The superhyperfine resonances were detected using optically detected NMR techniques
(Sec.3.5). This involved spectral holebuming using a Coherent 699-21 high resolution ring dye laser with a linewidth of ~1 MHz tuned to one of the hyperfine lines and hence caused bleaching of the absorption corresponding to a subgroup of Ho3+ ions within the sample. The RF field (in the 0-30 MHz frequency range) was applied to a five-turn 5 mm-diameter coil wrapped round
the sample immersed in a pumped liquid helium bath at ~ 2K. The depth of the hole is recorded as a function of the frequency of the radiofrequency by monitoring the intensity of the emission at 15513.5 cm '1 which corresponds to a transition from the lowest level of the 5F5 multiplet.
In principal, the superhyperfine spectrum can be obtained with holebuming in any of the transitions to 5F5 multiplet levels. However, in practice the best signal-to-noise was obtained with holebuming in the lower energy transitions, 5I8 (Aj, A2) —> 5F5(E), Figs.5.19 and 5.20.
The ODMR spectrum is dependent on the hyperfine level involved (as selected by the optical frequency) and, hence, can be used to identify the nuclear spin state associated with the various spectral lines. For example, holebuming in one of the extreme spectral lines associated with the
± - hyperfine levels, such as line A, three prominent resonances are detected at 23.49 MHz and a doublet at 17.74 and 17.54 MHz along with weaker signals at 3.20, 2.85, 1.94, 1.28, 1.19, 1.02, and 0.90 MHz. (Fig.5.19). By drawing analogy with the superhyperfine structure of the
C4v centre of Pr3+ in CaF2 (Burum et al 1982), the highest frequency signal can be assigned as due to the interstitial F ion, and the next two lower closely-separated frequency signals as due
to the two sets of four nearest-neighbours F ions. The four nearest-neighbour F ions between the Ho3+ ion and interstitial F ion are crystallographically equivalent and, therefore, give one
resonance. The other four nearest-neighbour F ions furthest from the interstitial F ion are, likewise, crystallographically equivalent to each other and give the other resonance. It is worth
noting that the observed larger intensity (~ 2 x) of the nearest-neighbour superhyperfine resonances, compared to the interstitial superhyperfine resonance, is to be expected given the
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14
18
22
rf frequency (MHz)
Figure 5.19 Superhyperfine resonances detected by RF-optical double-resonance techniques, (a) 0.5-6 MHz and (b) 6-26 MHz. Spectra shown correspond to holebuming at six different wavelengths A-F (as indicated), within the 5Ig(Aj)
in
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)
laser
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rf frequency (MHz)
Figure 5.20
Superhyperfine resonances associated with the 5I8(A2) —> 5F5(E) opticalBeyond the nearest-neighbour (NN) and interstitial F ions, it is expected, in the paramagnetic material, that the dipolar field from the magnetic moment of the Ho3+ ion becomes the dominant term compared to any exchange contribution. This assumption is supported by a comparison of
the observed superhyperfine resonances at 3.20, 2.85, and 1.94 MHz with the three next- nearest-neighbour (NNN) resonances reported for the tetragonal (C4v) centre in CaF9: Pr3+ by Burum et al (1982). There is a scaling factor of 2.21(6) between the NNN resonances of the two centres which correlates with the larger moment present in the CaF2: Ho3+ C4v centre. Thus, the 3.20 MHz resonance can be assigned to the four NNN F ions furthest from the
interstitial, the 2.85 MHz resonance to the four NNN F closest to the interstitial, while the 1.94 MHz resonance belongs to the other sixteen NNN F ions. The latter assignment, in particular, is strengthened by the observed larger intensity (~ 2 x) of the 1.94 MHz resonance compared to the 3.20 and 2.85 MHz resonances. Such a relative change in intensities is consistent with the assignment of sixteen, four, and four NNN F ions, respectively, to the above resonances, and
with the observed relative intensities of the NN and interstitial resonances mentioned earlier.
When the selected optical transition involves the ± ^ hyperfine levels, such as line B, there is a consistent reduction in the frequencies of the observed superhyperfine resonances which arises as a consequence of the change in the Ho3+ magnetic moment. The associated superhyperfine resonances are now observed at 18.35, 13.58, 13.48, 2.49, 2.22, 1.53, 1.02, 0.95, 0.82, and 0.70 MHz (Fig.5.19).
3
For the ± - hyperfine levels, the associated interstitial resonance is at 11.71 MHz and those for the nearest neighbour shell lie at 8.8 MHz (i.e. not resolved).
Finally, there are equivalent resonances when either one of the four hyperfine lines E, F, G, H
associated with the ± ^ hyperfine levels is optically selected, and they lie in the region of 4 MHz
and 3 MHz. Note from Fig.5.21, however, that these resonances have split. Ignoring this extra effect for the moment (treated in Sec.5.7) and assigning average values of 4.11 and 3.08 MHz
for the interstitial and nearest neighbour resonances, respectively, the change in the frequency of the hyperfine resonances associated with the different hyperfine levels is due to their
5.72:4.47:2.85:1. The frequencies of the nearest neighbour F ions are all lower than that of the interstitial and display approximately the same ratio for the resonances of the various hyperfine levels (5.74:4.40:2.86:1).
; i The observed superhyperfine resonances are all summarized in Table 5.5.
= + - T