Tuning the Effective Anisotropy in a Voltage-Susceptible Exchange-Bias Heterosystem

Voltage- and temperature-tuned ferromagnetic hysteresis is investigated by a superconducting quantum-interference device and Kerr magnetometry in a thin-film heterostructure of a perpendicular anisotropic Co/Pd ferromagnet exchange coupled to the magnetoelectric antiferromagnet ${\mathrm{Cr}}_2{\mathrm{O}}_3$. An abrupt disappearance of exchange bias with a simultaneous more than twofold increase in coercivity is observed and interpreted as a competition between the effective anisotropy of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and the exchange-coupling energy between boundary magnetization and the adjacent ferromagnet. The effective anisotropy energy is given by the intrinsic anisotropy energy density multiplied by the effective volume separated from the bulk through a horizontal antiferromagnetic domain boundary. Kerr measurements show that the anisotropy of the interfacial ${\mathrm{Cr}}_2{\mathrm{O}}_3$ can be tuned isothermally and in the absence of an external magnetic field by application of an electric field. A generalized Meiklejohn-Bean model accounts for the change in exchange bias and coercivity as well as the asymmetric evolution of the hysteresis loop. In support of this model, the reversal of the boundary magnetization is experimentally confirmed as a contribution to the magnetic hysteresis loop.

Figure 1

Hysteresis loops of the magnetization of perpendicular anisotropic Co/Pd for temperatures 180 K (squares), 193 K (circles), 198 K (triangles), and 203 K (diamonds). Diamagnetic background from the substrate is subtracted. Inset: Relative magnetization ($M/M_s^{+}$, magnetization/positive saturation magnetization) at 93 mT (dotted line in the main figure) during the ascending branch of the hysteresis loop.

Figure 2

Absolute value of the hysteresis loop’s exchange bias (squares) and coercivity (triangles). Coercive fields are determined from interception with the magnetic field axis. The inset shows a schematic of the sample including a cartoon of the spin structure. Lower layer of up and down arrows depicts the antiferromagnetic order, the second layer depicts the boundary magnetization, and the top layer depicts the spins of the ferromagnet. Angles $\alpha$ and $\beta$ are visualized relative to the common easy axis of the spins.

Figure 3

Total change in the magnetic moment between positive and negative saturation magnetization as a function of the temperature. The transition region is marked. Dotted lines are to guide the eye. Inset shows data of a second piece of the same sample measured with identical protocol.

Figure 4

(a) Hysteresis of sample 2 at 295 K after $-829\text{−}\mathrm{mT}$ and $+12\text{−}\mathrm{V}$, 0-V and $-12\text{−}\mathrm{V}$ pulses. (b) Detail of the active region shown in (a). (c) Detail of the active region after 0 mT and $-12\mathrm{V}$ is pulsed. Loops are taken in succession; each loop is measured in 50 s.