Water’s response to ultralow magnetic fields comes as a surprise

Perturbed by an external magnetic field, nuclear spins dance a two-step: They wobble, or precess, about the field axis and polarize in the field direction. In a 100-µT field—comparable to that of an ordinary bar magnet—it takes room-temperature water a little more than 10 seconds to relax, or achieve equilibrium polarization; in a 10-µT field, it takes roughly three-quarters of that time. Theorists chalk up the difference to proton exchanges among H₃O⁺, H₂O, and OH⁻ that assist spin relaxation only when the magnetic field is weak. Stefan Hartwig, Martin Burghoff, and colleagues at the National Metrology Institute of Germany have now discovered that in fields of 1 µT and lower, water responds to a magnetic perturbation even faster still. To see the increased relaxation rates, they had to measure water’s magnetization in fields less than 1/500 the intensity of Earth’s geomagnetic field—a feat made possible in part by a superconducting quantum interference device. Hartwig and company haven’t yet pinpointed the detailed mechanism underlying the fast spin dynamics, but they suspect that coupling between H⁺ and ¹⁷O nuclei plays a role. Their work could lead to improved strategies for using nuclear magnetic resonance to identify cancerous tissue, explosives, and other materials. (S. Hartwig et al., J. Chem. Phys. 135, 054201, 2011.)—Ashley G. Smart