Successive Magnetic-Field-Induced Transitions and Colossal Magnetoelectric Effect in ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$

We report the discovery of a metamagnetic phase transition in a polar antiferromagnet ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ that occurs at 52 T. The new phase transition accompanies a colossal magnetoelectric effect, with a magnetic-field-induced polarization change of $0.3\mu \mathrm{C}/{\mathrm{cm}}^2$, a value that is 4 times larger than for the spin-flop transition at 9 T in the same material, and also comparable to the largest magnetically induced polarization changes observed to date. Via density-functional calculations we construct a full microscopic model that describes the data. We model the spin structures in all fields and clarify the physics behind the 52 T transition. The high-field transition involves a competition between multiple different exchange interactions which drives the polarization change through the exchange-striction mechanism. The resultant spin structure is rather counterintuitive and complex, thus providing new insights on design principles for materials with strong magnetoelectric coupling.

Figure 1

(a) Crystal structure of ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$. Only Ni ions are shown. (b)–(e) Schematic spin structure along a $c$-axis chain at different magnetic fields applied along the $c$ axis. The spins in the buckled honeycomb planes are aligned ferromagnetically in zero magnetic field [13]. $J_i$ stands for the five nearest-neighbor exchange constants considered in the text. Dotted boxes indicate the magnetic unit cell along the $c$ axis, which is doubled for (b)–(d) but not for (e).

Figure 2

(a),(b) Magnetization and (c),(d) change of electric polarization ($\mathrm{\Delta }P$) of ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ for magnetic fields applied along different crystalline axes as indicated. (e) $c$-axis magnetostriction as a function of magnetic field applied along the $c$ axis. Solid lines are experimental data taken under isothermal condition and open squares are from model calculations based on parameters shown in Table 1. (f) $\mathrm{\Delta }P$ measured up to 92 T for $H\parallel ab$ and $H\parallel c$. (g), (h) The magnetic field dependence of $M$ and $\mathrm{\Delta }P$ along the $c$-axis in $H\parallel c$ configuration obtained from the model calculations described in text. Dashed lines in (a) and (b) are guides for the eyes. (a),(b) magnetization and (e) magnetostriction curves are shifted for clarity. (e) Jump in magnetostriction at $H_{c2}$ shows a slight difference in magnitude between the different types of magnets used, possibly due to the variation in magnetic field sweep rate. (f) Lines and open symbols denote data taken by using a capacitor-bank-driven and a hybrid pulse magnet, respectively. A single domain sample was used for (c) and (d) while a multidomain (as-grown) sample was used for (f).

Figure 3

$H\text{−}T$ phase diagram of ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ with magnetic field applied along the $c$ axis determined by peak positions in $dM/dH$, $\alpha \equiv dP/dH$, and specific heat (see Supplemental Material [18]) data. Dashed line is a guide for the eyes.