Magnetic Force Microscopy (MFM)

2.5 microns image of flux-closure self-assembled Co dots on a W(110) surface. [FRU2013]

Overview

Magnetic Force Microscopy (MFM) is a scanning-probe technique derived from Atomic Force Microscopy. Information about a magnetic sample is gathered through its interaction with a magnetic tip, in most cases an AFM tip capped with a few tens of nanometers of a magnetic material. MFM presents a good compromise between spatial resolution (50nm routine), versatility and cost.

Measurement scheme

MFM is most efficiently performed in a so-called oscillating scheme, where the cantilever holding the magnetic tip is driven mechanically close to its resonant frequency. In oscillating mode the phase shift is indicative of the force gradient along the oscillation direction, be it of topographic, magnetic or electric origin. MFM is often operated in a so-called double-pass : in a first pass the tip apex is in strong interaction with the sample surface (eg hard-sphere repulsion), revealing the topography. In a second pass the tip ’flies’ at a given height above the surface, canceling topographic forces, and thus revealing other forces only (such as magnetic).

Vortex head-to-head domain wall in a 15nm-thick permalloy stripe of width 500nm.

Applications

We are applying magnetic force microscopy to various systems, for various physical purposes :

- Magnetization processes inside domain walls and vortices, with physics such as domain-to-vortex transition, or switching of internal degrees of freedom.

- Field- and current-induced domain-wall motion in stripes and wires

- Dots and nanoparticles, tracking magnetization switching and dipolar interactions.

- Permanent magnets, to identify the microscopic origin of coercivity and design ways to improve their performance.

Illustration of our current move towards high spatial resolution : 2x2 microns image of 4nm-thick FePt film with out-of-plane magnetization. Resolution here is 15nm.

Our specific developments

We have been operating an NT-MDT microscope NTegra-Aura since 2008. The microscope is equipped with static magnetic field of to 250mT in-the-plane (built-in), and above 1T out-of plane (home-designed). We have also developed small coils and power electronics to deliver bursts of several Tesla (both in-plane and out-off-plane), for the sake of performing magnetization processes in permanent magnets. We use both commercial tips, and custom-developed ones for specific applications, such as ultra-low stray field for very soft magnetic materials, or sharp tips for reaching a high spatial resolution.

Some publications

- Growth and micromagnetism of self-assembled epitaxial fcc(111) cobalt dots, O. Fruchart, A. Masseboeuf, J. C. Toussaint, P. Bayle-Guillemaud, J. Phys. Condens. Matter. 25, 496002 (2013).

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