Ucko, Daniel Hans; (2001) Magnetic relaxation in nanoscale granular alloys. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Magnetic nanoscale granular alloys are of current interest from both a fundamental and a technological viewpoint. At a fundamental level, nanocrystalline magnetic materials exhibit fine-particle magnetic behaviour that is significantly different from the bulk behaviour. At a practical level, they are used as nanocrystalline soft magnetic alloys for electromagnet cores and transformers, and for magnetoresistive sensors and devices. Primarily to address the fundamental issues, a systematic series of magnetic relaxation measurements were performed on a suite of nanostructured iron-silver and iron-copper-silver alloys. Several experimental techniques (DC Magnetisation, Vibrating Sample Magnetometry, AC Susceptibility, Mossbauer Spectroscopy and Muon Spin Relaxation) were applied in order to observe superparamagnetic relaxation at a wide range of measurement timescales. Structural tests by X-ray Diffractometry and Rietveld analysis showed that the samples had grain sizes of 4 - 20 nm, and dispersion fractions of magnetic material of 5 - 40 % of the whole sample volume. Superparamagnetic relaxation transitions or blocking temperatures were observed to be a function of measurement time ?m. For a low concentration sample, Fe12Cu09Ag79, the blocking temperatures, after correction to compensate for the particle size distribution and interaction effects, were found to vary from 28 K to 69 K over the measurement time range 100 s to 0.1 ms. For more concentrated samples, e.g. Fe52Ag48, blocking temperatures at all timescales were higher than room temperature. From the relaxation behaviour, it was concluded that the magnetic grains experience strong inter-particle interactions that raise the effective energy barrier against spontaneous magnetic relaxation. These were shown to be mainly dipolar and RKKY in nature.

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