Can swapping bismuth for antimony flip a magnet's Hall signals?
Replacing bismuth with antimony in a magnetic topological insulator reverses the sign of both its anomalous Hall signal and its second-harmonic Hall signal, because the band structure's Berry curvature and spin texture change.
Source
Tunable chiral magneto-transport through band structure engineering in magnetic topological insulators Mn(Bi<sub>1-<i>x</i></sub>Sb<i><sub>x</sub></i>)<sub>2</sub>Te<sub>4</sub>
Study at a glance
- Design
- Other — Magneto-transport on Hall-bar devices from thin films with varied Sb content, interpreted with first-principles band-structure calculations
- N
- No sample N; five-septuple-layer films at several Sb fractions (x from 0 to 1) were grown and measured
- Population
- Five-septuple-layer Mn(Bi1-xSbx)2Te4 magnetic topological insulator thin films on sapphire
- Outcome
- Magnetoresistance, anomalous Hall resistance and conductance sign, and second-harmonic Hall response
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The authors grew thin films of MnBi2Te4 with increasing fractions of Sb replacing Bi, checked their structure with electron diffraction, microscopy and X-ray methods, and patterned them into Hall-bar devices. They measured magnetoresistance and Hall responses at low temperature in fields up to several tesla, including second-harmonic signals under a rotating in-plane field. Density functional theory calculations of Berry curvature and spin texture were used to interpret the sign changes.
What they found
Calculations showed the topological band gap closes at x = 0.35, marking a transition from a Chern number of one to zero. Experimentally, the anomalous Hall resistance at 8 T changed from negative for x up to 0.67 to positive for x of 0.9 and above, matching DFT where negative Berry-curvature-driven conductance pockets disappear at high Sb. The second-harmonic Hall signal kept one polarity up to x = 0.95 but flipped for pure MnSb2Te4, which calculations attribute to reversed spin chirality and surface potential gradient. Magnetoresistance also changed from antiferromagnetic-like to ferromagnetic-like shapes with more Sb.
The limits
What it doesn't show
The DFT comparison is only qualitative: the authors note extrinsic scattering contributions to the anomalous Hall conductance were not modelled. Only a handful of compositions were measured, all five layers thick and at low temperatures, so thickness and room-temperature behaviour are untested. The spin-chirality explanation for the second-harmonic flip rests on calculations and is described as a possible origin. No actual spin-orbit-torque switching device was demonstrated.
Key terms
- Anomalous Hall effect
- A Hall voltage in a magnetic material that arises from its magnetisation rather than only from the applied field.
- Berry curvature
- A property of electronic bands in momentum space that acts like an effective magnetic field and sets the intrinsic anomalous Hall conductance.
- Spin texture
- The pattern of electron spin directions around the Fermi surface, which can circulate clockwise or counterclockwise.
- Second-harmonic Hall response
- A Hall signal at twice the drive-current frequency that reveals nonreciprocal, direction-dependent transport.
- Topological phase transition
- A change in a material's band topology, such as closing and reopening of an inverted gap that changes the Chern number.
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Quiz yourself
At what Sb fraction does DFT predict the band gap closes?
Common questions
Why does the anomalous Hall sign flip with Sb content?
Adding Sb changes which sign of Berry curvature dominates the occupied bands, so the intrinsic Hall conductance changes from negative to positive.
Is the Hall sign change just from carriers switching type?
No. The authors note the Fermi-level shift sets the high-field Hall slope but does not contribute to the anomalous Hall polarity change.
Why does this matter for devices?
Controlling spin chirality by composition could let spin-orbit-torque devices be designed with a chosen switching direction.
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