Magnetic Lenz lenses improve the limit-of-detection in nuclear magnetic resonance

A high NMR detection sensitivity is indispensable when dealing with mass and volume-limited
samples, or whenever a high spatial resolution is required. The use of miniaturised RF
coils is a proven way to increase sensitivity, but situations may arise where space restrictions
could prevent the use of a small resonant coil, e.g., in the interior of the smallest practicable
micro-coils. We present the use of magnetic lenses, denoted as Lenz lenses due to
their working principle, to focus the magnetic flux of an RF coil into a smaller volume and
thereby locally enhance the sensitivity of the NMR experimentÐat the expense of the total
sensitive volume. Besides focusing, such lenses facilitate re-guiding or re-shaping of magnetic
fields much like optical lenses do with light beams. For the first time we experimentally
demonstrate the use of Lenz lenses in magnetic resonance and provide a compact mathematical
description of the working principle. Through simulations we show that optimal
arrangements can be found.