Magnetic Properties of Iron Implanted Polymers and Graphite
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MAGNETIC PROPERTIES OF IRON IMPLANTED POLYMERS AND GRAPHITE
N. C. KOON, D. WEBER, P. PEHRSSON, AND A. I. SCHINDLER Naval Research Laboratory, 4555 Overlook Avenue, Washington, DC 20375
ABSTRACT We have measured the magnetic properties of highly oriented pyrolytic graphite (HOPG), polyethylene (PE), and polyvinylidine fluoride (PVF,), implanted with fluences of 1 25 keV iron atoms ranging from 1016 to 1017 atoms/cm . The lowest fluence specimens were paramagnetic down to 2 K, with evidence for clusters of only a few spins, while the highest fluence specimens were clearly ferromagnetic, with magnetization curves resembling those of a set of randomly oriented soft magnetic planes. The critical fluence for formation of a ferromagnetic state appears to be between 1 and 3 x 1016 atoms/cm2 at 25 keV. These results can be qualitatively understood based on the critical density for percolation of near neighbor exchange interactions.
INTRODUCTION In recent years the modification of crystallographically layered materials by intercalation of foreign species, principally by chemical means, has been the subject of intense interest. Many species, however, cannot be intercalated by chemical means. Ion implantation offers an alternative method of inserting foreign species into host materials. For a variety of reasons it cannot really be considered equivalent to chemical intercalation. For ions implanted into polymers and graphite, one of the common results is an amorphous surface layer (1-3) which typically occurs for fluences greater than 1-5 x 101s ions/cm2. The purpose of the present work was to explore the magnetic properties of iron implanted into graphite and the polymers PE and PVF,. Because of the typically strong nature of the exchange coupling between iron spins, even in amorphous materials, it was expected that the magnetic propeties would provide useful information about the structure and separation of the implanted atoms.
EXPERIMENT The samples were prepared by implantation at a fixed energy of 25 keV normal to the graphite face or free standing polymer films. The penetra&ion depth of iron atoms in graphite at that energy is expecSed to be 147 A. The l/e width of the distribution is expected to be 47 A, assuming no sputtering atothe surface. The penetration into PE and PVF, is approximately 500 A. Samples were prepared with fluence levels of 1, 3, 6, and 10 x 1016 atoms/cm2. Peak concentrations of the iron atoms in the HOPG, for example, were expected to be 6.5, 17.3, 29.5, and 41.1 atomic percent, respectively. The ion implanted samples were removed in an inert atmosphere and sealed in quartz tubes under vacuum. Magnetic measurements were made using a vibrating sample magnetometer and a superconducting solenoid. For experimental reasons the data were limited to temperatures less than 150 K.
Mat. Res. Soc.Symp. Proc. Vol. 27 (1984) @Elsevier Science Publishing Co., Inc.
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HOPG: 1017 Fc Z
T =2
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0 H
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-5.0 MAGNETIC
0.0 FIELD
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FIGURE 1. Magnetization vs. applied magnetic field a
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