Size distribution of polymer-coated magnetite nanoparticles

A method for estimating the size distribution of magnetite nanoparticles from their magnetic properties is presented. The 10 nm diameter particles were coated with poly(acrylic) acid and prepared as a water-based suspension. A vacuum-dried sample was placed in a transmission electron microscope (TEM) so that the physical sizes of the particles could be estimated. The particle magnetization was measured by a superconducting quantum interference device (SQUID) in magnetic fields up to 25 kiloOersted and temperatures ranging from 5 to 370 Kelvin. The magnetic moments in the sample were estimated by fitting those measurements to a Langevin magnetization model, weighted by a log-normal distribution with unknown parameters.The best-fit procedure yielded particle volumes smaller than those observed by transmission electron microscopy, suggesting the existence of a magnetically inactive layer of atoms. In addition, our particles exhibited stronger spin-wave behavior than expected for particles of similar size, as evidenced by the lower saturation magnetization and higher Bloch coefficient.