Figure 3: Normalized magnetization (M) vs. external magnetic
field (H) hysteresis of 4 (a), 3 (b) and 2(c) at 2 K. Blue traces: naturally abundant Dy, undiluted; black traces:
naturally abundant Dy, diluted ∼5% in Y; red traces: ∼96.6%164Dy enriched, ∼5% diluted in Y. For all data,
except the blue traces for 3 and 4 , sweep rates are
110(20) Oe s−1 for |Hext|
> 20 kOe, 60(10) Oe s−1 for 10 kOe
< |Hext| < 20 kOe, 38(8) Oe
s−1 for 6 kOe < |Hext|
< 10 kOe, and 20(4) Oe s−1 for
|Hext| < 6 kOe. For the blue trace for 3 the
data are taken from Ref. 13a with a sweep rate of 50 Oe
s−1, and for 4 the data are taken from Ref.
35 with a sweep rate of 35 Oe s−1. Reproduced from
Ref. 29 with permission from The Royal Society of Chemistry.
Hyperfine interactions. A further improvement on the
performances of Dy(III) SMMs could be achieved by removing hyperfine
interactions between electrons and spin-active nuclei. The presence of
these interactions generate avoided crossings near zero-field, which
results in additional relaxation pathways.[32,33]Experimentally, this is a very hard task to achieve, as it involves the
separation of the naturally occurring isotopes of dysprosium:161Dy (I = 5/2, 18.9%),162Dy (I = 0, 25.5%), 163Dy
(I = 5/2, 24.9%) and 164Dy (I = 0,
28.3%). The effect of isotopic enrichment has been extensively studied
in recent years, especially by Pointilliart and co-workers who have
indeed been able to observe an improvement in magnetic properties when
measuring 164Dy-doped tetrathiafulvalene yttrium
complexes.[34] Since isotopic enrichment work had
previously been carried out on SMMs with relatively small barriers, we
decided to study a series of high-barrier systems to exclude thermally
activated relaxation pathways (Raman and Orbach) and thus focus on the
hyperfine effect on QTM. Thus, complexes 2-4 were used to
further investigate the causes behind the residual zero-field step in
high-performing SMMs.[29]
Our protocol involved the use of highly isotopically enriched
(~97% 164Dy)164Dy2O3, which was
then laboriously converted to anhydrous164DyCl3 in several steps. This was a
necessity as most high-performance SMMs reported to date require the
exclusion of air and moisture. With164DyCl3 in hand, it was diluted in
YCl3 (at ca. 5%) in order to also reduce dipolar
interactions (see above). This mixture could then be employed for the
synthesis of 2-4 . Hysteresis measurements of isotopically
enriched 2-4 showed that the zero-field step was still present
in all of them,[29] after the minimization of both
1) dipolar interactions through spin-dilution, and 2) near-elimination
of metal-spin hyperfine interaction by isotopic enrichment. This was
further corroborated via detailed field- and
temperature-dependent studies on 4 : these measurements show
that whilst below 6 K the dipolar and hyperfine interactions do indeed
affect the mJ = ±15/2 ground state, these effects do not
completely account for the zero-field step.[35]Altogether, these data suggest that the relative difference in the
zero-field step in these molecules is more related to CF and molecular
design than to dipolar and hyperfine interactions. The question as to
why this is the case remains open and it would entail development of a
theory of vibrational contributions to QTM.