Innovative Method to Detect Magnetic Fields of Nanoscale Particles

As if they were bubbles growing in a recently-opened champagne bottle, minute circular regions of magnetism can be quickly enlarged to offer a precise technique of measuring the magnetic properties of nanoparticles.

The method, exposed by scientists at the National Institute of Standards and Technology (NIST) and their collaborators, offers a deeper insight into the magnetic behavior of nanoparticles. As the technique is rapid, economical, and does not need special environments — measurements can happen at room temperature and under atmospheric pressure, or even in liquids — it offers manufacturers a practical way to measure and enhance their control of the properties of magnetic nanoparticles for a multitude of environmental and medical applications.

Magnetic nanoparticles can act as miniature actuators, magnetically pushing and pulling other small objects. Depending on this property, researchers have used the nanoparticles to clean up chemical spills and build and work nanorobotic systems. Magnetic nanoparticles even possess the prospect to treat cancer — quickly reversing the magnetic field of nanoparticles injected into a tumor produces sufficient heat to destroy cancer cells.

Individual magnetic nanoparticles produce magnetic fields similar to the north and south poles of familiar bar magnets. These fields form magnetic bubbles — flat circles with preliminary diameters less than 100 nm (billionths of a meter) — on the surface of a magnetically sensitive film created at NIST. The bubbles surround the nanoparticle pole that points opposite to the direction of the magnetic field of the film. Although they encrypt information about the magnetic orientation of the nanoparticles, the miniature bubbles are not easily spotted with an optical microscope.

However, like bubbles in champagne, the magnetic bubbles can be stretched to hundreds of times their original diameter. By applying a small external magnetic field, the team enlarged the diameter of the bubbles to tens of micrometers (millionths of a meter) — sufficiently big to view with an optical microscope. The brighter signal of the enlarged bubbles quickly exposed the magnetic orientation of individual nanoparticles.

After establishing the preliminary magnetic orientation of the nanoparticles, the scientists used the enlarged bubbles to monitor the variations in that orientation as they applied an external magnetic field. Recording the strength of the external field required to flip the north and south magnetic poles of the nanoparticles exposed the magnitude of coercive field, an important measure of the magnetic stability of the nanoparticles. This vital property had formerly been challenging to measure for separate nanoparticles.

Samuel M. Stavis of NIST and Andrew L. Balk, who carried out most of his research at the Los Alamos National Laboratory and NIST, together with colleagues at NIST and the Johns Hopkins University, explained their findings in a new issue of Physical Review Applied.

The team studied two types of magnetic nanoparticles — rod-shaped particles composed of a nickel-iron alloy and irregularly shaped particle clusters composed of iron oxide. The applied magnetic field that expanded the bubbles has a similar role to play as the pressure in a bottle of champagne, Balk said. Under high pressure, when the champagne bottle is corked, the bubbles are basically nonexistent, just as the magnetic bubbles on the film are very small to be spotted by an optical microscope when no external magnetic field is applied. When the cork is opened and the pressure reduces, the champagne bubbles expand, just as the external magnetic field puffed up the magnetic bubbles.

Each magnetic bubble exposes the orientation of the magnetic field of a nanoparticle at the instant that the bubble developed. To explore how the orientation differed with time, the scientists produced thousands of new bubbles per second. In this manner, the researchers measured variations in the magnetic orientation of the nanoparticles at the moment that they happened.

To improve the sensitivity of the method, the scientists altered the magnetic properties of the film. Specifically, the team tuned the Dzyaloshinskii-Moriya (DMI) interaction, a quantum-mechanical occurrence that enforces a twist in the bubbles within the film. This twist decreased the energy required to form a bubble, offering the high sensitivity needed to measure the field of the smallest magnetic particles in the research.

Other techniques to measure magnetic nanoparticles, which necessitate working in a vacuum chamber, cooling with liquid nitrogen, or measuring the field at only one location, do not allow such fast determination of nanoscale magnetic fields. With the new method, the team quickly imaged the magnetic fields from the particles over a large area at room temperature. The improvement in convenience, speed, and flexibility allows new experiments wherein scientists can track the behavior of magnetic nanoparticles instantly, such as during the assembly and working of magnetic microsystems with lots of parts.

The research is the latest example of a continuing effort at NIST to create devices that enhance the measurement capabilities of optical microscopes, an instrument available in several labs, said Stavis. This allows fast measurement of the properties of single nanoparticles for both major research and for engineering of nanoparticle, he explained.

A tiny magnetic rod is placed over a strip of magnetic film. This nanorod has a particular magnetic orientation, and a fringe field that interacts with the film, creating a bubble-shaped area where the direction of magnetism is reversed. By applying a second magnetic field, researchers can change the magnetic orientation of the nanorod, causing the magnetic bubble to shift from one end of the rod to the other. Measuring the location of the bubble can give scientists insight into the geometry and magnetic properties of the nanorod, and reveal if it is alone or clustered with other nanoparticles. (Credit: S. Kelley/NIST)

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