Tuning Magnetic Properties of Magnetoelectric Nanostructures
 
Investigators: Steven P. Crane, C. Bihler, M. S. Brandt, S. T. B. Goennenwein, M. Gajek, and R. Ramesh
 
Multifunctional complex oxide soft magnetic materials are interesting for their potential applications in microwave communications as tunable signal filters, phase shifters, and resonators. The use of magnetoelectric nanostructures comprised of magnetic pillars embedded in a ferroelectric matrix system allows ideal electric tunability of the magnetization orientation due to the strain-induced magnetoelectric coupling between the ferroelectric and ferrimagnetic phases. One limitation of previously studied systems has been the focus on CoFe2O4, a material with a large magnetocrystalline anisotropy component. This aspect prevents the ability to tune magnetic properties by changing thickness, a valued capability in such thin film systems. A nanostructured multiferroic thin film with a soft-magnetic ferrite can be obtained using NiFe2O4 (NFO) as the magnetic component, which permits use in high frequency applications.

The use of NFO also permits the ability to tune the magnetic properties of the films by adjusting only the thickness, allowing one to switch the easy magnetic direction from an in-plane axis to an out-of-plane axis as the aspect ratio of the NFO pillars is altered. This capability opens the possibility of tuning magnetic properties according to the device geometry required for a particular application by changing film thickness alone, an ideal property for reducing processing steps in device fabrication. Such magnetoelectric materials could lower both the size and power consumption if applied to current microwave communication devices, leading to higher operating efficiencies.
 
 
Fig. 3: Ferromagnetic resonance response for BiFeO3-NiFe2O4 thin films of varying thickness. Both angle-dependent measurements were performed varying the out-of-plane angle (θ). The FMR spectra of the (a) 40-nm (1:2 aspect ratio) film show an in-plane anisotropy, whereas the spectra of (b) the 800-nm (10:1 aspect ratio) film show an out-of-plane anisotropy. This switch is denoted by the change in direction of superimposed arrows that compare the relative resonance fields found in the out-of-plane and in-plane directions.