The magnitude of the ordered moment has been precisely determined. Above the magnetic ordering temperature short-range magnetic fluctuations are observed. Based on the high-field magnetization data, we refined the parameters of the recently proposed microscopic spin model describing the multiferroic phase of Ba 2 CoGe 2 O 7 .

Direct observation of spin-quadrupolar excitations in Sr2CoGe2O7 by high-field electron spin resonance

“…The associated spin shortening is S ≈ 1.3 to 1.35, about 10% of the full spin value of 3/2 -the magnetization at the saturation field is about 10% smaller from the fully saturated value in very high magnetic field [24]. In comparison, the spin shortening in SCGO is S = 1.491, less than 1%.…”

“…The magnetization curves are nearly isotropic, and the difference in the saturation fields is mainly explained by the g-tensor anisotropy. In otheråkermanite compounds, like Sr 2 CoSi 2 O 7 [8] and Ba 2 CoGe 2 O 7 [24], the slope of in-plane magnetization is almost twice the slope of the magnetization measured perpendicular to the cobalt plane. This large difference signals the presence of large easy-plane anisotropy.…”

Manifestation of magnetic quantum fluctuations in the dielectric properties of a multiferroic

“…Earlier studies of BCGO suggested a tetragonal crystal symmetry, and thus the fourfold or U(1) symmetry for describing the in-plane magnetization 26,28 . However, recent detailed investigations indeed revealed the presence of the twofold magnetization in the plane and an orthorhombic Cm 0 m2 0 magnetic space group 30,31 . The twofold symmetry is likely to be reinforced by the small but unavoidable in-plane magnetic field component, coming from the tilting of the magnetic field from the c-axis.…”

“…1), inelastic neutron diffraction 28 , electron spin resonance and far infrared spectroscopy 29 . However, recent studies on the lattice and magnetic structure 30,31 show that BCGO actually has a lower symmetry with weak orthorhombic distortion in the plane and result in magnetic space group Cm 0 m2 0 . A change in the FE polarization appears below the AF ordering temperature (T N ¼ 6.7 K, Supplementary Fig.…”

Magnetic structure of the magnetoelectric material Ca2CoSi2O7

“…In Ba 2 CoGe 2 O 7 , the room temperature crystal structure is indeed tetragonal P42 1 m with a = b ≈ 8.39Å and c ≈ 5.56Å (Ref. [11]), with possible symmetry lowering to orthorhombic Cmm2 at low temperatures [11,12]. The room temperature crystal structure of Sr 2 CoSi 2 O 7 is also tetragonal P42 1 m with a = b ≈ 8.03Å and c ≈ 5.16Å (Ref.…”