Structural and Microanalytical Studies of CrO₂ Thin Films on c-Sapphire by High Resolution Electron Microscopy Methods

Abstract

Chromium dioxide (CrO 2 ) has been extensively used in the magnetic recording industry. However, it is its ferromagnetic half-metallic nature that has more recently attracted much attention, primarily for the development of spintronic devices. CrO 2 is the only stoichiometric binary oxide theoretically predicted to be fully spin polarized at the Fermi level. It presents a Curie temperature of ~ 396 K, i.e. well above room temperature, and a magnetic moment of 2 μ B per formula unit [1][2][3][4][5]. However an antiferromagnetic native insulating layer of Cr 2 O 3 is always present on the CrO 2 surface which enhances the CrO 2 magnetoresistance and might be used as a barrier in magnetic tunnel junctions.Here we report on high resolution electron microscopy structural and analytical studies of CrO 2 films deposited onto Al 2 O 3 (0001) by atmospheric pressure CVD from CrO 3 precursor. As reported previously [6] films were grown within a broad range of deposition temperatures from 320 to 410 ºC, and oxygen carrier gas flow rates of 50 -500 sccm. Herein we will show the results obtained for two films grown at 340 and 360 ºC with 100 sccm oxygen.Cross-sectional specimens for TEM were prepared by mechanical grinding, dimpling and argon ion-milling of the samples in a liquid-nitrogen-cooled holder with an acceleration voltage of 5 keV and an incidence angle of 8º. Analytical TEM and HREM studies were carried out using a Philips Tecnai 20F FEG microscope operating at 200 keV, equipped with an Energy Dispersive X-Ray (EDX) analysis system and a scanning transmission electron microscopy modulus (STEM) with a high angle annular detector (HAAD) for Z-contrast imaging and composition mapping. Electron diffraction pattern (EDP) and simulations using Fast Fourier transform (FFT) of the HREM images were used to analyze the crystal structure.Bright field images of the sapphire substrate and of the chromium oxide film deposited at 360 ºC are shown in figure 1. Two different growth areas were identified. At the first stages of deposition a three-dimensional (3D) growth forming pyramidal structures of Cr 2 O 3 is observed. Defects and distortions at the substrate-film interface can be seen. The corresponding high resolution image is also shown in figure 1, as well as the simulated patterns using FFT which indicate that both aluminium and chromium oxides show corundum structure with an orientation relationship between the substrate and the 3D layer of the type (0001) Al 2 O 3 || (0001) Cr 2 O 3 . The top part of the layer that appears with a clear contrast is a mixture of Cr 2 O 3 and tetragonal rutile CrO 2 . The observed contrast in the top layer suggests a columnar growth and the defects observed along the layer indicate epitaxial stress during growth.