Energy barrier distributions of maghemite nanoparticles

Abstract

A recently introduced method for the characterization of magnetic nanoparticles (MNPs) based on the analysis of the temperature-dependent Néel relaxation signal (TMRX) has been applied to characterize maghemite particles with different particle size distributions. The samples were made using an improved magnetic fractionation method for a ferrofluid with a broad particle size distribution. The temperature range of the measurement set-up has been extended from 315 K down to 4 K to detect even the smallest particles in the fractions. A mean magnetically relevant particle size has been derived from TMRX and low temperature coercivity measurements and has been compared to the physical size determined by atomic force microscopy (AFM) investigations.

Characterization of energy barrier and particle size distribution of lyophilized ferrofluids by magnetic relaxation measurements

“…In such a way we can not only reconstruct the energy barrier distribution of the investigated mixture of nanoparticles from the temperature-dependent measurements of the Ne´el relaxation signal, but also determine the particle size and the particle size distribution. In former investigations we have shown that the calculated particle size distribution for Co-and Fe 2 O 3 -based systems agreed very well with the direct physical measurement [8,9].…”

“…The original ferrofluid with a broad particle size distribution was fractionated in an inhomogeneous magnetic field using an electromagnet (Bruker B-E 10v) with variable coil current I Coil and a magnetic separation column (MACS XS, Miltenyi Biotech) [5,9]. The coil current was set to 1 A for the first fraction and successively decreased for the other fractions.…”

Hydrodynamic and magnetic fractionation of superparamagnetic nanoparticles for magnetic particle imaging

“…Therefore, the isolation of this large sized particle population responsible for the high MPI signal is of great interest. Several attempts were made to separate Resovist s magnetically [5,6] and characterize the obtained fractions with regard to MPI performance [7][8][9]. Furthermore, asymmetric flow field-flow fractionations (A4F) were performed to gain insight into the distribution of hydrodynamic sizes of Resovist s as this method gently produces fractions of narrow hydrodynamic size distribution and permits highly reproducible and accurate size evaluations [4,10,11].…”

Examination of magnetite nanoparticles utilising the temperature dependent magnetorelaxometry

“…The set-up for TMRX measurements has been described in detail elsewhere [12,13]. In brief the measurement set-up for the temperature dependent determination of the magnetic relaxation consists of a second order SQUID gradiometer to measure the magnetic relaxation signal and Helmholtz coils to magnetise the sample.…”

“…The TMRX method is based on the analysis of the dependence of the magnetic relaxation signal on the sample temperature which in special cases directly delivers the energy barrier distribution of the magnetic system. The magnetic properties of multi-particle systems are strongly influenced by the distribution density of the energy barriers E. To measure the distribution of these energy barriers over the course of temperature the TMRX method was developed [12,13]. In principle, the TMRX method measures a relaxation of a magnetic moment.…”