Imprinting Vortices into Antiferromagnets

The effect of imprinting symmetric and displaced vortex structures into an antiferromagnetic material is investigated in micron-sized disks consisting of exchange coupled ferromagnetic-antiferromagnetic bilayers. The imprint of displaced vortices manifests itself by the occurrence of a new type of asymmetric hysteresis loops characterized by curved, reversible, central sections with nonzero remanent magnetization. Such an imprint is achieved by cooling the disks through the blocking temperature of the system in small fields. Micromagnetic simulations reveal that asymmetric vortexlike loops naturally result from the competition between the different energies involved in the system.

Figure 1

Hysteresis loops corresponding to: (a) Py (●) and Py-IrMn (○) disks FC along ${\theta }_{\mathrm{FC}}=0°$ in ${\mu }_0{H}_{\mathrm{FC}}=250\mathrm{mT}$; (b) Py-IrMn disks after ZFC (gray ●) and FC in ${\mu }_0{H}_{\mathrm{FC}}=2.5\mathrm{mT}$ (red ×) and ${\mu }_0{H}_{\mathrm{FC}}=10\mathrm{mT}$ (blue ■) along ${\theta }_{\mathrm{FC}}=0°$; (c) longitudinal (●) and transverse (○) components of Py-IrMn disks FC along ${\theta }_{\mathrm{FC}}=90°$ in ${\mu }_0{H}_{\mathrm{FC}}=10\mathrm{mT}$.

Figure 2

(a) Micromagnetic simulations of the hysteresis loops corresponding to Py-IrMn disks (solid line) and Py (dotted line) after ZFC; the micromagnetic simulations predict no transverse signal (dashed-dotted line) for both Py and Py-IrMn; (b) calculated longitudinal (solid line) and transverse (dashed-dotted line) hysteresis loops for ${\theta }_{\mathrm{FC}}=0°$ and ${\mu }_0{H}_{\mathrm{FC}}=10\mathrm{mT}$; (c) calculated longitudinal (solid line) and transverse (dashed-dotted line) hysteresis loops for Py-IrMn for ${\theta }_{\mathrm{FC}}=90°$ and ${\mu }_0{H}_{\mathrm{FC}}=10\mathrm{mT}$; (d) calculated positions ($x$, $y$) of the vortex core center in Py and Py-IrMn ZFC; (e) calculated positions of the vortex core in Py-IrMn, for ${\theta }_{\mathrm{FC}}=0°$ and ${\mu }_0{H}_{\mathrm{FC}}=10\mathrm{mT}$; (f) calculated positions of the vortex core in Py-IrMn, for ${\theta }_{\mathrm{FC}}=90°$ and ${\mu }_0{H}_{\mathrm{FC}}=10\mathrm{mT}$.

Figure 3

Micromagnetic calculations of the spin configurations imprinted to the AFM when cooling in (a) zero field, (b) ${\theta }_{\mathrm{FC}}=0°$ and ${\mu }_0{H}_{\mathrm{FC}}=2.5\mathrm{mT}$, (c) ${\theta }_{\mathrm{FC}}=0°$ and ${\mu }_0{H}_{\mathrm{FC}}=10\mathrm{mT}$, and (d) ${\theta }_{\mathrm{FC}}=90°$ and ${\mu }_0{H}_{\mathrm{FC}}=10\mathrm{mT}$. (e)–(h) are the spin configurations of the FM at remanence calculated for the imprinted configurations of (a)–(d), respectively. The shading represents the magnitude of the magnetization vector along the perpendicular to the field-cooling direction. The direction of the applied field during the hysteresis loops (${\mu }_0{H}_{\mathrm{appl}}$) is also indicated. The lines illustrate the core trajectories.

Figure 4

Evolution of the measured (●) and calculated normalized remanent magnetization $M_R/M_S$ (continuous line) and the calculated core position at remanence $y_R$ (dotted line) from the FC disks (${\theta }_{\mathrm{FC}}=0°$), as a function of cooling field ${\mu }_0{H}_{\mathrm{FC}}$.