New spin echo techniques in the earth's magnetic field range
Free precession, the fundamental phenomenon of spin echo techniques, is essentially nonresonant. The spin echoes include always 1) a coherent transitory state as starting point; 2) a dispersion of the elementary moments in the plane perpendicular to the magnetic field; 3) a rotation by an angle cc of the dispersion plane of the moments with respect to the magnetic field direction, and, 4) the observation of one or several echoes. Stages 2) and 4), caused by the spreading of Larmor frequencies, are passive; in general, stages 1) and 3) are obtained by pulsed magnetic fields, in a suitable direction and of a certain duration, at the resonance frequency w0 of the nuclei in the magnetic field H0. In this sense, spin echo techniques are derived from magnetic resonance. We show phenomenologically, and illustrate by our experiments that stages 1) and 3) can easily be obtained in weak fields by nonresonant methods. We can use the method of pulses at zero frequency, and have realized experimentally rotary echoes in the laboratory reference frame. These first results allowed us to analyze the various spin echo methods which can be deduced by a suitable choice of the pulse axis and by rotation of the reference systems (dispersion plane of the moments and axis perpendicular to this plane) of a hypothetical experiment in which the initial finite magnetization is subjected to a 'pure gradient' (zero average field, but strongly inhomogeneous) whose axis of symmetry can be varied. Some new techniques of spin echoes are proposed and discussed. The principal perturbations caused by a bad choice of pulse parameters or by the presence of residual permanent fields can, in this perspective, be quite simply analyzed and evaluated with precision.