Observation of anomalous single-magnon scattering in half-metallic ferromagnets by chemical pressure control

Temperature variation of resistivity and specific heat have been measured for prototypical half-metallic ferromagnets, R_0.6Sr_0.4MnO_3, with controlling the one-electron bandwidth W. We have found variation of the temperature scalings in the resistivity from T^2 (R = La, and Nd) to T^3 (R = Sm), and have interpreted the $T^3-law in terms of the anomalous single-magnon scattering (AMS) process in the half-metallic system.Comment: To appear in Phys. Rev. Lett., 3 pages + 4 EPS figure

Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems

We present a theory of Andreev reflection in a ferromagnet/superconductor/ferromagnet double junction system. The spin polarized quasiparticles penetrate to the superconductor in the range of penetration depth from the interface by the Andreev reflection. When the thickness of the superconductor is comparable to or smaller than the penetration depth, the spin polarized quasiparticles pass through the superconductor and therefore the electric current depends on the relative orientation of magnetizations of the ferromagnets. The dependences of the magnetoresistance on the thickness of the superconductor, temperature, the exchange field of the ferromagnets and the height of the interfacial barriers are analyzed. Our theory explains recent experimental results well.Comment: 8 pages, 9 figures, submitted to Phys. Rev.

A spin triplet supercurrent through the half-metallic ferromagnet CrO2

In general, conventional superconductivity should not occur in a ferromagnet, though it has been seen in iron under pressure. Moreover, theory predicts that the current is always carried by pairs of electrons in a spin singlet state, so conventional superconductivity decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. It has been predicted that this rapid spatial decay would not occur when spin triplet superconductivity could be induced in the ferromagnet. Here we report a Josephson supercurrent through the strong ferromagnet CrO2, from which we infer that it is a spin triplet supercurrent. Our experimental setup is different from those envisaged in the earlier predictions, but we conclude that the underlying physical explanation for our result is a conversion from spin singlet to spin triplets at the interface. The supercurrent can be switched with the direction of the magnetization, analogous to spin valve transistors, and therefore could enable magnetization-controlled Josephson junctions.Comment: 14 pages, including 3 figure

Measurements of Spin Polarization of Epitaxial SrRuO3 Thin Films

We have measured the transport spin-polarization of epitaxial thin films of the conductive ferromagnetic oxide, SrRuO3, using Point Contact Andreev Reflection Spectroscopy (PCAR). In spite of the fact that spin-up and spin-down electronic densities of states at the Fermi level for SrRuO3 calculated from band structure theory are practically the same, the experimental transport spin polarization for these films was found to be about 50%. This result is a direct consequence of the Fermi velocity disparity between the majority and minority bands and is in good agreement with our theoretical estimates.Comment: 12 pages pdf onl

Spin accumulation induced resistance in mesoscopic ferromagnet/ superconductor junctions

We present a description of spin-polarized transport in mesoscopic ferromagnet-superconductor (F/S) systems, where the transport is diffusive, and the interfaces are transparent. It is shown that the spin reversal associated with Andreev reflection generates an excess spin density close to the F/S interface, which leads to a spin contact resistance. Expressions for the contact resistance are given for two terminal and four terminal geometries. In the latter the sign depends on the relative magnetization of the ferromagnetic electrodes.Comment: RevTeX 10 pages, 4 figures, submitted to Phys.Rev. Let

Electron and hole transmission through superconductor - normal metal interfaces

We have investigated the transmission of electrons and holes through interfaces between superconducting aluminum (Tc = 1.2 K) and various normal non-magnetic metals (copper, gold, palladium, platinum, and silver) using Andreev-reflection spectroscopy at T = 0.1 K. We analyzed the point contacts with the modified BTK theory that includes Dynes' lifetime as a fitting parameter G in addition to superconducting energy gap 2D and normal reflection described by Z. For contact areas from 1 nm^2 to 10000 nm^2 the BTK Z parameter was 0.5, corresponding to transmission coefficients of about 80 %, independent of the normal metal. The very small variation of Z indicates that the interfaces have a negligible dielectric tunneling barrier. Fermi surface mismatch does not account for the observed transmission coefficient.Comment: 9 pages, 4 figures, submitted to Proceedings of the 19th International Conference on Magnetism ICM2012 (Busan 2012

The effect of the spin-orbit interaction on the band gap of half-metals

The spin-orbit interaction can cause a nonvanishing density of states (DOS) within the minority-spin band gap of half-metals around the Fermi level. We examine the magnitude of the effect in Heusler alloys, zinc-blende half metals and diluted magnetic semiconductors, using first-principles calculations. We find that the ratio of spin-down to spin-up DOS at the Fermi level can range from below 1% (e.g. 0.5% for NiMnSb) over several percents (4.2% for (Ga,Mn)As) to 13% for MnBi.Comment: 5 pages, 3 figure

Spin-Imbalance and Magnetoresistance in Ferromagnet/Superconductor/Ferromagnet Double Tunnel Junctions

We theoretically study the spin-dependent transport in a ferromagnet/super- conductor/ferromagnet double tunnel junction. The tunneling current in the antiferromagnetic alignment of the magnetizations gives rise to a spin imbalance in the superconductor. The resulting nonequilibrium spin density strongly suppresses the superconductivity with increase of bias voltage and destroys it at a critical voltage Vc. The results provide a new method not only for measuring the spin polarization of ferromagnets but also for controlling superconductivity and tunnel magnetoresistance (TMR) by applying the bias voltage.Comment: 4pages, to be published in Phys. Rev. Let

Spin diffusion and injection in semiconductor structures: Electric field effects

In semiconductor spintronic devices, the semiconductor is usually lightly doped and nondegenerate, and moderate electric fields can dominate the carrier motion. We recently derived a drift-diffusion equation for spin polarization in the semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics and identified a high-field diffusive regime which has no analogue in metals. Here spin injection from a ferromagnet (FM) into a nonmagnetic semiconductor (NS) is extensively studied by applying this spin drift-diffusion equation to several typical injection structures such as FM/NS, FM/NS/FM, and FM/NS/NS structures. We find that in the high-field regime spin injection from a ferromagnet into a semiconductor is enhanced by several orders of magnitude. For injection structures with interfacial barriers, the electric field further enhances spin injection considerably. In FM/NS/FM structures high electric fields destroy the symmetry between the two magnets at low fields, where both magnets are equally important for spin injection, and spin injection becomes locally determined by the magnet from which carriers flow into the semiconductor. The field-induced spin injection enhancement should also be insensitive to the presence of a highly doped nonmagnetic semiconductor (NS$^+$) at the FM interface, thus FM/NS$^+$/NS structures should also manifest efficient spin injection at high fields. Furthermore, high fields substantially reduce the magnetoresistance observable in a recent experiment on spin injection from magnetic semiconductors

Orbital character of O 2p unoccupied states near the Fermi level in CrO2

The orbital character, orientation, and magnetic polarization of the O 2$p$ unoccupied states near the Fermi level ($E_F$) in CrO$_2$ was determined using polarization-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) from high-quality, single-crystal films. A sharp peak observed just above $E_F$ is excited only by the electric field vector ($\bf E$) normal to the tetragonal $c$-axis, characteristic of a narrow band ($\approx$ 0.7 eV bandwidth) constituted from O 2$p$ orbitals perpendicular to $c$ (O 2$p_y$) hybridized with Cr 3$d_{xz-yz}$ $t_{2g}$ states. By comparison with band-structure and configuration-interaction (CI) cluster calculations our results support a model of CrO$_2$ as a half-metallic ferromagnet with large exchange-splitting energy ($\Delta_{exch-split}$ $\approx$ 3.0 eV) and substantial correlation effects.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. B Rapid Com