[Paper Review] Direct-write of free-form 3D nanostructures with controlled magnetic frustration

L. Keller, Mohanad Al Mamoori+10 more|arXiv (Cornell University)|Sep 18, 2017

Advanced Condensed Matter Physics53 citations

TL;DR

The paper demonstrates direct-write fabrication of freestanding 3D ferromagnetic nanostructures using FEBID, characterizes their microstructure, and analyzes magnetization switching and magnetic frustration via micro-Hall measurements and micromagnetic/macro-spin simulations.

ABSTRACT

Building nanotechnological analogues of naturally occurring magnetic structures has proven to be a powerful approach to studying topics like geometry-induced magnetic frustration and to provide model systems for statistical physics. Moreover, it practically allows to engineer novel physical properties by realizing artificial lattice geometries that are not accessible via natural crystallization or chemical synthesis. This has been accomplished with great success in the field of two-dimensional artificial spin ice systems with important branches reaching into the field of magnetic logic devices. Although first proposals have been made to advance into three dimensions (3D), established nanofabrication pathways based on electron beam lithography have not been adapted to obtain free-form 3D nanostructures. Here we demonstrate the direct-write fabrication of freestanding ferromagnetic 3D nano-architectures with full control over the degree of magnetic frustration. By employing micro-Hall sensing, we have determined the magnetic stray field generated by our free-form structures in an externally applied magnetic field and we have performed micromagnetic and macro-spin simulations to deduce the spatial magnetization profiles in the structures and analyze their switching behavior. Furthermore we show that the magnetic 3D elements can be combined with other 3D elements of different chemical composition and intrinsic material properties.

Motivation & Objective

• Motivate and enable 3D nanomagnetic architectures to study geometry-induced magnetic frustration and novel magnetic states.
• Develop and validate a FEBID-based route to fabricate free-form 3D magnetic nano-structures with controlled composition.
• Characterize microstructure, composition, and magnetic switching using complementary experimental and simulations.
• Explore integration of 3D magnetic elements with different materials for complex 3D magnetic systems.

Proposed method

• Use FEBID with precursor HCo3Fe(CO)12 to directly write 3D Fe-Co/Cobalt-rich nanostructures on micro-Hall sensors.
• Characterize microstructure and composition with TEM, EELS, and EDXS to identify metallic core and oxide shell (core-shell model).
• Measure magnetic stray fields with micro-Hall magnetometry during external field sweeps at various angles.
• Perform macro-spin simulations to reproduce switching behavior and interpret features in Hall curves.
• Conduct micromagnetic simulations with both all-metal and core-shell (Co metal core, Co2FeO4 shell) scenarios to understand discrepancies with macro-spin results.
• Demonstrate 3D arrays and the possibility to replace magnetic vertices with non-magnetic material to build 3D lattices.

Experimental results

Research questions

• RQ1 Can direct-write FEBID fabricate freestanding 3D magnetic nano-architectures with controlled magnetic frustration?
• RQ2 How do the microstructure and composition (core-shell vs. pure metal) influence magnetization switching and stray-field signatures in 3D nanostructures?
• RQ3 To what extent do macro-spin and micromagnetic models reproduce the observed switching behavior across different field orientations?
• RQ4 Can 3D magnetic elements be integrated into composite 3D architectures or arrays, including non-magnetic vertex substitutions, for artificial spin-ice-like studies?

Key findings

• FEBID enables direct writing of freestanding 3D Fe-Co/Cobalt-rich nano-structures with detectable magnetic frustration effects.
• Deposits are crystalline and consist of a metallic Co-Fe core with a ferrimagnetic oxide shell, affecting magnetic behavior.
• Micro-Hall measurements reveal complex stray-field hysteresis that depends on the external field angle; macro-spin models capture many features, but micromagnetic simulations are needed for detailed agreement in some cases.
• Core-shell magnetic structure (Co3Fe core / Co2FeO4 shell) yields good agreement with experimental stray-field data across multiple angles, especially for challenging angles where macro-spin alone fails.
• Macro-spin models provide a scalable first-pass interpretation of switching, while micromagnetic simulations validate and refine understanding, highlighting the role of near-surface oxide in real samples.
• Demonstrations include 3D nano-tree and nano-cube geometries, with potential to replace vertices by non-magnetic segments to form 3D arrays for Ising/Heisenberg-like studies.