Spin-Flop Coupling and Exchange Bias in Embedded Complex Oxide Micromagnets

The magnetic domains of embedded micromagnets with $2\mu \mathrm{m}\times 2\mu \mathrm{m}$ dimensions defined in epitaxial ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ (LSMO) thin films and ${\mathrm{LaFeO}}_3/\mathrm{LSMO}$ bilayers were investigated using soft x-ray magnetic microscopy. Square micromagnets aligned with their edges parallel to the easy axes of LSMO provide an ideal experimental geometry for probing the influence of interface exchange coupling on the magnetic domain patterns. The observation of unique domain patterns not reported for ferromagnetic metal microstructures, namely divergent antiferromagnetic vortex domains and “$Z$”-type domains, suggests the simultaneous presence of spin-flop coupling and local exchange bias in this system.

Figure 1

 (a) X-PEEM measurement geometry; X-PEEM images showing (b) AFM domain structure at $T>T_C$ ($T=300\mathrm{K}$), (c) FM domain structure at $T<T_C$ ($T=100\mathrm{K}$), and (d) corresponding AFM domain structure for an unpatterned region of the LFO/LSMO bilayer. The legend in the lower left insets (b)–(d) illustrates the AFM or FM spin orientations associated with each contrast value. Fe XMLD-PEEM images were taken with $\omega =90°$.

Figure 2

(a) Cross section of embedded micromagnets in an implanted matrix. (b) AFM domain structure at $T>T_C$ ($T=300\mathrm{K}$) of $2\mu \mathrm{m}\times 2\mu \mathrm{m}$ bilayer micromagnets oriented with edges parallel to $⟨110⟩$ substrate directions; (c) FM domain structure at $T<T_C$ ($T=100\mathrm{K}$), and (d) corresponding AFM domain structure. Arrows denote AFM or FM spin orientations associated with each contrast value. Insets indicate the experimental geometry.

Figure 3

X-PEEM images of $2\mu \mathrm{m}\times 2\mu \mathrm{m}$ micromagnets oriented with edges parallel to $⟨110⟩$ substrate directions. (a) FM domain structure at $T<T_C$ ($T=100\mathrm{K}$) and (b) corresponding AFM domain structure in the bilayer micromagnets. (c) FM domain structure at $T<T_C$ ($T=100\mathrm{K}$) for a patterned single-layer LSMO film. (d) AFM domain maps deduced from polarization dependent measurements of micromagnets outlined in (b) and schematics of the FM domain structure deduced from micromagnets outlined in (a). Insets in the top right corner of (a)–(c) and in (d) show the mapping of image contrast to spin directions. Insets in the bottom left of (a)–(c) depict the experimental geometry.

Figure 4

oommf simulations for $2\mu \mathrm{m}\times 2\mu \mathrm{m}$ micromagnets with edges parallel to $⟨110⟩$ substrate directions and $H$ applied along the [100] substrate direction. For $\mathit{H}>9\mathrm{mT}$, the domain pattern transitions from a flux-closure state with a shifted core to a $Z$ domain. (c) and (d) Simulations reproducing the two variants of $Z$ domains.