Preparation of magnetic nanoparticles by pulsed plasma chemical vapor synthesis

FePt nanoparticle is expected as a candidate for the magnetic material of the high density recording media. We attempted to synthesize FePt alloy nanoparticles using 13.56 MHz glow discharge plasma with the pulse operation of a square-wave on/off cycle of plasma discharge to control the size of nanoparticles. Vapors of metal organics, Biscyclopentadienyl iron (ferrocene) for Fe and (Methylcyclopentadienyl) trimethyl platinum for Pt, were introduced into the capacitively coupled flow-through plasma chamber, which consisted of shower head RF electrode and grounded mesh electrode. Synthesis experiments were conducted at room temperature under the conditions of pressure 0.27 Pa, source gas concentration 0.005 Pa, gas residence time 0.5 s and plasma powers 60 watts. Pulse width for plasma duration was chosen from 0.5 to 30 s and plasma off period was 4 s to each pulse operation. Visual observations during the particle growth showed plasma emission in the bulk region was increased with the particle growth. These were theoretically explained by using the model for both transient particle charging in the plasma and single particle behavior in the stationary plasma as well as assuming the similarity between the negative charged particle and negative gas containing plasma. Synthesized nanoparticles were directly collected onto TEM grid, which was placed just below the grounded mesh electrode in the plasma reactor downstream. TEM pictures showed two kinds of particles in size, one of which was nanometer size and isolated with crystal structures and the other appeared agglomerate of nanometer size particles. The size of agglomerated particle was controlled in the 10–120 nm range by varying the plasma-on time from 0.5 to 30 s, although the nanometer size particles did not change. The composition of FePt alloy particles could be altered by adjusting the source gas feed ratio. Also magnetization of FePt nanoparticles was measured by use of SQUID (superconducting quantum interference device) magnetometry measurements. As-synthesized FePt nanoparticles did not exhibit loop-shape characteristic, which indicated superpamagnetic property. Annealed nanoparticles with the composition of Fe58Pt42 at 650°C in atmospheric hydrogen showed clear hysterisis loop with the coercivity as large as 10 KOe.