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 γ = 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.
Highlights
When planar structures are extended to 3D by changing their curvature, the magnetic behavior of the corresponding surface can change significantly, leading for example to the appearance of new magnetic configurations that are not energetically favorable in the planar case[1,2] and to the induction of anisotropic and chiral effects that merely originate from the curvature of the surface[3,4,5,6,7]
Different theoretical and simulation approaches have been used in order to study the thermodynamic properties of magnetic hollow nanocylinders, including micromagnetic studies[15,45], scaling techniques[46,47], many-body Green’s functions[48,49,50], ab-initio calculations[51,52,53], and Monte Carlo (MC) simulations[54]
Direction onto a cylindrical geometry in order to get a zig-zag ended tube that can be characterized by Nz layers of rings stacked along the z axis with N spins per layer
Summary
When planar structures are extended to 3D by changing their curvature, the magnetic behavior of the corresponding surface can change significantly, leading for example to the appearance of new magnetic configurations that are not energetically favorable in the planar case[1,2] and to the induction of anisotropic and chiral effects that merely originate from the curvature of the surface[3,4,5,6,7]. This observation indicates that zig-zag tubes with spins in a vortex configuration are minimum energy states, whereas the formation of tubes with AA stacking (consecutive rings staked onto each other, with spins forming columns) is not energetically favored by dipolar interactions.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
