Exchange interactions and magnetic relaxation mechanisms in lanthanide single-molecule magnets

The work contained in this thesis focusses on magnetic exchange interactions in lanthanide (Ln) dimetallic molecules, and the investigation of novel high-performance single-molecule magnets (SMMs). It combines a range of experimental techniques (Far-IR, SQUID magnetometry, and EPR and INS spectroscopies) to obtain a comprehensive picture of the magnetic properties of such molecular materials. Due to the complexity of describing exchange interactions between pairs of strongly spin-orbit coupled spin systems, the goal is to build a reliable experimental dataset before comparisons against model Hamiltonians. To this end, herein we study an asymmetric dimetallic molecule [hqH2][Dy2(hq)4(NO3)3]-MeOH (hqH = 8-hydroxyquinoline) and a symmetric molecule [Ln2(HMeQ)4(NO3)6] (HMeQ = 2-methyl-8-hydroxyquinoline). Describing the exchange interaction of two Kramer's Ln ions using an effective Seff = 1/2 model proved successful in reproducing the highly detailed experimental spectra for both systems. Therefore, the resultant exchange coupled spectrum is representative of the true exchange coupled system. These results, along with previous work within the group,1 build a data-base of spectroscopically verified exchange coupled Ln systems. As more molecules are studied in this way, it is hopeful that trends in magnetic interactions can be identified and synthetic targets can be designed with specific magnetic interactions in mind. This work also provides detailed data sets which can be used in the development of new theoretical models for complicated exchange-coupled molecules. Concurrently, work on new high-performance SMMs prepared at Manchester was undertaken, performing magnetic investigations of [Dy(DiMeQ)2(H2O)Cl3] (DiMeQ = 5,7-dimethyl-8-hydroxyquinoline) and several [Dy(SCS)2]- variants. These studies allowed the identification of trends in literature SMMs and revealed the importance of the Raman relaxation process in influencing SMM blocking temperatures. I present a detailed study of high performing literature SMMs, with the assignment of t_switch as a new parameter which correlates with SMM blocking behaviour.