Appl. Phys. A 74 [Suppl.], S710–S712 (2002) / Digital Object Identifier (DOI) 10.1007/s003390201574 Applied Physics A Materials Science & Processing Neutron depolarisation in ferrofluids during magnetising/demagnetising cycles V.N. Zabenkin 1,∗ , L.A. Axelrod 1 , G.P. Gordeev 1 , W.H. Kraan 2 , I.M. Lazebnik 1 , D.N. Orlova 1 , A.A. Vorobiev 1,3 1 Petersburg Nuclear Physics Institute, 188300, Gatchina, Russia 2 Interfacultair Reactor Institute, TU-Delft, 2629 JB Delft, The Netherlands 3 Max-Planck-Institut für Metallforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany Received: 18 July 2001/Accepted: 13 November 2001 –  Springer-Verlag 2002 Abstract. 3-D neutron depolarisation data were taken from fer- rofluids of several concentrations along magnetic loops with the field both parallel and perpendicular to the neutron direction. The data taken at a concentration of 10 vol % of Fe 3 O 4 indi- cate a strong dependence of the organisation of the ferrofluid particles on the magnetic history over several cycles of the field. PACS: 71.15.Dx; 75.50.Mm Ferrofluids (FF’s) consist of ferromagnetic nanoparticles (e.g. Fe 3 O 4 ) surrounded by an organic surfactant to avoid cluster- ing, dispersed in some fluid (e.g. H 2 O). They are of techno- logical interest in bearings, during the production of record- ing media and in certain types of printing processes. The or- ganisation of the nanoparticles is determined by a very subtle interplay between applied magnetic field, gravity, and vel- ocity field (if present), therefore distinctive structures have been observed near surfaces of the FF and in its bulk [1–3]. A parameter which plays a decisive role, is the nano- particle concentration. Basically, two states of particle or- ganisation are distinguished, referred to as the “drop” model and the “chain” model [4, 5]. It is a very intriguing question, whether at sufficiently high concentration of FF particles, their distribution over space might give rise to “frustration” of dipolar interactions as is found in spin glass systems. Neutron techniques (reflection, SANS,depolarisation) can contribute to understanding FF structures, because FF samples up to 10 mm thickness can readily be transmitted by neutrons. The present experiment aimed to study the FF’s nanoparticles structure which may arise at several concentra- tions under influence of a small applied magnetic field (up to 200 Oe), both parallel and perpendicular to gravity. 1 Experimental setup and samples Our experimental technique is full 3D analysis of the po- larisation vector of a polarised neutron beam (λ = 0.23 nm) ∗ Corresponding author. (Fax: +7-812/713-9023, E-mail: zabenkin@mail.pnpi.spb.ru) after transmission through the FF. To magnetise the FF in rizontal direction (parallel to neutron fly direction), we use an electromagnet with horizontal field,having permendur poles (diameter 45 mm; gap 14 mm), each containing a hol (diameter 24 mm) for the neutron beam (diameter 3 mm) (s Fig. 1). The field profile as a function of position along the beam (x-axis) had FWHM equal to 20 mm; the variation of the field over the sample thickness (5 mm) was less than 2% in the directions perpendicular to the beam (y- and z-axis; see Fig. 1) the variation was less than 0.2% over the beam diameter. To magnetise along the z-axis, we used a solenoid placed around the sample holder. The flux was short circuited in a yoke of amorphous ribbon, pressed against the top and bo tom of the sample holder. FF’s of concentration (vol % of Fe 3 O 4 ) equal to 4, 5, 7 and 10% were prepared at PNPI. In all samples D 2 0 was used as dispersive medium. The Fe 3 O 4 particle diameter (measured from electromicrographs of dry powder and light scattering from FF) was 10 nm; as surfactant sodium aleat was used. Y D 1 D 2 X Z Si FF K o C C Fig. 1. Magnetand geometry of the experiment. C: coil; D 1 , D 2 : dia- phragms; K o is the wave vector of ingoing neutrons