Highlights

Structure-Property Relationship of CoFe₂O₄ Thin Films

Scientific Achievement

Multiferroic tunnel junctions consisting of a ferroelectric tunnel barrier sandwiched between two ferromagnetic electrodes, have aroused intense interest because of the interactions between the ferroelectric and ferromagnetic layers that can lead to new spin-dependent electron tunneling phenomena.  In order to couple the properties across the interfaces, high quality epitaxial growth is required, while maintaining good magnetotransport behavior.  We are addressing this by using metalorganic chemical vapor deposition (MOCVD) to grow suitable heterostructures, and using TEM and AFM to study the microstructure.

We have grown CoFe₂O₄ (CFO) thin films epitaxially on (001) SrTiO­₃ despite the large lattice mismatch.  Electron diffraction patterns and HREM images confirm that the epitaxial relationship is CFO₍₀₀₄₎//STO₍₀₀₂₎ and CFO_<100>//STO_<100>.  The magnetic anisotropy measured in the CFO thin films can thus be ascribed to lattice strain.  The TEM bright field images and AFM topography images show that the CFO films deposited at high temperature (>600°C) have very rough surfaces (~22 nm) and a columnar structure resulting from an island growth mode (generally observed in MOCVD growth).  However, for CFO films grown at lower temperature (400-600°C), we observed that the surface roughness drops two orders of magnitude to 0.2 nm.  This suggests a change to a layer-by-layer growth mode, which occurs because the lower substrate temperature does not provide enough energy for the precursors to congregate to form large islands.  The magnetic properties are relatively independent of deposition temperature although a rougher surface can result in pinning sites during the magnetization reversal process.

Significance

We have shown that we can use MOCVD to grow epitaxial CFO thin films, and that we can control surface roughness without degrading magnetic properties over a wide range of deposition temperatures.  This deposition temperature window will enable further development of multiferroic systems in the future, and also makes this potentially an industry-compatible and economical deposition technique.  In the future, we will focus on fabricating multiferroic heterostructures with CFO as the ferromagnetic material and correlating the magnetic/ferroelectric properties with microstructure.

Performers

M. Pan (Northwestern U.; Argonne-MSD); G. Bai, Yuzi Liu, A. Petford-Long (Argonne-MSD); V. Dravid (Northwestern U.)