R. Yamamoto, S. Mayr, A. Hariki, S. Finizio, K. Sakurai, E. Weschke, K. Litzius, M. T. Birch, L. A. Turnbull, E. Zhakina, M. Di Pietro Martínez, J. Reuteler, F. Schulz, M. Weigand, J. Raabe, G. Schütz, S. S. P. K. Arekapudi, O. Hellwig, W. H. Campos, L. Šmejkal, J. Kuneš, C. Donnelly, S. Wintz
7 min
Abstract
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 Néel 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 $α$-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 Néel 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.
Sam: They switched to unpolarized X-ray absorption spectroscopy, summing images from two linear polarizations to ignore dichroic effects. This picks up shifts in absorption from spins pointing in-plane or out-of-plane, due to interactions between the X-ray-created core hole and electrons. High- and low-temperature spectra differed clearly, matching models. Scanning transmission X-ray microscopy, or STXM, focused synchrotron X-rays to nanoscale spots for maps at maximum contrast energies.
Alex: So this unpolarized method acts like a neutral lens to spot orientation without polarization tricks?
Sam: Precisely. At low temperature, STXM showed a bright line about 160 nanometers wide near a metal lead—a domain wall where spins reoriented in-plane amid out-of-plane surroundings. There, XMCD and XMLD signals reappeared, confirming local flat spins allow detection, just like above the transition.
Alex: Huh. So these walls become like tiny switches for spintronic functions, even in the bulk phase.
Sam: Yes. In the high-temperature phase, STXM showed rotationally symmetric patterns around dark cores—merons where the center tilts out-of-plane, while surroundings stay active. Line profiles confirmed real tilt, not imaging blur. This mix suggests stable control of altermagnetic properties in dense devices using abundant hematite.
Alex: How did the images reveal the continuous spin twist in these merons?
Sam: They imaged at iron's absorption peaks around 724 eV. Unpolarized light showed a dark 140-nanometer spot—the out-of-plane core absorbing less—surrounded by brighter in-plane areas. Line scans showed gradual rotation from center to edges, matching a half-knot spin pattern called a meron. XMCD highlighted in-plane domains; XMLD confirmed surroundings. Together, they prove distinct zones in a mostly suppressed bulk.
Alex: But why does that matter for devices? You mentioned spintronic signals.
Sam: The crystal rules enabling XMCD also drive the anomalous Hall effect—a sideways push on electrons from spin textures, like traffic veering in a patterned road. In walls, non-zero XMCD means non-zero Hall signals between quiet domains. Strain or fields could switch it on and off for high-density devices.
Alex: Huh. So textures create built-in switches for that Hall push, where bulk can't.
Sam: Yes. Nanoscale imaging proves these spots exist and respond as expected.
Alex: What does the crystal's symmetry say about why hematite supports such stable patterns?
Sam: The study uses spin group theory—a math framework for how spins transform under rotations or mirrors, like checking if a puzzle pattern holds after flipping. Hematite is a g-wave altermagnet. In its repeating unit cell, four spin groups balance overall but have non-canceling densities. This creates spin-split electron paths with flat band touches—like smooth highway merges—and a curved one, stabilizing responses in textures.
Alex: So symmetry forces non-canceling patterns, like built-in grooves directing magnetism?
Sam: Precisely. Models matching images confirm this topology stabilizes twists. Single-crystal purity via Laue diffraction ruled out defects.
Alex: Clean crystals and symmetry back the views?
Sam: Exactly. A key limit is synchrotron STXM needing thin samples and heavy computations blending electron density maps with interaction models. These ensure accuracy but confine it to big facilities.
Alex: So scaling needs easier tools?
Sam: Correct. The paper points to walls for single-spin Hall sensors or logic gates in dense memory, toggled locally with strain or fields—feasible with abundant hematite.
Alex: Overall, this maps controllable spots in a common material, despite tool limits—a clear step for denser spintronics.
Sam: Precisely. Nanoscale XMCD in walls and merons unlocks altermagnetic control below the Morin transition, backed by models and clean crystals. This advances spintronics meaningfully.
Alex: That's a solid takeaway, Sam. Thanks for listening to ResearchPod.