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.