AFM Cantilever Magnetometry for Measuring Femto-Nm Torques Generated by Single Magnetic Particles for Cell Actuation

TL;DR

This paper introduces a method using Atomic Force Microscopy to measure femto-Newton-meter torques generated by single magnetic nanoparticles, enabling precise mechanobiology applications like cell membrane rupture.

Contribution

Developed a straightforward AFM-based technique to quantify the torque of single synthetic antiferromagnetic nanoparticles with high magnetic anisotropy.

Findings

Measured torque of 1.6±0.6×10⁻¹⁵ Nm on a SAF nanoplatelet.

Demonstrated the method's ability to estimate forces capable of rupturing cell membranes.

Showcased tunability of magnetic anisotropy for controlled mechanical stimuli.

Abstract

Particles with high anisotropy in their magnetic properties and shape are of increasing interest for mechanobiology, where transducing a remotely applied magnetic field vector to a local mechanical response is crucial. An outstanding challenge is quantifying the mechanical torque of a single nanoparticle, typically in the range of atto- to femto-Newton-meters (Nm). The magneto-mechanical torque manifests due to a misalignment of the external magnetic field vector with the built-in magnetic anisotropy axis, as opposed to a magnetic force, and complicates the measurement scheme. In this work, we developed a method using a commercially available Atomic Force Microscopy setup and cantilevers to quantify the torque generated by a single synthetic antiferromagnetic (SAF) nanoplatelet with high perpendicular magnetic anisotropy. Specifically, we measured 1.6$\pm$0.6$\cdot$10$^{-15}$ Nm torque while applying 373$\pm$5 mT field at 12$\pm$2 degrees to the built-in anisotropy axis exerted by a single circular SAF nanoplatelet with 1.88 $\mu$m diameter and 72 nm thickness, naively translating to a $\approx$ 1.7 nN maximum force at the nanoplatelet apex. This measured torque and derived force of the SAF nanoplatelets is strong enough for most applications in mechanobiology; for example, it can be used to rupture (cancer) cell membranes. Moreover, SAF nanoplatelets open a route for easy tuning of the built-in magnetic anisotropy and size, reducing the torque and allowing for small mechanical stimuli for ion channel activation. This work presents a straightforward and widely applicable method for characterizing magnetic particles' mechanical transduction, which is applied to SAF nanoplatelets with a high PMA.