Magnetic properties of FePt nanoparticles

We have prepared FePt nanoparticles with concentrations in the vicinity of the stoichiometric Fe50Pt50 composition by means of a gas-phase-based process. The preparation technique employed allows to anneal ('sinter') the particles in the gas-phase prior to their deposition. Depending on the gas-pressure during preparation, the structure of the unsintered particles is icosahedral or polycrystalline face-centered cubic (FCC). Temperature-dependent magnetization measurements reveal that these particles are superparamagnetic at room temperature. Gas-phase sintering of polycrystalline FCC FePt nanoparticles results in the formation of predominantly single crystal particles which are face-centered tetragonal due to an increased degree of L1(0) order at elevated sintering temperatures T-S. Concurrently, we observe an increase of both the blocking temperature T-B and the coercivity H-C with increasing T-S. This enhancement of T-B and H-C is more pronounced in iron-rich off-stoichiometric samples, for which the concentrations are still within the L1(0) stability range of the FePt phase diagram. FC62Pt38 particles, which are sintered at T-S = 1273 K, reach T-B = 530 K and H-C0 = H-C(T = 0) = 4.7 kOe. Whereas the elevated blocking temperature is mainly due a somewhat increased particle size at elevated sintering temperatures, the higher coercivity is attributed to the enhanced degree of L1(0) order in the gas-phase sintered particles. (C) 2003 Elsevier B.V. All rights reserved.