Effects of germanium on the electronic transport mechanism in Co 20 (Cu 1-x Ge x ) 80 nanogranular ribbons
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D. Zhang Shenyang National Laboratory for Materials Science and International Center for Materials Physics, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China
J.P. Liu Department of Physics and Astronomy and Center for Materials Research and Analysis, University of Nebraska, Lincoln, Nebraska, 68588-0113, and Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana 71272
D.J. Sellmyer Department of Physics and Astronomy and Center for Materials Research and Analysis, University of Nebraska, Lincoln, Nebraska 68588-0113 (Received 26 February 2002; accepted 27 August 2002)
The dependency of giant magnetoresistance (GMR) on the nonmagnetic matrix in nanogranular Co20(Cu1−xGex)80 ribbons was studied. When the matrix Cu is substituted with semiconductor Ge, the magnetoresistance transitioned from negative to positive at low temperatures. The positive GMR effect is closely related to the quantity of Co/Co3Ge2/Co junctionlike configurations. This result provides evidence for the competition between two types of electronic transport mechanisms in the magnetic granular ribbons: (i) electronic spin-dependent scattering, inducing a negative magnetoresistance and (ii) Coulomb blockade of the electronic tunneling, inducing a positive magnetoresistance.
I. INTRODUCTION
Extensive studies of spin-dependent transport properties of magnetically heterogeneous multilayers1–3 and granular films4–9 have been performed in the past decade since the remarkable discovery of the giant magnetoresistance (GMR) effect. GMR is a change in the electrical resistance of magnetically inhomogeneous systems when an applied field causes a realignment of magnetic moments in different parts of the materials. The resistance usually decreases with increasing magnetic field. Basically, GMR is considered to be mainly a manifestation of the dependence of electronic spin-dependent scattering on local magnetic configurations in metallically nonmagnetic matrix. In recent years, much importance has been attached to the observation of GMR in tunnel-type nanostructures such as the ferromagnetic nanograins embedded in a nonmagnetic insulator matrix,7,10–12 an effect called tunneling magnetoresistance. The electronic tunneling conductance also depends on the relative orientation of magnetic moments among different magnetic layers or 3050
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J. Mater. Res., Vol. 17, No. 12, Dec 2002 Downloaded: 14 Mar 2015
granules. Mitani et al. reported the enhanced magnetoresistance in tunnel-type nanostructures Co–Al–O granular films.10 The anomalous enhancement of GMR at low temperatures is the result of a higher-order process of spin-dependent tunneling between large magnetic grains through intervening small ones. 1 0 These small grains tend to present a strong charging effect during the spin-dependent tunneling, resulting in Coulomb blockade.13 The applied field decreases the tunneling resistance by alignment of the relative orientation of magnetic moments between different
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