Role of charge carriers for ferromagnetism in cobalt-doped rutile TiO2

Electric and magnetic properties of a high temperature ferromagnetic oxide semiconductor, cobalt-doped rutile TiO2, are summarized. The cobalt-doped rutile TiO2 epitaxial thin films with different electron densities and cobalt contents were grown on r-sapphire substrates with laser molecular beam epitaxy. Results of magnetization, magnetic circular dichroism, and anomalous Hall effect measurements were examined for samples with systematically varied electron densities and cobalt contents. The samples with high electron densities and cobalt contents show the high temperature ferromagnetism, suggesting that charge carriers induce the ferromagnetism.Comment: 14 pages, 12 figure

Bulk and Surface Magnetization of Co atoms in Rutile Ti_[1-x]Co_xO_[2-delta] Thin Films Revealed by X-Ray Magnetic Circular Dichroism

We have studied magnetism in Ti_[1-x]Co_xO_[2-\delta] thin films with various x and \delta by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L_[2,3] absorption edges. The estimated ferromagnetic moment by XMCD was 0.15-0.24 \mu\beta/Co in the surface, while in the bulk it was 0.82-2.25 \mu\beta/Co, which is in the same range as the saturation magnetization of 1.0-1.5 \mu\beta/Co. Theseresults suggest that the intrinsic origin of the erromagnetism. The smaller moment of Co atom at surface is an indication of a magnetically dead layer of a few nm thick at the surface of the thin films.Comment: This Paper is accepted in J. of Phys: Conds. Matte

Mass Outflows from Dissipative Shocks in Hot Accretion Flows

We consider stationary, axisymmetric hydrodynamic accretion flows in Kerr geometry. As a plausible means of efficiently separating a small population of nonthermal particles from the bulk accretion flows, we investigate the formation of standing dissipative shocks, i.e. shocks at which fraction of the energy, angular momentum and mass fluxes do not participate in the shock transition of the flow that accretes onto the compact object but are lost into collimated (jets) or uncollimated (winds) outflows. The mass loss fraction (at a shock front) is found to vary over a wide range (0 - 95%) depending on flow's angular momentum and energy. On the other hand, the associated energy loss fraction appears to be relatively low (<1%) for a flow onto a non-rotating black hole case, whereas the fraction could be an order of magnitude higher (<10%) for a flow onto a rapidly-rotating black hole. By estimating the escape velocity of the outflowing particles with a mass-accretion rate relevant for typical active galactic nuclei, we find that nearly 10% of the accreting mass could escape to form an outflow in a disk around a non-rotating black hole, while as much as 50% of the matter may contribute to outflows in a disk around a rapidly-rotating black hole. In the context of disk-jet paradigm, our model suggests that shock-driven outflows from accretion can occur in regions not too far from a central engine. Our results imply that a shock front under some conditions could serve as a plausible site where (nonthermal) seed particles of the outflows (jets/winds) are efficiently decoupled from bulk accretion.Comment: 25 pages, 10 black&white figures, Accepted to Ap

Spin-phonon coupling in multiferroic RCrO3_3 (R-Y, Lu, Gd, Eu, Sm): A Raman study

Raman study on a select few orthochromites, RCrO$_3$ (R = Y, Lu, Gd, Eu and Sm) shows that the phonon behavior at TN in compounds with magnetic R-ion (Gd and Sm) is remarkably different from that of non-magnetic R-ion (Y, Lu and Eu). While anomalies in most of the observed phonon frequencies in all these compounds may result from the distortion of CrO$_6$ octahedra due to size effect and magnetostriction arising from Cr-ordering, the anomalous behavior of their linewidths observed at TN for the compounds with only magnetic R-ion suggests spin-phonon coupling. The presence of spin-phonon coupling and the anomalies in the low frequency modes related to R-ion motion in orthochromites (R = Gd and Sm) support the suggestion that the coupling between 4f-3d moments play important role in inducing switchable electric polarization.Comment: 6 pages (two column format), 7 figures; The updated version of the manuscript can be found at Euro. Phys. Lett. 101, 17008 (2013

Magnetic oxide semiconductors

Magnetic oxide semiconductors, oxide semiconductors doped with transition metal elements, are one of the candidates for a high Curie temperature ferromagnetic semiconductor that is important to realize semiconductor spintronics at room temperature. We review in this paper recent progress of researches on various magnetic oxide semiconductors. The magnetization, magneto-optical effect, and magneto-transport such as anomalous Hall effect are examined from viewpoint of feasibility to evaluate the ferromagnetism. The ferromagnetism of Co-doped TiO2 and transition metal-doped ZnO is discussed.Comment: 26 pages, 5 tables, 6 figure

Giant Planar Hall Effect in Epitaxial (Ga,Mn)As Devices

Large Hall resistance jumps are observed in microdevices patterned from epitaxial (Ga,Mn)As layers when subjected to a swept, in-plane magnetic field. This giant planar Hall effect is four orders of magnitude greater than previously observed in metallic ferromagnets. This enables extremely sensitive measurements of the angle-dependent magnetic properties of (Ga,Mn)As. The magnetic anisotropy fields deduced from these measurements are compared with theoretical predictions.Comment: 3 figure

Isothermal Shock Formation in Non-Equatorial Accretion Flows around Kerr Black Holes

We explore isothermal shock formation in non-equatorial, adiabatic accretion flows onto a rotating black hole, with possible application to some active galactic nuclei (AGNs). The isothermal shock jump conditions as well as the regularity condition, previously developed for one-dimensional (1D) flows in the equatorial plane, are extended to two-dimensional (2D), non-equatorial flows, to explore possible geometrical effects. The basic hydrodynamic equations with these conditions are self-consistently solved in the context of general relativity to explore the formation of stable isothermal shocks. We find that strong shocks are formed in various locations above the equatorial plane, especially around a rapidly-rotating black hole with the prograde flows (rather than a Schwarzschild black hole). The retrograde flows are generally found to develop weaker shocks. The energy dissipation across the shock in the hot non-equatorial flows above the cooler accretion disk may offer an attractive illuminating source for the reprocessed features, such as the iron fluorescence lines, which are often observed in some AGNs.Comment: 22 pages with 11 figures, presented at 5th international conference on high energy density laboratory astrophysics in Tucson, Arizona. accepted to Ap

Intrinsic Tunneling in Cuprates and Manganites

The most anisotropic high temperature superconductors like Bi2Sr2CaCu2O8, as well as the recently discovered layered manganite La1.4Sr1.6Mn2O7 are layered metallic systems where the interlayer current transport occurs via sequential tunneling of charge carriers. As a consequence, in Bi2Sr2CaCu2O8 adjacent CuO2 double layers form an intrinsic Josephson tunnel junction while in in La1.4Sr1.6Mn2O7 tunneling of spin polarized charge carriers between adjacent MnO2 layers leads to an intrinsic spin valve effect. We present and discuss interlayer transport experiments for both systems. To perform the experiments small sized mesa structures were patterned on top of single crystals of the above materials defining stacks of a small number of intrinsic Josephson junctions and intrinsic spin valves, respectively.Comment: 6 pages, 8 figure