[Paper Review] Mechanical magnetometry of Cobalt nanospheres deposited by focused electron beam at the tip of ultra-soft cantilevers

Hugo Lavenant, V. V. Naletov+4 more|arXiv (Cornell University)|Jan 1, 2014

Advanced Electron Microscopy Techniques and Applications45 references23 citations

TL;DR

This study demonstrates mechanical magnetometry of individual cobalt nanospheres, grown via focused electron beam-induced deposition (FEBID) at the tips of ultra-soft cantilevers, to measure their magnetic hysteresis. By tracking field-dependent shifts in the cantilever's mechanical resonance frequency, the authors extract a saturation magnetization of 430 emu/cm³, independent of particle size, and identify the magnetic vortex state as the equilibrium configuration at remanence.

ABSTRACT

Using focused-electron-beam-induced deposition, Cobalt magnetic nanospheres with diameter ranging between 100 nm and 300 nm are grown at the tip of ultra-soft cantilevers. By monitoring the mechanical resonance frequency of the cantilever as a function of the applied magnetic field, the hysteresis curve of these individual nanospheres are measured. This enables to evaluate their saturation magnetization, found to be around 430 emu/cm^3 independently of the size of the particle, and to infer that the magnetic vortex state is the equilibrium configuration of these nanospheres at remanence.

Motivation & Objective

• To develop a method for in-situ integration of high-quality magnetic nanospheres at the tip of ultra-soft cantilevers for use in magnetic force microscopy (MFM) and magnetic resonance force microscopy (MRFM).
• To characterize the magnetic properties of individual Co nanospheres (100–300 nm diameter) grown via focused-electron-beam-induced deposition (FEBID) on ultra-soft cantilevers.
• To determine the saturation magnetization and magnetic configuration (e.g., vortex state) of these nanospheres at remanence using mechanical detection techniques.
• To assess the feasibility of using FEBID-grown Co nanospheres as high-sensitivity, high-resolution probes for future MRFM and MFM applications.

Proposed method

• Focused-electron-beam-induced deposition (FEBID) was used to grow Co nanospheres (nominal radii: 100, 200, 300 nm) directly at the apex of ultra-soft silicon nitride cantilevers with spring constants < 0.01 N/m.
• The cantilevers were fabricated using a dual-beam Focused Ion Beam (FIB) system at the Universidad de Zaragoza, with Co2(CO)8 as the precursor and optimized beam parameters (5 kV, 25–50 pA) to achieve ~75% Co purity.
• Magnetic characterization was performed via mechanical magnetometry: the cantilever's resonance frequency was monitored as a function of applied magnetic field using laser deflection and phase-locked loop feedback.
• Measurements were conducted in a vacuum chamber (P < 10⁻⁵ mbar) with an electromagnet, using a magnetic cylinder to generate field gradients and enhance sensitivity.
• The frequency shift was attributed to magnetic torque on the nanomagnet, enabling extraction of the hysteresis loop and saturation magnetization via calibration and modeling.
• Micromagnetic simulations were performed using experimentally derived parameters to interpret the observed hysteresis behavior and confirm the presence of a vortex state.

Experimental results

Research questions

• RQ1 What is the saturation magnetization of FEBID-grown Co nanospheres (100–300 nm) when attached to ultra-soft cantilevers?
• RQ2 What is the equilibrium magnetic configuration of these nanospheres at remanence, and how does it depend on particle size?
• RQ3 Can mechanical magnetometry via frequency shift measurements on ultra-soft cantilevers provide quantitative magnetic characterization of individual nanomagnets?
• RQ4 How does the magnetic hysteresis behavior of Co nanospheres compare to that of bulk or reference magnetic materials?
• RQ5 To what extent can FEBID be optimized to improve the Co content and magnetic quality of 3D nanomagnets for MRFM applications?

Key findings

• The saturation magnetization of the Co nanospheres was measured to be 430 ± 80 emu/cm³, independent of particle size (100–300 nm), indicating consistent magnetic quality across the size range.
• The hysteresis loops of the nanospheres exhibit weak hysteresis and linear behavior at low fields, with saturation fields of ~1.8 kOe, consistent with the measured magnetization.
• Micromagnetic simulations confirm that the magnetic vortex state is the ground state at remanence, with characteristic magnetization jumps observed at ±0.1 kOe and ±0.4 kOe fields, indicating vortex core nucleation and reversal.
• The 100 nm particle exhibits a more square hysteresis loop than the 300 nm particle, suggesting stronger finite-size effects on magnetization reversal dynamics.
• The Co content of the nanospheres is currently ~75% at., which is below bulk Co (100%); optimization of FEBID parameters (e.g., beam current, dwell time, vibration control) could improve purity and magnetization.
• The results demonstrate that FEBID-grown Co nanospheres on ultra-soft cantilevers are viable, high-sensitivity probes for future MRFM and MFM applications with sub-100 nm spatial resolution.