A. Barry

Production and magnetotransport properties of CrO2 films

CrO2 films were deposited on Al2O3 and TiO2 by high-pressure decomposition of CrO3. They are metallic, with a residual resistivity ρ0 in the range 0.2–10 μΩ m. The resistivity below Tc varies as ρ0+αT2+βT7/2 as expected for a correlated fully spin-polarized d band. At Tc the resistivity changes slope but the sign remains positive. There is a small negative magnetoresistance (∼−0.5%/ T) in a wide temperature range below Tc. Coercivity of the films varies according to the substrates from 7 mT for Al2O3 (110) to 15 mT for TiO2 (110).

Andreev reflection: A new means to determine the spin polarization of ferromagnetic materials

We have developed a simple method to measure the transport spin polarization of ferromagnetic materials. This technique relies on the fact that the Andreev reflection process at the interface between a superconductive and normal is influenced by the spin polarization P of the normal metal. In a very short time we have been able to measure the spin polarization of several metals: NixFe1−x, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2, whose spin polarization ranges from 35% to 90%. Our results compare well with other methods for measuring P.

Spin flop in goethite

A spin-flop transition is observed when a field of 20 T is applied parallel to the b axis of a specimen of natural goethite at 4.2 K. The goethite orders in a four-sublattice antiferromagnetic structure with the sublattice magnetizations inclined at +or-13 degrees to b, and an iron moment of 3.90 mu B. There is a small net ferromagnetic moment of 0.004 mu B along b which has the effect of more than doubling the threshold field for spin flop. The anisotropy field is deduced to be 0.11 T, corresponding to a uniaxial anisotropy energy K=6*104 J m-3. The origin of the weak moment is discussed, and it is suggested that mode superparamagnetism may arise in non-collinear antiferromagnets when two transverse spin configurations are effectively degenerate.

A CrO2-based magnetic tunnel junction

A tunnel junction based on the half-metallic oxide CrO2 uses a native oxide barrier layer and a cobalt top electrode. The I :V characteristic is fitted to the Simmons model with = 0.76 eV and t = 2.0 nm. The magnetoresistance is positive with R /R = 1.0% at 77 K.

Evidence for a gap in the excitation spectrum of CrO2

Magnetization and resistivity of CrO2 powders and oriented CrO2 films on different TiO2 single-crystal substrates are reported, and the low-temperature specific heat is measured on powders. The M(T) data follow a Bloch T3/2 law, with no sign of a spin-wave gap. The residual resistivity depends strongly on film texture, but a T2 temperature dependence of the resistivity is found in all samples above a cut-off temperature of the order of 70 K. The behavior is attributed to electron-magnon spin-flip scattering which is suppressed at low temperature in a half-metallic ferromagnet.

Raman Spectra of the Half-Metallic Ferromagnet CrO2

Chromium dioxide, CrO2, attracts significant interest as it has a unique electronic band structure, resulting in half-metallic ferromagnetism (TC ≈ 390 K) with completely spin-polarized electrons at EF. In this work we report the polarized Raman spectra of CrO2 between 5 and 420 K, as obtained from (110) and (001) oriented thin films on TiO2 substrates of the same orientation. The Raman lines corresponding to the four Raman allowed phonon modes of A1g, B1g, B2g and Eg symmetry were identified. The two low frequency Raman lines are very narrow, in particular at low temperatures, indicating weak scattering of the corresponding B1g and Eg phonons. The Raman intensities are not strongly affected by the magnetic ordering, whereas the line position versus T dependencies change their slope near TC for the A1g and Eg modes. At low temperatures the variations with T of the phonon linewidths follow the dependence expected for a phonon–phonon scattering. The additional broadening, which appears with approaching TC is tentatively assigned to the scattering of phonons from collective spin fluctuations.

Conduction across the interface between a superconductor and a spin polarized metal

We have studied Andreev reflection in point contacts formed between several low T/sub c/ superconductors and various spin polarized normal metals. We have also developed a theoretical model for the phenomenon. There is good accordance between theory and experiment.