High frequency magnetic behavior through the magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered ferromagnetic thin films

We studied the dynamics of magnetization through an investigation of the magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered thin films grown by magnetron sputtering. Impedance measurements were analyzed in terms of the mechanisms responsible for their variations at different frequency intervals and the magnetic and structural properties of the multilayers. Analysis of the mechanisms responsible for magnetoimpedance according to frequency and external magnetic field showed that for the CoFeB/Cu multilayer, ferromagnetic resonance (FMR) contributes significantly to the magnetoimpedance effect at frequencies close to 470 MHz. This frequency is low when compared to the results obtained for CoFeB/Ta and CoFeB/Ag multilayers and is a result of the anisotropy distribution and non-formation of regular bilayers in this sample. The MImax values occurred at different frequencies according to the used non-magnetic metal. Variations between 25% and 30% were seen for a localized frequency band, as in the case of CoFeB/Ta and CoFeB/Ag, as well as for a wide frequency range, in the case of CoFeB/Cu.Comment: 14 pages, 5 figure

Current and power spectrum in a magnetic tunnel device with an atomic size spacer

Current and its noise in a ferromagnetic double tunnel barrier device with a small spacer particle were studied in the framework of the sequential tunneling approach. Analytical formulae were derived for electron tunneling through the spacer particle containing only a single energy level. It was shown that Coulomb interactions of electrons with a different spin orientation lead to an increase of the tunnel magnetoresistance. Interactions can also be responsible for the negative differential resistance. A current noise study showed, which relaxation processes can enhance or reduce fluctuations leading either to a super-Poissonian or a sub-Poissonian shot noise.Comment: 12 pages, 4 figure

Linear response conductance and magneto-resistance of ferromagnetic single-electron transistors

The current through ferromagnetic single-electron transistors (SET's) is considered. Using path integrals the linear response conductance is formulated as a function of the tunnel conductance vs. quantum conductance and the temperature vs. Coulomb charging energy. The magneto-resistance of ferromagnet-normal metal-ferromagnet (F-N-F) SET's is almost independent of the Coulomb charging energy and is only reduced when the transport dwell time is longer than the spin-flip relaxation time. In all-ferromagnetic (F-F-F) SET's with negligible spin-flip relaxation time the magneto-resistance is calculated analytically at high temperatures and numerically at low temperatures. The F-F-F magneto-resistance is enhanced by higher order tunneling processes at low temperatures in the 'off' state when the induced charges vanishes. In contrast, in the 'on' state near resonance the magneto-resistance ratio is a non-monotonic function of the inverse temperature.Comment: 10 pages, 6 figures. accepted for publication in Phys. Rev.

Structural, Magnetic And Transport Properties Of Discontinuous Granular Multi-layers

Results of structural, magnetic and transport properties of magnetic Co/SiO2 discontinuous multi-layers produced by sequential deposition are presented. Transmission electron microscopy (TEM) images show that the samples that are close to metal-insulation transition are composed by a connected network of metallic paths, and display an enhanced Hall Effect. The granular samples are composed by an almost periodic array of Co nanoparticles, and after annealing these samples show a clear evolution in the nanostructure, with increasing average Co grain sizes and decreasing size dispersion. Relationships between the nanostructure and magnetotransport properties are discussed and compared with previous results obtained in cosputtered films. © 2005 Elsevier B.V. All rights reserved.2942206212Abeles, B., Sheng, P., Coutts, M.D., Arie, Y., (1975) Adv. Phys., 24, p. 40Vázquez, M., Hernando, A., (1995) Adv. Mater., 7, p. 1021Herzer, G., (1993) Phys. Scripta T, 49, p. 307Hernando, A., Kulik, T., (1994) Phys. Rev. B, 49, p. 7064Denardin, J.C., Brandl, A.L., Knobel, M., Panissod, P., Pakhomov, A.B., Liu, H., Zhang, X.X., (2002) Phys. Rev. B, 65, p. 064422Milner, A., Gerber, A., Groisman, B., Karpovsky, M., Gladkikh, A., (1996) Phys. Rev. Lett., 76, p. 475Slawska-Waniewska, A., Gutowski, M., Lachowicz, H.K., Kulik, T., Matyja, H., (1992) Phys. Rev. B, 46, p. 14594Pakhomov, A.B., Yan, X., Zhao, B., (1995) Appl. Phys. Lett., 67, p. 3497Jing, X.N., Wang, N., Pakhomov, A.B., Fung, K.K., Yan, X., (1996) Phys. Rev. B, 53, p. 14032Denardin, J.C., Knobel, M., Pakhomov, A.B., Zhang, X.X., (2003) J. Magn. Magn. Mater., 262, p. 15Aronzon, B.A., Rylkov, V.V., Kovalev, D.Y., Meilikhov, E.Z., Ryl'Kov, V.V., Sedova, M.V., (2000) Phys. Stat. Solidi, 218, p. 169Aronzon, B.A., Granovskii, A.B., Kovalev, D.Y., Meilikhov, E.Z., Ryl'Kov, V.V., Sedova, M.V., (2000) JETP Lett., 71, p. 469Zhang, X.X., Wan, C.C., Liu, H., Li, Z.Q., Sheng, P., Lin, J.J., (2001) Phys. Rev. Lett., 86, p. 5562Wan, C.C., Sheng, P., (2002) Phys. Rev. B, 66, p. 075309Maurice, J.L., Briatico, J., Carrey, J., Petroff, F., Schelp, L.F., Vaures, A., (1999) Philos. Mag. A, 79, p. 2921Skumryev, V., Stoyanov, S., Zhang, Y., Hadjipanayis, G., Givord, D., Nogues, J., (2003) Nature, 423, p. 850L.M. Socolovsky, C.L.P. Oliveira, J.C. Denardin, M. Knobel, I.L. Torriani, unpublishedDa Silva, F.C.S., Ferrari, E.F., Knobel, M., (1998) J. Appl. Phys., 84, p. 5366Denardin, J.C., Knobel, M., Socolovsky, L.M., Brandl, A.L., Zhang, X.X., (2003) IEEE Trans. Magn., 39, p. 276

Structural And Magnetotransport Properties Of Discontinuous Co/sio 2 Multilayers

Results of structural, magnetic and transport properties of magnetic Co/SiO2 granular multilayers, consisting of successive planes of nanosized cobalt clusters embedded in SiO2 and produced by sequential deposition, are presented. Microscopy shows that samples can be composed of a periodical array of Co nanoparticles. Relationships between the nanostructure and magnetotransport properties are discussed. © 2004 Elsevier B.V. All rights reserved.1122-3 SPEC. ISS.120122Abeles, B., Sheng, P., Coutts, M.D., Arie, Y., (1975) Adv. Phys., 24, p. 40Vázquez, M., Hernando, A., (1995) Adv. Mater., 7, p. 1021Herzer, G., (1993) Phys. Scripta, T49, p. 307Hernando, A., Kulik, T., (1994) Phys. Rev. B, 49, p. 7064Denardin, J.C., Brandl, A.L., Knobel, M., Panissod, P., Pakhomov, A.B., Liu, H., Zhang, X.X., (2002) Phys. Rev. B, 65, p. 064422Milner, A., Gerber, A., Groisman, B., Karpovsky, M., Gladkikh, A., (1996) Phys. Rev. Lett., 76, p. 475Slawska-Waniewska, A., Gutowski, M., Lachowicz, H.K., Kulik, T., Matyja, H., (1992) Phys. Rev. B, 46, p. 14594Maurice, J.L., Briatico, J., Carrey, J., Petroff, F., Schelp, L.F., Vaures, A., (1999) Philos. Mag. A, 79, p. 2921Schelp, L.F., Fert, A., Fettar, F., Holody, P., Lee, S.F., Mourice, J.L., Petroff, F., Vaures, A., (1997) Phys. Rev. B, 56, pp. R5747Skumryev, V., Stoyanov, S., Zhang, Y., Hadjipanayis, G., Givord, D., Nogue's, J., (2003) Nature, 423, p. 85