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Séminaire Magnétisme non conventionnel

Jeudi 18 Mai à 14h00,
Salle Remy Lemaire, K223

Orateur : Vladimir S. MIRONOV (Shubnikov Institute of Crystallography, Moscow)
"Molecular engineering of high-temperature single-molecule magnets : New building blocks, principles and approaches."

Abstract

New principles and approaches for molecular engineering of high-temperature single-molecule magnets (SMM) are discussed. SMMs are high-spin molecules exhibiting slow magnetic relaxation and magnetic hysteresis of purely molecular origin below characteristic blocking temperature TB. Since the discovery of SMMs in the 1990s, they attract extensive interest as promising materials for high-density information storage, quantum computing and molecular spintronics [1]. SMM behavior originates from a combined effect of negative uniaxial magnetic anisotropy D and large ground-state spin S resulting in an energy barrier Ueff = |D|S2 between the two lowest degenerate |+S> and |–S> quantum spin states. However, the blocking temperature TB is still too low for practical applications of SMMs (TB < 5 K in most cases). In recent years, considerable progress in increasing Ueff and TB values has been achieved for lanthanide-based SMMs exhibiting the record barrier (Ueff = 1815 K) and blocking temperature (TB = 20 K), which are far superior to those of 3d-based SMMs (Ueff = 90 K and TB = 4.5 K) [1,2]. However, further progress is difficult since these advances exploit single-ion magnetic anisotropy, which is already close to the physical limits for 4f and 3d ions. Alternative approaches toward high-TB SMMs are discussed with a special emphasis on pair-ion magnetic anisotropy resulting from anisotropic spin coupling. In this respect, especially promising are orbitally-degenerate pentagonal-bipyramidal 4d and 5d complexes (such as [MoIII(CN)7]4–) which provide highly anisotropic (Ising-type) spin coupling with attached high-spin 3d ions, such as –JzSz5dSz3d – Jxy(Sx5dSx3d + Sy5dSy3d) with |Jz| > |Jxy|. In this case, the barrier Ueff is controlled by exchange parameters : namely, Ueff is directly proportional to |Jz–Jxy| and to the number n of exchange-coupled 5d–3d pairs in a SMM (Figure) [3]. This provides a very efficient straightforward strategy for scaling Ueff and TB values by means of enhancing exchange parameters Jz, Jxy and increasing the number of pentagonal 5d complexes in a SMM molecule [3]. New building blocks for the development of high-TB SMMs are presented, which are based on orbitally-degenerate pentagonal 5d complexes with planar pentadentate macrocyclic ligands. General principles of engineering of large Ueff barriers are formulated and some specific 5d-3d based molecules with expected high-TB SMM behavior are discussed.

[1] S. Gao, Molecular Nanomagnets and Related Phenomena, Springer : Berlin, 2015.
[2] J. Tang, P. Zhang, Lanthanide Single-Molecule Magnets, Springer : Berlin, 2015.
[3] V.S. Mironov, Inorg. Chem., 54 (2015) 11339-11355.

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