Keiichiro Nakamura

NMR study of formate dihydrate at millikelvin temperatures

Abstract We have observed 59 Co spin echo signals in Co(HCOO)2·2H2O by pulsed NMR without external magnetic field at 90 and 75 mK using a dilution refrigerator. Co(HCOO)2·2H2O has a magnetic sandwich structure in which the Co2+ spins on one plane are antiferromagnetically ordered while the Co2+ spins on another plane remain paramagnetic below TN (5.1 K ) . There are two peaks in NMR spectra below 90 mK . It is concluded that the higher-frequency peak corresponds to the signal from antiferromagnetically ordered planes and the lower-frequency peak from paramagnetic planes.

NMR study of two-dimensional ferromagnet K2CuF4 below 1 K

Abstract Two-dimensional ferromagnet K2CuF4 is an ideal compound to study two-dimensional magnetic systems because the interplane exchange coupling constant is much smaller than the intraplane exchange coupling constant. We measured the temperature dependence of the NMR frequencies of the 1 2 ↔ 3 2 transition for 63 Cu and 65 Cu nuclei in K2CuF4 single crystal far below T c =6.25 K . The NMR measurements were made by spin-echo method at zero magnetic field. The experimental results are discussed in the framework of the two-dimensional spin wave theory at millikelvin temperatures.

A Vortex Refrigerator for NMR Experiments

Publisher Summary In NMR experiments at very low temperatures, the two points are important––there must be a good thermal contact between a sample and the cryostat. Since thermal contact resistance between two cold bodies increases rapidly as the temperature falls, it becomes increasingly important how to transfer heat from one place to another, and there must be little or no eddy current heating by applied rf pulses. In the presences of some metals, eddy current heating occurs when applying rf pulses in the NMR experiment. Therefore it is advantageous to make cryostat by non-metal substances and to immerse samples directly into cold fluid. In this way good thermal contact between the samples and the fluid is achieved.. This chapter helps in constructing a vortex refrigerator especially designed for NMR measurements around I K. The main parts of the refrigerator have been made by non-metal substances to reduce eddy current heating due to applied rf pulses. The sufficient cooling power for continuous measurements has been obtained. The mutual friction between superfluid component and normal fluid component in He II is also discussed.

Coupling of many NMR modes at very low temperatures

Abstract At very low temperatures where the thermal energy kT is comparable with the hyperfine interaction energy, nuclear spins begin a coupled motion through hyperfine interactions. Thus, many NMR modes are mixed and make other resonance modes. These phenomena are observed in the NMR experiments of K2CuF4, V3O7 and CsMnBr3 at the temperatures down to 30 mK.