Size-dependent magnetic parameters of fcc FePt nanoparticles: Applications to magnetic hyperthermia

For nominal 3 and 9 nm FePt nanoparticles coated with oleylamine/oleic acid and having a face-centred-cubic (fcc) structure, temperature variations (5-300 K) of magnetization M, ac susceptibility χ′ and χ″ for the frequency range f_m = 0.1-1000 Hz and electron magnetic resonance (EMR) spectra at 9.28 GHz are reported. X-ray diffraction of the samples shows fcc structure with a lattice constant a = 3.84 and TEM characterization yields log-normal distributions of the particle sizes with average D = 3.15(0.16) nm and D = 8.70(0.12) nm for the 3 nm and 9 nm samples, respectively. M versus T data for the zero-field-cooled and field-cooled modes yield a blocking temperature T_B = 15 K (85 K) for the 3 nm (9 nm) samples whereas the hysteresis loops at 5 K yield a coercivity H_c = 0 Oe (1.4 kOe). Analysis of the data of T_B at different f_m determined from the peaks in χ″ in ac susceptibility and the temperature variation of the EMR spectra are used to determine the following parameters of the Vogel-Fulcher relaxation for the 3 nm (9 nm) samples respectively: the attempt frequency f_o = 8 × 10¹⁰ Hz (2 × 10 ¹² Hz); inter-particle interaction temperature T_o = 3 K (33 K) and anisotropy K_a = 1.96 × 10⁶ ergs cm ^-3 (4.3 × 10⁵ ergs cm^-3). The use of the above parameters for the calculations of the optimum size for magnetic hyperthermia is analysed and discussed.