H. R. Foster

The extraction of weak nuclear electric quadrupole splittings of dilute impurities in ferromagnets using modulated Adiabatic Passage NMR/ON

A novel extension of the Single or Adiabatic Fast Passage NMR/ON technique is described. Its principal advantages in extracting the mode magnitudes and distribution widths of weak nuclear electric quadrupole splittings much less than the magnetic inhomogeneous broadening are its simplicity and generality requiring no assumptions on the degree of adiabaticity of the nuclear spin motion during sweep through the quadrupolar split subresonances. The technique is applied to a concentrated single crystal sample of60CoFe which failed to yield a well resolved mid passage signal to conventional single passage NMR/ON. The result is an asymmetric frequency distribution of quadrupole frequencies with a mode value of P=3e2qQ/4I (2I-1)=+4.5 ±1.0 kHz and half maxima of +2.5 kHz and +7.0 kHz.

Nuclear orientation — Pulsed magnetic resonance studies of60Co nuclei in iron

The observation of nuclear magnetic resonance of oriented60Co nuclei in iron using the techniques of pulsed single passages and single unswept radiofrequency pulses is reported. For large rf fields resonant destructions as high as 80% of the γ-radiation anisotropy were observed.

Discrepancies in NMRON Knight Shifts of impurity nuclei in ferromagnets

Precision NMRON field shift studies have been carried out up to 8T on a54MnNi single crystal along a hard [100] direction and on a125SbFe single crystal along a hard [110] direction. For both systems, high field (Bapp>-1T) and low field (0.3T<Bapp<0.8T) data sets are obtained. The analysis reveals a significant discrepancy between the apparent Knight Shift dependent on whether the low field or high field data set is utilised. For both systems, consideration of the high field data sets yield a zero Knight Shift K(54MnNi)=+0.0(0.2)% and K(125SbFe)=+0.2(1.4)%, whereas the low field data sets yield K(54MnNi)=+7.5(3.9)% and K(125SbFe)=−5.4(3.3)% respectively. The field range dependence of K suggests that only Knight shifts measured in large fields (>-1T) are meaningful for establishing systematics. This casts some doubt on the greater bulk of the literature’s NMRON Knight shift studies, where predominantly low fields have been used to determine K.

Transient NMR/ON of56CoFe

Adiabatic Fast Passage NMR/ON measurements on single crystal56CoFe indicate a unique electric field gradient when the applied field is parallel to the 〈100〉 direction.A well defined spin echo has been observed with pulsed NMR/ON using the 847 keV gamma and the echo amplitude decay followed for pulse separations out to 75 ms. The decay of the echo is non exponential, being essentially constant for pulse separations of 75 μs, 750 μs and 7.5 ms. The echo is significantly attenuated at 75 ms. This may be contrasted with an effective nuclear spin lattice relaxation time of T1′ ∼30 s.

Single and double passage AFP NMR/ON studies of54MnNi

High resolution conventional AFP NMR/ON studies provide evidence for very small efgs of predominantly negative sign at the nucleus of the nominal S-state ion54Mn in single crystal nickel when the applied field is paralled to the 〈111〉 direction. The form of the mid passage signals for opposing sweep directions indicates a unique efg super-imposed upon a random component of comparable magnitude. The advantages of a second analyzing sweep performed during spin lattice relaxation of a conventional post-passage AFP NMR/ON sweep are demonstrated.

Pulsed fm NMR/on studies of single crystal54MnNi and125SbFe

Precision field shift studies of Bappl-8T using pulsed FM NMR/ON along hard directions (eg <100>54MnNi, <112>125SbFe and the easy axis (<111>54MnNi, <100>125SbFe) yield no measurable Knight shifts within the uncertainty allowed by current accuracy of the nuclear moments. This is in striking contrast to earlier measurements with the same apparatus that yielded K=+1.5 (4)% for60CoFe.The modulation frequency dependences of the pulsed FMNMR/ON signals are investigated for a variety of rf parameters and compared with model predictions.

Nuclear Techniques for Directed Emission and Switched Operation of Grasers

The high frequency limitation of gamma ray lasers (grasers), the laser condition for feasible densities and the use of polarized nuclei in grasers is discussed. It is shown that, with polarized nuclei, directed emission and switched graser control are feasible. Experimental techniques and results for both resonant and non- resonant notations of the γ-radiation distributions from polarized radioactive nuclei are presented.