Germanium‐based alloys hold great promise for future spintronics applications, due to their potential for integration with conventional Si‐based electronics. High‐quality single phase Mn5Ge3(0001) films, grown by solid‐phase epitaxy on Ge(111) and GaAs(111), exhibit strong ferromagnetism up to the Curie temperature TC ∼ 296 K. Point Contact Andreev Reflection (PCAR) measurements on Mn5Ge3 epilayers reveal a spin‐polarization P = 42 ± 5% for both substrates. We also calculate the spin polarization of bulk Mn5Ge3 in the diffusive and ballistic regimes using density‐functional theory (DFT). The measured spin polarization exceeds the theoretical estimates of PDFT = 35 ± 5% and 10 ± 5% for the diffusive and ballistic limits, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
We investigate the applicability of spin polarization measurements using Andreev reflection in a point contact geometry in heavily doped dilute magnetic semiconductors, such as (Ga,Mn)As. While we observe conventional Andreev reflection in non-magnetic (Ga,Be)As epilayers, our measurements indicate that in ferromagnetic (Ga,Mn)As epilayers with comparable hole concentration the conductance spectra can only be adequately described by a broadened density of states and a reduced superconducting gap. We suggest that these pair-breaking effects stem from inelastic scattering in the metallic impurity band of (Ga,Mn)As and can be explained by introducing a finite quasiparticle lifetime or a higher effective temperature. For (Ga,Mn)As with 8% Mn concentration and 140 K Curie temperature we evaluate the spin polarization to be 83 ± 17%.
We present evidence for spin polarized charge carriers in In2O3 films. Both In2O3 and Cr doped In2O3 films exhibit room temperature ferromagnetism after vacuum annealing, with a saturation moment of approximately 0.5 emu/cm 3 . We used Point Contact Andreev Reflection measurements to directly determine the spin polarization, which was found to be approximately 50±5% for both compositions. These results are consistent with suggestions that the ferromagnetism observed in certain oxide semiconductors may be carrier mediated. 15 , the precise relationship between the spin transport properties of the charge carriers and the net ferromagnetic moment remains unclear.In this Letter we demonstrate that the charge carriers in undoped In 2 O 3 films have a significant spin polarization at helium temperatures. Furthermore, measurements on Cr doped In 2 O 3 samples yield quantitatively similar results to measurements on undoped samples, suggesting that transition metal dopants may not play any significant role in the development of ferromagnetic order.We prepared ceramic samples of In 2 O 3 (with a base purity of 99.99%) and In 2 O 3 doped with 2 at% Cr using a standard solid state process 16 . The powder samples were pressed into 2" diameter sputtering targets, then annealed in air at 1100• C for 6 hours. In 2 O 3 and Cr:In 2 O 3 thin films were deposited by reactive magnetron sputtering of this target using an RF power source. High-purity argon was used as the sputtering gas and a small partial pressure of oxygen was maintained to obtain stoichiometric films. Oxygen at a partial pressure of 10 −3 torr and argon at a partial pressure of 1.4x10 −2 torr were used as reactive and sputtering gases respectively. The films were deposited onto (0001) oriented single crystal sapphire substrates. While the as-prepared samples were insulating and non-magnetic, the films became conducting and ferromagnetic when annealed in vacuum for 6-8 hours.The X-Ray Diffraction (XRD) spectra for the In 2 O 3 and Cr:In 2 O 3 samples, are shown in Figure 1a. The polycrystalline films are textured, with strong diffraction peaks indicating a preferred orientation along (222) or (400). There is no evidence for secondary phase formation. We show high resolution (HR) and cross-sectional transmission electron microscope (TEM) images of a Cr:In 2 O 3 film in Figs 1b and 1c. The HRTEM image shows the absence of defects, secondary phases, or clusters in these high-quality samples. Extensive SEM EDS mapping of the In 2 O 3 films (not shown) gave no indication of any transition metal dopants, including Cr, Co, Fe, Ni, and Mn, thus ruling out the possibility of accidental contamination with magnetic transition metal impurities.We plot the temperature dependent electrical resistivity for both the In 2 O 3 and Cr:In 2 O 3 samples in Fig. 2a. These data were obtained on 1.1 µm thick samples after vacuum annealing.
We use point contact Andreev reflection spectroscopy to measure the transport spin polarization of MnAs epitaxial films grown on ͑001͒ GaAs. By analyzing both the temperature dependence of the contact resistance and the phonon spectra of lead, acquired simultaneously with the spin polarization measurements, we demonstrate that all the contacts are in the ballistic limit. A ballistic transport spin polarization of approximately 49% and 44% is obtained for the type A and type B orientations of MnAs, respectively. These measurements are consistent with our density functional calculations, and with recent observations of a large tunnel magnetoresistance in MnAs/AlAs/͑Ga,Mn͒As tunnel junctions.
Double perovskites such as Sr2FeMoO6 (SFMO) have been predicted to be half-metallic (100% spin polarized). However, this conclusion is reached under the assumption that SFMO has a perfect crystalline structure. We measure the values of spin polarization P for SFMO single crystals with 11%, 15%, and 16% of B-site disorder using the point contact Andreev reflection technique. The highest spin polarization of P≈70% was determined. Using local spin density approximation, we compute P≈53% for the degree of B-site disorder of 12.5% in good agreement with the experimental results. While our results show that imperfect SFMO is not a half-metal, the observed correlation between the degree of antisite disorder, the magnetic properties, and the values of spin polarization implies that a perfectly ordered SFMO crystal might be half-metallic.
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