Magnetoelectric Switching of Exchange Bias

The perpendicular exchange bias field, $H_{\mathrm{EB}}$, of the magnetoelectric heterostructure ${\mathrm{Cr}}_2{\mathrm{O}}_3(111)/(\mathrm{Co}/\mathrm{Pt}{)}_3$ changes sign after field cooling to below the Néel temperature of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ in either parallel or antiparallel axial magnetic and electric freezing fields. The switching of $H_{\mathrm{EB}}$ is explained by magnetoelectrically induced antiferromagnetic single domains which extend to the interface, where the direction of their end spins controls the sign of $H_{\mathrm{EB}}$. Novel applications in magnetoelectronic devices seem possible.

Figure 1

Normalized hysteresis curves of ${\mathrm{Cr}}_2{\mathrm{O}}_3(111)/\mathrm{Pt}0.5\mathrm{nm}/[\mathrm{Co}0.3\mathrm{nm}/\mathrm{Pt}1.5\mathrm{nm}{]}_3/\mathrm{Pt}1.5\mathrm{nm}$ measured after magnetic field cooling in ${\mu }_0{H}_{\mathrm{fr}}=0.6\mathrm{T}$ and $E_{\mathrm{fr}}=0$ from $T=350$ to 298 K (1) and after magnetoelectric field cooling to 250 K in ${\mu }_0{H}_{\mathrm{fr}}=0.6\mathrm{T}$ and $E_{\mathrm{fr}}=-500\mathrm{kV}/\mathrm{m}$ (2) and $E_{\mathrm{fr}}=+500\mathrm{kV}/\mathrm{m}$ (3), respectively. The lines are to guide the eyes. The exchange bias fields ${\mu }_0{H}_{\mathrm{EB}}$ referring to the loops 2 and 3 are indicated by arrows.

Figure 2

Schematic sketches of a FM/AFM bilayer with freezing fields $(H_{\mathrm{fr}},\mp E_{\mathrm{fr}})$, order parameters $(\mathbf{M},{\mathbf{l}}_{\pm })$, and magnetic moments $(S_{\mathrm{FM}},S_{\mathrm{AFM}})$, where the AFM layer is single domained ($A$ and $B$, respectively) after MEFC (see text).

Figure 3

Magnetic moment of ${\mathrm{Cr}}_2{\mathrm{O}}_3(111)/\mathrm{Pt}0.7\mathrm{nm}/[\mathrm{Co}0.3\mathrm{nm}/\mathrm{Pt}1.2\mathrm{nm}{]}_3/\mathrm{Pt}3.1\mathrm{nm}$ in its remanent state, $m_r=1.8\times {10}^{-8}\mathrm{A}{\mathrm{m}}^2$, as a function of the applied electric field $E$ at $T=150\mathrm{K}$ after MEFC from $T=350$ to 150 K in ${\mu }_0{H}_{\mathrm{fr}}=0.5\mathrm{T}$ and $E_{\mathrm{fr}}=0$ (line 1), $-460\mathrm{kV}/\mathrm{m}$ (line 2) and $+425\mathrm{kV}/\mathrm{m}$ (line 3).

Figure 4

Exchange bias field ${\mu }_0{H}_{\mathrm{EB}}$ vs electric freezing field $E_{\mathrm{fr}}$ as obtained from hysteresis loops after MEFC from $T=350$ to 298 K in $E_{\mathrm{fr}}$ and magnetic fields ${\mu }_0{H}_{\mathrm{fr}}=0.1$ (curve 1), 0.3 (curve 2), and 0.6 T (curve 3) (main panel and upper inset at enhanced scale). The lines are guides to the eye. The lower inset shows the experimental data $E_0$ vs $({\mu }_0{H}_{\mathrm{fr}}{)}^{-1}$ of the electric threshold field $E_0=E_{\mathrm{fr}}({\mu }_0{H}_{\mathrm{EB}}=0)$. The straight line indicates the best linear fit to Eq. (3).