B. Kresse

NMR field-cycling at ultralow magnetic fields.

The paper describes some significant technical improvements of a home built NMR field cycling relaxometer [O. Lips, A. Privalov, S. Dvinskikh, F. Fujara, J. Magn. Reson. 149 (2001) 22-28] now allowing for fast switching of polarization fields (up to more than 1T) to evolution fields down to the sub-μT range. The most important instrumental details such as the description of an involved 3-dimensional resistive coil setup are given. Fields below about 5 μT can only be stabilized by incorporation of an active field drift and fluctuation compensation tool. In this way, the smallest 1H Larmor frequency obtained and measured so far has been 12 Hz.

Simultaneous measurement of very small magnetic fields and spin-lattice relaxation

A field cycling (FC) NMR experiment is presented which allows for the simultaneous determination of very small magnetic fields down to about 3 μT and the concomitant measurement of nuclear spin-lattice relaxation times in these fields. The technique will enable broadband spin-lattice relaxation dispersion experiments down to about 100 Hz (1)H Larmor frequency. Limitations of its applicability are discussed.

Combining 7Li NMR field-cycling relaxometry and stimulated-echo experiments: A powerful approach to lithium ion dynamics in solid-state electrolytes

We use (7)Li NMR to study lithium ion dynamics in a (Li2S)-(P2S5) glass. In particular, it is shown that a combination of (7)Li field-cycling relaxometry and (7)Li stimulated-echo experiments allows us to cover a time window extending over 10 orders of magnitude without any gaps. While the (7)Li stimulated-echo method proved suitable to measure correlation functions F2(t) of lithium ion dynamics in solids in recent years, we establish the (7)Li field-cycling technique as a versatile tool to ascertain the spectral density J2(ω) of the lithium ionic motion in this contribution. It is found that the dynamic range of (7)Li field-cycling relaxometry is 10(-9)-10(-5)s and, hence, it complements in an ideal way that of (7)Li stimulated-echo experiments, which amounts to 10(-5)-10(1)s. Transformations between time and frequency domains reveal that the field-cycling and stimulated-echo approaches yield results for the translational motion of the lithium ions that are consistent both with each other and with findings for the motional narrowing of (7)Li NMR spectra of the studied (Li2S)-(P2S5) glass. In the (7)Li field-cycling studies of the (Li2S)-(P2S5) glass, we observe the translational ionic motion at higher temperatures and the nearly constant loss at lower temperatures. For the former motion, the frequency dependence of the measured spectral density is well described by a Cole-Davidson function. For the latter phenomenon, which was considered as an universal phenomenon of disordered solids in the literature, we find an exponential temperature dependence.

1H NMR at Larmor frequencies down to 3 Hz by means of Field-Cycling techniques

Field-Cycling (FC) NMR experiments were carried out at 1H Larmor frequencies down to about 3Hz. This could be achieved by fast switching a high polarizing magnetic field down to a low evolution field which is tilted with respect to the polarization field. Then, the low frequency Larmor precession of the nuclear spin magnetization about this evolution field is registered by means of FIDs in a high detection field. The crucial technical point of the experiment is the stabilization of the evolution field, which is achieved by compensating for temporal magnetic field fluctuations of all three spatial components. The paper reports on some other basic low field experiments such as the simultaneous measurement of the Larmor frequency and the spin-lattice relaxation time in such small fields as well as the irradiation of oscillating transversal magnetic field pulses at very low frequencies as a novel method for field calibration in low field FC NMR. The potential of low field FC is exemplified by the 1H relaxation dispersion of water at frequencies below about 2kHz stemming from the slow proton exchange process.