Cheap electronic components industrially manufactured in their millions every year are also smart materials that can sense magnetic fields without any external power supply, UK scientists have discovered.

Neil Mathur from the University of Cambridge announced his team's serendipitous finding on 25 November at the Materials Research Society Fall meeting, in Boston, US. A ceramic capacitor made by AVX Corporation, he explained, costs just one cent to make - yet it can turn a weak (0.1Tesla) room temperature magnetic field into 7mV of electricity. As no power supply is needed to help generate this current, the device effectively harvests the energy of stray magnetic fields. 

Possible applications range from underwater devices which report on ships entering harbours (detecting their hulls' magnetic fields); to toys which light up when a magnet is brought near them. Magnetic sensors could even be used in the body.

The trick is not new: chemists and physicists have spent decades at the lab bench investigating so-called magnetoelectric transducers. But it was a surprise, Mathur explained, to find that components churned out in a factory unintentionally did the same thing all along. 

The researchers only made the discovery because capacitor manufacturers have recently switched from platinum to nickel electrodes to cut down on costs. When a magnetic field approaches nickel, the metal deforms as its magnetised domains shift around - a property known as magnetostriction that is not possessed by platinum. In capacitors, nickel electrodes encase a barium titanate insulator, which is strained by the electrode deformations and - being a piezoelectric material - generates charge.

Many researchers have coupled magnetostrictive and piezoelectric materials in thin films or composites - notably Dwight Viehland at Virginia Tech University, US, who uses terfenol and lead zirconate titanate.¹ But coupling the two together to get a cheap, reliable detector was considered to be the principal hurdle - until the Cambridge team discovered that capacitor manufacturers had already accidentally made a commercial sensor. Mathur hasn't investigated full details of the capacitor's magnetic sensing properties, but said he hoped others would investigate further.

Richard Van Noorden

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