Electric Field Control of Local Ferromagnetism Using a Magnetoelectric Multiferroic
 
Investigators: Ying-Hao Chu, Lane W. Martin, Mikel B. Holcomb, Martin Gajek, Shu-Jen Han, Qing He, Nina Balke, Chan-Ho Yang, Donkoun Lee, Wei Hu, Qian Zhan, Pei-Ling Yang, Arantxa Fraile-Rodríguez, Andreas Scholl, Shan X. Wang, and R. Ramesh
 
Electric field control of ferromagnetism is an exciting new area of condensed matter research with the potential to impact magnetic data storage, spintronics, and high frequency magnetic devices. Magnetoelectric multiferroics, or materials that simultaneously show some form of magnetic and ferroelectric order, such as BiFeO3 (BFO), have piqued the interest of researchers worldwide with the promise of coupling between magnetic and electric order parameters. BFO is an antiferromagnetic, ferroelectric multiferroic with a Curie temperature of ~820°C and a Neél temperature of ~370°C. Researchers have recently used this material to explore a particular manifestation of this multiferroic order in enabling electrical control of ferromagnetism through exchange coupling with a ferromagnet. The approach utilizes two types of electromagnetic coupling phenomena that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BFO. The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BFO film that leads to an electric field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between the ferromagnet (Co0.9Fe0.1) and the antiferromagnet. The inter-dependent coupling between these order parameters has been probed using a combination of piezoresponse force microscopy and photoemission electron spectromicroscopy. The researchers have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the co-linear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Experiments using a simple in-plane test structures reveal the possibility to control ferromagnetism with an electric field which would allow for new and novel devices that would greatly impact memory and logic applications.

Researchers developed a simple device structure to enable deterministic control of ferromagnetism with an electric field. This was achieved by controlling the switching of ferroelectric domains in the underlying BFO film with a device that consists of in-plane electrodes that allow for the application of in-plane electric fields to the BFO layer. Investigating in-plane piezoresponse force microscopy (PFM) images of the BFO layer for this device structure in the as-grown state and after the application of electrical field pulses reveals the presence of a set of two stripe-like domains (black and brown contrast in the images) running at 45° to the SRO IP contacts that switch by 90° back-and-forth in a repeatable fashion.

Photoemission electron microscopy (PEEM) images of the CoFe features on such a BFO surface taken at the Co L-edge in the as-grown state, after the first electrical switch, and following a second electrical switch reveal the ability to control ferromagnetism with an applied electric field. Analysis of the intensity distribution in the PEEM image enables us to reveal the direction of the local magnetization in the CoFe. Our observations strongly indicate that the average magnetization direction in the ferromagnet rotates by 90° upon the application of the electric field. Upon switching the BFO once again, the average magnetization direction changes back to the original state. We note that these promising results notwithstanding, our observations are still early in our understanding of the details of the coupling in such heterostructures. Furthermore, the exact details of factors such as the BFO surface roughness, shape and thickness of the CoFe layer, the magnitude of the applied magnetic field during the growth process and most importantly the magnitude of the coupling energy between the AFM and FM vis-à-vis other energy scales in this coupled system (magnetostatic energy, magnetocrystalline anisotropy energy) need to be carefully examined in future experimental and theoretical studies. In summary, we have presented direct evidence for electric field control of ferromagnetism through the coupling of the multiferroic BFO and a ferromagnet. This represents a significant advancement in the field of multiferroics as it marks the first demonstration of a possible room temperature application to utilize multiferroic materials in a novel new device.
 
Fig. 3: Electrical control of local ferromagnetism. PEEM images of ferromagnetic domain structure of a CoFe feature taken in the as-grown state (a), after the first electrical switch (b), and after the second electrical switch (c). d-f, Schematic descriptions of the observed magnetic contrast (gray, black, and white) in the corresponding PEEM images reveals that the net magnetization of the CoFe feature rotations by 90° upon application of an electric field.