A superconducting point contact is used to determine the spin polarization at the Fermi energy of several metals. Because the process of supercurrent conversion at a superconductor-metal interface (Andreev reflection) is limited by the minority spin population near the Fermi surface, the differential conductance of the point contact can reveal the spin polarization of the metal. This technique has been applied to a variety of metals where the spin polarization ranges from 35 to 90 percent: Ni0.8Fe0.2, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2.

Using the point contact Andreev reflection technique, we have carried out a systematic study of the spin polarization in the colossal magnetoresistive manganite, La0.7Sr0.3MnO3 (LSMO). Surprisingly, we observed a significant increase in the current spin polarization with the residual resistivity. This counterintuitive trend can be understood as a transition from ballistic to diffusive transport in the contact. Our results strongly suggest that LSMO does have minority spin states at the Fermi level. However, since its current spin polarization is much higher than that of the density of states, this material can mimic the behavior of a true half-metal in transport experiments. Based on our results we call this material a transport half-metal.A half-metallic ferromagnet is a metal that has an energy gap at the Fermi level, E F , in one of the two spin channels. Only the other channel has states available for transport, and thus the electric current is fully spin-polarized. Finding half-metallic or other highly spin-polarized metals would bring about major advances in magnetoelectronics, since device performance improves dramatically as the spin polarization of the metal approaches 100%.1 Although half-metallicity has been predicted in quite a number of materials, the experimental situation is still controversial, especially for the manganese perovskite, La 0.7 Sr 0.3 MnO 3 (LSMO). Theoretical 2 and experimental values 3-6 of the spin polarization of this fascinating material with highly unusual structural, magnetic and electronic properties, obtained by different techniques vary from 35% to 100%. Not surprisingly, when Park et al. concluded from their spin-resolved photoemission spectroscopy measurement that LSMO is completely spin-polarized 3 it attracted immediate attention. This result was important not only from a practical viewpoint, but also as a potential new insight into the microscopic physics of this system, since the values of the spin polarization are extremely sensitive to the band structure of LSMO. Importantly, the measured value of the spin polarization, P n , depends on the experimental technique. It is often possible 10 to define P n in the following form:where N ↑ (E F ), N ↓ (E F ) and v F ↑ , v F ↓ are the majority and minority spin DOS and the Fermi velocities, respectively. This definition allows a direct comparison between different experiments and the theory, since all the quantities in Eq. 1 can be evaluated from the band structure. The spin polarization P 0 (n =0) measured by spinresolved photoemission measurements is determined only by the DOS at the Fermi level.11 Transport experiments measure a different spin polarization, which includes the Fermi velocities (Eq.1). In the ballistic, or Sharvin, limit (mean free path, L, larger than the contact size, d) the DOS is weighted linearly with v F , and P 1 is measured. 12In the diffusive, or Maxwell regime (L < d), as in the classical Bloch-Bolzmann theory of transport in metals, the weighting is quadratic in v F (n=2) and P 2 is measured (a...

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.

We present a systematic analysis of point-contact Andreev reflection (PCAR) spectra for ferromagnetic materials, using both modeling and experimental data. We emphasize the importance of consistent data analysis to avoid possible misinterpretation of the data. We consider the relationship between ballistic and diffusive transport, the effect of different transport regimes on spin polarization measurements, and the importance of unambiguous identification of the type of transport regime. We find that in a realistic parameter range, the analysis of PCAR spectra of purely diffusive character by a ballistic model yield approximately the same (within ~3%) values of the spin polarization and the barrier strength Z larger by ~ 0.5-0.6. We also consider the dependence of polarization values on Z, and have shown by simple modeling that letting the superconducting gap vary as an adjustable parameter can result in a spurious dependence of the spinpolarization P c on Z. At the same time we analyzed the effects of finite Z on the apparent value of P c measured by the PCAR technique, using a large number of examples from both our own measurements and from the literature. We conclude that there is a systemdependent variation in P c (Z), presumably due to spin-flip scattering at the interface.However, the exact type of this dependence is hard to determine with any statistical certainty.

HdiT) has been measured for thin BSCO films over a larger combined range of magnetic field and reduced temperature than for any other superconductor. H c i(T) diverged anomalously as the temperature decreased: At the lowest temperature, it was 5 times that expected for a conventional superconductor. Such a strong divergence cannot be explained by any conventional model.

Rechargeable lithium-ion batteries are the preferred power source for consumer electronic devices, but the cost and toxicity of many cathode materials limit their scale-up. Worldwide research efforts are addressing this concern by transitioning from conventional Co-and Ni-based intercalation hosts towards Fe-and Mn-based alternatives. The unfavorable energetics of the Fe 2+/3+ redox couple and limited Li-insertion capacities render the use of iron oxides impractical. We address this limitation with the defect spinel ferrite g-Fe 2 O 3 as a model structure for Li-ion insertion by replacing a fraction of the Fe 3+ sites with highly oxidized Mo 6+ to generate cation vacancies that shift the onset of Li-ion insertion to more positive potentials as well as increase capacity. In the present study, native and Mo-substituted iron oxides are synthesized via base-catalyzed precipitation in aqueous media, yielding nanocrystalline spinel materials that also exhibit short-range disorder characteristic of a proton-stabilized structure. The Mo-substituted ferrite reported herein is estimated to have $3Â as many cation vacancies as g-Fe 2 O 3 with a corresponding increase in the Li-ion capacity to >100 mA h g À1 between 4.1 and 2.0 V vs. Li/Li + . This dual enhancement in capacity and insertion potential will enable these and related defect spinel ferrites to be explored as positive electrode materials for lithium batteries, while retaining the cost advantages of a material whose metal composition is still predominately iron based.

We have demonstrated a general framework for realizing and modulating perpendicular magnetic anisotropy in a rare-earth-element and heavy-metal -free material system. Using GaAs(001)/Fe(001) template, we have developed a synthesis scheme to produce epitaxial body center tetragonal Fe-N with (001) texture. By varying the N doping concentration, the crystal tetragonality (c/a) can be tuned in a relatively wide range. It is found that the Fe-N layer developed a strong perpendicular magnetic crystalline anisotropy (MCA) as it approaches the iron nitride interstitial solubility limit. Further annealing process significantly improves the MCA due to the formation of chemically ordered Fe 16 N 2 . In addition to realize an MCA up to 10 7 erg/cm 3 , the spin polarization ratio (P~0.52), as probed directly by a Point Contact Andreev Reflection (PCAR) method, even shows a moderate increase in comparison with normal metal Fe (P~0.45). These combined properties make this material system a promising candidate for applications in spintronic devices and also potential rare-earth-element free magnets.

A recent proposal that the metamaterial approach to dielectric response engineering may increase the critical temperature of a composite superconductor-dielectric metamaterial has been tested in experiments with compressed mixtures of tin and barium titanate nanoparticles of varying composition. An increase of the critical temperature of the order of ΔT ~ 0.15 K compared to bulk tin has been observed for 40% volume fraction of barium titanate nanoparticles. Similar results were also obtained with compressed mixtures of tin and strontium titanate nanoparticles.