Local force mapping of an NdFeB micro-magnet arrays using a custom-made Micro- particle MFM probe



Permanent NdFeB or SmCo micro-magnet arrays fabricated by a thermo-magnetic process [1] produce high

stray magnetic field gradients (up to 10e6 T/m). They have been implemented successfully into microfluidic

devices as a key component to separate magnetic and non-magnetic microspheres [2] as well as to trap bio-

logical cells tagged with magnetic nanoparticles [3]. In biology and medicine, the interest for this kind of

devices is growing because of their practical importance in stem cell research, cancer diagnostics and cell

therapeutics [3] for isolation and sorting of target cells from heterogeneous suspensions.

Since the magnetic force exerts on an object is determined by both its magnetic properties (moment and

volume) and the magnetic field it experiences, reliable magnetic force measurements at micro and nanoscale

are required to study the impact of the sample magnetic fields properties (intensity, homogeneity and spatial

extension) on the object according to its magnetic properties. In addition, as the magnetic tag can have toxicity

effects on living biological species, its optimization is crucial according to the design of the sorting device.

In this work, we report quantitative measurements of the forces exerted by a micro-magnet array on a mag-

netic microparticle, based on Magnetic Force Microscopy. Firstly, two methods were developed to fix a single

super-paramagnetic microparticle (polystyrene sphere functionalized with FeO nanoparticles) on the tip apex

of silicon AFM probe. The first method exploits AFM capabilities in frequency measurements, weak force

interaction detection and imaging, to adjust the quantity of glue on the tip and to localize, select and fix a

single magnetic microsphere to form a so-called particle probe [4]. The second method exploits capabilities

of a dual beam FIB/SEM machine equipped with a micromanipulator. As a result, smart MFM probes (fig.1)

where all the magnetic material concentrated in the microparticle, are obtained.

MFM measurements were then performed in air to localize the magnetic traps (i.e. the interfaces between

areas with initial and reversed orientation of magnetization) and to measure the force and its gradient induced

by the micro-magnets on the magnetic microsphere. For the MFM operation, a standard two-pass technique

has been used [5] in dynamic and in contact modes, where the offset elevation (often called “lift height”) was

varied from 300nm up to 3500nm. MFM maps obtained in dynamic mode reveal that the force gradient is

always positive while MFM maps obtained in contact mode reveal attractive forces with an intensity ranging

from 7- 25nN and 0.5-5nN, for microspheres of diameter 3.5um and 1.45um, respectively. These experiments

constitute the first direct and quantitative characterization of the magnetic interaction between a superpara-

magnetic microsphere and the micro-magnets [4]. Similar measurements have been carried out using ferro-

magnetic microparticles to study the impact of high magnetic moment on the trapping process.

Such MFM characterization combined with simulations can feed back into the design of micro-magnet arrays

and microfluidic devices developed for the trapping of objects functionalized with magnetic nanoparticles.

Figure 1. SEM image of a particle


Figure 2. Topography and MFM images of a micromagnet array

obtained with a 3.5um colloidal probe at a Lift Height of 500nm.

[1] F. Dumas-Bouchiat et al, “Thermomagnetically patterned micromagnets”, Appl. Phys. Lett. 96, 102511


[2] L. F. Zanini et al, “Autonomous micro-magnet based systems for highly efficient magnetic separation”,

Appl. Phys. Lett. 99, 232504 (2011).

[3] O. Osman et al, “Microfluidic immunomagnetic cell separation using integrated permanent micromag-

nets”, Biomicrofluidics 7, 054115 (2013).

[4] S. Ponomareva et al, “Measuring the force gradient acting on a magnetic microsphere above a micro-

magnet array”, Adv. Mater. Res. 872, 167–173 (2013).

[5] D. Rugar et al, “Magnetic force microscopy: General principles and application to longitudinal recording

media”, J. Appl. Phys. 68, 1169 (1990).

Short CV

Florence Marchi is associated professor in Physics at the University of Grenoble Alpes (France) since 2001

and leads her research activity at Néel Institute in the Nano Optics and Force group. She is a specialist of

force measurements at micro/nano-scale using mechanical prehensor and specially Atomic force Microscope.

Her present activity belongs on two main fields: Local force measurement on micro-magnet arrays by

AFM/MFM and interactive manipulation in 3D of objects at micro and nanoscale.

Since the beginning of her career, Florence Marchi has been involved in the development of

innovative practical works in Nanosciences and Nanotechnology.

More information is available:

Eingeladen von: Prof. Dr. Sergej Fatikow


22. Februar 2018 13:45


Florence Marchi, Grenoble Alpes University and Néel Institute/CNRS, Grenoble, FRANCE


A1 3-330

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