Nanoscale imaging of spin textures with locally varying altermagnetic response in $\alpha$-Fe$_2$O$_3$
Altermagnetism is a recently identified magnetic state in which time-reversal symmetry is broken despite a collinear compensated spin structure. The response of altermagnets is determined not only by their $d$-, $g$-, or $i$-wave spin order, but also the orientation of their N\'eel vector $\mathbf{L}$. Therefore, accessing a response that fundamentally depends on the orientation of $\mathbf{L}$, such as the anomalous Hall effect, remains experimentally challenging in particular at the nanoscale. Here, we harness nano-spectroscopic X-ray magnetic circular dichroism (XMCD) to investigate nanoscale modulated altermagnetic responses in $\alpha$-Fe$_2$O$_3$ (Hematite). By performing spectroscopy across the temperature-induced $\mathbf{L}$-reorientation Morin transition, we observe the on-and-off switching of XMCD, in agreement with our theoretical calculations. Although the bulk XMCD vanishes below the Morin temperature, we confirm the reorientation of $\mathbf{L}$ by harnessing polarization-independent X-ray absorption spectroscopy. Moreover, we observe a finite XMCD signal in nanoscale domain walls with locally modulated N\'eel vectors, while the surrounding domains exhibit no XMCD. At room temperature, we instead identify altermagnetic meron spin textures that exhibit XMCD in their planar regions but no XMCD in their nanoscopic cores. Our results establish a pathway to harness complex spin textures with nanoscale functionalities in a broader class of altermagnets with various $\mathbf{L}$-orientations and using light, earth-abundant elements.