Anisotropic crystallite growth and magnetic properties of MFe2O4 (M = Co, Mg, Cu) thin films prepared by spin-coating of N,N-dimethylformamide/acetic acid-solutions

Thin films of MFe2O4 (M = Co, Mg, Cu) were prepared on platinum coated silicon wafers by spin coating of solutions of the metal nitrates in N,N-dimethylformamide and acetic acid with subsequent heating at different temperatures. X-ray diffraction and Raman spectroscopy revealed the formation of phase-pure spinel ferrites. For medium synthesis temperatures, scanning electron microscopy and atomic-force microscopy investigations reveal smooth and uniform surfaces with root mean square roughness values in the nm range for all ferrites. Heating at higher temperatures leads to an increase of the grain sizes and in turn to higher surface roughnesses. For CoFe2O4 and CuFe2O4 the grains show a columnar growth perpendicular to the substrate surface, while the MgFe2O4 grains grow plate-like parallel to it. Rocking curve measurements revealed a highly oriented growth for all three ferrites above a specific synthesis temperature with the (111) lattice plane being parallel to the film surface. This orientation and the mosaicity of the ferrite films are induced by the underlying Pt-layer. Magnetic properties were measured with H perpendicular and parallel to the film surface. We observed a magnetic anisotropy for the three ferrites, where the easy axis is perpendicular to the film surface for the CoFe2O4 films and parallel in the case of MgFe2O4 and CuFe2O4. For the CoFe2O4 films a reorientation of the magnetic easy axis in the [111] direction and a very strong anisotropy was found with the remanent magnetization perpendicular to the film surface being about twice as large as parallel to it. (C) 2017 Elsevier B.V. All rights reserved.