Estimating the Relative Concentration of Superparamagnetic and Stable Single Domain Particles in Geological, Biological, and Synthetic Materials

Obtaining an estimate of the relative proportion of superparamagnetic (SP) to stable single-domain (SSD) particle sizes in a material can be useful in evaluating environmental conditions in natural materials, or in understanding the homogeneity of particle size and the degree of agglomeration in synthesized particles. Frequency dependent magnetic susceptibility is one of the most common methods used to identify SP particles in a material. The ability to detect SP particles, however, will be dependent on the field frequencies that can be applied. This study is concerned with evaluating three methods to estimate the SP content in a mixture of SSD and SP magnetite. We examine the use of the Day-Dunlop plot, first-order reversal curves (FORC) and principal component analysis (PCA), and the relationship between the reversible and irreversible magnetization as methods to evaluate qualitatively the relative contributions of SSD and SP magnetite in a material. Two series of mixtures of coated nanoparticles with a mean diameter of 20 and 11 nm are used as the SP end member and magnetosomes or intact magnetotactic bacterium of Magnetospirillum gryphiswaldense as the SSD end member. The Day-Dunlop plot tracks the progressive change in hysteresis properties with growing SP concentration. PCA of FORC data is sensitive in detecting differences in the SP component, when the SP particle size are not too small; otherwise the ratio between the reversible and irreversible magnetization can better assess differences. The results from the series are used to evaluate the relative SP content in three further sets of samples: biological tissue, synthetic nanoparticles, and samples from natural environments, to assess the strengths and weaknesses in each approach.

First Order Reversal Curve Study of SmFe2 Melt-Spun Ribbons

First-order reversal curves (FORC) and the FORC distribution provide a detailed characterization of the relative proportions of reversible and irreversible components of the magnetization of a material, revealing the dominant interactions in the system. Alloys with the nominal composition SmFe2 were obtained by melt-spinning with a cooper wheel velocity of 30 m/s. X-ray powder diffraction analysis showed a greater part consisting of an amorphous phase and a very small amount of SmFe2 crystalline phase with an average crystallite size of 8 nm. A constant acceleration Mössbauer spectrum, measured at room temperature in transmission mode, was fitted to a continuous distribution of effective fields at the nucleus of the amorphous phase (about 84% of the total area), plus two sextets for the non-equivalent sites of Fe in the SmFe2 crystalline phase. 91 first-order reversal curves were collected in a Quantum Design PPMS-VSM with reversal fields from –800 mT to +800 mT and using a calibration field of 850 mT. The obtained FORC diagrams showed a combined effect of a local interaction field and a mean interaction field, and showed that the reversible magnetization is a function of both, the applied magnetic field and the irreversible magnetization.