Colloids and nanostructured magnetic materials

Small Angle Neutron Scattering (SANS) is a technique probing fluctuations of composition and magnetic moment at the nanoscale (1-100 nm). Available instrumentation at the HZB allows users to fully characterize their magnetic colloids, with size and shape determination, precise knowledge of interactions and super-structures formation with or without magnetic field, segregation of chemical composition and magnetic properties (core/shell systems), and dynamic response to periodic field stress leading to values for relaxation times down to 0,1ms. All experiments can be performed at any temperature ranging between ~1K and ~3000K for static measurements, and between ~30K and ~300K for kinetic measurements. In contrast to direct imaging techniques, a macroscopic sample is analyzed over its entire volume.

Magnetic colloids are stable dispersions of nanoscopic particles made of ferromagnetic materials, either solid (alloys) or liquid (ferrofluids, “FF”). In the latter case, nanoparticles are typically stabilized by surfactants to prevent irreversible aggregation using Van der Waals repulsion (in apolar medium), and electrostatic interactions (in water). The use of polymer coatings, allowing chemists to play with functionalities, is increasing. Biomolecules are used to improve tolerance inside living organisms.

Ferrofluids are of great importance in today's industry, with large amounts used as liquid hermetic seals in mechanical systems with moving parts (O-ring for computer hard drives), highly solicited mechanical dampers and heat conductor (loudspeakers), and other more specific applications using friction, optical, catalytic or rheological properties. The use of bio-compatible FF in hyperthermia therapy and magnetic-driven drug targeting also attracts considerable attention. Ferrogels, where polymers are added to a ferrofluid to get a physical (reversible) or chemical (persistent) gel are foreseen as artificial muscles.

The relation between the nanoscale (shape, size, polydispersity, coating, interactions) and macroscopic properties in various conditions of stress is the key to design improved FF, in particular through an in-depth understanding of structure formation (size, organization and reversibility of aggregates) explaining the aging and the mechanical behavior of these liquids.

Resolution of structures under field

Thanks to a large panel of sample environments, many stress conditions can be reproduced at the sample position, to study the effect of external forces on the organization of nanoparticles. This insight is invaluable as the non-destructive technique of neutron scattering gives a statistical average view of particles in the “bulk”, the geometry of the sample being about 100 000 times larger than the individual particles (no wall effect).

Variation of temperature, magnetic field and particle size on monodisperse magnetite-based FF demonstrated the formation of long 2-dimensional bands of spherical nanoparticles with hexagonal packing, both the distances between particles inside sheets and between sheets being resolved by SANS. As a definitive proof of previous cryo-TEM observations, where the required processing of the sample could have caused bias to the results, these findings could be explained by SANS qualitatively and quantitatively by depletion forces caused by the excess of free surfactant forming micelles.

Literature:

M. Klokkenburg et al., Phys. Rev. E 75 (2007) 051408

Combined contrast variation

SANS possesses two unique advantages when it comes to ferrofluids:

· The classical possibility of nuclear contrast-variation via isotopic substitution. This method utilizes the fact that hydrogen and deuterium reveal scattering length densities (SLD) almost at both extremities of the SLD scale, thus allowing to match the SLD of virtually any material by mixing H-based with D-based components.

· The magnetic sensibility of neutrons. This adds a magnetic SLD depending on the polarization of the incoming neutron beam.

Combining the analysis of the scattering of (+) and (-) polarized neutrons with an isotopic variation of the nuclear SLD allows to cover a wide range of contrast matching to identify uniquely all individual domains of a multiphasic system. A typical example is the study of a Co FF where the magnetic cobalt core, the non-magnetic oxidized CoO external layer, the surfactant coating, and the micelles from surfactant in excess could be unambiguously isolated.

Literature:

A. Wiedenmann et al., Phys. Rev. E 68 (2003) 031203

Time-resolved analysis of relaxation mechanisms

Due to relatively weak flux, until recently neutron scattering was considered as being unsuitable to probe fast relaxations processes. However, technical improvements allows short-time processes to be analysed, providing that they are repeatable. Acquisition is periodically reproduced until sufficient statistics is obtained.

The kinetics of ordering and relaxation processes in magnetic colloids can be studied by triggering the time and position dependent acquisition of neutrons at the detector by an oscillating magnetic field at the sample position. This stroboscopic technique gives access to millisecond resolution of the relaxation of FF. However, the wavelength distribution of the neutrons (typically 10% FWHM) leads to a time-of-flight spread, which limits the applicable sample field frequency to a few hundred Hz. This limitation can be overcome by adding a chopper in the neutron path. A careful timing of the chopper, sample and data acquisition frequencies considering the geometrical distances between these parts allows to group neutrons originating from different chopper pulses but scattered at the same magnetic field, avoiding the time-of-flight smearing. This technique, called TISANE, has successfully been tested at the NEAT instrument of HZB and at the ILL, and is currently being installed at the HZB.

Slow and fast relaxation processes probed on cobalt and magnetite FF at the HZB demonstrated the existence of large living structures, the volume fraction of which depends mainly on the temperature, with an arrested system observed well below the freezing temperature of the pure solvent.

Dynamical scattering intensities lead to the determination of volume fraction of mobile particles and the time-dependent value of a dynamical inter-particle structure factor, which scale with the square of the Langevin function. The alignment of particle moments along the applied field could therefore be assessed as governed by the fast Brownian rotation of individual nanoparticles and small aggregates, while the magnetic relaxation of longer dipolar chains (Co FF) and local hexagonal domains (magnetite FF), easily distinguished by the technique, was much slower.

Literature:

U. Keiderling and A. Wiedenmann, J. Appl. Cryst. (2007) 40, s62-s67

A. Wiedenmann et al., Phys. Rev. B 77 (2008) 184417

A. Wiedenmann et al., Phys. Rev. Lett. 97 (2006), 057202