A century-old assumption about the Hall effect just fell, and a single nanometer-scale device can now sense magnetic fields along multiple axes

For roughly a century, the Hall effect has come with an unspoken rule written into nearly every physics textbook: to get a Hall voltage out of a material, you point the magnetic field at it perpendicular to the current, and the charge carriers get shoved sideways. That is the geometry that Edwin Hall documented in 1879, and it has been the working picture of how a Hall signal works ever since. Now a team at Carnegie Mellon University has shown that the rule is not as universal as it looked. According to the CMU release, researchers in the university’s LIQUID lab built atomically thin heterostructures of the layered semimetal TaIrTe₄ paired with the magnetic insulator Cr₂Ge₂Te₆, and observed an unconventional Hall signal tied to in-plane magnetization — a response that tracks the component of a magnetic field lying flat in the plane of the material, not crossing it.

The geometry is what makes the result genuinely surprising. A perpendicular field breaks the symmetry of a 2D sheet in an obvious way: the field has a component out of the page, and the Lorentz force shoves electrons sideways as they flow. An in-plane field does not, on the face of it, break anything — the field is parallel to the current and to the plane of the material, so the textbook Lorentz-force argument predicts no transverse signal at all. For decades the working assumption has been that any in-plane Hall response was either a measurement artifact, a misaligned sample, or something that only existed in toy models. The ScienceDaily write-up of the result frames it as a direct overturning of that long-held assumption: a magnetic field lying in the plane of a material can now produce a real Hall voltage.

The mechanism the CMU group identifies sits in the interface between the two layered crystals. TaIrTe₄ is a non-magnetic topological semimetal with strong spin-orbit coupling; Cr₂Ge₂Te₆ is a 2D ferromagnet whose magnetization can sit in-plane. Stacking them produces a heterostructure whose symmetry is reduced enough that the in-plane component of the magnetization is no longer invisible to the electrons flowing through it. Interfacial spin-orbit coupling and exchange interactions together convert that in-plane magnetization into a transverse voltage. The Nature Materials paper describing the work pins down the underlying mechanism and notes that the in-plane Hall response is gate-tunable, meaning the size and sign of the signal can be adjusted by applying a voltage to the device, and that the effect emerges in a low-dimensional system whose reduced symmetry is what makes it possible in the first place. The CMU release is explicit that this particular effect had been theoretically proposed before but had not been experimentally demonstrated until now.

The practical consequence is that the same device can pick up magnetic fields pointing in more than one direction. A conventional Hall sensor is essentially a one-axis magnetometer — it is sensitive to the component of the field perpendicular to the chip, because that is the only component that drives a signal. The CMU group’s heterostructure produces both the conventional perpendicular Hall response and the new in-plane response in a single nanometer-scale device. The ScienceDaily piece points out that this combination enables simpler vector magnetometry: a single chip can now resolve magnetic fields along multiple axes, without stacking multiple sensors or rotating the device in the field to recover the in-plane components. For applications that need to know the direction of a magnetic field — navigation, hard-disk read heads, magnetic imaging — that is a structural simplification of the kind that tends to ripple outward into sensor design for years.

There is also something worth sitting with about the timescale here. Hall’s original paper was published in 1879, the Lorentz-force explanation of the ordinary Hall effect came within a decade of that, and the anomalous Hall effect — the version that depends on magnetization rather than an applied field — was first reported in 1881 and only adequately explained nearly a century later. The version CMU has now measured is, in a real sense, a third member of the family: a Hall signal whose direction and whose underlying symmetry breaking are both different from the original. The Nature Materials paper places the result in that lineage: it is the first experimental realization of the in-plane anomalous Hall effect in a low-dimensional system. Symmetry arguments had suggested such a response should exist in materials with sufficiently low symmetry; what the heterostructure provides is a system low-symmetry enough — and clean enough at the interface — that the response is large enough to actually measure.

A useful framing: Hall’s 1879 experiment is one of those measurements that looks, in retrospect, like it should have been the end of the story. A magnetic field, a current, a perpendicular voltage. Done. A century and a half of condensed-matter physics has been spent discovering that the story keeps reopening. The CMU group’s result is one more opening, and unlike most of them, it has a payoff that is easy to point at — a single chip that reads a magnetic vector instead of a single component, built out of two atomically thin crystals stacked on top of each other.

Sources

• Scientists just overturned a century-old physics assumption — ScienceDaily (sourced from Carnegie Mellon University)
• CMU Physicists Take Hall Effect in a New Direction — Carnegie Mellon University Mellon College of Science
• In-plane anomalous Hall effect in a low-dimensional system — Nature Materials (peer-reviewed journal)