title: Size-dependent nonlinear weak-field magnetic behavior of maghemite nanoparticles.
abstract: The magnetic behavior at room temperature of maghemite nanoparticles of variable sizes (from 7 to 20 nm) is compared using a conventional super quantum interference device (SQUID) and a recently patented technology, called MIAplex. The SQUID usually measures the magnetic response versus an applied magnetic field in a quasi-static mode until high field values (from -4000 to 4000 kA m(-1)) to determine the field-dependence and saturation magnetization of the sample. The MIAplex is a handheld portable device that measures a signal corresponding to the second derivative of the magnetization around zero field (between -15 and 15 kA m(-1)). In this paper, the magnetic response of the size series is correlated, both in diluted and powder form, between the SQUID and MIAplex. The SQUID curves are measured at room temperature in two magnetic field ranges from -4000 to 4000 kA m(-1) (-5T to 5T) and from -15 to 15 kA m(-1). Nonlinear behavior at weak fields is highlighted and the magnetic curves for diluted solutions evolve from quasi-paramagnetic to superparamagnetic behavior when the size of the nanoparticles increases. For the 7-nm sample, the fit of the magnetization with the Langevin model weighted with log-normal distribution corresponds closely to the magnetic size. This confirms the accuracy of the model of non-interacting superparamagnetic particles with a magnetically frustrated surface layer of about 0.5 nm thickness. For the other samples (10-nm to 21-nm), the experimental weak-field magnetization curves are modeled by more than one population of magnetically responding species. This behavior is consistent with a chemically uniform but magnetically distinct structure composed of a core and a magnetically active nanoparticle canted shell. Accordingly the weak-field signature corresponds to the total assembly of the nanoparticles. The impact of size polydispersity is also discussed.
authors.affiliation: CSPBAT laboratory, UMR 7244 CNRS, University of Paris 13, 93017 Bobigny, France.
authors.forename: Caroline
authors.initials: C
authors.lastname: de Montferrand
authors.forename: Yoann
authors.initials: Y
authors.lastname: Lalatonne
authors.forename: Dominique
authors.initials: D
authors.lastname: Bonnin
authors.forename: Nicole
authors.initials: N
authors.lastname: Lièvre
authors.forename: Marc
authors.initials: M
authors.lastname: Lecouvey
authors.forename: Philippe
authors.initials: P
authors.lastname: Monod
authors.forename: Vincent
authors.initials: V
authors.lastname: Russier
authors.forename: Laurence
authors.initials: L
authors.lastname: Motte
mesh_terms: D000465:Algorithms; D001709:Biotechnology / Q000379:methods*; D004563:Electrochemistry / Q000379:methods; D005290:Ferric Compounds / Q000737:chemistry*; D057446:Hydrodynamics; D008280:Magnetics; D053768:Metal Nanoparticles / Q000737:chemistry; D046529:Microscopy, Electron, Transmission / Q000379:methods; D053758:Nanoparticles / Q000737:chemistry*; D036103:Nanotechnology / Q000379:methods; D055095:Optics and Photonics; D010316:Particle Size; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D005290:Ferric Compounds; C000499:ferric oxide