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Understanding Magnetism in Multiferroics Using X-rays
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Investigators: Mikel Barry Holcomb, L. W. Martin, Q. He, Y.-H. Chu, M. Gajek, A. Scholl, R. Ramesh
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Magnetoelectric multiferroics – materials that exhibit both electronic
and magnetic order—present a fascinating opportunity to study coupling
phenomena in complex correlated oxides. The promise of
electrically-tunable magnetic properties has led such materials to be
the focus of much research. Yet the creation of the wide range of
devices that would take advantage of this type of functionality has
yet to be achieved. This stems from the fact that the fundamental
nature of the underlying order parameters in these materials is
difficult to study and often highly complex. BiFeO3 (BFO), both a
ferroelectric and an antiferromagnet, is a model system to study
magnetoelectric coupling because it is the only single-phase room
temperature magnetic ferroelectric currently known. X-ray linear
dichroism images of this material were obtained using a high spatial
resolution photoelectron emission microscope (PEEM), allowing
elemental specificity and surface sensitivity. Careful analysis of
linear dichroism images at critical angles before and after poling
with an electric field allows determination of magnetic directions in
both BFO and coupled FM layers. These measurements show that not
only the antiferromagnetism of BFO is coupled to the ferroelectric
directions before and after electric field switching, but that the
ferromagnetic directions of a thin layer of CoFe on top of BFO is also
coupled. |
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Fig. 1: Electrical control of BiFeO3 (BFO) antiferromagnetism and coupling of
CoFe ferromagnetic domains to BFO. We used the PEEM (top) to compare
the BFO ferroelectric domains taken by PFM. In the as grown film
(left), the PEEM images match very nicely with the in-plane projection
of the ferroelectric domains. We used a PFM to apply a vertical
electric field to changed the polarization directions inside the box.
The magnetism changes to match the new ferroelectric domains (middle).
The fact that we see only two reversible colors in the linear images
and that the temperature dependence is the same both with or without
poling confirms that the antiferromagnetism in BFO is coupled to the
ferroelectric direction. We observe in many samples that the CoFe
magnetic domains (top right) match quite nicely with the BFO
ferroelectric domains (bottom right) that have been switched with an
electric field. Further work on related samples has demonstrated
reversibility in ferromagnetic domains with electric field. |
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