A tiny magnetic needle just 10 μm long could be used to create a magnetic-field sensor that – if built – would be 1000 times more sensitive than the best available magnetometers today. That’s the claim of physicists in the US, who say that this performance could be achieved by having the needle wobble in the presence of very weak magnetic fields – in the same way that atoms wobble during nuclear magnetic resonance (NMR) measurements.
A compass needle is a bar magnet that will simply align itself along the Earth’s magnetic field. Particles such as electrons, muons and some atoms also behave like bar magnets, but when they are in a magnetic field their magnetic moments rotate around the direction of the field – much like how a wobbling top rotates around the direction of Earth’s gravitational field. This process is called Larmor precession and it plays an important role in nuclear magnetic resonance. By measuring this precession frequency, atom magnetometers are able to detect magnetic fields as tiny as one trillionth of the Earth’s magnetic field.
Falling out of phase
While atom magnetometers are extremely sensitive, quantum mechanics imposes a “standard quantum limit” – or SQL – on their performance. In an atom magnetometer, measurements are made on many atoms in a gaseous sample to boost the signal and reduce the noise. However, because each atom is independent, the atoms will fall out of phase with each other and this puts an SQL on the precision of the measurement. One way around this is to ensure that the individual magnetic moments are strongly coupled to each other and are therefore not able to fall out of phase. This strong coupling is exactly what exists in a compass needle, in which all of the magnetic moments point in the same direction.
While conventional compass needles don’t wobble, Derek Jackson Kimball of California State University, East Bay, Alexander Sushkov of Boston University and Dmitry Budker of the University of California, Berkeley, have done calculations that show that a tiny needle will undergo Larmor precession as long as the spin angular momentum associated with its magnetic moment is greater than the orbital angular moment associated with the precession. The trio reckon that a needle of cobalt that is about 10 μm long with a radius of about 1 μm would do the trick. Such a needle would comprise a single magnetic domain in which all atomic spins add together to give a very large spin angular momentum. The orbital angular moment of the needle would, however, be relatively low because of its small size.
The researchers have calculated that the precession frequency of the needle could be measured using a superconducting quantum interference device (SQUID), which itself is a very sensitive detector of magnetic fields. According to the team, the sensitivity of their needle magnetometer would be limited by the performance of the SQUID, rather than that of the needle. But even so, it would be about 1000 times more sensitive than the best existing atom magnetometers and capable of detecting fields smaller than about 1 fG, which is less than one hundred trillionth of the Earth’s magnetic field.
Daunting technical challenge
However, Jackson Kimball and colleagues admit that building a practical magnetometer would be a significant challenge. The tiny ferromagnetic needle would have to be cooled to near absolute zero (0.1 K) while being suspended in a vacuum chamber. Writing in Physical Review Letters, they say: “Perhaps the most daunting technical challenge…is the problem of suspension.” Their calculations suggest that hanging the needle from a very fine wire or levitating it using light would both result in the magnetometer being overwhelmed by external noise. One possibility, they say, would be to levitate the needle above a superconductor using the Meissner effect. Another solution would be to have the needle free-fall in a drop tower or in microgravity on a satellite – although they admit that a laboratory-based set-up would be more convenient.
One potential application for the magnetometer could be high-precision testing of some aspects of the Standard Model of particle physics. The device could, in principle, look for exotic spin-dependent interactions between electrons at much lower energy scales than possible today. The researchers also point out that their calculations may be of interest to astrophysicists. This is because micron-sized ferromagnetic needles could be present in the interstellar medium, where it may be possible to observe them wobbling around interstellar or intergalactic magnetic fields.
