Realizing Magnetoelectric Coupling with Hydrogen Intercalation

Materials with a coexistence of magnetic and ferroelectric order (i.e., multiferroics) provide an efficient route for the control of magnetism by electric fields. Unfortunately, a long-sought room temperature multiferroic with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings is still lacking. Here, we propose that hydrogen intercalation in antiferromagnetic transition-metal oxides is a promising way to realize multiferroics with strong magnetoelectric coupling. Taking brownmillerite ${\mathrm{SrCoO}}_{2.5}$ as an example, we show that hydrogen intercalated ${\mathrm{SrCoO}}_{2.5}$ displays strong ferrimagnetism and large electric polarization in which the hydroxide acts as a new knob to simultaneously control the magnetization and polarization at room temperature. We expect that ion intercalation will become a general way to design magnetoelectric and spintronic functional materials.

Figure 1

(a) Illustration of the general idea of using hydroxide as a knob to simultaneously control the magnetization and polarization. Longer gray arrows and shorter blue arrows represent local magnetic moment of magnetic ions (${\mathrm{B}}^{n+}$ and ${\mathrm{B}}^{(n-1)+}$), respectively. For clarity, only the two B ions near the hydroxide are shown. (b) ME coupling in ${({\mathrm{SrCoO}}_{2.5})}_8{\mathrm{H}}_1$. Depending on the orientation of the hydroxide, two different configurations (left, tet1 configuration; right, oct1 configuration) have different net magnetization ($M$) and polarization ($P$). The local magnetic moments for each Co ion are displayed. The rhombus and triangle represent the oxygen octahedron and tetrahedron, respectively. Hydroxide and hydrogen bond are represented by solid and dotted line, respectively.

Figure 2

Origin of the electric polarization difference between tet1 and oct1 configurations. (a) Simple estimation of the polarization change due to the hydroxide rotation and charge transfer associated with the charge ordering. Tetrahedral Co1 and octahedral Co2 are labeled in Fig. 1. (b) The partial DOS plot of the tetrahedral site Co1 (gray line) and the octahedral site Co2 (blue line) in the oct1 configuration (left) and tet1 configuration (right), evidencing the charge transfer associated with the charge ordering. In the DOS calculations, the $G$-type magnetic ordering is adopted. The positive and negative values represent majority component and spin minority component, respectively.

Figure 3

Schematics of four configurations in ${({\mathrm{SrCoO}}_{2.5})}_8{\mathrm{H}}_2$. (a) TT configuration: both hydroxides point toward oxygen tetrahedron, (b),(c) TO and OT configuration: one hydroxide points toward the octahedron, while the other points toward the tetrahedron, (d) OO configuration: both hydroxides point toward oxygen octahedron. All four configurations adopt the $G$-type magnetic structure with the easy axis along the $a$ axis as the magnetic ground state. Arrows denote the spin directions of the Co ions. Relative energies ($\mathrm{meV}/2\mathrm{H}$), net magnetization, and direction of total electric polarization are given for different configurations.