Multiferroics: the two-in-one crystals that could revolutionise data-storage

Figure 1. Relationship between magnetically and electrically polarizable materials (showing the multiferroic and magnetoelectric subsets). [6]

Picture a house where one light-switch simultaneously flips on the lamps and starts your playlist. Handy, right? Nature has built an atomic-scale version of that trick — it’s called a multiferroic crystal. Flick an electric field and the crystal’s magnetism responds; apply a magnetic field and its internal electric dipoles shuffle. Two levers, one material. Researchers think these double-duty solids could one day shrink an entire server-farm onto a postage stamp [2].

So what exactly is a multiferroic?

Crystals like BiFeO₃ (bismuth ferrite) or carefully stacked oxide superlattices grown on sapphire wafers combine two normally incompatible traits:

• Ferroelectricity – tiny positive and negative charges shift off-centre, giving each unit cell a built-in electric dipole that you can flip with a modest voltage.
• Ferromagnetism – unpaired electron spins line up, producing a permanent magnetization.

Most materials manage one or the other. Doing both at once is like patting your head and rubbing your tummy — hard, but not impossible [3].

Figure 2: Ordering of the magnetic dipoles in magnetic materials.[3]

Why should I care?

Because a multiferroic bit can be read magnetically (fast, contactless) and written electrically (low-energy). That unlocks:

• Ultra-compact “racetrack” memory — magnetic domains race along nanowires while electric pulses set the pace, promising terabytes in a thumbnail [1].

• Brain-inspired chips — charged domain walls behave like adjustable synapses inside neuromorphic circuits [1, 4].

• Quantum & neuromorphic hybrids — room-temperature multiferroics with exotic vortex textures could host robust qubits or artificial neurons [2, 5].

• THz cameras and modulators — swirling “polar vortices” interact with terahertz light in ways normal crystals can’t [2, 5].

• Domain-wall nano-electronics — engineers can draw conducting lines only a few atoms wide, then erase or move them with a battery-powered pen [4].

The catch: chemistry’s civil war

Those unpaired spins that create magnetism live in d-orbitals. Unfortunately, the same d-electrons tend to lock atoms in place, preventing the off-centre shift that ferroelectricity needs — genuine magneto-electric multiferroics are therefore as rare as blue diamonds [3].

Scientists are fighting this war with clever hacks:

• Strain-engineering — stretching thin films on mismatched substrates bends the energy rules [4].

• Layer cakes — alternating a few atomic planes of different oxides forces interfaces to compromise, birthing brand-new phases [5].

• Topology tricks — letting the polarization curl into vortices or skyrmions avoids the usual energy penalties [2, 5].

Laboratory breakthroughs

• In 2016, researchers at Berkeley zoomed in on a PbTiO₃/SrTiO₃ superlattice only 10 nm thick and saw perfect checkerboards of clockwise and anticlockwise polar vortices [5].

• Teams have shown that certain domain walls in hexagonal manganites conduct electricity a thousand times better than the bulk crystal — movable, re-writeable wiring [4].

• Here in Ireland, Dr Lynette Keeney’s group has discovered room-temperature multiferroics peppered with charged domain walls and polar vortices that may serve as ultra-dense, brain-inspired memory cells [1].

What’s left to crack?

• Operating temperature – many prototypes misbehave above 200 K; engineers crave room-temperature reliability.

• Signal size – the magneto-electric coupling is often faint; it must beat today’s flash memory.

• Fabrication cost – growing flawless oxide films layer-by-layer is slow and expensive.

• Integration – marrying brittle oxide films with everyday silicon chips remains an art.

Outlook: flipping the (dual) switch

Every decade our appetite for data doubles. If multiferroics live up to their promise, tomorrow’s smartwatch could store your entire video library and run for days on a single charge. The switch that controls both your lights and your speakers may remain sci-fi, but the atomic-scale version is already humming in university clean-rooms. Keep an eye out: the next revolution in computing might whirl out of a crystal lattice near you.

References

1. Moore, K., O’Connell, E. N., Griffin, S. M., Downing, C., … Keeney, L., & Conroy, M. (2022). Charged domain wall and polar vortex topologies in a room-temperature magnetoelectric multiferroic thin film. ACS Applied Materials & Interfaces, 14(4), 5525-5536. https://doi.org/10.1021/acsami.1c17383

2. Martin, L. W. (2021). Whirls and swirls of polarization. Science, 371(6533), 992-993. https://bookcafe.yuntsg.com/ueditor/jsp/upload/file/20210325/1616677613715039681.pdf

3. Hill, N. A. (2000). Why are there so few magnetic ferroelectrics? The Journal of Physical Chemistry B, 104(29), 6694-6709. https://doi.org/10.1021/jp000114x

4. Nataf, G. F., Guennou, M., Gregg, J. M., Meier, D., Hlinka, J., Salje, E. K. H., & Kreisel, J. (2020). Domain-wall engineering and topological defects in ferroelectric and ferroelastic materials. Nature Reviews Physics, 2, 634-648. https://doi.org/10.1038/s42254-020-0235-z

5. Yadav, A. K., Nelson, C. T., Hsu, S. L., Hong, Z., Clarkson, J. D., … Ramesh, R. (2016). Observation of polar vortices in oxide superlattices. Nature, 530, 198-201. https://doi.org/10.1038/nature16463

6. Eerenstein, W., Mathur, N. D., & Scott, J. F. “Multiferroic and magnetoelectric materials.” Nature 442 (7104), 759 – 765 (2006), Fig. 1. https://doi.org/10.1038/nature05023. © 2006 Springer Nature.