Communication Capacitor-based detection of nuclear magnetization: Nuclear quadrupole resonance of surfaces Alan Gregorovic ˇ a,⇑ , Tomaz ˇ Apih a , Ivan Kvasic ´ a , Janko Luz ˇnik b , Janez Pirnat b , Zvonko Trontelj b , Drago Strle c , Igor Muševic ˇ a,d a Joz ˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia b Institute For Mechanics, Physics and Mathematics, Jadranska 19, 1000 Ljubljana, Slovenia c Faculty of Electrical Engineering, University of Ljubljana, Trz ˇaška 25, 1000 Ljubljana, Slovenia d Faculty for Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia article info Article history: Received 28 September 2010 Revised 3 December 2010 Available online 15 December 2010 Keywords: NQR Thin samples Surfaces E-field detection RF sensors abstract We demonstrate excitation and detection of nuclear magnetization in a nuclear quadrupole resonance (NQR) experiment with a parallel plate capacitor, where the sample is located between the two capacitor plates and not in a coil as usually. While the sensitivity of this capacitor-based detection is found lower compared to an optimal coil-based detection of the same amount of sample, it becomes comparable in the case of very thin samples and even advantageous in the proximity of conducting bodies. This capac- itor-based setup may find its application in acquisition of NQR signals from the surface layers on conduct- ing bodies or in a portable tightly integrated nuclear magnetic resonance sensor. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Wire-wound coils are the most common elements for the crea- tion of localized RF magnetic fields in a number of applications. They are easy to build and optimize for a variety of geometries. However, coil efficiency to create large magnetic fields diminishes substantially in the immediate neighborhood of conducting bodies due to the generation of unwanted eddy currents. For techniques such as nuclear magnetic resonance (NMR) and nuclear quadru- pole resonance (NQR), where the coil is the principal detection element, this usually implies an inefficient observation of bulk metals [1–3]. Often the sensitivity can be improved by changing the sample form from bulk to either powders or rolls of very thin foils. Similarly, NMR/NQR suffers in the observation of protecting surfaces of metallic objects, e.g. anti-corrosion layers, anti-abrasive coatings, and other functional layers [4]. Here we propose an NMR/NQR detection technique for the observation of such surfaces employing a capacitor instead of the coil for the excitation and the detection of nuclear magnetization. The capacitor, built here from two copper parallel plates, serves only as a model, whereas the foreseen application is to use a single electrode with a conducting body under investigation forming the other electrode as shown in Fig. 1a. The method presented here should not be confused with an earlier presented method of Prance and Aydin [5], where an electrode was used to indirectly detect coil-excited nuclear magnetization. However, in that particular case, the detection of the associated electric field was substantially modified by the proximity of a tuned coil. A loopless antenna for magnetic resonance imaging was also discussed by Ocali and Atalar [6]. 2. Theory In solids, the observation of nuclear magnetization in the MHz range [7] requires the application of a strong RF magnetic field pulse with a magnitude of 1 mT at the sample defined frequency (x) followed by a detection of an extremely small nuclear magne- tization (M). Depending on a particular application, this is achieved either by a single coil, or by a separate excitation/detection coil pair [8]. Although the process of detection occurs after the excita- tion, the two processes are related by the antenna reciprocity theorem [9–11]; the coil’s sensitivity for the detection of M is proportional to the magnetic field produced by the same coil per unit current. This is why the goal of a sensitive coil (probe) is to create as high magnetic field as possible. For a coil adjacent to a conducting body this becomes very difficult, as the induced eddy currents oppose the coil’s original magnetic field and therefore lower the final sensitivity. The reduction of the excitation magnetic field can be rather easily compensated by the increase of the 1090-7807/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2010.12.002 ⇑ Corresponding author. E-mail address: alan.gregorovic@ijs.si (A. Gregorovic ˇ). Journal of Magnetic Resonance 209 (2011) 79–82 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr