Pulsed NMR study in nuclear ordered solid³He

Abstract

The nuclear spin dynamics in nuclear spin ordered solid³He in low magnetic fields on the melting curve has been studied by pulsed NMR down to 0.6 mK. The free induction decay signals (FID) were measured in single crystals of solid³He at three operating frequencies of 920, 1380, and 1840 kHz. The FIDs were nonexponential and dependent on the rf pulse strength β _p ≡γH ₁ t _w , where γ is the gyromagnetic ratio,H ₁ is the rf field strength, andt _w is the pulse width. At small β _p they decayed almost linearly in time with a small exponential tail at the end. When β _p was further increased they became shorter and neither exponential nor linear in time. At large β _p they decayed very rapidly and sometimes could not be observed at all because of the dead time of the NMR detection system. Such behavior of the FID was observed in many different single crystals in the given temperature range at 920 kHz. Tsubota and Tsuneto have shown by solving the nonlinear equations of motion numerically that the motion of the nuclear spin becomes chaotic when the tipping angle exceeds a critical value. Comparing their result with our experimental results, we concluded that some of the results of the rapid decay of the FID at large β _p might be attributed to the onset of the chaotic motion. At 1840 kHz it is expected that the nonlinear effects in the equations of motion become less effective than that at 920 kHz. In fact, at this operating frequency the FIDs even at large β _p and the tipping angle-dependent frequency shift could be observed. These frequency shifts were in rather good agreement with Namaizawa's theory provided an effective tipping angle was taken into account.