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Updated: Jun 3, 2026

Magnetic Tweezers for the Measurement of Twist and Torque
Published on: May 19, 2014
Elastic torsion effects in magnetic nanoparticle diblock-copolymer structures
L Schulz1, W Schirmacher, A Omran
1Physik-Department E13, Technische Universität München, Garching, Germany.
Abstract:
Magnetic properties of thin composite films, consisting of non-interacting polystyrene-coated γ-Fe(2)O(3) (maghemite) nanoparticles embedded into polystyrene-block-polyisoprene P(S-b-I) diblock-copolymer films are investigated. Different particle concentrations, ranging from 0.7 to 43 wt%, have been used. The magnetization measured as a function of external field and temperature shows typical features of anisotropic superparamagnets including a hysteresis at low temperatures and blocking phenomena. However, the data cannot be reconciled with the unmodified Stoner-Wohlfarth-Néel theory. Applying an appropriate generalization we find evidence for either an elastic torque being exerted on the nanoparticles by the field or a broad distribution of anisotropy constants.
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