Change in the magnetic configurations of tubular nanostructures by tuning dipolar interactions

TL;DR

This study uses atomistic Monte Carlo simulations to explore how varying dipolar and exchange interactions affect the magnetic states of nanotubes, revealing stable helical configurations and phase transitions influenced by geometry.

Contribution

It introduces a detailed phase diagram of magnetic states in nanotubes based on dipolar and exchange interaction ratios, highlighting the stability of helical states.

Findings

Helical states are stable at intermediate interaction ratios.

Phase diagrams depend on nanotube aspect ratio.

Metastability of helical structures affects reversal modes.

Abstract

We have investigated the equilibrium states of ferromagnetic single wall nanotubes by means of atomistic Monte Carlo simulations of a zig-zag lattice of Heisenberg spins on the surface of a cylinder. The main focus of our study is to determine how the competition between short-range exchange (J) and long-range dipolar (D) interactions influences the low temperature magnetic order of the nanotubes as well as the thermal-driven transitions involved. Apart from the uniform and vortex states occurring for dominant J or D, we find that helical states become stable for a range of intermediate values of g = D=J that depends on the radius and length of the nanotube. Introducing a vorticity order parameter to better characterize helical and vortex states, we find the pseudo-critical temperatures for the transitions between these states and we establish the magnetic phase diagrams of their stability regions as a function of the nanotube aspect ratio. Comparison of the energy of the states obtained by simulation with those of simpler theoretical structures that interpolate continuously between them, reveals a high degree of metastability of the helical structures that might be relevant for their reversal modes.