Anisotropic Superparamagnetism of Monodispersive Cobalt-Platinum Nanocrystals

Based on the high-temperature organometallic route (Sun et al. Science 287, 1989 (2000)), we have synthesized powders containing CoPt_3 single crystals with mean diameters of 3.3(2) nm and 6.0(2) nm and small log-normal widths sigma=0.15(1). In the entire temperature range from 5 K to 400 K, the zero-field cooled susceptibility chi(T) displays significant deviations from ideal superparamagnetism. Approaching the Curie temperature of 450(10) K, the deviations arise from the (mean-field) type reduction of the ferromagnetic moments, while below the blocking temperature T_b, chi(T) is suppressed by the presence of energy barriers, the distributions of which scale with the particle volumes obtained from transmission electron microscopy (TEM). This indication for volume anisotropy is supported by scaling analyses of the shape of the magnetic absorption chi''(T,omega) which reveal distribution functions for the barriers being also consistent with the volume distributions observed by TEM. Above 200 K, the magnetization isotherms M(H,T) display Langevin behavior providing 2.5(1) mu_B per CoPt_3 in agreement with reports on bulk and thin film CoPt_3. The non-Langevin shape of the magnetization curves at lower temperatures is for the first time interpreted as anisotropic superparamagnetism by taking into account an anisotropy energy of the nanoparticles E_A(T). Using the magnitude and temperature variation of E_A(T), the mean energy barriers and 'unphysical' small switching times of the particles obtained from the analyses of chi''(T,omega) are explained. Below T_b hysteresis loops appear and are quantitatively described by a blocking model, which also ignores particle interactions, but takes the size distributions from TEM and the conventional field dependence of E_A into account.Comment: 12 pages with 10 figures and 1 table. Version accepted for publication in Phys. Rev. B . Two-column layou

Investigation by Mössbauer spectroscopy and small angle neutron scattering of the precipitation process in Fe76 Mn12.5 Co8.5 Mo3

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SUPERPARAMAGNETIC-PARAMAGNETIC TRANSITION IN SMALL PARTICLES

The paramagnetic state and the superparamagnetic regime of small particles of the granular system Fe64Cr16B20-Al2O3 have been studied by mean of magnetization and susceptibility and compared to the properties of the corresponding bulk metal. The evolution of the magnetic structure near and above Tc is the same for both compounds. Deductions upon the particle size have been attempted

Mössbauer effect evidence for disordering induced by mechanical alloying in the Fe-Si system

Based on X - ray diffraction (XRD) patterns, differential scanning calorimetry (DSC) investigations, the phase transitions induced by mechanical alloying (MA) on the Fe rich side of the Fe - Si system has been studied. Starting from a mixture of Fe and Si powders, MA leads to an expansion (up to 27.5 at. % Si) of the A2 crystalline disordered solid solution phase domain. In this composition field, an amorphous phase is also detected. The expansion of the disordered A2 phase domain up to 27.5 at.% Si has been confirmed by Mössbauer spectroscopy investigations. An annealing of the mechanically alloyed powders at 800°C for 48 hours leads exactly to the thermodynamically stable structures consistent with the sample stoechiometry. A high coercive force value of 17.2 kA/m at 1 kHz frequency and 0.15 T magnetic induction was reported for the 9.5 at. % Si

Magnetocrystalline anisotropy of YFe11−xCoxTiC

International audienc

Effect of carbonation on the structure and magnetic properties of YFe11−xCoxTiCy alloys

International audienc