Spin–Echo NMR in Pulsed High Magnetic Fields up to 48 T

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Application of high magnetic fields is playing increasing role in scientific researches, since a magnetic field has strong effects on the motion of electronic spin as well as on electronic orbits. Among various measurements under high magnetic fields, nuclear magnetic resonance (NMR) is one of the major techniques in physics, chemistry and biology. It has also developed into the diagnostic medicine of magnetic resonance imaging (MRI). With the installation of resistive magnets and the 45 T hybrid magnet, NMR experiments have been conducted at high fields up to 43 T. In addition to the apparent merit of enhanced signal-to-noise ratio, high-field NMR has advantage of providing unique insights in the quest of the phenomena such as field-induced phase transition, superconductivity and quantum-spin magnetism. However, in many cases, the currently available highest static magnetic field is not enough. For example, the upper critical field for the copper-oxide high transition-temperature (Tc) superconductors is in the range of 100 T. Therefore, a still higher field is required, for example, to access the lowtemperature normal state of the high-Tc superconductors. Pulsed high magnetic fields offer such route. Haase and coworkers were the first to show that it is possible to conduct NMR experiments at pulsed high magnetic field using the free induction decay (FID) method. They have succeeded in observing FID signal of H up to 56 T. Here we report the first NMR observation by the spin– echo method at pulsed high magnetic fields up to 48 T. It is important to establish the spin–echo experimental method, since the NMR spectrum is broad and the FID is not possible in most functional materials such as superconductors and magnetically ordered substances. It is also important to establish the technique for nuclei of moderate NMR signal sensitivity, such as Co or Cu. We report the magnet characteristics, Co-NMR experimental setups, and discuss the advantage and limitation of pulsed-field NMR compared to conventional static-field NMR. The pulsed magnet used is a 55 T, 22mm bore resistive coil magnet with an inductance of 7.59mH. The coil material is Cu–24wt% Ag wire (Showa Electric Wire and Cable). The magnet consists of an inner multiple-layered coil backed up with high strength polymer fibers Zylon and the outer has about ten layers and a Maraging ring made of YAG-300 steel (Hitachi Metal). The magnet coil is placed in a dewar filled with liquid nitrogen for cooling. The magnet is energized with a 1.02MJ condenser bank at KYOKUGEN, Osaka University. The time profile of the magnetic field was monitored by a one-turn pick-up coil wound outside the NMR coil. The magnitude of the magnetic field was calibrated by measuring the ESR signal of (C6H5)2NNC6H2(NO2)3 (1,1-diphenyl-2-picrylhydrazyl, or DPPH for short), which gives a field accuracy better than 0.01 T. Examining several shots at the same charged voltage shows that the magnet has a reproducibility of 0.3%. Figure 1 shows the spatial distribution of the magnetic field along the vertical direction. In the most homogeneous part within 1mm, the magnet has a homogeneity of 10 . Figure 2 shows the time profile of the magnetic field after discharging the capacitor bank (C 1⁄4 11:28mF) which was initially charged with a voltage of 9.39 kV. A home-built phase-coherent NMR spectrometer was used to collect data. The NMR coil is made of 3 turn Cu wire with a diameter of 1.1mm. The NMR probe is inserted in a glass dewar and kept at 4.2K. Figure 3 shows three NMR observing RF pulse sequences, each followed by the spin– echo signal of Co, which were recorded near the maximal field at a frequency of 495.4MHz. The sample is Na0:3CoO2 5.80 5.82 5.84 5.86 5.88 5.90 5.92 5.94