Historical research guide · 0503 · 2023
Electrically tunable moiré magnetism in twisted double bilayers of chromium triiodide
Guanghui Cheng · Mohammad Mushfiqur Rahman · Andres Llacsahuanga Allcca · Avinash Rustagi · Xingtao Liu · Lina Liu · Lei Fu · Yanglin Zhu · Zhiqiang Mao · Kenji Watanabe · Takashi Taniguchi · Pramey Upadhyaya · Yong P. Chen
In one minute
Take a crystal of chromium triiodide two atoms thick, lay a second identical two-layer flake on top, and rotate it by a fraction of a degree. The mismatch makes a moiré pattern — the interference figure you get through two overlapping window screens — and across that pattern the atoms of the two halves sit in different alignments from place to place. Guanghui Cheng and colleagues at Purdue show that this geometry alone rewrites the magnetism. Ordinary chromium triiodide bilayers are antiferromagnets with no net moment. The twisted stacks show a ferromagnetic loop alongside the antiferromagnetic behaviour, meaning ferromagnetic and antiferromagnetic regions coexist in one flake. The effect appears between about a tenth of a degree and twenty degrees of twist, strengthens and then weakens as the sample warms, and — the part that makes it a device rather than a curiosity — can be switched with a voltage on a back gate. A model of competing exchange and anisotropy energies reproduces the phase diagram they measure.
Connection
The thesis says the bottleneck on using the vacuum is material science, and this is what modern material science looks like when it is working: a knob nobody had thirty years ago — the angle between two sheets — plus a gate voltage, and the magnetic order of a solid is yours to set. Chapter 11 collects the engineered solids that give a field something to act on, and chapter 2 gets the general lesson that the structure of the medium, not its chemistry, is the adjustable quantity.
From the source
Stacking one van der Waals layer on another with a small twist creates a moiré superlattice whose period is the lattice constant divided by twice the sine of half the twist angle — about 20 nm for the samples measured here. Scanning transmission electron microscopy and its Fourier transform confirm the superlattice, giving a measured twist of about 1.9 degrees for a 1.42 degree target, with the split Bragg peaks absent in a natural, untwisted bilayer.Section Moiré superlattice of tDB CrI3, Figure 1b to 1d
In twisted double bilayer chromium triiodide the magneto-optical Kerr effect shows a ferromagnetic hysteresis loop at about plus or minus 0.2 T alongside the antiferromagnetic spin-flip transitions at about plus or minus 0.7 T. Antiferromagnetic and ferromagnetic order coexist with a nonzero net magnetisation, which is the hallmark of moiré magnetism, and neither a natural bilayer nor a natural four-layer crystal shows it.Abstract, and section Twist angle and temperature dependence, Figures 2b and 2c
The twist angle alone controls the effect. The ferromagnetic fraction is nonzero across the range 0.14 degrees to 20 degrees, reaching a maximum near 0.3, and the ferromagnetic loop disappears at about zero degrees and above 20 degrees, indicating phase transitions across critical angles.Section Twist angle and temperature dependence, Figure 2f
The temperature dependence is nonmonotonic: the ferromagnetic loop amplitude first grows and then falls as the sample warms, while the saturation magnetisation falls monotonically as in an ordinary magnet, with all transitions gone by 40 to 50 K. The authors read the anomaly as a temperature-driven crossing between noncollinear phases, since anisotropy and the two exchange energies weaken at different rates.Section Twist angle and temperature dependence, Figures 3a to 3c, and the Discussion
Electrical control was demonstrated on a back-gated 0.14 degree device: both the ferromagnetic coercive field and the antiferromagnetic spin-flip field fall as the gate voltage rises, and with the magnetic field held just short of the coercive field a gate sweep flips the magnetic state outright. Voltage-assisted switching was reproduced in further samples at 5 and 20 degrees.Section Electrical control of the magnetic behaviors, Figures 4c to 4e
A continuum free energy with intralayer exchange stiffness, easy-axis anisotropy and a stacking-dependent interlayer exchange yields a phase diagram of collinear and noncollinear ground states, and identifies the twisted-A one-domain-wall state as the most likely source of the measured loops. What to watch is named in the paper: probes sensitive to in-plane magnetisation, such as longitudinal Kerr microscopy or spatially resolved spin probes, would identify the twisted-S phases, and would settle the one-step versus two-step reversal difference between this group’s samples and another team’s.Section Theoretical analysis of moiré magnetism, Figures 5a to 5c, and the Discussion
The way in
https://doi.org/10.1038/s41928-023-00978-0How to cite it
Guanghui Cheng, Mohammad Mushfiqur Rahman, Andres Llacsahuanga Allcca, Avinash Rustagi, Xingtao Liu, Lina Liu, Lei Fu, Yanglin Zhu, Zhiqiang Mao, Kenji Watanabe, Takashi Taniguchi, Pramey Upadhyaya, Yong P. Chen (2023) Electrically tunable moiré magnetism in twisted double bilayers of chromium triiodide doi:10.1038/s41928-023-00978-0