Orientational bistability and field-controlled switching of a superparamagnetic dimer

TL;DR

This study investigates the orientational bistability and field-controlled switching behavior of superparamagnetic dimers, revealing how combined induced and permanent magnetic moments influence their dynamic response under varying magnetic fields.

Contribution

It introduces a comprehensive analysis of the complex energy landscape and bistable dynamics of superparamagnetic dimers with both induced and permanent moments, including a method to characterize their internal magnetic structure.

Findings

Dimers exhibit bimodal steady-state orientation distributions in static fields.

Reversing the magnetic field induces a transition from small hops to full rotations.

The internal magnetic structure can be characterized by analyzing the orientational response.

Abstract

We study the orientational dynamics of superparamagnetic colloidal dimers that carry both an induced magnetic moment, proportional to the applied field, and an effective permanent moment. In a static, uniform magnetic field, dimers that are permanently fixed together hop between two preferred in-plane angles, developing a bimodal steady-state orientation distribution. When the same field is periodically reversed, we observe a sharp, field-controlled change in the dynamical response from small hopping events with $\Delta \theta\ll \pi$ to full $\Delta\theta \approx \pi$ rotations on each field flip. We show that both the static bistability and the switching bifurcation can be rationalised by a magnetic response in the dimer that consists of both a strong induced and weak body-fixed component. This leads to a complex orientational energy/potential landscape, with coupled roll-yaw rotations of the dimer responsible for the bistable dynamics. By combining the misorientation between dimer axis and field, bifurcation field strength and short-time orientational variance, we determine the magnitude and orientation of the net permanent dipole, thereby characterising details of the internal magnetic structure of the particles via microscopy.