Chromium Dioxide Thin Films Research Articles

Ferromagnetic resonance of patterned chromium dioxide thin films grown by selective area chemical vapour deposition

A selective area chemical vapour deposition technique has been used to fabricate continuous and patterned epitaxial CrO2 thin films on (100)-oriented TiO2 substrates. Precessional magnetization dynamics were stimulated both electrically and optically, and probed by means of time-resolved Kerr microscopy and vector network analyser ferromagnetic resonance techniques. The dependence of the precession frequency and the effective damping parameter upon the static applied magnetic field were investigated. All films exhibited a large in-plane uniaxial anisotropy. The effective damping parameter was found to exhibit strong field dependence in the vicinity of the hard axis saturation field. However, continuous and patterned films were found to possess generally similar dynamic properties, confirming the suitability of the deposition technique for fabrication of future spintronic devices.

Planar Hall effect and magnetic anisotropy in epitaxially strained chromium dioxide thin films

We have measured the in-plane anisotropic magnetoresistance of 100nm thick CrO2 thin films at liquid He temperatures. In low magnetic fields H, both the longitudinal and the transverse (planar Hall) resistance show abrupt switches, which characteristically depend on the orientation of H. All the experimental findings consistently demonstrate that the magnetic anisotropy in these CrO2 thin films is biaxial. We show that the biaxial magnetic anisotropy is due to epitaxial coherency strain, and that it naturally explains the complex magnetic switching behavior reported recently in CrO2 films with thicknesses of 50nm⩽d⩽250nm.

Optimization of Cr8O21 targets for Pulsed Laser Deposition

AbstractThis work describes the preparation of Cr8O21 pellets with optimised mass density to be used as targets for Pulsed Laser Deposition (PLD) of chromium dioxide thin films. Cr8O21 is synthesised by thermal decomposition of CrO3, at 270 °C. An attempt to reduce the grain size of the Cr8O21 powder to the nanometer scale has been made in order to increase the density of the pressed and sintered pellets serving as targets. The morphology of those starting fine powders as well as of targets (before and after laser ablation) and the corresponding thin films were characterized and studied using X‐ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Raman spectroscopy. The composition of the films is a mixture of crystallised Cr2O3 and amorphous CrO2. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetotransport study of granular chromium dioxide thin films prepared by the chemical vapor deposition technique

The temperature-dependent magnetotransport properties of granular chromium dioxide (CrO 2) films with grain size about 1 μm grown by chemical vapor deposition on Si(0 0 1) substrates has been studied over a temperature range from 2 to 300 K. The magnetoresistance (MR) of about 20% at T=2 K decreases very fast with increasing temperature as a result of the rapid increase of the spin-independent hopping channel, whilst the temperature-dependent MR is almost the same for samples after 100 days annealing in air at room temperature, and its electrical resistivity at room temperature increases from roughly 50 to >75 μΩ m. The contribution to the conductance at T<6 K can be attributed to the intergranular tunneling and that at 6 K<T<25 K is the second order inelastic hopping, which is amenable to σ∼ T 1.33. It becomes a poor metal at temperatures >160 K and its resistance follows R∼T 2 exp(−Δ/T) . We have shown experimentally that the room temperature annealing results in the decomposition of CrO 2 into Cr 2O 3 mainly near the surface of the films.

Switching behavior and its strain dependence in epitaxial CrO/sub 2/ thin films

Chromium dioxide thin films grown epitaxially on TiO/sub 2/ substrates exhibited a strong in-plane uniaxial anisotropy. Field dependence of high frequency (10/sup 6/ Hz) transverse susceptibility (/spl chi//sub T/) measurements were used to probe the magnetic anisotropy and switching in CrO/sub 2/ films for varying temperature (10 to 300 K) and different orientations of the applied field with respect to the magnetic easy axis. Singular peaks in /spl chi//sub T/ were observed and are associated with the anisotropy and switching fields. The /spl chi//sub T/ data at low temperatures reveal an unusual variation of the singular peaks. This can be described by including magnetoelastic contributions resulting from the in-plane tensile strain in the film due to lattice mismatch with the substrate.

High frequency impedance spectra on the chromium dioxide thin film

We report on the study of high frequency magnetotransport properties of the chromium dioxide (CrO2) thin films, grown on Si substrate using chemical vapor deposition. The film exhibits a ferromagnetic transition with a Curie temperature near 390 K. The temperature dependent spontaneous magnetization follows Bloch’s law. The impedance spectra, being analyzed based on the fundamental electrodynamics, are demonstrated to be in a low-loss dielectric limit along with the occurrence of dielectric relaxation and magnetization response. The specific features of impedance spectra, distinct from the usual metallic ferromagnet, are attributed to the half metallic nature of CrO2. The results explore the possibility for high frequency device applications.

Probing magnetic anisotropy effects in epitaxialCrO2thin films

The magnetic anisotropy in ferromagnetic chromium dioxide thin films ${(T}_{c}\ensuremath{\sim}393\mathrm{K})$ grown epitaxially on (100) ${\mathrm{TiO}}_{2}$ substrates, is probed using precise rf transverse susceptibility $({\ensuremath{\chi}}_{T})$ measurements. Singular peaks in ${\ensuremath{\chi}}_{T}$ are observed that are associated with the anisotropy $(\ifmmode\pm\else\textpm\fi{}{H}_{K})$ and switching $(\ifmmode\pm\else\textpm\fi{}{H}_{s})$ fields in ${\mathrm{CrO}}_{2}.$ Theoretical calculations of ${\ensuremath{\chi}}_{T}$ based on a simple coherent rotation model display remarkable agreement with the experimental data, indicating that these thin films behave like single-domain magnetic particles. Magnetoelastic contributions to the total anisotropy energy are needed to describe the evolution of ${\ensuremath{\chi}}_{T}$ peaks at lower temperatures.

Highly spin-polarized chromium dioxide thin films prepared by chemical vapor deposition from chromyl chloride

Highly spin-polarized chromium dioxide (CrO2) thin films were deposited on (100) TiO2 substrates by chemical vapor deposition using chromyl chloride as a precursor. The spin polarization, as measured by the point contact Andreev reflection technique, was 81±3%. X-ray diffraction θ/2θ scans indicated the films grew completely (100) oriented, in registry with the (100) oriented TiO2 substrate. X-ray diffraction φ scans on the CrO2 (110) reflection indicated the expected twofold symmetry, with no evidence of misaligned material. The resistivity at room temperature was 240 μΩ cm and decreased to 10 μΩ cm at 5 K, consistent with metallic behavior. The films were ferromagnetic with a Curie temperature of 395 K and a coercivity of ∼100 Oe at 298 K. The use of chromyl chloride as a precursor resulted in efficient and controlled CrO2 film growth.

Magnetic and transport properties of epitaxial and polycrystalline chromium dioxide thin films (invited)

The magnetic and transport properties of epitaxial and polycrystalline chromium dioxide (CrO2) thin films have been investigated. They are grown epitaxially on single crystal TiO2 (100) substrates, and with multiple grain orientations on polycrystalline TiO2 substrates, by chemical vapor deposition. The films have a Curie temperature (TC) of 390–395 K, with the epitaxially grown CrO2 (100) films exhibiting in-plane uniaxial magnetic anisotropy. While the epitaxial samples display metallic characteristics, the polycrystalline films are semiconducting with a dominant grain boundary contribution to the resistance at low temperatures. The magnetoresistance (MR) properties have also been measured with the magnetic field applied in the plane. For the epitaxial films, the MR is negative at temperatures near TC and is positive at low temperatures. A negative MR is also observed near TC for the polycrystalline samples. However, unlike the epitaxial films, the MR is found to be negative also at low temperatures, with a significant low field component. The latter is attributed to spin-polarized transport of electrons across grain boundaries.

Strain dependence and magnetic anisotropy in chromium dioxide thin films

AbstractField-dependent transverse susceptibility measurements based on a novel tunnel-diode oscillator (TDO) method were used to probe the magnetic anisotropy and switching in epitaxially grown CrO2 films on TiO2 substrates. The static field was applied in three different orientations (0°, 45° and 90° ) with respect to the magnetic easy axis. Singular peaks in the transverse susceptibility were observed that are associated with the anisotropy and switching fields in CrO2. Theoretical calculations based on a coherent rotation model display remarkable agreement with the experimental data. A new peak emerges at low temperatures and is identified to be a consequence of temperature-dependent magnetoelastic contributions resulting from the in-plane tensile strain in the films due to varying coefficients of thermal expansion in the film and the substrate.