Depletion-driven antiferromagnetic, paramagnetic, and ferromagnetic behavior in quasi-two-dimensional buckled colloidal solids

We investigate quasi-two-dimensional buckled colloidal monolayers on a triangular lattice with tunable depletion interactions. Without depletion attraction, the experimental system provides a colloidal analog of the well-known geometrically frustrated Ising antiferromagnet [Y. Han et al., Nature 456, 898-903 (2008)]. In this contribution, we show that the added depletion attraction can influence both the magnitude and sign of an Ising spin coupling constant. As a result, the nearest-neighbor Ising spin interactions can be made to vary from antiferromagnetic to para- and ferromagnetic. Using a simple theory, we compute an effective Ising nearest-neighbor coupling constant, and we show how competition between entropic effects permits for the modification of the coupling constant. We then experimentally demonstrate depletion-induced modification of the coupling constant, including its sign, and other behaviors. Depletion interactions are induced by rod-like surfactant micelles that change length with temperature and thus offer means for tuning the depletion attraction in situ. Buckled colloidal suspensions exhibit a crossover from an Ising antiferromagnetic to paramagnetic phase as a function of increasing depletion attraction. Additional dynamical experiments reveal structural arrest in various regimes of the coupling-constant, driven by different mechanisms. In total, this work introduces novel colloidal matter with magnetic features and complex dynamics rarely observed in traditional spin systems.

Automated summary

In this study, we investigate the effects of depletion interactions on quasi-two-dimensional buckled colloidal monolayers. The results show that depletion attraction can modify the magnitude and sign of the Ising spin coupling constant, leading to a variation in the nearest-neighbor Ising spin interactions from antiferromagnetic to para- and ferromagnetic. By using a simple theory, we calculate an effective Ising nearest-neighbor coupling constant and demonstrate experimentally the depletion-induced modification of the coupling constant, including its sign and other behaviors. Additionally, we observe a crossover from an Ising antiferromagnetic to paramagnetic phase with increasing depletion attraction, and structural arrest in different regimes of the coupling constant driven by different mechanisms.