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The data storage on which out smartphones, tablets and computers rely works via electrical fields (e.g. flash drives) or magnetic fields (e.g. hard drives), but research into magnetoelectric materials could lead to a new generation of multifunctional devices that use a combination of the two.
Magnetoelectric materials have both electrical and magnetic functionality, and changing one induces a change in the other – it’s called ‘cross-coupling’. Developing an understanding of cross-coupling involves studying how the magnetic properties change when an electric field is applied, but most magnetoelectric materials have very complicated structures. Researchers simplified the process by simplifying the materialitself,developinga unique process for making a simplier magnetoelectric material.
Staying ahead of Moore’s Law – which predicts a doubling of the number of transistors in microelectric circuits every two years – is driving the search for new device functionalities, which in turn require the discovery of materials with new properties that can be controlled by applying a voltage or current. Multiferroic materials, especially 'magnetoelectrics' that show coupled magnetic and ferroelec tric properties in the same phase, offer great promise to satisfy these requirements1,2. Thin films are necessary for any device that should eventually be integrated into a large-scale circuit, and epitaxial growth of films also offers the opportunity to control and even improve certain properties of a material ascompared to its bulk counterpart. For example, in silicon films the electron mobility can increase under certain conditions of epitaxial strain (i.e. squeezing or stretching the crystal in two directions, leaving the third direction free to adjust), thus allowing higher operating speeds. Epitaxial strain can also improve the properties of ferroelectric films influenced by the size and distribution of domains, adjacent volumes in which the polarisation points in different directions. For BiFeO3, the deleterious effect of multiple domains was eliminated by creating large-area BiFeO3 films possessing only a single ferroelectric domain – 'monodomain' films, with the desirable property that there is only one, well-defined direction of the polarisation P in either the 'up' and 'down' states3. In magnetoelectric BiFeO3, the magnetic organisation of spins on the Fe sites is antiferromagnetic, i.e. moving from one Fe site to the next, the magnetic spins point in opposite directions. In bulk BiFeO3, the spins have the added complexity that they rotate slightly going from one site to the next, completing a full rotation over a distance of about 62 nm in bulk (the period of the so-called 'spin–cycloid'). The cycloid is expected to create problems for any BiFeO3–based magnetic device, since over large areas there is no single direction of the magnetisation, as there is in simple collinear antiferromagnets. To complicate matters even more, in BiFeO3 crystals there are 8 possible ferroelectric domains, each with 3 cycloids of different orientations – 24 different configurations in all, making multidomain BiFeO3 a complex material difficult to incorporate in a well-behaved, deterministic magnetic device.
One of the building blocks of spintronic devices is an 'exchange–coupled' bilayer, based on an effect discovered in the 1950s but still not well understood, in which the spins in a simple antiferromagnetic layer are coupled to the spins in a ferromagnetic overlayer. Since the spin axis L in a collinear antiferromagnet is fixed, exchange coupling across the interface serves to effectively fix the direction of the ferromagnetic layer. New functionality can be introduced by creating a magnetoelectric exchange–coupled bilayer, in which the spin axis L in the antiferromagnetic layer rotates when the ferroelectric polarisation is switched; then exchange–coupling between L and the spins in the ferromagnetic overlayer would cause the ferromagnetic spins to rotate as well, in a deterministic and controllable way4.
Precisely this new type of magnetoelectric exchange coupling is demonstrated by the XPEEM data (Figs. 1-3). Several improvements to the samples constructed for these experiments were crucial to achieving unambiguous interpretation of the data. Most important is the use of monodomain BiFeO3 films under epitaxial strain: neutron diffraction confirms that each ferroelectric monodomain is correlated to a single antiferromagnetic domain, even after hundreds of ferroelectric switching cycles. Thus monodomain BiFeO3 films eliminate the 24 different polarisation–cycloid configurations mentioned previously for bulk BiFeO3. In the presence of a strong exchange coupling to the ferromagnetic overlayer, the stability of the monodomains ensures (a) one-to-one correlation between the BiFeO3 ferroelectric state and the overlayer spin orientation and (b) deterministic control over the spins orientation of the overlayer.
Saenrang W, Davidson BA, Maccherozzi F, Podkaminer JP, Irwin J, Johnson RD, Freeland JW, Íñiguez J, Schad JL, Reierson K, Frederick JC, Vaz CAF, Howald L, Kim TH, Ryu S, Veenendaal MV, Radaelli PG, Dhesi SS, Rzchowski MS, Eom CB. Deterministic and robust room–temperature exchange coupling in monodomain multiferroic BiFeO3 heterostructures. Nature Communications 8(1583), 1-8, doi:10.1038/s41467-017-01581-6 (2017).
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