Magnetoelectric effect in simple collinear antiferromagnetic spinels

We report the discovery of the linear magnetoelectric effect in a family of spinel oxides, $\mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4$ and $\mathrm{Mn}{B}_2{\mathrm{O}}_4$ $(B=\mathrm{Al},\text{Ga})$ with simple collinear antiferromagnetic spin structure. An external magnetic field induces a dielectric anomaly at $T_{\mathrm{N}}$, accompanied by the generation of electric polarization that varies linearly with magnetic field. Magnetization and magnetoelectric measurements on a single crystal of $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$ together with a phenomenological theory suggest that the easy axis direction is [111] with the corresponding magnetic symmetry $R{\overline{3}}^{\prime }{m}^{\prime }$. The proposed theoretical model of single-ion contribution of magnetic ions located in a noncentrosymmetric crystal environment stands for a generic mechanism for observing magnetoelectric effects in these and other similar materials.

Figure 1

(a) Schematic representation of cubic crystal structure of $A$-site magnetic spinel oxide $\left(A{B}_2{\mathrm{O}}_4\right)$. Magnetic structures of $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$ with spin moments along (b) [001] and (c) [111] directions.

Figure 2

Temperature dependencies of dc magnetization of (a) $\mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4$ and (b) $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$. Insets of (a) and (b) show the $C_p/T$ as a function of temperature for the respective compounds. Temperature-dependent dielectric constant measured in the presence of different magnetic fields for (c) $\mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4$ and (d) $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$.

Figure 3

Temperature-dependent magnetoelectric polarization induced by magnetic field for (a) $\mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4$ and (b) $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$. (c) Switching of magnetoelectric polarization in the presence of an oscillating magnetic field of ±80 kOe for $\mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4$ at 20 K.

Figure 4

Theoretically calculated magnetic-field dependent dM($H$)/dH in $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$ for the easy axis along (a) [111] and (b) [001] directions. (c) Magnetic-field dependent dM($H$)/dH measured along [001], [011], and [111] on a single crystal of $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$. Inset of (c) shows the crystal of $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$. (d) Temperature-dependent magnetic field $\left(H=90\mathrm{kOe}\right)$ induced electric polarization along [011] with $H$‖[100], $H$‖[011], $H$‖[111] on a single crystal of $\mathrm{MnG}{\mathrm{a}}_2{\mathrm{O}}_4$.