Field-induced phase transitions in ferro-antiferromagnetic diblock copolymers

TL;DR

This paper explores the phase behavior of magnetic diblock copolymers under external magnetic fields, revealing complex phases and hybrid states through mean-field and Monte Carlo methods, advancing understanding of field-controlled self-assembly.

Contribution

It introduces a minimal model combining magnetic interactions and spatial organization, demonstrating rich phase diagrams and hybrid phases not previously characterized.

Findings

Identification of multiple distinct phases including swollen, mixed, and segregated states.

Quantitative agreement between mean-field theory and Monte Carlo simulations.

Discovery of a hybrid 'tadpole' phase with coexisting extended and collapsed blocks.

Abstract

We study the equilibrium properties of a model of magnetic diblock copolymer where each monomer is decorated with an Ising-like spin. Spins interact ferromagnetically within each block and antiferromagnetically across blocks, generating frustration between magnetic ordering and spatial organization. By employing a mean-field approach and Monte Carlo simulations for self-avoiding walks on the cubic lattice, we investigate the system's response to an external magnetic field. We discover a rich phase diagram that includes: a swollen phase with both filaments magnetically disordered and spatially extended; a mixed compact phase characterized by a single globule in which the two filaments are strongly intertwined; a segregated compact phase composed of two globular, magnetically ordered, and spatially separated blocks. Further, if the magnitude of the intra-block ferromagnetic interaction differs between the two blocks, we observe a hybrid segregated (``tadpole'') phase where one extended block coexists with a collapsed one. Mean-field predictions are in quantitative agreement with Monte Carlo results for the location of the phase boundaries. These findings provide a minimal statistical-mechanical framework for field-controlled self-assembly of tunable patterns by magnetically heterogeneous polymers. They may also serve as a simple platform to investigate the coupling between internal epigenetic-like states and chromatin folding.