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[en] Semi-adiabatic approximation which accounts fairly well for the macroscopic fluctuational magnetodynamics of an isolated superparamagnetic particle in the intermediate-to-low temperature range is employed to describe ZFC and FC magnetization tests
[en] A new magnetic adsorbent PAMAMG_3-Fe_3O_4/P(GMA-AA-MMA) was prepared by growing third generation poly(amido) amine (PAMAMG_3) on the surface of superparamagnetic polymer microspheres Fe_3O_4/P(GMA-AA-MMA). The maximum sorption capacity of U(VI) (395.2 mg g"-"1) on the sorbent was much higher than those of U(VI) on many other reported materials. The sorption of U(VI) on the magnetic adsorbent obeyed the Langmuir mode. The selectivity of the sorbent could reach up to 91.8 % at pH 4.5. Furthermore, PAMAMG_3-Fe_3O_4/P(GMA-AA-MMA) could also be reused at least five times without any noticeable loss of its sorption capacity. (author)
[en] The exchange bias effect in nano-objects is modelled by adding to the magnetic energy two terms allowing for two kinds of anisotropy, viz. a fixed-axis and a rotatable one. The signature of the second contribution is an isotropic shift of the ferromagnetic resonance field. The temperature dependence of HRA that is due to the superparamagnetic theory is obtained in the framework of kinetic theory. Our analysis predicts that the temperature behaviors of are qualitatively different depending on the possible symmetry type of the rotatable anisotropy energy term: unidirectional or uniaxial.
[en] Dynamic magnetic hysteresis in a nanoparticle is treated with the aid of Brown's kinetic equation. The case in question is the one where the period of the external field falls in between the short intrawell and long interwell magnetic relaxation times. Dependencies of the hysteresis loop on the frequency and amplitude of the field and on temperature are found and analyzed
[en] Nuclear magnetic relaxation dispersion profile representing the evolution of the longitudinal nuclear magnetic relaxation rate of water protons is a valuable tool for the characterization of superparamagnetic nanoparticles. The analysis of this 'relaxation spectrum' unravels some important physical parameters of the superparamagnetic crystals as their size and specific magnetization and is, therefore, a powerful method to monitor magnetic filtration of magnetic colloids
[en] Complete text of publication follows. Although magnetic hysteresis is usually modeled as a set of equilibrium curves with jumps at critical fields, jumps will always occur before the critical field is reached because of thermal fluctuations. The probability of a jump at a given field depends on the network of connections between stable states by way of saddle points. The most difficult part of modeling such connections is finding the saddle points. A new method finds all the saddle points using a special tool for finding roots of polynomial systems. All equilibrium states are found and classified as stable, saddle, or other, and a two-stage solver finds the downhill paths from each saddle point to the nearest minima. A master equation is solved to get the time dependence of the magnetic moment. This method is applied to a chain of magnetostatically interacting single-domain particles such as those in magnetotactic bacteria. If there are N particles in a chain, there are up to 2N saddle points. The number of saddle points decreases until there are at most two for particles in contact with each other. In zero field, the relaxation mode for chains of 5 or more particles is a new mode that I call the two-domain fanning mode. This mode has only a limited domain of stability and is replaced by a symmetric fanning mode in larger fields. If the critical size for the transition to superparamagnetism is expressed as the cube root of the volume, it approaches about 10 nanometers as the number of particles increases, independent of the shapes of the particles.
[en] A direct probe of superparamagnetism was used to determine the complete anisotropy energy distribution of Co nanoparticle films. The films were composed of self-assembled lattices of uniform Co nanoparticles of 3 or 5nm in diameter, and a variable temperature scanning-SQUID microscope was used to measure temperature-induced spontaneous magnetic noise in the samples. Accurate measurements of anisotropy energy distributions of small volume samples will be critical to magnetic optimization of nanoparticle devices and media
[en] One-dimensional core–shell nanostructures consisting of a ferromagnetic cobalt core and a multiferroic BiFeO_3 (BFO) shell were fabricated by an artificial two-step methodology. The coupling between the ferromagnetic core and multiferroic shell manifests a significant exchange bias effect which gives a clear demonstration of the anti-ferromagnetic functionality of the BFO shell material. Exchange biases of 30 Oe and 60 Oe are observed at 300 K and at 5 K, respectively. Superparamagnetic contributions at lower temperatures play an important role in contributing to overall magnetic behavior. Dominant shape anisotropy causes parallel alignment of the easy magnetization axis along the axis of core–shell nanowires. A coherent mode of the magnetization reversal mechanism is observed by the angular dependence of coercivity (H _c). This versatile two-step methodology can be employed to fabricate and investigate many other hybrid nanostructures leading to a vast scope of investigation for researchers. (paper)