American scientists have discovered a new way to order the tiny hydrogen proton magnets in water molecules, which in our drinking glass are completely disordered. Mohsen Arabgol and Tycho Sleator of New York University (NYU) have shown that they can align the spins of protons by spinning them at very high speeds. They realized that a phenomenon known as the Barnett effect, first demonstrated in electrons in 1915, should also apply to protons. “What we expected to find, we found eventually,” says Sleator world of chemistry. “Our idea was correct.”
Sleator explains that the idea originated from a project funded in part by the US Defense Advanced Research Projects Agency. He was looking to make a simpler and more portable brain imaging probe than is possible with MRI. The idea was to send circularly polarized X-rays to spin molecules in the brain to try to align the spins of their hydrogen protons.
MRI machines and NMR spectroscopy instruments commonly used by chemists are bulky because they contain large magnets to align protons. This alignment, or polarization, is possible because protons have a magnetic moment, reflecting the strength and orientation of their magnetic field, the latter better known as spin. This orientation only ever takes one of the two values. The spins are therefore always either aligned in the same direction as the magnetic field used to measure them, or aligned opposite this field.
The same is true for electrons, only their magnetic moments are about 700 times larger than protons. This is important when it comes to rotating samples, because there is a fundamental relationship between the magnetic moments of particles and the angular momentum of their rotation. At the start of the 20th century, Albert Einstein and Wander Johannes de Haas realized that this meant that changing the magnetic moment of a body could cause it to rotate. Samuel Barnett demonstrated the opposite by showing that metal rods rotating on their axes could magnetize them by polarizing the spins of the electrons.
Polarizing the spins in this way requires giving the particles enough angular momentum. “If you have a sample of, say, protons in water, which is spinning very fast, if a spin flips from a direction parallel to the direction of spin to the opposite of the direction of spin, there is an exchange of angular momentum between the returned spin and the bulk sample,” says Sleator. Spinning a sample fast enough can provide enough energy to partially align the spins in one direction, partly overcoming the jamming effect of thermal energy.
Because the magnetic moments of protons are so small, the sample must be rotated very quickly. Sleator acquired a spinner from Colorado-based Revolution NMR that can reach up to 15,000 rpm. By comparison, the crankshaft of a Formula 1 racing car’s engine spins at 18,000 rpm. Sleator admits he and Arabgol were afraid to push the limit of the spinner, so stuck at 13,500 rpm.
NYU scientists detected the result using NMR measurements, but rather than the usual large magnets, they placed the spinner between two inexpensive low-field ring magnets. The field was enough for them to detect the spin-activated magnetization, but not to overwhelm it.
Meghan Halse of the University of York, UK, says it’s “interesting to be able to generate bias” normally produced by a strong magnetic field by simply rotating a sample. “It’s an unusual effect,” she adds. She adds that the polarization generated, even at very high rotational speeds, is “quite weak and therefore this effect cannot easily replace the strong magnetic fields used in NMR/MRI”.
Sleator acknowledges that the nuclear Barnett effect is unlikely to revolutionize MRI, calling the idea “pie in the sky”. The applications of this effect are not obvious at the moment, but he points out that it was the same initially for electromagnetic induction, which today allows a large part of our electricity production.