Magnetic order and fluctuations in the presence of quenched disorder in the kagome staircase system (Co1−xMgx)

Abstract

Co3V2O8 is an orthorhombic magnet in which S=3/2 magnetic moments reside on two crystallographically inequivalent Co 2+ sites, which decorate a stacked, buckled version of the two dimensional kagome lattice, the stacked kagome staircase. The magnetic interactions between the Co 2+ moments in this structure lead to a complex magnetic phase diagram at low temperature, wherein it exhibits a series of five transitions below 11 K that ultimately culminate in a simple ferromagnetic ground state below T∼6.2 K. Here we report magnetization measurements on single and polycrystalline samples of (Co1−xMgx)3V2O8 for x<0.23, as well as elastic and inelastic neutron scattering measurements on single crystals of magnetically dilute (Co1−xMgx)3V2O8 for x=0.029 and x=0.194, in which non-magnetic Mg 2+ ions substitute for magnetic Co 2+ . We find that a dilution of 2.9% leads to a suppression of the ferromagnetic transition temperature by ∼15% while a dilution level of 19.4% is sufficient to destroy ferromagnetic long-range order in this material down to a temperature of at least 1.5 K. The magnetic excitation spectrum is characterized by two spin-wave branches in the ordered phase for (Co1−xMgx)3V2O8 (x=0.029), similar to that of the pure x=0 material, and by broad diffuse scattering at temperatures below 10 K in (Co1−xMgx) 3V2O8 (x=0.194). Such a strong dependence of the transition temperatures to long range order in the presence of quenched non-magnetic impurities is consistent with two-dimensional physics driving the transitions. We further provide a simple percolation model that semi-quantitatively explains the inability of this system to establish long-range magnetic order at the unusually-low dilution levels which we observe in our experiments.

Spin-orbit transitions in $α$ and $γ$-CoV$_{2}$O$_{6}$

“…Co 3 V 2 O 8 has a similar local framework based on a distorted octahedra, but two different Co 2+ sites giving a "distorted-Kagome" type magnetic lattice which also displays magnetic field induced transitions. [73][74][75][76][77][78] Single crystal work on that compound has revealed also two low-energy branches with similar energy scales to that observed here for CoV 2 O 6 . 79 The similarity in the energy scales indicates a predominately anisotropic model may provide a consistent description.…”

Three-dimensional magnetism and the Dzyaloshinskii-Moriya interaction in S = 3/2 kagome staircase Co 3 V 2 O 8

“…Previous work on CVO has described the material as quasi-twodimensional (2D) (27,25), where the dominant magnetic interactions were entirely within the buckled kagome planes. The spin wave dispersion along the [H 0 0] and [0 0 L] directions has been measured (22), and two spin wave branches were observed, with the higher energy branch significantly weaker in intensity and broader in energy.…”

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

“…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. The phase diagram is highly anisotropic, and the susceptibility in the ferromagnetic state along the easy a-axis is almost two orders of magnitude larger than that along the hard b-axis [16,24,28]. The susceptibility along c is intermediate between that along a and b, and we therefore expect CVO to approximate a quasi-2D Ising system.…”