Tuneable hysteresis loop and multifractal oscillations of magnetisation in weakly disordered antiferromagnetic–ferromagnetic bilayers

Abstract

Coupled antiferromagnetic–ferromagnetic bilayers have been intensively investigated as low-dimensional memory materials. However, the connection between their architecture and emergent hysteresis loop phenomena remains elusive. Here, we revealed this relation through low-temperature simulations of the field-driven Ising spin reversal dynamics in heterostructures of coupled ferromagnetic and antiferromagnetic layers of varied thicknesses and a weak random-field disorder. The hysteresis loop exhibits the fractional-magnetisation plateaus, where their number, the height of the central loop, and the structure of side sub-loops strictly depend on the antiferromagnetic layer thickness. Meanwhile, the interlayer coupling chiefly determines the coercive field values, modified by the magnetic disorder, the thickness of the ferromagnetic layer and the system size, in agreement with the derived theoretical formula. The magnetisation fluctuations are modulated with peaks at the transitions between successive plateaus, reflecting the active groups of spins with different levels of (anti)ferromagnetic couplings arising at the interplay of antiferromagnetic sublattices and disorder. The cyclic trend drives the magnetisation fluctuations within limited time intervals; multifractal fluctuations occur on larger time scales. These findings shed new light on the tuneable hysteresis loop properties and the related magnetisation fluctuations in thin antiferromagnetic–ferromagnetic bilayers. The described hysteresis-loop phenomena are not limited to these model systems but should be present in different antiferromagnetic materials with complex morphology. • The AFM-FM bilayer architecture determines the emergent hysteresis-loop properties. • Simulations of the magnetisation-reversal processes reveal the hysteresis-loop shape and the coercive fields. • Magnetisation fluctuations on the hysteresis loop exhibit cycles and multifractality. • Cycles indcate distinct spin groups activated by the field, geometry and weak disorder.