Determination of the magnetic order and the crystal symmetry in the multiferroic ground state of Ba2CoGe2O7

Abstract

Detailed structural investigation of Ba 2 CoGe 2 O 7 was performed in its low-temperature multiferroic state combining neutron diffraction with magnetization measurements and the optical study of lattice vibrations on single crystals. The crystal structure above (10.4 K) and the crystal and magnetic structures below (2.2 K) the antiferromagnetic transition temperature of T N = 6.7 K were determined using neutron diffraction. The tetragonal space group (SG) P -42 1 m, corresponding to the average structure at room temperature, was found to also describe the structure at low temperatures well. Neutron diffraction data and infrared phonon mode analysis imply no structural phase transition down to 2.2 K. Orthorhombic polar SG Cmm2 is proposed as a true crystal structure. Below T N , the spins of the Co 2+ ions form a square-lattice Néel order within the (a,b) plane, while their alignment is ferromagnetic along the c axis. The magnitude of the ordered moment, fully lying within the (a,b) plane, is found to be 2.9(1) μ B /Co 2+ and the easy axis of the sublattice magnetizations corresponds to the [110] direction. A noncollinear spin structure due to small canting is allowed.

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

“…High quality single crystals of Ba 2 CoGe 2 O 7 were grown by floating-zone technique and characterized in previous studies [9,11,13,18]. SNP measurements were performed at 4 K with a Cryopad on the polarized single-crystal diffractometer POLI@HEiDi at the hot source of the FRM II reactor in Garching, Germany [19,20].…”

“…[5] the ground state is collinear and magnetic structure become canted only under the influence of external magnetic field. Recently, using both conventional unpolarized neutron diffraction data [13] and magnetic symmetry analysis [3,4] we have studied the magnetic structure of Ba 2 CoGe 2 O 7 at 2.2 K, below T N ≈ 6.7 K. The results showed an antiferromagnetic (AFM) order of the Co magnetic moments within the (a,b) plane, while neighboring planes stacked along the c axis are ordered ferromagnetically (FM). Throughout the paper we index the momentum-space coordinates q = (h,k,l) in the corresponding reciprocal lattice units (r.l.u.)…”

“…of the orthorhombic Cmm2 crystallographic unit cell proposed previously in Ref. [13], where the two mirror planes are the (100) and (010) planes and the two-fold axis points along the [001] direction. The relation between the unit cells based on the space groups P42 1 m and Cmm2 is illustrated in Ref.…”

“…The relation between the unit cells based on the space groups P42 1 m and Cmm2 is illustrated in Ref. [13]. The direction of the Co magnetic moments was assumed to be parallel to the [100] direction of the Cm m2 cell, based on bulk magnetization measurements in our former work [13], while it was tentatively assigned to be parallel to the [110] axis in early neutron diffraction studies [14].…”

“…[13]. The direction of the Co magnetic moments was assumed to be parallel to the [100] direction of the Cm m2 cell, based on bulk magnetization measurements in our former work [13], while it was tentatively assigned to be parallel to the [110] axis in early neutron diffraction studies [14]. Nevertheless, the precise moment direction within the (a,b) plane cannot be determined unambiguously by unpolarized neutron diffraction due to the presence of energetically equivalent magnetic domains.…”