Understanding Magnetism in Multiferroics Using X-rays
 
Investigators: Mikel Barry Holcomb, L. W. Martin, Q. He, Y.-H. Chu, M. Gajek, A. Scholl, R. Ramesh
 
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.
 
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.