Complex magnetic order in the kagomé staircase compoundCo3V2O8

Abstract

Co 3 V 2 O 8 ͑CVO͒ has a different type of geometrically frustrated magnetic lattice, a kagomé staircase, where the full frustration of a conventional kagomé lattice is partially relieved. The crystal structure consists of two inequivalent ͑magnetic͒ Co sites, one-dimensional chains of Co͑2͒ spine sites, linked by Co͑1͒ cross-tie sites. Neutron powder diffraction has been used to solve the basic magnetic and crystal structures of this system, while polarized and unpolarized single crystal diffraction measurements have been used to reveal a rich variety of incommensurate phases, interspersed with lock-in transitions to commensurate phases. CVO initially orders magnetically at 11.3 K into an incommensurate, transversely polarized, spin density wave state, with wave vector k = ͑0,␦ ,0͒ with ␦ = 0.55 and the spin direction along the a axis. ␦ is found to decrease monotonically with decreasing temperature and then locks into a commensurate antiferromagnetic structure with ␦ = 1 2 for 6.9Ͻ T Ͻ 8.6 K. In this phase, there is a ferromagnetic layer where the spine site and cross-tie sites have ordered moments of 1.39 B and 1.17 B , respectively, and an antiferromagnetic layer where the spine-site has an ordered moment of 2.55 B , while the cross-tie sites are fully frustrated and have no observable ordered moment. Below 6.9 K, the magnetic structure becomes incommensurate again, and the presence of higherorder satellite peaks indicates that the magnetic structure deviates from a simple sinusoid. ␦ continues to decrease with decreasing temperature and locks in again at ␦ = 1 3 over a narrow temperature range ͑6.2Ͻ T Ͻ 6.5 K͒. The system then undergoes a strongly first-order transition to the ferromagnetic ground state ͑␦ =0͒ at T c = 6.2 K. The ferromagnetism partially relieves the cross-tie site frustration, with ordered moments on the spine-site and cross-tie sites of 2.73 B and 1.54 B , respectively. The spin direction for all spins is along the a axis ͑Ising-like behavior͒. A dielectric anomaly is observed around the ferromagnetic transition temperature of 6.2 K, demonstrating that there is significant spin-charge coupling present in CVO. A theory based on group theory analysis and a minimal Ising model with competing exchange interactions can explain the basic features of the magnetic ordering.

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

“…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.…”

Heat switch effect in an antiferromagnetic insulator Co3V2O8

“…The various phases at lower temperatures are distinguished by the commensurability of the spin density wave vector. 17,22 The b-component of the magnetic modulation vector, δ, decreases continuously from δ = 0.55 and it locks in at a commensurate value of δ = 1/2 at T c1 = 8.9 K. Below T c2 = 7.0 K, δ decreases continuously again locks into another commensurate value, δ = 1/3, at T c3 = 6.6 K. At T c4 = 6.2 K, δ becomes zero and the Co 2+ spins on both sites become ferromagnetically ordered with the spin alignment along the a axis.…”

“…The presence of two different Co 2+ sites along with competing interactions such as the single-ion anisotropy, the nearest-neighbor and the next nearest-neighbor exchanges, and Dzyaloshinskii-Moriya (DM) interactions lead to fascinating magnetic behaviors at low temperatures. [1][2][3][16][17][18][19][20]22,23 In zero field, there are successive magnetic transitions from the paramagnetic (PM) to incommensurate antiferromagnetic (ICAF), commensurate antiferromagnetic (CAF) and commensurate ferromagnetic phases (CF). 1,19 The geometric frustration results in an antiferromagnetic order of Co 2+ spins at a rather low temperature of T N = 11.4 K. 2,16,18 At T N , only the Co 2+ spins locating in the spine site order antiferromagnetically along the a axis.…”

“…The various phases at lower temperatures are distinguished by the commensurability of the spin density wave vector. 17,22 The b- …”

Magnetic structure of the Kagomé mixed compound (Co0.5Ni0.5)3V2O8

“…Due to the reduced symmetry of the Kagomé staircase geometry with respect to the ideal plane net the degree of frustration is lowered leading to interesting long range ordered magnetic structures. Magnetization and neutron diffraction experiments on a (Co 0.52 Ni 0.48 ) 3 V 2 O 8 powder sample [3] revealed only one magnetic phase transition into an antiferromagnetic ground state in contrast to the richness of magnetic phase transitions of its parent compounds [4,5]. The magnetic structure is modulated by a composition dependent propagation vector k=(δ, 0, 0) with δ being 0.491(4) for (Co 0.52 Ni 0.48 ) 3 V 2 O 8 where a similarity to the NVO type magnetic structure was assumed [3].…”