Endohedral metallofullerene single molecule magnets

Abstract

Nanoscale magnetic materials have been attracting enormous interests during the past few decades due to the rapid development of high-density magnetic storage devices. A breakthrough in nanomagnetism is the discovery of single molecule magnets (SMMs), which exhibit slow relaxation and quantum tunneling of the magnetization resulted from a molecular-based blocking anisotropy, and thus have potential applications in high-density data storage, molecular spintronics and quantum computing. Since the report of Mn12-complex as the first SMM by Sessoli et al. in 1993, a large number of SMMs based on organometallic coordination compounds have been synthesized. Compared to the traditional bulk magnets, the magnetic properties of SMMs are determined by their instrinsic spin structures resulting in slow magnetic relaxation. SMM is characterized by a bi-stable magnetic ground state and a certain spin-reversal barrier ( U eff). Additionally, hysteresis loops can be observed below the blocking temperature ( T B). Therefore, SMMs with excellent magnetic properties always have high T B and U eff. U eff depends strongly on spin ground state ( S ) and magneto-crystalline anisotropy ( D ), and can be expressed with the formula U eff =| D|S 2. For transition metal-based SMMs, their low magneto-crystalline anisotropies ( D ) result in relative low U eff. In recent years, lanthanide based SMMs have been studied intensively and become the most promising high-performance SMMs, owning to their strong single ion anisotropy. Encapsulating metal atoms or metallic clusters into fullerene cages generate a special class of fullerenes, the so-called endohedral metallofullerenes (EMFs), for which the encapsulation of metal atoms endow EMFs more fascinating properties compared to empty fullerenes. For instance, EMFs are paramagnetic when the entrapped metal ions have unpaired electrons. Furthermore, EMFs were able to show excellent SMM properties when the encapsulated clusters induce appropriate ligand-fields. EMFs with lanthanide open up a new avenue in the field of SMMs in 2012 when single molecule magnetism was proved for the metal nitride clusterfullerene (NCF) DySc2N@ I h-C80. Fullerene cages are stable in air and thus they can protect the magnetic ions from ambient conditions, and the negatively charged nonmetallic atom(s) within the cluster and the negatively charged carbon atoms of fullerene cages provide magnetic anisotropy which is necessary for single molecule magnetism. Moreover, the molecular structures of EMFs can be modulated by varying the encapsulated metal ions, metal cluster and cage isomer; hence the research on EMF-SMMs has been developed rapidly since 2012. Up to now, metal nitride clusterfullerenes (NCFs), metal carbide clusterfullerenes (CCFs), metal oxide clusterfullerenes (OCFs), metal sulfide clusterfullerenes (SCFs), metal cyanide clusterfullerenes (CYCFs) and dimetallofullerenes (di-EMFs) have been confirmed as SMMs. In this paper, we present an exhaustive review on all kinds of EMF-SMMs reported to date including their 1D, 2D and 3D assemblies. We mainly discuss the influences of the types and the numbers of the encapsulated metal ions and non-metal atoms on the magnetic properties of EMF-SMMs. Finally, the prospects and challenges of EMF-SMMs are also discussed.