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Title: Probing SMM behaviour in heterometallic 3/4d-4f complexes
Author: McNab, Robbie
ISNI:       0000 0004 8509 5546
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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In recent years the field of molecular magnetism has focused its attention on creating novel magnetic molecules that can retain their magnetisation in the absence of a magnetic field, below a blocking temperature. These complexes are known as Single-Molecule Magnets (SMMs). Initial focus on the use of transition metal ions swiftly moved on to lanthanide metal ions on account of their larger anisotropies and larger spin values. The latter have now shown barriers to magnetization reversal far superior to the former. Recent studies have proved that coordination chemistry and structural modification can be exploited to manipulate magnetic properties of Ln species to maximise energy barrier heights and minimising under barrier relaxation processes such as Quantum Tunnelling of the Magnetisation (QTM). QTM, which represents a loss of magnetisation through degenerate Ms levels, is a limiting factor in designing Ln containing SMMs with larger barriers to magnetisation reversal. The complexity of the energy level spectrum of individual Ln ions is further convoluted by the addition of exchange interactions between neighbouring metals. These create low lying excited states which are easily accessible from the ground state, and result in thermally activated QTM (TA-QTM). Extensive research in the field has shown that these problems can be overcome and the SMM properties can be optimised with careful modification of the geometry of the lanthanide ions, the topology of the molecule and the nature of the exchange interactions. However, even with these recent insights it remains a challenge to correlate magnetic behaviour with structural properties. Each chapter of this thesis aims to elucidate the correlation between the structure and magnetic properties of novel lanthanide containing molecular systems exploiting the proligands 2-(hydroxymethyl)pyridine (hmpH) and 6-methyl-2-(hydroxymethyl)pyridine (mhmpH). Chapter 2 describes the structural and magnetic studies of six analogous, highly symmetric triangle-in-triangle 3d-4f molecules with general formula [NiII 3LnIII 3(hmp)12](ClO4)3·3MeCN (where Ln = Gd, Tb, Dy, Ho, Er, Y). Magnetic susceptibility measurements reveal that they do not act as SMMs. Fitting of experimental susceptibility data for the [Ni3Gd3] analogue afforded JGdGd = -0.02 cm-1 and JGdNi = -0.32 cm-1; the antiferromagnetic exchange interactions leading to a vast increase in the number of potential relaxation pathways through the presence of low-lying excited states. Chapter 3 describes the structural and magnetic studies of two structurally similar, butterfly-like molecules, namely [MII2LnIII2(hmp)6(NO3)4(MeCN)2]·MeOH (BF1) and[MII2LnIII 2(mhmp)6(NO3)4]·MeCN (BF2); where M = Ni, Zn; Ln = Gd, Tb, Dy, Y; hmpH = 2- (hydroxymethyl)pyridine and mhmpH = 6-methyl-2-pyridinemethanol. The subtle changes in the molecular structure and lanthanide geometry results in a change in the 3d-4f exchange interaction which in turn affect the SMM properties. Fitting of the susceptibility data of the [Ni2Gd2] complex afforded JNiNi = 1.09 cm-1 and JNiLn = 0.7325 cm-1 for BF1 and JNiNi = -0.32 cm-1 and JNiLn = 0.52 cm-1 for BF2. The [Zn2Dy2] analogue of BF1 and BF2 exhibits SMM behaviour with Ueff = 38.58 cm-1 and 72.45 cm-1, respectively. The [Ni2Dy2] analogue of BF2 possesses a Ueff = 11.96 cm-1 whereas the analogue in BF1 displays no SMM behaviour. The improvement in SMM behaviour in BF2 is due to the improvement of single-ion behaviour of the lanthanide ions, paired with a weaker JNiLn value. Chapter 4 describes the structural and magnetic studies of the heptanuclear disc-like structure [CdII4(DyIII(3-n)YIIIn)(hmp)12(NO3)3](ClO4)2·3MeCN (where n = 0, 1, 2 or 3), in which the diamagnetic CdII act as a spacer between the DyIII ions thereby removing JDyDy interactions. The [Cd4Dy3] analogue shows a Ueff = 295.87 cm-1 and butterfly hysteresis loops at T ⪝ 2 K. Simulation of the experimental data shows that JDyDy (Jtotal) = -0.13 cm-1. Site dilution (replacing paramagnetic DyIII with diamagnetic YIII) further decreases the JDyDy interactions, increasing the coercivity of the hysteresis loops by decreasing QTM and TA-QTM processes.
Supervisor: Brechin, Euan ; Inglis, Ross Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: molecular magnetism ; Single-Molecule Magnets ; Quantum Tunnelling of the Magnetisation ; QTM ; TA-QTM