Even-Odd-Layer-Dependent Symmetry Breaking in Synthetic Antiferromagnets

TL;DR

This study investigates how the magnetic states of synthetic antiferromagnets depend on whether they have an even or odd number of layers, revealing symmetry-breaking phenomena and different evolution paths under external magnetic fields.

Contribution

The paper introduces a macrospin model and experimental validation showing layer-dependent symmetry breaking in synthetic antiferromagnets with varying numbers of layers.

Findings

Even-layered structures evolve symmetrically from antiferromagnetic to canted states.

Odd-layered structures start ferrimagnetic and break mirror symmetry at a critical field.

Distinct magnetic evolution paths depend on layer parity.

Abstract

In this work we examine synthetic antiferromagnetic structures consisting of two, three, and four antiferromagnetic coupled layers, i.e., bilayers, trilayers, and tetralayers. We vary the thickness of the ferromagnetic layers across all structures and, using a macrospin formalism, find that the nearest neighbor exchange interaction between layers is consistent across all structures for a given thickness. Our model and experimental results demonstrate significant differences in how the magnetostatic equilibrium states of even and odd-layered structures evolve as a function of the external field. Even layered structures continuously evolve from a collinear antiferromagnetic state to a spin canted non-collinear magnetic configuration that is mirror-symmetric about the external field. In contrast, odd-layered structures begin with a ferrimagnetic ground state; at a critical field, the ferrimagnetic ground state evolves into a non-collinear state with broken symmetry. Specifically, the magnetic moments found in the odd-layered samples possess stable static equilibrium states that are no longer mirror-symmetric about the external field after a critical field is reached. Our results reveal the rich behavior of synthetic antiferromagnets.