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Slow Magnetic Relaxation in Single-Chain and Light Actuated Single-Molecule Magnets

  • Author(s): Feng, Xiaowen
  • Advisor(s): Long, Jeffrey R
  • et al.
Abstract

The work presented in this dissertation describes the synthesis and characterization of single chain and single molecule magnets that display slow magnetic relaxation at low temperatures. A wide variety of techniques and spectroscopic methods are covered, including SQUID measurements, high field and high frequency EPR spectroscopies, structural analysis, infrared spectroscopies. Chapter one provides a brief introduction to single chain and single molecule magnets, characterization method, and synthetic strategies toward these materials.

In Chapter Two Cyano-bridged single-chain magnets of the type L4FeReCl4(CN)2, where L = diethylformamide (DEF) (1), dibutylformamide (DBF) (2), dimethylformamide (DMF) (3), dimethylbutyramide (DMB) (4), dimethylpropionamide (DMP) (5), and diethylacetamide (DEA) (6), have been synthesized to enable a systematic study of the influence of structural perturbations on magnetic exchange and relaxation barrier. Across the series, varying the amide ligand leads to Fe-N-C bond angles ranging from 154.703(7)° in 1 to 180° in 6.Variable-temperature dc magnetic susceptibility data indicate ferromagnetic exchange coupling in all compounds, with the strength of exchange increasing linearly, from J = +4.2(2) cm–1 to +7.2(3) cm–1, with increasing Fe-N-C bond angle. Ac magnetic susceptibility data collected as a function of frequency reveal that the relaxation barriers of the chain compounds rises steeply with increasing exchange strength, from 45 cm–1 to 93 cm–1. This examination demonstrates that subtle tuning of orbital overlap, and thus exchange strength, can engender dramatic changes in the relaxation barrier. Indeed, the perfectly linear Fe-N-C bond angle in 6 leads to one of the highest barriers and coercive fields yet observed fora single-chain magnet.

Chapter Three briefly discusses model compounds (NBu4)2[ReCl4(CN)2] (1), (DMF)4ZnReCl4(CN)2 (2), and [(PY5Me2)2Mn2ReCl4(CN)2]-(PF6)2 (3) synthesized to probe the origin of the magnetic anisotropy barrier in the one-dimensional coordination solid (DMF)4MnReCl4(CN)2 (4). High-field EPR spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in compounds 1- 3. These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy-plane, resulting in magnetic bistability and slow relaxation behavior. Within this picture, it is the transverse E term associated with the ReIV centers that determines the easy-axis and the anisotropy energy scale associated with the relaxation barrier. The results demonstrate for the first time that slow magnetic relaxation can be achieved through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy, thus providing a new direction in the design of single-molecule and single-chain magnets.

In Chapter Four, molecules exhibiting bistability have been proposed as elementary binary units (bits) for information storage, potentially enabling fast and efficient computing. In particular, transition metal complexes can display magnetic bistability via either spin-crossover or single-molecule magnet behavior. I now show that the octahedral iron(II) complexes in the molecular salt [Fe(1-propyltetrazole)6](BF4)2, when placed in its high-symmetry form, can combine both types of behavior. Light irradiation under an applied magnetic field enables fully reversible switching between an S = 0 state and an S = 2 state with either up (MS = +2) or down (MS = –2) polarities. The resulting tristability suggests the possibility of using molecules for ternary information storage in direct analogy to current binary systems that employ magnetic switching and the magneto-optical Kerr effect as write and read mechanisms.

In Chapter Five, the mononuclear complex trans-(Bu4N)2[ReIVCl4(CN)2] ⋅2DMA (DMA = N,N-dimethylacetamide) with D = +11.5 cm-1 (and |E| = 3.1 cm–1) is shown to display Orbach-type slow relaxation of magnetization with an energy barrier for spin-reversal of Ueff = 26.7 cm-1 in well agreement with the spectroscopically and magnetically derived energy gap. This energy barrier represents the highest record for 4d/5d mononuclear single-molecule magnets yet observed.

In Chapter Six, one-dimensional chain solid Co(hfac)2(NITPhenOMe)(DMF)x is shown to display stronger intrachain interaction between Co(II) and radical centers with J = -97 cm-1, compared to that of -76 cm-1 in the original Co(hfac)2(NITPhOMe) compound. Further magnetic measurements revealed a energy barrier of Δτ = 251 cm-1 with magnetic blocking up to 12 K. This energy barrier and blocking temperature represents the highest record yet for any single-chain magnets reported.

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