Spin waves in the ferromagnetic ground state of the kagome staircase systemCo3V2O8

Abstract

Inelastic neutron scattering measurements were performed on single crystal Co3V2O8 wherein magnetic cobalt ions reside on distinct spine and cross-tie sites within kagomé staircase planes. This system displays a rich magnetic phase diagram which culminates in a ferromagnetic ground state below TC ∼ 6 K. We have studied the low-lying magnetic excitations in this phase within the kagomé plane. Despite the complexity of the system at higher temperatures, linear spin-wave theory describes most of the quantitative detail of the inelastic neutron measurements. Our results show two spin-wave branches, the higher energy of which displays finite spin-wave lifetimes well below TC, and negligible magnetic exchange coupling between Co moments on the spine sites. PACS numbers: 75.30.Ds, 75.50.Dd, 75.10.Dg Magnetic materials in which the constituent magnetic moments reside on networks of triangles and tetrahedra have been of great interest due to their propensity for exotic ground states, a consequence of geometrical frustration [1]. While ferromagnetically-coupled moments on such lattices generally do not result in such ground states, ferromagnets, and materials which display both ferromagnetic (FM) and antiferromagnetic (AFM) interactions, on such lattices remain of great interest, in part due to the relative scarcity of well-studied examples, and in part due to intriguing spin ice [2] and multiferroic phenomena [3] which characterize some of these ground states. The kagomé lattice is comprised of a two-dimensional network of corner-sharing triangles. Several realizations of magnetic moments on stacked kagomé lattices with varying degrees of crystalline order have been extensively studied. Recently studied examples include jarosites, such as KFe 3 (OH) 6 (SO 4 ) 2 [4] and herbertsmithite ZnCu 3 (OH) 6 Cl 2 [5], both of which show evidence of strong magnetic frustration. The stacked kagomé staircase materials M 3 V 2 O 8 (M=Ni,Co) display orthorhombic crystal structures with space group Cmce [6]. Their kagomé layers are buckled and composed of edge-sharing M 2+ O 6 octahedra. These layers are separated by non-magnetic V 5+ O 4 tetrahedra. The buckled kagomé layers are perpendicular to the orthorhombic b-axis and form what is known as a stacked kagomé staircase structure. Figure 1 shows the projection of the kagomé staircase onto the a-c plane. The two inequivalent M sites are known as spines (M1) and crossties (M2). The superexchange interaction between spine and cross-tie sites and between two adjacent spine sites : Co (1) : Co (2) a c R+δ1 R-δ2 R-δ1 R+δ2 R JSS JSC JSC (a) (b) FIG. 1: [color online] (a) A schematic diagram of the kagomé staircase structure as reduced to 2D in the a-c plane. The cobalt ions are represented by open and solid circles for spine (M1) and cross-tie sites (M2), respectively. Chains of spine sites running parallel to the a-direction are alternatively above (⊙) and below the plane (⊗). (b) The basis used in the linear spin-wave theory calculation.are denoted by J sc and J ss , respectively.One member...

Magnetic anisotropy in geometrically frustrated kagome staircase lattices

“…Such measurements have been performed for the first time on Co 3 V 2 O 8 single crystals using inelastic neutron scattering technique [19]. The results yielded, Another (indirect) way to determine crystal field parameters acting on magnetic ions is to study the electron paramagnetic resonance (EPR) spectra of magnetically diluted kagome staircase compounds.…”

Magnetic order and fluctuations in the presence of quenched disorder in the kagome staircase system (Co1−xMgx)Fritsch1, Yamani2, Chang3 et al. 2012Phys. Rev. BSelf Cite7130View full textAdd to dashboardCite

“…For example, Ni 3 V 2 O 8 displays 4 different magnetically ordered states below 10 K and has attracted much interest due to the discovery of multiferroic behavior in one of its incommensurate phases at low temperature 3,7,8 . The low-temperature phase diagram of Co 3 V 2 O 8 (CVO) has been studied by neutron diffraction in both zero [9][10][11][12] and in finite applied magnetic fields 13,14 . It has also been studied by optical spectroscopy 15,16 , by heat capacity and magnetization 3,17 as well as by µSR 18 and by NMR techniques 19,20 .…”

“…These ultimately culminate in a simple ferromagnetic ground state below T∼6.2 K. All five of the magnetic states display a preferred direction of the spins parallel to the a-axis, the easy axis of this system. From previous inelastic neutron scattering measurements, the exchange parameters of the microscopic Hamiltonian in the ground state of CVO have been extracted to some approximation, using linear spin wave theory 11 . These calculations approximate the CVO system as two-dimensional and reveal that the exchange between magnetic moments on the cross-tie and spine sites is ferromagnetic and J cs ∼1.25meV, while the exchange between adjacent spine sites (J ss in Fig.…”

Quantum phase transitions and decoupling of magnetic sublattices in the quasi-two-dimensional Ising magnetCo3V2O8in a transverse magnetic field

“…Within the ferromagnetic phase, the ordered moments on the cross-tie sites are 1.8µ B , almost a factor of two smaller than the 3µ B moments on the spine sites [29]. The spin Hamiltonian is approximately known [18] from earlier zero field neutron measurements which treated CVO as a 2D system and employed linear spin wave theory. The resulting exchange parameters between magnetic moments on the cross-tie and spine sites were found to be ferromagnetic with J cs ∼ 1.25 meV, while the exchange between adjacent spine sites (J ss in Fig.…”

“…The low temperature phase diagram of CVO has been studied by neutron diffraction in both zero [16][17][18][19] and finite applied magnetic fields [20,21], by optical spectroscopy [22,23], heat capacity and magnetization [12,24] as well as µSR [25] and NMR [26,27] measurements. In zero field, CVO displays four different incommensurate and commensurate antiferromagnetic phases below 11.2 K that ultimately terminate in a ferromagnetic ground state below T ∼6.2 K. All five of the magnetic states display a preferred direction of the spins parallel to the a-axis, the easy axis of this system.…”