Role of substrate clamping on anisotropy and domain structure in the canted antiferromagnet $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$

Antiferromagnets have recently been propelled to the forefront of spintronics by their high potential for revolutionizing memory technologies. For this, understanding the formation and driving mechanisms of the domain structure is paramount. In this work, we investigate the domain structure in a thin-film canted antiferromagnet $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$. We find that the internal destressing fields driving the formation of domains do not follow the crystal symmetry of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, but fluctuate due to substrate clamping. This leads to an overall isotropic distribution of the Néel order with locally varying effective anisotropy in antiferromagnetic thin films. Furthermore, we show that the weak ferromagnetic nature of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ leads to a qualitatively different dependence on the magnetic field compared to collinear antiferromagnets such as NiO. The insights gained from our work serve as a foundation for further studies of electrical and optical manipulation of the domain structure of antiferromagnetic thin films.

Figure 1

(a) Long-range (006)/(0012) symmetric scan of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ (10 nm) on $c$-axis (001)-oriented ${\mathrm{Al}}_2{\mathrm{O}}_3$. (b) (1 0 10)-reciprocal space map of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ (10 nm) on $c$-axis (001)-oriented ${\mathrm{Al}}_2{\mathrm{O}}_3$. (c) Phi scan of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ (20 nm) on $c$-axis (001)-oriented ${\mathrm{Al}}_2{\mathrm{O}}_3$ with inset of hexagonal crystal structure. (d) Raman spectrum of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ (10 nm) thin film on a ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate. (e) Cross-sectional HRTEM image of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ (20 nm) on $c$-axis (001)-oriented ${\mathrm{Al}}_2{\mathrm{O}}_3$. The inset shows the Fourier transform of the image.

Figure 2

(a) Angle-dependent SMR signal for magnetic field varying from 50 mT (light orange) to 1 T (dark red). Inset shows a schematic of the SMR measurement geometry. (b) SMR signal as a function of field at $\theta ={45}^{\circ }$. The gray line shows the initial field sweep from a demagnetized state $0_d$ to $+H_{\text{sat}}$, and the dark- and light-red lines show the hysteresis between the $+H_{\text{sat}}\to -H_{\text{sat}}$ and $-H_{\text{sat}}\to +H_{\text{sat}}$ field sweeps, respectively. The inset illustrates the canting of the sublattice magnetizations ${\mathrm{M}}_A$ and ${\mathrm{M}}_B$.

Figure 3

(a) Positive field loop at ${45}^{\circ }$. The SMR signal shows small hysteresis below $H_{\text{irr}}\approx 200$ mT and is fully reversible above $H_{\text{irr}}$. Insets show scanned XMLD images of the $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ domain structure at 0 and 250 mT. Dark/bright contrast corresponds to the horizontal/vertical orientation of the Néel vector. (b) Transverse remanent resistance (dark red) after $H$ has been relaxed from a saturated state at 0.6 T (light red) to zero field as a function of the angle of the applied magnetic field. The inset shows the orientation of the magnetocrystalline easy axes within the hexagonal unit cell.

Figure 4

(a),(b) Schematic illustrating the deflection of the anisotropy axis away from the magnetocrystalline easy axis in the central domain depending on the local surroundings. (c) Schematic of distribution of orientation of Néel vectors ranging from strongly aligned with a magnetocrystalline anisotropy (dark gray) to isotropic (red). (d) Slope of the linear SMR signal at small $H$ as a function of the angle of the field calculated using the modified destressing model based on a sixfold anisotropy (black line) and isotropic distribution of Néel vector orientations (red line). The isotropic model shows good agreement with the experimental data for a field sweep from a demagnetized state and saturated state (dots and triangles, respectively).

Figure 5

(a) Vector map of the $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ domain structure constructed from angle-dependent XMLD XPEEM images. The orientation of the Néel vector represented by different colors is shown in the histogram in (b). The height of the bars in the circular histogram marks the frequency of occurrence of the given orientation of the Néel vectors on a linear scale. Pixels for which the uncertainty is larger than ${15}^{\circ }$ are marked in gray.