All That Matters

In the absence of GPS, a compass is the best option to find your way around. However, although the earth’s magnetic field is a great way to find your own position, doing the reverse, measuring magnetic fields with a high accuracy — on an atomic scale — remains a challenge. Sure, there are electron microscopes, which are great instruments that can image single atoms and other physical objects. However, when it comes to measuring magnetic fields, the achievable resolution is much worse than the size of an atom.

The schematic of a Lorentz microscope. Differently oriented magnetic domains deflect an electron beam in opposite directions, creating an image contrast at the border between magnetic domains.

The problem is even bigger in the fourth dimension, time, when we like to know how magnetic fields evolve over time on a microscopic scale. Then, the best resolution that can be achieved is about 500 times worse than what is possible in the imaging of atoms using state-of-the-art electron microscopes. The group of Ahmed Zewail at Caltech in Pasadena, California has now developed a technique that could significantly enhance the resolution of time-resolved measurements of magnetic fields.

There are of course a number of methods to measure a magnetic field with high spatial resolution. One is magnetic force microscopy, where essentially the tip of a magnetic needle gets moved across the surface of a magnetic material. The force between tip and sample is then a measure of the magnetic field. If the tip is atomically sharp, resolutions of a couple of tens of nanometers can be achieved.

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Fridge magnets, and related magnets used in homes and offices, are made from the ceramic BaFe₁₂O₁₉, whose annual commercial production reaches 830,000 tons a year. Contrary to what their mundane use suggests, the physics of these magnets is rather unusual. They belong to a rare class of materials whose magnetism can be controlled with electric voltages and vice versa, which offers a new way of controlling magnetic fields in applications such as information storage (other than putting notes up on your fridge!).

Unfortunately, in most of these ceramics such effects are confined to low temperatures and have been mainly of interest to physicists only. Tsuyoshi Kimura and colleagues from Osaka University may now have changed this. They have discovered that in a close relative of fridge magnets, Sr₃Co₂Fe₂₄O₄₁, coupled interactions between magnetic and electric properties occur even at room temperature. “This demonstrates that such magnets can be used for other practical usages,” says Kimura. Their work is published online this week in Nature Materials.

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