Stroboscopic Small Angle Neutron Scattering Investigations of Microsecond Dynamics in Magnetic Nanomaterials

Time resolved Small Angle Neutron Scattering SANS techniques have recently been developed that allow ordering and relaxation processes of magnetic moments in nanoparticles to be monitored. In stroboscopic experiments, time frame data acquisition has been synchronized with a periodic external magnetic field. Slow relaxation of magnetic particle moments onto equilibrium has been studied in periods of the order of 30 s after switch off a static field. By applying a sine wave modulated magnetic field at frequencies above 50 Hz, the time resolved SANS response to a forced oscillation could be analyzed. When a continuous neutron flux was used in conventional SANS, the shortest accessible time range was limited to about 3 ms resulting from the wavelength spread. A breakthrough of time resolution into the micro second range was achieved with the pulsed frame overlap TISANE technique, which allows us to exploit a dynamical range similar to that of X ray photon correlation spectroscopy. Here we present a combination of these stroboscopic neutron techniques on surfactant stabilized ferrofluids with nearly monodisperse Cobalt and Fe3O4 nanoparticles. Results are compared to a solid CuCo alloy with superparamagnetic nanosized Cobalt precipitates. The SANS scattering response was measured stroboscopically in an oscillating applied magnetic field at frequencies up to 2800 Hz. As long as the magnetic moments followed the applied field, the 2D scattering patterns alternated between fully isotropic and strongly anisotropic. The analysis of time dependent SANS data as a function of frequency, field and temperature allowed us i to proof the validity of the Langevin statistics describing the particle moment orientation, ii to extract the effect of field induced interparticle correlations, iii to monitor the slowing down of the dynamics of moment rotation with decreasing temperature, iv to study the effect of freezing of the solvent on the dynamics of the particle moments, and v to decide between the possible relaxation mechanisms N el and Brownia