Metamagnetism and soliton excitations in the modulated ferromagnetic Ising chain CoV${}_2$O${}_6$

We report a combination of physical property and neutron scattering measurements for polycrystalline samples of the one-dimensional spin-chain compound CoV${}_2$O${}_6$. Heat capacity measurements show that an effective $S=1/2$ state is found at low temperatures and that magnetic fluctuations persist up to $\sim$6$T_N$. Above $T_N=6.3$ K, measurements of the magnetic susceptibility as a function of $T$ and $H$ show that the nearest-neighbor exchange is ferromagnetic. In the ordered state, we have discovered a crossover from a metamagnet with strong fluctuations between 5 K and $T_N$ to a state with a 1/3 magnetization plateau at $2<T<5$ K. We use neutron powder diffraction measurements to show that the antiferromagnetic state has incommensurate long-range order and inelastic time-of-flight neutron scattering to examine the magnetic fluctuations as a function of temperature. Above $T_N$, we find two broad bands between 3.5 and 5 meV and thermally activated low-energy features which correspond to transitions within these bands. These features show that the excitations are deconfined solitons rather than the static spin reversals predicted for a uniform ferromagnetic Ising spin chain. Below $T_N$, we find a ladder of states due to the confining effect of the internal field. A region of weak confinement below $T_N$, but above 5 K, is identified which may correspond to a crossover between two- and three-dimensional magnetic ordering.

Figure 1

(a) Structure of triclinic CoV${}_2$O${}_6$ showing edge-sharing CoO${}_6$ chains, with the two crystallographically independent sites indicated. For clarity, the V${}_2$O${}_5$ blocks are omitted. The principle exchange interactions in the chain are indicated in (b).

Figure 2

(a) Magnetic susceptibility and inverse susceptibility of CoV${}_2$O${}_6$: the black line shows the Curie-Weiss fit to the high-temperature region described in text. Inset shows $\chi T$ as a function of temperature. (b) Magnetic specific heat of CoV${}_2$O${}_6$: line shows integrated magnetic entropy, inset shows total specific heat and estimated lattice contribution.

Figure 3

(a) Magnetization isotherms measured for CoV${}_2$O${}_6$ above magnetic ordering temperature; (b) magnetization isotherms measured for CoV${}_2$O${}_6$ below magnetic ordering temperature. A constant $x$-axis offset of 1500 Oe has been applied to the results in (b) for clarity. Lines in all plots are guides to the eye.

Figure 4

Observed, calculated, and difference plots for Rietveld fit to (a) 2.8 Å and (b) 1.79 Å neutron powder diffraction profiles of CoV${}_2$O${}_6$ at 2 K. Insets show unindexed magnetic satellite reflections and fit to high-angle data, respectively. The star in (a) marks the magnetic reflections followed to determine the order parameter using E6.

Figure 5

Temperature dependence of the elastic magnetic diffraction peaks recorded using E6. The black squares are elastic data recorded using IN5 with 5-Å incident neutrons. The solid line marks a power-law fit, and the dashed line marks the approximate extent of the critical region.

Figure 6

Inelastic neutron scattering from CoV${}_2$O${}_6$ for $7<T<20$ K. Data have been summed for all momentum transfers. The inset shows the temperature dependence of the integrated inelastic scattering above and below 1.5 meV. All lines are fits as described in the text.

Figure 7

(a) Inelastic neutron scattering from CoV${}_2$O${}_6$ just above $T_N$ at 7 K. Data have been summed for all momentum transfers, the lines are fits as discussed in the text, and the horizontal bar shows the instrumental resolution. (b) Energy-integrated inelastic neutron scattering data for CoV${}_2$O${}_6$. The solid line shows the squared Co${}^{2+}$ form factor. The insets show the $Q$ dependencies of the two principle excitations.

Figure 8

The results of the inelastic neutron scattering experiment using the low-energy configuration of IN5 for a range of temperatures. The data are presented in the form of constant $Q=1.3\pm 0.1$ Å${}^{-1}$ and have been corrected for the effects of temperature using Eq. (3). The lines show the results of fitting Lorentzian functions and a sloping background.

Figure 9

Momentum transfer summed inelastic scattering of CoV${}_2$O${}_6$ at 1.5 K and 7.08 meV incident energy showing the confining effect of the internal field in the magnetically ordered state. Lines are fits to Lorentzian functions. The inset shows the energy of the bound states with a linear fit.

Figure 10

The temperature-magnetic field phase diagram of CoV${}_2$O${}_6$ established from the physical property measurements in this work. The various phases are labeled as FM (fully polarized ferromagnetic), 1/3 (plateau in magnetization at 1/3 of saturation), and AFM (antiferromagnetic phase). The region of hysteresis found at the lower bound of the 1/3 plateau is shown in magenta (unlabeled dark shading).