Spin-flop driven magneto-dielectric effect in Co4Nb2O9

Abstract

Co4Nb2O9 becomes antiferromagnetic (AFM) below 27.4 K with a spin-flop transition at a critical field, Hc, of 12 kOe. Room-temperature dielectric properties are dominated by finite electronic conductivity. Below 125 K, the charge carriers are frozen-out and the dielectric constant is controlled by the lattice phonons. A large (12%) spin flop-driven enhancement in dielectric constant is found in the very narrow temperature interval (Δ T = 1.6 K) in the vicinity of the AFM phase transition. Magneto-dielectric anomaly shows low-frequency dispersion; therefore, the H-induced changes in the phonon eigenfrequencies are unlikely. Other possible reasons for unusual magneto-dielectric effect in Co4Nb2O9 are discussed.

Large magnetoelectric coupling in Co4Nb2O9

“…The unit cell consists of two formula units built up by three unit cells of the corundum type. The magnetic structure of Co 4 Nb 2 O 9 has been settled by Bertaut et al and recently confirmed by Schwarz et al282930 According to the result of neutron diffraction, two non-equivalent octahedral sites are occupied by Co I and Co II . The magnetic moments of the Co ions are parallel to c direction and form chains along the line (, , z ) (spin up) and (, , z ) (spin down), which is plotted in Fig.…”

“…The magnetic moments of the Co ions are parallel to c direction and form chains along the line (, , z ) (spin up) and (, , z ) (spin down), which is plotted in Fig. 1(b), resulting in a collinear antiferromagnetic spin structure with a magnetic point 30. This spin configuration is similar to that of Cr 2 O 3 , which breaks both the space-inversion and time-reversal symmetries and then allows a linear ME effect19.…”

Magnetodielectric and magnetoelastic coupling in Co4Nb2O9 below Néel temperature

“…Thus the magnetodielectric effect exists in the entire temperature region below T N , not only near T N as reported previously. At the same time, the strain exhibits a drop below T N , and the magnetic field suppresses this anomaly, demonstrating magnetoelastic coupling in Co 4 Co 4 Nb 2 O 9 is a collinear antiferromagnet with a spin-flop transition at a critical field of about 12 kOe, and a dielectric peak around Néel temperature (T N ) under the magnetic field larger than this critical field is found by Kolodiazhnyi et al 1 Subsequently, magnetic field induced electric polarization and electric field control of magnetism is observed below T N by Fang et al 2 More recently, Khanh et al investigated magnetic structure and magnetoelectric response in single crystal sample, and the results suggest the in-plane antiferromagnetic order and the formation of a ferrotoroidic order in Co 4 Nb 2 O 9 . 3 However, none of them get much attention to the dielectric property below T N .…”

“…The magnetic field has no effect on ε ′ in the high temperature region, but induce a sharp peak around T N in the ε ′ ∼T curve as reported previously. 1 The magnetic field also shifts up the plateaus of ε ′ in the low temperature region under zero magnetic field.…”

Nonlinear magnetoelectric effect in paraelectric state of Co4Nb2O9 single crystal

“…According to previous report by Kolodiazhnyi et al . 6 , the structural space group of CNO is . And a magnetodielectric peak attributed to magnetically-driven spin-flop phase transition was observed in polycrystalline CNO near the antiferromagnetic phase transition temperature (T N  = 27.5 K) under a high external magnetic field (>12 kOe).…”