[Paper Review] Direct writing of CoFe alloy nanostructures by focused electron beam induced deposition from a heteronuclear precursor

Fabrizio Porrati, Merlin Pohlit+6 more|arXiv (Cornell University)|Jun 24, 2015

Advanced Materials Characterization Techniques11 references3 citations

TL;DR

This study demonstrates direct fabrication of CoFe alloy magnetic nanostructures via focused electron beam induced deposition (FEBID) using a single heteronuclear precursor, HFeCo₃(CO)₁₂. The method yields high-metal-content deposits (up to 84 at% metal, ~80 at% CoFe phase) with ferromagnetic behavior up to 250 K, enabling high-resolution, direct-writing of functional magnetic nanostructures for spintronics applications.

ABSTRACT

Recently, focused electron beam induced deposition has been employed to prepare functional magnetic nanostructures with potential in nanomagnetic logic and sensing applications by using homonuclear precursor gases like Fe(CO)5 or Co2(CO)8. Here we show that an extension towards the fabrication of bi-metallic compounds is possible by using a single-source heteronuclear precursor gas. We have grown CoFe alloy magnetic nanostructures from the HFeCo3(CO)12 metal carbonyl precursor. The compositional analysis indicate that the samples contain about 80 at% of metal and 10 at% of carbon and oxygen. Four-probe magnetotransport measurements are carried out on nanowires of various sizes down to a width of 50 nm, for which the room temperature resistivity of 43~$μΩ$cm is found. Micro-Hall magnetometry reveals that 50~nm$ imes$250~nm nanobars of the material are ferromagnetic up to the highest measured temperature of 250 K. Finally, the TEM microstructural investigation shows that the deposits consist of a bcc Co-Fe phase mixed with a FeCo2O4 spinel oxide phase with nanograins of about 5 nm diameter.

Motivation & Objective

• To develop a direct-writing method for bi-metallic magnetic nanostructures using a single-source precursor.
• To overcome the limitations of dual precursor gas systems in FEBID, which suffer from poor compositional control and low metal content.
• To achieve high metal content and reproducible composition in CoFe alloy nanostructures via a heteronuclear carbonyl precursor.
• To characterize the magnetic, electrical, and microstructural properties of the fabricated nanostructures for spintronic applications.

Proposed method

• Employed the heteronuclear precursor HFeCo₃(CO)₁₂ for FEBID, enabling simultaneous deposition of Co and Fe in a single molecular source.
• Used a scanning electron microscope (SEM) with electron beam rastering to induce precursor decomposition and nanostructure growth.
• Conducted four-probe magnetotransport measurements on nanowires down to 50 nm width to assess electrical resistivity.
• Performed micro-Hall magnetometry on 50 nm × 250 nm nanobars to evaluate ferromagnetic behavior up to 250 K.
• Utilized transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) for phase and composition analysis.
• Applied Pt-C FEBID to protect samples during TEM preparation and to induce controlled crystallization for microstructural analysis.

Experimental results

Research questions

• RQ1 Can a single heteronuclear precursor enable the direct fabrication of CoFe alloy nanostructures with high metal content via FEBID?
• RQ2 What is the magnetic behavior of FEBID-grown CoFe nanostructures, and up to what temperature is ferromagnetism retained?
• RQ3 What are the microstructural phases present in the deposits, and what is the relative abundance of metallic Co-Fe versus oxide phases?
• RQ4 How does the elemental composition and resistivity of the nanostructures depend on the electron beam parameters and geometry?
• RQ5 What causes the characteristic wasp-waist shape in the hysteresis loop observed in micro-Hall magnetometry?

Key findings

• The FEBID process using HFeCo₃(CO)₁₂ produced CoFe alloy nanostructures with a metal content of up to 84 at%, significantly higher than typical FEBID deposits from dual precursors.
• Compositional analysis revealed approximately 80 at% metal (with Co:Fe ratio of 3:1), 10 at% carbon, and 10 at% oxygen, indicating a mixture of bcc Co-Fe phase and FeCo₂O₄ spinel oxide phase.
• Four-probe measurements showed a room temperature resistivity of 43 μΩ·cm for 50 nm-wide nanowires, indicating metallic behavior.
• Micro-Hall magnetometry confirmed ferromagnetic order in 50 nm × 250 nm nanobars up to 250 K, with a characteristic wasp-waist hysteresis loop.
• TEM analysis identified nanocrystalline bcc Co-Fe phase (dominant, ~72.5 at% of metal) and a minor FeCo₂O₄ spinel oxide phase (~7.5 at% of metal), with grain sizes around 5 nm.
• The wasp-waist hysteresis loop is attributed to a complex reversal mechanism involving both the multi-phase nature of the material and metastable magnetic states in confined geometries.