Beyond the Magnetic Domain Matching in Magnetic Exchange Coupling

We report a new macroscopic first-field-induced magnetic anisotropy for $\mathrm{Co}/\alpha \mathrm{\text{−}}{\mathrm{Fe}}_2{\mathrm{O}}_3(0001)$ layers, a prototypical ferromagnetic-antiferromagnetic interface for which the antiferromagnetic film has small in-plane magnetic anisotropy as compared to the interface coupling. We demonstrate that the effect is due to a first-field-induced irreversible magnetic domain motion in the antiferromagnetic layer, dragged by the ferromagnetic Co one. Whereas the initial domain matching is lost, the macroscopic manifestations of the exchange coupling remain stable. Therefore, the initial domain matching probably has only a marginal role in the explanation of the magnetic exchange coupling.

Figure 1

Longitudinal Kerr hysteresis loop at room temperature for Co films evaporated on hematite layer (a) sample 1, 1 nm of Co, (b) sample 2, 2 nm of Co. The corresponding ${\alpha }_H$ angles are indicated. (c) $H_C$ and squareness ($M_R/M_S$) of sample  2 as function of ${\alpha }_H$. (d) Polar representation of $M_R/M_S$. Sample similar to (c) with same composition. The radial scale extends from 0.3 to 0.9.

Figure 2

(a) Squareness of hysteresis loop recorded on 2 nm Co on hematite films as function of the azimuth from the first field: ${\alpha }_H=95°$ (straight line), ${\alpha }_H=5°$ (dotted line), and ${\alpha }_H=4°$ (dashed line); Co evaporated on alumina (circles). (b) Virgin magnetic curves obtained on $2\mathrm{nm}\mathrm{Au}/1.4\mathrm{nm}\mathrm{Co}/{\mathrm{Fe}}_2{\mathrm{O}}_3$ for the as-grown sample and after demagnetization. FM and AFM XPEEM domain structures for $1.6\mathrm{nm}\mathrm{Co}/{\mathrm{Fe}}_2{\mathrm{O}}_3(20\mathrm{nm})/\mathrm{Pt}(111)$ in (1) virgin state and (2) after demagnetization.

Figure 3

Dichroic XPEEM images recorded at the Co ${\mathrm{L}}_3$ (a),(d),(g), Fe ${\mathrm{L}}_3$ (b),(e),(h), and Fe ${\mathrm{L}}_{2A,B}$ (c),(f),(i) edges for a $2.5\mathrm{nm}\mathrm{Au}/3.5\mathrm{nm}\mathrm{Co}/20\mathrm{nm}{\mathrm{Fe}}_2{\mathrm{O}}_3/\mathrm{Pt}(111)$ sample. Virgin state (a)–(c) and effect of increasing magnetic fields. The regions are different since the sample had to be moved to apply the magnetic field.

Figure 4

LMOKE (top) and TMOKE (bottom) measurement performed on $10\mathrm{ML}\mathrm{Co}/10\mathrm{nm}$ annealed ${\mathrm{Fe}}_2{\mathrm{O}}_3$ (see text) for different azimuthal angles with respect to the first-field direction. Straight line, experiment; dotted line, model with $t_{\mathrm{FM}}=2\mathrm{nm}$, $t_{\mathrm{AFM}}=10\mathrm{nm}$, $M_{\mathrm{FM}}=1400\mathrm{emu}/{\mathrm{cm}}^3$, and $K_{\mathrm{AFM}}=2.5\times {10}^5\mathrm{erg}/{\mathrm{cm}}^3=2.5\times {10}^4\mathrm{J}/{\mathrm{m}}^3$ (see text).