Abstract
In an external magnetic field, concentrated Co-ferrofluids form an inter-particle order revealing coexistence of a pseudo-crystalline hexagonal ordering of core-shell nanoparticles and segments of dipolar chains. The decay of this order after switching off the magnetic field is characterized by time constants of a few seconds. This relaxation behaviour has been investigated using a newly developed stroboscopic small-angle neutron scattering technique based on listmode data acquisition. This technique utilizes the fact that the process of creation and decay of the inter-particle order is reversible, thus allowing very good scattering statistics to be gained by stroboscopically repeating the process and superimposing the data from the individual cycles. This paper presents time constants determined for the decay of inter-particle order states created by different external magnetic fields. The decay is generally characterized by an exponential course, assigned to Brownian rotation of locally ordered domains, although in the very beginning it appears to take place faster, indicating a different decay mechanism. The time constants at fields > 0.1 T significantly increase, indicating a larger size of the ordered domains resulting from enhanced dipolar interactions.
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