Magnetic insights

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A new MRI technique has been developed to allow physicists to see deep within tiny magnets. The technique could improve our understanding of magnetism at the fundamental level and lead to better computer hard drives and perhaps even new small-scale MRI instruments.

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In work funded by the US Department of Energy, project leader Chris Hammel, an Ohio Eminent Scholar in Experimental Physics at Ohio State University and colleagues Yuri Obukhov, a research associate, Thomas Gramila, associate professor of physics, Denis Pelekhov, a research scientist in the university's Institute for Materials Research, post-doctoral researcher Palash Banerjee, together with graduate students Jongjoo Kim and Sanghun An are working with Ivar Martin, Evgueni Nazaretski and Roman Movshovich of Los Alamos National Laboratory (LANL) and Sharat Batra of Seagate Research, the research and development division of hard drive manufacturer Seagate Technologies. They hope to reveal what there is to see within a ferromagnet.

"The magnets we study are basically the same as a refrigerator magnet, only much smaller," explains Hammel. In a recent issue of Physical Review Letters, Hammel and colleagues report the first-ever magnetic resonance image of the inside of an extremely tiny magnet. The disk-shaped magnets in their study measured a mere two micrometres in diameter.

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"Because ferromagnets generate such strong magnetic fields, we cannot study them with standard MRI," Hammel adds, "A related technique, ferromagnetic resonance, or FMR, would work, but it is not sensitive enough to study individual magnets that are this small." Instead, the team has, in the parlance of hybrid computing technologies and popular music, "mashed up" three different analytical technologies: MRI, FMR, and AFM (atomic force microscopy) to allow them to observe the inner secrets of their microscopic magnets.

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Success with this hybrid technique on such a scale bodes well for creating much smaller magnetic storage devices. Additionally, the approach could open up a whole new area of medicine to MRI that is not amenable to conventional instrumentation. For instance, it could be possible to image the microscopic structure of the thin layers of arterial and brain plaques that form in the body. This new approach to magnetic resonance could eventually become a tool for biomedical research.

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The researchers in contravention of decades of acronymic naming of NMR techniques have called their technique the unpronounceable SPFMRFM, for scanned probe ferromagnetic resonance force microscopy. The approach involves detecting a magnetic signal using a tiny silicon bar with an even tinier magnetic probe on its tip.

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Hammel explains that as this probe passes over a material, it captures a bowl-shaped image: a curved cross-section of an object. The magnetic signal is more intense in the middle, the "bottom" of the bowl; the signal fades towards the edges of the bowl. The researchers admit that this may sound like a peculiar experimental setup for any of the techniques, but it is this unique configuration that makes the technique work.

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Standard, large-scale MRI machines get around the problem of localising the radio signal by varying the magnetic field by precise amounts as it sweeps over a sample, organ, or patient. The computer controlling the MRI knows that where the magnetic field equals X, the location equals Y. Sophisticated software combines the data, and this produces the familiar three-dimensional view of conventional MRI.

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Until now, there was no way to scan objects as small as the tiny magnets in Hammel's work, moreover, there was no known method for precise localization. The new probe system generates a magnetic field that varies naturally, so that the physicists found that they could sweep the probe over an array of magnets and obtain a flat two-dimensional view similar to a medical MRI, but without depth. They were able to produce an image with a resolution of 250 nanometres.

With this technique available to them, Hammel and his team are now recording the properties of many different kinds of tiny magnets, which they explain is a critical first step towards developing them for novel computer magnetic storage devices. Indeed, researchers in computing have suggested that magnetic storage could one day be incorporated into a computer's central processing unit (CPU) chip rather than it being contained in a standalone hard drive unit. Such a chip could give us boot-free computers that switch on as quickly as a modern television and are ready to operate instantly because system data could be recorded on the CPU rather than it having to be accessed independently and loaded into random access memory.