Indirect detection of nuclear magnetic resonance via geometrically induced long-range dipolar fields.

We report the indirect detection of the magnetization of one spin species via the NMR signal of a second species. Our method relies on the control of long-range dipolar fields between two separate objects, in this case, a water droplet (sensor) immersed in a tube containing mineral oil (sample). Unlike prior experiments, no gradient pulses are used; rather, the setup geometry is exploited to select the part of the sample to be probed and modulate the spin alignment in the sensor. Our results are discussed in the context of Dipolar Field Microscopy, a proposed strategy in which the detector is a hyperpolarized tip.