Anisotropic magnetization plateaus in $S_{\mathrm{eff}}=1/2$ skew-chain single-crystal $\mathrm{C}{\mathrm{o}}_2{\mathrm{V}}_2{\mathrm{O}}_7$

We report anisotropic magnetization plateaus in the skew-chain antiferromagnet ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ with $S_{\mathrm{eff}}=1/2$ and a large $g$ factor. When the field is applied along the easy $b$ axis, a 1/2-like magnetization plateau is observed within $5.4\sim 11.6\mathrm{T}$, followed by a 3/4 plateau at 15.7 T. For the hard $a$ and $c$ axes, however, there is only a sign of 1/2-like magnetization plateau. These observations are quite different from the isostructural ${\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ [Z. W. Ouyang et al., Phys. Rev. B 97, 144406 (2018)]. Theoretically, the first-principles calculations reveal the large interchain coupling, which leads to classical antiferromagnetic ordering and spin-flop-like transition. With this large interchain coupling, the exact diagonalization analysis yields 1/2 and 3/4 plateaus, showing the quantum origin of magnetization plateaus. Thus, ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ is an interchain-coupled system showing both classical and quantum behaviors.

Figure 1

(a) Skew chain with alternating $\mathrm{Co}\left(\mathrm{I}\right){\mathrm{O}}_6$ (red) and $\mathrm{Co}\left(\mathrm{II}\right){\mathrm{O}}_6$ (blue) octahedra along the $c$ axis. There are four Co(I) atoms numbered by 1, 3, 6, and 8 and four Co(II) atoms numbered by 2, 4, 5, and 7 in a unit cell (4 formula unit). The arrows denote the spin orientations of Co ions for the ground-state AFM-1 derived from the first-principles calculations. (b) Three types of superexchange interactions: $J_1$—3.053(3) Å (green bond, Co5-O1/O2-Co6), $J_2$—3.033(5) Å (blue bond, Co6-O3/O4-Co7), and $J_3$—2.972(7) Å (black bond, Co3-O5/O6-Co6) as well as the Co–O–Co bond angles.

Figure 2

(a) Magnetic susceptibility $\chi \left(T\right)$ curves measured at 0.1 T. The inset is the corresponding ${\chi }^{-1}\left(T\right)$ curves. The dashed lines represent Curie-Weiss fit. (b) Specific heat curves for $H//b$ measured at different fields. The curves are shifted up by various magnitudes in relation to the 7-T curve.

Figure 3

(a) High-field $M\left(H\right)$ curves (left) measured at 1.7 K and $dM/dH$ for the $b$ axis (right). The absolute values of magnetization are calibrated with SQUID data (symbols). The dashed straight lines represent the linear extrapolation of the curves to zero field. (b) $M\left(H\right)$ (left) and normalized $M/M_{\mathrm{s}}$ curves (right) corrected for the Van Vleck term. The dashed straight lines mark the magnetization between $H_{\mathrm{c}1}$ and $H_{\mathrm{c}2}$, at $H_{\mathrm{c}3}$, and above $H_{\mathrm{c}4}$. (c) A comparison of the experimental $M\left(H\right)$ curve at 0.7 and 1.7 K for the $b$ axis with the calculated $M\left(H\right)$ curve with $g=3.95, J_1=-17.1\mathrm{K}, J_2=-30.1\mathrm{K}$, and $J_3=-13.9\mathrm{K}$. The arrows show the field direction and possible spin structures at various states.

Figure 4

Field-increasing and decreasing $M\left(H\right)$ curves for $H//b$ measured from 0.7 to 7 K.

Figure 5

(a) ESR spectra of single crystal measured at 170 GHz and 40 K. The sharp DPPH line ($g=2.0$) is used for a field marker. (b) Frequency-field ($f-H$) relation at 2 K as well as several representative ESR spectra of powder sample.

Figure 6

Partial DOS of $\mathrm{Co}3d$ and $\mathrm{O}2p$ for the ground-state AFM-1. The Fermi level is at zero energy.

Figure 7

(a) Isosurface of valence electron density ($\mathrm{e}{\AA }^{-3}$) at the plane determined by Co5, Co7, Co4, and Co2 containing $J_1, J_2$, and $J_3$ superexchange paths. (b)–(d) Isosurface of valence electron density for each Co-O-Co plane:$J_1$—Co6-O-Co5, $J_2$—Co6-O-Co7, and $J_3$—Co6-O-Co3.

Figure 8

Observed and calculated powder XRD patterns and the difference between them. The green vertical bars indicate the expected Bragg reflection positions.

Figure 9

Energy-dispersive spectrum measured by an EPM for single-crystal and powder samples. The inset shows a photograph of the single crystal.

Figure 10

The $M\left(H\right)$ curve measured at 2 K for powder sample.