Anisotropic superparamagnetism of monodispersive cobalt-platinum nanocrystals

Abstract

Based on the high-temperature organometallic route (Sun et al. Science 287, 1989(2000), we have synthesized powders containing CoPt3 single crystals with mean diameters of 3.3(2) nm and 6.0(2) nm and small log-normal widths σ=0.15(1). In the entire temperature range from 5 K to 400 K, the zero-field cooled susceptibility χ(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 , χ(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 χ ′′ (T, ω) 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) µB per CoPt3 in agreement with reports on bulk and thin film CoPt3. 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 EA(T ). Using the magnitude and temperature variation of EA(T ), the mean energy barriers and 'unphysical' small switching times of the particles obtained from the analyses of χ ′′ (T, ω) 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 EA into account.

Size-induced enhanced magnetoelectric effect and multiferroicity in chromium oxide nanoclusters

“…1b,c). Very much like the magnetization isotherms of assemblies of monodispersive magnetic nanocrystals 18 , the resistance loop then contains both reversible and irreversible contributions.…”

Antiferromagnetic dipolar ordering in[Co2MnGe∕V]Nmultilayers

“…Experimentally the magnetization isotherms fall progressively below the commonly supposed Langevin function. Essentially we attribute this behaviour to cluster interactions [215] and to anisotropy of the clusters [216]. Furthermore the clusters probably exhibit a size distribution leading to deviations from Eq.…”

Spin wave dynamics in two- and three-dimensional superlattices of nanosized ferromagnetic spheres

“…By the application of new technologies it is possible to produce self-assembled two-dimensional ͑2D͒ and three-dimensional ͑3D͒ superlattices with various and well defined long-range translational order and with only small deviations from the average size of nanoparticles. 1,2,4 We find a considerably different effect of the dipolar interaction in 2D and 3D superlattices and calculate the specific parameters of the 3D hexagonal-closepacked ͑hcp͒ system. Due to the competition between crystalline anisotropy within every particle and the dipolar interaction between the particles, the SW dispersion suggests a phase transition from the initial ferromagnetic ͑FM͒ order into probably 6 a helicoidal order.…”

“…10. Based on existing experimental specimen preparation methods, 2 we choose the effective distance between particles to be 2 nm. Therefore, although the principal results only depend on the ratio R / L ͑with L being the distance between the centers of the spheres͒, we based our concrete calculations on the explicit case of nanospheres with R = 5 nm and L = 12 nm.…”

“…This barrier primarily depends on the value of the nanoparticle anisotropy constant and there are already considerable achievements concerning the enhancement of this factor. 2 The second parameter that changes the energy barrier ͑and, consequently, the relaxation time and the blocking temperature͒ is the interparticle interaction of dipolar origin. Much attention has been paid to this question, both theoretically and experimentally.…”