Fig.1: (a) Arrangement of magnetic Co atoms at two sites resembles “water strider” structure, with alternating links in the form of dimers (red spheres, Co1) and monomers (blue spheres, Co2). The resulting water striders exhibit helical symmetry and form chiral chains by interconnected feet that helical symmetrically along the b direction. The bottom is a simplified representation of the chiral symmetric chains, in which the spin propagation vectors of the left-hand (L) and right-hand (D) chains are derived and translated along the b axis. (b) High-field magnetization processes of Co₂V₂O₇ measured at 1.7 K and 4.2 K for H // c. The insets in the lower right area are the corresponding dM/dH curves. (c) The H dependence of electric polarization along the a axis, DP_a, for H // c at 2 K and 4.2 K. (d) The H dependence of electric polarization along the c axis, DP_c, for H // c at 2 K, 3 K, and 4.5 K. The solid (dashed) curve denotes the field-ascending (descending) sweep, also as seen by the solid (dashed) arrow. (e) H-T phase diagrams of P // c for H // c mapped by plotting the strength of polarization using DP(H) curves for H ascending at different temperatures. The symbols denote the phase-transition points extracted from magnetization and specific heat C_p(T) curves. Dashed lines are provided as guides for the eye. (f) Top: Magnetocaloric effect in Co₂V₂O₇ measured at various initial temperatures in pulsed magnetic fields (H // c). The red symbol denotes the multiferroic quantum critical point (QCP). Gray shadow shows the half magnetization plateau evolved from QCP. The light yellow and light blue backgrounds represent FE and PE phases, respectively. The dashed arrows are the evolution of phase transition points provided as guides for the eye and separate the half plateau in magnetization, FE-II, and PM phases. Bottom: Magnetic Grüneisen parameter obtained from the MCE curves. The color of curves corresponds to that in the upper panel of (f).

Background

Magnetic field control of ferroelectric polarization is a hot topic in the field of multiferroic materials. Previous studies have found that Co₂V₂O₇ has some exotic effects in the H || b direction, such as quantum magnetization plateaus, magnetically induced ferroelectricities and H-induced continuous polarization switches.

What we discover?

In this work, we present isothermal ferroelectric (FE) and quasiadiabatic phase diagrams for single crystal Co₂V₂O₇ with magnetic fields along the c axis, obtained through magnetization, polarization, and magnetocaloric effect measurements under pulsed magnetic fields up to 30 T. At the range of 3-5 K, two ferroelectric phases FE-I and FE-II is observed under the magnetic field. Between them, a paraelectric phase is obtained corresponding to a half magnetization plateaus. As the temperature increases, the FE-II and FE-I phases gradually disappear; With the temperature decreasing, these two ferroelectric phases approach and overlap with each other. At 2 K, the electric polarization of FE-II and FE-I (P || c) exhibit opposite polarity driven by different magnetic fields.

Why is this important?

For Co₂V₂O₇, we propose a chiral symmetric water-strider type spin configuration, elucidating the origin of dynamic ferroelectricity and the reversal of the polarization signal during the magnetization process. In addition, the novel dynamic ferroelectricity and multiferroic quantum critical behavior are revealed.

Who did the research?

R. Chen^1,3,*, H. J. Hu¹, Q. K. Lei¹, C. B. Liu^2,†, C. Dong³, X. Y. Yue⁴, Z. Qu¹, H. W. Wang³, Y. Qiu¹, D. Chen¹, X. L. Yi¹, Z. W. Ouyang³, and J. F. Wang³

(1) College of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China

(2) College of Physics and Electronic Engineering, Nanyang Normal University, Nanyang 473061, China

(3) Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China

(4) Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, China

Physical Review B 108, 224405 (2023)

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.108.224405

Funding

This work was supported by the National Natural Science Foundation of China (Grants No. 12104388, No. 12074135, and No. 12104351), the Natural Science Foundation of Henan (Grant No. 232300420121), the Henan Provincial Department of Science and Technology Research Project (Grant No. 222102230105), the Hubei Province Natural Science Foundation of China (Grant No. 2021CFB027), and the Nanhu Scholars Program for Young Scholars of Xinyang Normal University.