Can ferroelectric interfaces host tunable chiral spin textures and topological Hall signals?

Two topological Hall signals flank the coercive field

The paper's central observation is that the Hall response of a few-layer Fe3GeTe2/α-In2Se3 (FGT/IS) device cannot be described by a single anomalous Hall loop. After subtracting the ordinary Hall background, the authors find two groups of humps during magnetization reversal: one just below and one just above the coercive field, labeled THE1 and THE2. In pristine FGT the same subtraction leaves a square hysteresis loop with a coercive field of μ0Hc = 0.34 T at 4 K, so the extra structure appears when the ferroelectric IS layer is part of the stack.

The authors treat these humps as topological Hall effect (THE) contributions rather than as two anomalous Hall loops of opposite sign. They report excluding the latter interpretation through analyses of parallel conduction channels and magnetic inhomogeneities, then extracting the THE components by using reflectance magneto-circular dichroism (RMCD) measured on the same region to fix the anomalous Hall background. The residual signal decomposes into two THE components with opposite signs.

The physical picture is that conduction electrons crossing a chiral spin texture acquire a real-space Berry phase, which acts like an emergent magnetic field and produces the THE. Because IS is described as highly resistive and nonmagnetic, the authors attribute the effect to interfacial chiral textures in FGT, such as skyrmions or skyrmion bubbles.

Magnetic bubbles with opposite contrast match the two signals

Low-temperature magnetic force microscopy (MFM) provides the real-space counterpart to the transport data. Near the coercive field the images reveal two types of magnetic bubbles with opposite magnetic contrast, and the paper associates each type with a distinct topological Hall signal. In the MFM measurement, red and blue contrast indicate attractive and repulsive tip–sample interaction, respectively, and the signal is proportional to the out-of-plane stray-field gradient.

This matters because the two THE components are not merely fitting artifacts of a non-monotonic Hall curve. The pairing of two contrast types with two transport features is the paper's main evidence that the Hall structure tracks distinct spin textures evolving through magnetization reversal. The authors also support the interpretation with atomistic spin-dynamics simulations and first-principles calculations, which they use to argue that a finite interfacial Dzyaloshinskii-Moriya interaction (DMI) stabilizes the textures.

Thickness dependence points to an interfacial origin

The DMI in this system is expected to arise at the interface, where inversion symmetry is broken and spin-orbit coupling is present. To test that expectation, the authors compare 3L-, 4L-, and 5L-FGT/IS devices. Both THE1 and THE2 are observed in all three thicknesses, but their critical temperatures fall sharply as the FGT layer grows. THE1 disappears at approximately 80 K in the 3L device but below 20 K in both the 4L and 5L devices. THE2 critical temperatures are 92 K, 84 K, and 24 K for 3L, 4L, and 5L, respectively.

The 3L device also shows three temperature regimes: below 20 K, THE1 decreases with increasing temperature while THE2 is absent; between 20 K and 65 K both signals are nearly unchanged; above 65 K both weaken. The authors read the rapid suppression with thickness as evidence for an intrinsically interfacial origin, reflecting competition between interfacial DMI and bulk ferromagnetism.

First-principles calculations for 1L–5L FGT/IS support that reading: with increasing FGT thickness the DMI magnitude decreases continuously while the exchange interaction increases markedly between 3L and 4L. The authors combine these into a ratio κN of DMI to exchange and anisotropy, which decreases with thickness. This is a model-based explanation, not a direct measurement of the interfacial DMI, and it is the main place where the paper's causal story rests on calculation rather than transport alone.

THE2 peaks in the four-layer device

Thickness does not act monotonically on both signals. THE2 reaches its maximum amplitude in the 4L device, which the authors say cannot be explained by the ordinary Hall coefficient alone: at 20 K the ratio |R_THE2/R0| is about 6.81 T in 4L, compared with about 2.26 T in 3L and about 4.12 T in 5L. They instead attribute the enhancement to the interplay between thickness-dependent energetics of chiral textures and the nonequilibrium evolution of domains during magnetization reversal.

This is a useful distinction for readers comparing the two signals. THE1 behaves like a more direct interfacial marker, fading as the FGT layer thickens, whereas THE2 depends on a combination of energetics and domain history and therefore peaks at an intermediate thickness. The paper does not reduce both signals to a single thickness trend.

Polarization switching reprograms the Hall response

The ferroelectric layer supplies the electrical control knob. Switching the polarization of α-In2Se3 nonvolatilely modulates both the anomalous and the topological Hall effects. The authors explain the anomalous Hall modulation through polarization-dependent interfacial electronic reconstruction: the P↑ and P↓ states produce different wavefunction overlap across the interface and electron doping levels of about 0.029 e/f.u. and 0.067 e/f.u., respectively.

Their first-principles calculation of the intrinsic anomalous Hall conductivity σxy varies strongly over this doping range, with a peak near n ≈ −0.04 e/f.u. arising from band crossings close to the Fermi level. The calculated trend is consistent with the measured modulation of the remanent anomalous Hall resistance, supporting a mechanism in which polarization changes the interfacial charge distribution and thereby the Berry curvature, without an appreciable change in total magnetization. The paper presents this as a supported mechanism rather than a directly imaged charge-transfer measurement.

Four nonvolatile states exist below about 40–50 K

Combining the independently switchable ferroelectric and magnetic bistabilities yields four distinguishable remanent Hall resistance states, which the authors describe as single-cell two-bit storage. Voltage pulses of ±8 V and magnetic-field pulses of ±0.2 T reproducibly switch the device among the four levels. The levels are unequally spaced, and the paper reports retention over thousands of seconds.

The operating window is the key qualification. The four-state response emerges below approximately 40–50 K. The authors state that this limitation does not mean ferroelectric or magnetic order is lost; rather, electrical readout becomes less efficient at higher temperatures, where altered transport mechanisms reduce the resistance contrast. They note that the enhanced low-temperature contrast likely stems from hopping transport, while the exact microscopic origin requires further study. At still higher temperatures, reduced stability of the chiral textures and the finite Curie temperature of few-layer FGT add intrinsic magnetic constraints.

The paper's own route to room-temperature operation therefore requires lifting both limits at once: the transport-readout limit and the magnetic limit. Its suggested strategies are strain, electrostatic doping, proximity coupling, or pairing IS with alternative high-TC van der Waals magnets. These are proposals, not demonstrations in this work.

What the evidence supports and what it does not

The strongest part of the case is convergent: two THE components appear in transport, two bubble types with opposite MFM contrast appear near coercivity, and thickness-dependent transport plus calculations point to an interfacial DMI origin. The weakest link is that the DMI itself is inferred from calculations and from the thickness trend rather than measured directly, and the assignment of the Hall humps to THE relies on excluding alternative anomalous Hall superpositions through supporting analyses.

For readers weighing this platform, the paper's distinctive result is not simply that a THE appears. It is that the ferroelectric polarization gives a nonvolatile, electrically programmable handle on both the anomalous and topological Hall responses, and that the two magnetic bistabilities together produce four resolvable states. The practical threshold is temperature: the four-state function is demonstrated below about 40–50 K, while the chiral-texture signals themselves already weaken well before that in thicker FGT layers. The most decision-relevant comparison is therefore between the 3L device, where THE1 survives to about 80 K, and the 4L device, where THE2 is largest but the topological signals fade below about 20–24 K.

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