An ultrathin multiferroic film keeps its order at four unit cells
Illustrative image: Indra Projects on Unsplash
Researchers led by Brown University have retained multiferroic behaviour in a bismuth ferrite film just four unit cells, or about 1.6 nm, thick. Their measurements also reveal signatures of altermagnetism in the ultrathin material.
The Nature Materials study uses a dysprosium scandate substrate and a strontium ruthenate electrode. Together they help control strain and screen electric charge in the film, preserving an ordered state at a thickness where functional behaviour can otherwise disappear.
“We’ve established that we can get rid of the dead layer,” says Brown’s Lucas Caretta. As a film becomes thinner, a region that has lost the desired properties can occupy an increasing proportion of it, eventually leaving too little active material to use.
The substrate and electrode help hold the state
Polarisation produces bound charge at the surfaces of a ferroelectric. If it is poorly screened, the resulting depolarising field can oppose the polarised state. The substrate also constrains the crystal’s distortion, changing the structure that supports its electrical and magnetic order. Reducing the film thickness therefore makes its interfaces more influential, rather than simply producing a smaller version of the bulk material.
The thickness at which the behaviour disappears depends on those surroundings as well as the composition. Brown’s result comes from a particular combination of film, substrate and electrode that stabilises the ultrathin state.
The paper reports altermagnetic signatures and topological textures. Altermagnetism combines compensated magnetic order with a spin dependent electronic structure, opening possibilities for spin electronics that differ from those of a conventional ferromagnet. Structural analysis and electrical and magnetic measurements support the researchers’ account of the state formed in the film.
Using it in a memory device would mean adding electrodes that can write and read the state without disrupting the strain and screening that sustain it. Those electrodes and the access circuitry could occupy much more area than the active material, even with a film only a few unit cells deep. Retention, endurance and switching energy would then need to be measured in that assembled device; the study has established the material behaviour rather than the performance of a memory array.
