W. R. Branford

Structural, magnetic, and transport properties of thin films of the Heusler alloy Co2MnSi

Thin films of Co2MnSi have been grown on a-plane sapphire substrates from three elemental targets by dc magnetron cosputtering. These films are single phase, have a strong (110) texture, and a saturation magnetization of 4.95μB/formula unit at 10 K. Films grown at the highest substrate temperature of 715 K showed the lowest resistivity (47 μΩ cm at 4.2 K) and the lowest coercivity (18 Oe). The spin polarization of the transport current was found to be of the order of 54% as determined by point contact Andreev reflection spectroscopy. A decrease in saturation magnetization with a decrease in film thickness and different transport behavior in thinner films indicate graded disorder in these films grown on nonlattice matched substrates.

Structural and transport studies of stoichiometric and off-stoichiometric thin films of the full Heusler alloy Co2MnSi

Co2MnSi Heusler alloy thin films have been grown by cosputtering from three elemental targets. Changes in growth temperature and stoichiometry affect the film texture and the temperature dependence of the resistivity. However, the spin polarization of the transport current is insensitive to these changes, being of the order of 54% as determined by point contact Andreev reflection spectroscopy. Stoichiometric films are single phase, have a strong (110) texture, and a saturation magnetization of 1007 emu/cc (4.95 μB/formula unit) at 10 K.

Geometric Manipulation of the High-Field Linear Magnetoresistance in InSb Epilayers on GaAs(001)

We address the inherent high-field magnetoresistance (MR) of indium antimonide epilayers on GaAs (001), studying the modification of the MR when processed into a set of geometries. The changes produced by the geometries are quite subtle. The extraordinary MR geometry produces the highest low-field MR while the Corbino geometry produces the largest high-field magnetoresistance. We demonstrate that any material with an unsaturating linear intrinsic MR, will also have an unsaturating linear Corbino MR, and that the ideal material for linear MR sensors in conventional geometries would have a high mobility and a small, linear intrinsic MR.

Highly aligned, spin polarized thin films of Sr2FeMoO6 by a chemical vapor process

Highly oriented films of Sr2FeMoO6 were fabricated by ultrasonic spray pyrolysis. A combined organic/inorganic solution was developed and the films were deposited on single-crystal LaAlO3 between 600 °C and 900 °C followed by postannealing at 850 °C or 1200 °C in Ar/5% H2. Optimum films showed a metallic resistivity behavior with less than a 0.25% magnetoresistive effect at 50 K, 1 T, indicative of highly quality intra- and intergranular material. At 4.2 K, the lower bound saturation magnetization, Ms was ∼2.5 μB/f.u. and the transport spin polarization was 60±3%.

Cation distribution in Ga-doped Li1.02Mn2O4

Abstract The structure of Ga-doped spinel oxides with the general formula, Li 1.02 Mn 2− x Ga x O 4 , has been studied by neutron and X-ray powder diffraction. Rietveld refinement of the neutron data confirms that the Ga 3+ ions selectively occupy the octahedral (16d) sites of the lattice. The absence of gallium ions from the tetrahedral (8a) or the additional octahedral (16c) is thought to be an important factor in the materials increased performance as a cathode in a rechargeable lithium ion battery.

Synthesis and physical properties of arc melted NiMnSb

Several polycrystalline samples of the half-Heusler alloy NiMnSb were grown by arc melting of stoichiometric and nonstoichiometric amounts of high-purity constituent elements. The structure and the phase-purity of the prepared samples were examined systematically by powder x-ray diffraction. The transport properties of the best sample, with saturation magnetization M-s(5 K)=4 mu(B)/formula unit, were studied by measuring electrical resistivity, thermal conductivity, and thermopower. Features in both magnetic and transport data are consistent with NiMnSb being in a half-metallic state at low temperatures, i.e., the conduction electrons are fully spin polarized. However, point-contact Andreev reflection measurements on the same sample at 4.2 K demonstrate only similar to45% spin polarization.

Enhanced intergrain magnetoresistance in bulk Sr2FeMoO6 through controlled processing

Enhanced low field magnetoresistance (LFMR) values have been obtained in bulk Sr2FeMoO6 through controlled processing. MR values, R(H)/R(0) of ∼60% [(R(H)−R(H=0)/R(H=0)=40%] were achieved at 50 K, in 1 T, and R(H)/R(0)∼8% [(R(H)−R(H=0)/R(H=0)=92%] at 290 K, in 1 T. For a ∼1 μm starting particle size, the optimum sintering temperature was 1300 °C, 100 °C higher than normally used. The optimum sintering time in Ar/H2 was only 5 h, much shorter than normally used. The magnitude of the LFMR appears to depend strongly on grain boundary oxygen content, controlled through optimization of sintering temperature and time.

The spin polarization of CrO2 revisited

Here, we use Andreev reflection spectroscopy to study the spin polarization of high quality CrO2 films. We study the spin polarization as a function of growth temperature, resulting in grain size and electrical resistivity. In these films low temperature growth appears to be a necessary but not sufficient condition to guarantee the observation of high spin polarization, and this is only observed in conjunction with suppressed superconducting gap values and anomalously low interface properties. We suggest that this combination of observations is a manifestation of the long range spin triplet proximity effect.

Tuning the inherent magnetoresistance of InSb thin films

We have investigated the 300 K inherent magnetoresistance of undoped InSb epilayers grown on GaAs(001) by molecular-beam epitaxy. The magnetoresistance of these films can be described well using a simplified model that incorporates gradation of properties away from the InSb/GaAs interface and the interplay between conduction and impurity bands. Although there is no significant intrinsic contribution in InSb bulk crystalline (001) materials due to its isotropic Fermi surface and mobility tensor, the linear and quadratic terms in the magnetoresistance as well as the overall magnitude can be tuned by varying the film thickness from 100 to 2000 nm.

The role of impurity band conduction in the low temperature characteristics of thin InSb films grown by molecular beam epitaxy

We report on the temperature-dependent electrical properties of high-quality undoped InSb layers of various thicknesses grown on GaAs(100) substrates by molecular beam epitaxy. The layers are found to be n-type in the measured temperature range (77-297 K). A differential Hall approach was employed to characterize the depth dependence of the electrical properties of the InSb films. The temperature variations of these data were then modelled with the inclusion of conduction through an impurity band, formed by the overlap of the wavefunctions of the dislocation-related donors. These analyses suggest that the contribution of the impurity band is dominant close to the interface even at room temperature, but its effect falls with increasing thickness, until at similar to 1000 nm, its contribution is only comparable to that of the conduction band at low temperatures. The dislocation donor densities deduced from this modelling follow an approximately reciprocal trend with increasing distance from the interface.