A new method for preforming adiabatic demagnetization in the rotating reference frame in NMR experiments is presented in this paper. In this new method the frequency of a crystal oscillator rather than the actual magnetic field is pulsed off resonance. Using simple circuits which are described in this paper, the effective field in the rotating frame can be pulsed by as much as 50 G, depending on the nucleus studied. This method completely avoids the problem of external magnetic field correction due to a pulsed field in the magnetic gap.

Topics
Oscillators, Signal processing, Nuclear magnetic resonance spectroscopy
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11.
In ultraslow motion experiments it is usually desirable to have H1 comparable to HL (see Ref. 4).
12.
The field pulsing is usually accomplished by pulsing the current through an extra set of coils wrapped around a Dewar in the magnet gap.
13.
Of course, when H1 = HL, the magnetization will be 0.707M0 (see Ref. 1).
14.
In principle arbitrarily large frequency changes Δω can be obtained by using a variable frequency oscillator which is stabilized by a crystal. The crystal would be removed suddenly. Simultaneously the frequency of the VFO would be kicked to a new level and later allowed to return adiabatically to the original crystal frequency.
15.
These crystals and circuits are quite inexpensive and can be purchased from International Crystal Mfg. Co., Inc., 18 North Lee, Oklahoma City, Okla. 73102.
16.
There is no difficulty in using phase sensitive detection since the reference frequency is at all times equal to the basic frequency of the oscillator from which it is derived. In fact, we employed phase sensitive detection and a boxcar integrator to maximize our sensitivity.
17.
For strongly magnetic nuclei, such as protons, it would be difficult to pull the crystals a sufficient amount in effective field; accordingly, an alternative method for increasing further the frequency modulation may be preferable.14
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© 1970 The American Institute of Physics.
1970
The American Institute of Physics
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