Magnetic Rare Earth (Gd) Implanted Tetrahedral Amorphous Carbon ( ta- C)
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0941-Q08-22
Magnetic Rare Earth (Gd) Implanted Tetrahedral Amorphous Carbon (ta-C) Li Zeng1,2, Erik Helgren2, Hayo Zutz3, Carsten Ronning3, and Frances Hellman2 1 Materials Science and Engineering Program, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA, 92093 2 Physics Department, University of California, Berkeley, 366 Le Conte, Berkeley, CA, 94720 3 II. Institute of Physics, University of Göttingen, Friedrich-Hund-PLatz 1, Gottingen, 37077, Germany
ABSTRACT Tetrahedrally bonded amorphous carbon (ta-C) thin films were prepared by mass selected ion beam deposition (MSIBD) using 100 eV carbon ions at room temperature. Gadolinium, a magnetic rare earth element, was implanted as a dopant into ta-C with two different fluences. The peak doping level of the ta-C:Gdx layers was 4 or 7 at.%, respectively. The Gd is believed to be electrically activated in as-implanted films, although contributions from damage-induced sp2 sites cannot be ruled out. A characteristic crossover temperature (T’) was found, below which there is a large negative magnetoresistance (MR) with strong temperature dependence. Thermal annealing greatly increases the sample conductivity due to the increase of sp2 sites. However, the MR persists at least up to an annealing temperature of 500°C. Magnetically, the Gd dopants behave like non-interacting local moments. Similarities and differences in physical properties between the ta-C:Gdx films and other Gd doped amorphous semiconductors are compared. INTRODUCTION The general interest in magnetically doped semiconductors arises from the combination of the spin degree of freedom of the magnetic dopant and the charge degree of freedom of the carriers in the semiconductor host. Discovering new materials and designing new devices with novel functionality based on this concept requires deep understanding of the interactions between the magnetic moments (spins) and the charge carriers. Gd doped group IV amorphous semiconductors are a simple and elegant system which provides a unique platform to study the fundamentals of these interactions. Gd has a half filled f electron shell (J=S=7/2), which provides a large local magnetic moment. It is also capable of providing 3 electrons as charge carriers. The amorphous elemental semiconductor matrix eliminates any doping sites and crystal structure dependence of the physical properties. It provides an isotropic environment and can accommodate high magnetic doping concentration beyond the thermodynamic solubility. The extensively studied Gd doped amorphous Si (a-GdxSi, x = 4-25 at.%) has shown remarkable physics for compositions near the three dimensional metal-insulator (MI) transition: many orders of magnitude negative magnetoresistance (MR) at low temperatures [1-2], and a high onset characteristic temperature (T*) where the effect of magnetic dopants “turns on” [3]. Both MR and T* are significantly reduced when doping Gd into amorphous Ge (a-Ge) and sp2 rich sputtered amorphous carbon (a-C), both of which have a narrower band gap and consequently a
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