Static and dynamic correlations in magnetic nanomaterials studied by small angle neutron scattering techniques

Abstract

Abstract. The performance of new techniques developed for Small Angle Neutron Scattering (SANS) is illustrated by investigations on magnetic colloids. Polarised neutrons ("SANSPOL") are introduced as a special type of contrast variation for magnetic systems. Examples of diluted magnetic systems are reviewed where low magnetic contrasts had to be analysed beside strong nuclear contributions or vice versa. In Ferrofluids magnetic core-shell composite particles and magnetic aggregates could be precisely evaluated beside non-magnetic micelles and free surfactants of similar sizes. In more concentrated Ferrofluids an external magnetic field induces a pseudo-crystalline ordering which coexists with chain like arrangements of particles. Ordering and relaxation processes of magnetic moments in nanoparticles have been monitored by time-resolved SANS. Slow relaxation of magnetic particle moments onto equilibrium has been studied after switch off a static field. In stroboscopic experiments, time-frame data acquisition has been synchronized with a periodic external magnetic field. 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. The analysis of time-dependent SANS data as a function of frequency, field and temperature allowed i) to determine the underlying statistics describing the particle moment orientation, ii) to extract the effect of field-induced inter-particle 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 work out the possible relaxation mechanisms (Néel and Brownian).

Magnetic small-angle neutron scattering

“…In particular, the approach of combining micromagnetic and SANS simulations complements experiments, which provide a weighted sum of Fourier components, a fact which often hampers the straightforward interpretation of experimental SANS data. While it is in principle possible to determine some Fourier coefficients separately, e.g., through the application of a saturating magnetic field or by exploiting the neutron polarization degree of freedom via SANSPOL or POLARIS methods (e.g., Honecker et al, 2010 andWiedenmann, 2010), it is difficult to unambiguously determine a particular scattering contribution without "contamination" by unwanted Fourier components. For instance, when the applied field is not large enough to completely saturate the sample, then the scattering of unpolarized neutrons along the field direction does not represent the pure nuclear SANS, but contains also the magnetic SANS due to the misaligned spins (Bischof et al, 2007).…”

Analysis of magnetic neutron-scattering data of two-phase ferromagnets

“…[4][5][6][7] The measurable quantity in a magnetic SANS experiment-the (energyintegrated) macroscopic differential scattering cross section d /d -depends on the Fourier coefficients of M(r). These Fourier coefficients M(q) depend in a complicated manner on the magnetic interactions, the underlying microstructure (e.g., particle-size distribution and crystallographic texture), and on the applied magnetic field.…”

Magnetic Small-Angle Neutron Scattering

“…The standard references for nuclear (nonmagnetic) SANS and SAXS are the well-known textbooks by Guinier and Fournet (1955), Glatter and Kratky (1982), Feigin and Svergun (1987), Svergun et al (2013), and Gille (2014). For reviews on various topics of small-angle scattering, for instance, on polymers, disordered and porous materials, colloidal systems, ferrofluids, magnetic materials, superconductors, ceramics, biological structures, and precipitates in metallic alloys and composites, see Schmatz et al (1974), Jacrot (1976), Gerold and Kostorz (1978), Higgins and Stein (1978), Schelten and Hendricks (1978), Chen and Lin (1987), Martin and Hurd (1987), Bates (1988), Chen et al (1988), Hayter (1988), Page (1988), Kostorz (1991Kostorz ( , 2014, Schmidt (1991), Pedersen (1997), Wiedenmann (2002Wiedenmann ( , 2010, Fratzl (2003), Svergun and Koch (2003), Thiyagarajan (2003), Fitzsimmons et al (2004), Radlinski et al (2004), Stuhrmann (2004), Allen (2005), Wagner and Kohlbrecher (2005), Wignall and Melnichenko (2005), Fritz and Glatter (2006), Melnichenko and Wignall (2007), Michels and Weissmüller (2008), Avdeev and Aksenov (2010), Hammouda (2010), Eskilds...…”

“…In particular, the approach of combining micromagnetic and SANS simulations complements experiments, which provide a weighted sum of Fourier components, a fact which often hampers the straightforward interpretation of experimental SANS data. While it is in principle possible to determine some Fourier coefficients separately, e.g., through the application of a saturating magnetic field or by exploiting the neutron-polarization degree of freedom via SANSPOL or POLARIS methods [see, e.g., Honecker et al (2010) and Wiedenmann (2010)], it is difficult to unambiguously determine a particular scattering contribution without "contamination" by unwanted Fourier components. For instance, when the applied field is not large enough to completely saturate the sample, then the scattering of unpolarized neutrons along the field direction does not represent the pure nuclear SANS, but contains also the magnetic SANS due to the misaligned spins (Bischof et al, 2007).…”