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Lanthanide-based single-molecule magnets: A rational design by chemical intuition



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Izuogu, David Chukwuma 


A model complex of dysprosium, Dy(N2O2C7H11)3 (Dy2) was built by successive capping of the peripheral aromatic ring of a butterfly-shaped dysprosium complex of a schiff base ligand, Dy(N4O5C14H11)3 (Dy1). The structural modifications were carried out in order to investigate the effect of the chemical surroundings like aromaticity on the observation of single-molecule magnet (SMM) behaviour in a lanthanide-based complex (Dy1). Experimental techniques were combined with theoretical tools to investigate the dynamics of magnetic properties of the Dy1 and compared with the theoretical results for Dy2 to gain insight on the contribution of covalency, crystal field effect and the role of aromaticity in stabilizing the excited magnetic levels of a single-molecule magnet. Dy1 showed frequency-dependent slow magnetic relaxation characteristics of a single-molecule magnet with and without applied dc field and a blocking temperature of about 8 K. The obtained results showed that despite f−electrons having weak interactions with ligand field, it is possible to tune the magnetic properties of lanthanide-based complexes using ring currents, control of covalency and peripheral ligand substitution. Furthermore, di-nuclear acetate bridged lanthanide complexes with two different structural motifs, Er1 = [Er(CH3COO)(CH3COO)2(H2O)2]2 · 4H2O and [Ln(CH3COO)(CH3COO)(CH3COO)(H2O)CH3COOH]2 · 2CH3COOH (Ln = Er (Er2), Y0.8Er0.2 (Er3)) were characterised. The solvent effects on the structure, electronic and magnetic properties were studied by experiments and theoretical methods. The tetraacetate-bridged erbium (Er2) showed a shorter intra-molecular Er – Er distance of 3.878 Å compared to the doubly-bridged counterpart (Er1) with Er – Er intra-molecular separation of 4.152 Å. Er2 exhibited weak ferromagnetic ordering at very low temperature in the dc magnetic measurement due to the short proximity of the Er centres. A field-induced slow magnetic relaxation for spin reversal characteristics of single-molecule magnet behaviour with relaxation dynamics dominated by Orbach process was observed for Er2. Er1 showed very fast slow magnetic relaxation dominated by quantum tunnelling of magnetization as evidenced in the Cole-Cole plot and the observed plateau in the susceptibility curve. The role of inter-molecular interactions between Er centres was investigated using yttrium diluted sample (Er3). The enhanced magnetic property of Er2 and Er3 over Er1 is attributed to the structural changes accompanying the choice of synthetic solvents used as well as the stronger interactions between Er and oxygen donor atoms of the acetate/acetic acid over that of water molecule as ligands. The present study presents an interesting result on solvent effects in the design of single-molecule magnets. In addition to the peripheral, aromatic, solvent effect and accompanying lanthanide-lanthanide coupling investigated, the role of hetero-metallic Pd-Ln bonding interaction was investigated using a set of tetranuclear acetate-bridged palladium-lanthanide complexes of the formula [Pd2Ln2(H2O)2(AcO)10] · 2AcOH (AcO = CH3COO– , Ln = Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb) by theoretical means and compared with experimental results for the lanthanide series to establish an interesting alternating trend in the observation of slow magnetic relaxation between Kramers and non-Kramers ions along the series. The role of axial perturbation, electron-cloud distortion and utility of metal as a ligand to upset the electronic properties of lanthanide complexes and their implication to the observation of slow magnetic relaxation were investigated. Results showed that transition metal–lanthanide bonding interaction presents a unique way to control the ligand field of lanthanide at varying degrees while confirming the role of rigidity in SMM design.





Thom, Alexander JW


Single-molecule magnets, slow magnetic relaxation, magnetism, Molecular magnet, information storage, hysteresis, solvent effect, intermolecular interaction, Intramolecular interaction, Blocking temperature, Energy barrier, spin Hamiltonia, spin reversal


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Cambridge Trust, Cambridge- Africa, IsDB