Size Effect Study of the L1 0 Phase Transformation in FePt Nanoparticles
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Size Effect Study of the L10 Phase Transformation in FePt Nanoparticles Yi Ding and Sara A. Majetich Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213-3890, U.S.A. ABSTRACT The relation between particle size and anisotropy in the fcc to L10 phase transformation in FePt nanoparticles is described. After annealing to partially sinter the particles, the volume distribution was found by transmission electron microscopy, and related to the magnetic switching field distribution. With the assumption that larger particles have higher switching fields, we find a consistent size threshold, d*, of about 12 nm for high anisotropy, independent of the annealing conditions. Our analysis shows that the phase transformation is limited by the lack of nucleation sites in small particles rather than by the chemical inhomogeneity.
INTRODUCTION FePt nanoparticle arrays have attracted much interest in recent years, because of their potential use in high density magnetic storage media [1]. However, as-made the particles are in the fcc phase, and the particles must be annealed to transform the FePt into the L10 phase. In the L10 phase, there are alternating layers of Fe and Pt atoms, leading to a small tetragonal distortion and a large increase in the magnetocrystalline anisotropy, K, and coercivity, Hc. While annealing, the particles sinter together, destroying the advantage of uniformity in the size and spacing. The annealing process changes K and V simultaneously, making it difficult to determine whether small nanoparticles in arrays can have coercivities large enough for use in data storage media. So far, high coercivities have only been observed in samples containing larger particles [2-12]. We developed an over-coat method to prevent the sintering during annealing [13]. However, even after annealing at 660°C for as long as 10 hours, Hc was only around 250 Oe for 8 nm FePt nanoparticles. A fundamental size threshold for phase transformation has been suggested, along with limitations due to chemical inhomogeneity [14], or kinetics [15]. To understand the connection between particle size and anisotropy, we studied the size distribution, and correlated it to the switching field distribution.
EXPERIMENTAL 8 nm FePt nanoparticles were synthesized by a variation of standard high temperature chemical methods [1]. The surfactant coating was a 1:1 molar ratio mixture of oleic acid and oleylamine. The amount of surfactant was varied to control the particle size. With a 10:1 molar surfactant to metal precursor ratio, we obtained 8 nm particles. The particles were then washed with ethanol, centrifuged, and redispersed in hexane several times to improve the monodispersity. The average chemical composition of particles was determined by calibrated xray fluorescence (XRF) to be Fe53Pt47.
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Monolayers of the particles were prepared and annealed to sinter the particles to different degrees. Si3N4 membrane transmission electron microscopy (TEM) grids were used as substrates. The particles were first dissolved in 1
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