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Introduction to Single-Molecule Magnets

Malgorzata Holynska

Philipps University Marburg, Department of Chemistry, Hans-Meerwein Straße, 35043 Marburg, Germany

This chapter will answer the following questions:

• What are single-molecule magnets (SMMs)?
• What are the related materials - single-chain magnets, single-ion magnets, single-toroid magnets?
• What is the history of SMMs?
• What applications are possible?
• What are the general and recent trends in SMM science?

1.1 What Is a Single-Molecule Magnet?

We will start with a definition crucial to the whole book, namely of that of a single-molecule magnet (). The term is self-explanatory, meaning a single molecule which is a magnet. But what does it take to be a magnet? Normally, we consider bulk magnets, e.g. materials in which the magnetic moment stems from the presence of unpaired electrons. Their spins tend to form parallel/antiparallel arrangements within areas called domains, separated by the so-called Bloch walls. The currently known most powerful "classical" magnets include SmCo5 and Nd2Fe14B. On the other hand, if we consider SMMs, we must assume that a single molecule is a "magnetic domain." The molecule can be magnetized and the attained magnetization relaxes slowly on removal of the external magnetic field. This new concept of molecular magnetism has experimental consequences, such as out-of-phase signals in ac studies of the magnetic properties. A detailed discussion of these will be included in Chapter 2 while an overview of the basic knowledge of them is addressed in this chapter. The unusual quantum phenomena found in SMMs open the possibilities of unprecedented applications, e.g. in molecular spintronics/quantum computing. These applications will also be reviewed in the following sections, underlining the interdisciplinary character of SMM science.

As the famous physicist Richard Feynman stated, "there's plenty of room at the bottom." "Bottom" is the world of atoms and molecules, also magnetic molecules being SMMs. Starting with this "bottom" to create a magnetic material represents the so-called "bottom-up" approach [1]. On the other hand, magnetic macromaterials, such as Prussian blue, may be taken to the "bottom" as a result of a "top-down" approach [2].

Molecular architecture is the key to understanding the magnetic properties of SMMs; therefore, X-ray diffraction studies have played a crucial role in the characterization of SMMs and require optimization of their synthesis/crystallization conditions, as reviewed in Chapter 2. Gaining insight into their magnetic properties through superconducting quantum interference device () magnetometry or electron paramagnetic resonance () studies requires theoretical interpretation and often advanced computations, as described in Chapters 3 and 4. Further characterization methods include paramagnetic nuclear magnetic resonance (), magnetic circular dichroism or, in special cases, Mössbauer spectroscopy, as described in Chapter 5.

There are other magnetically interesting molecules related to SMMs, such as single-chain magnets (s), single-ion magnets (s), and single-toroid magnets (s). These materials will be briefly described in the next three sections. In addition, there will be some brief explanation of possible future applications and how they could work.

1.1.1 Single-Chain Magnets (SCMs)

SCMs comprise ferromagnetic chain-like molecules that do not interact magnetically with each other. The modeling of their magnetic properties is different than that for SMMs: classical spin Heisenberg chain or Ising chain equations finds application in this case.

The first prediction of SCM behavior based on their magnetic properties was published by Glauber in 1963 [3]. The possibility of slow relaxation of magnetization in zero-dimensional systems was postulated. The necessary conditions to enable this phenomenon are:

• J/J´?>?104 (where J describes the magnetic interaction within the chain and J´ between the chains),
• 1D Ising ferromagnetism or ferrimagnetism.

After coining the term SMM in the 1990s, Gatteshi and coworkers [4] were the first researchers to show that these conditions could be realized experimentally. They demonstrated this by using the example of a coordination polymer comprising Co(hfac)2 and radical (NITPhOMe?= 4´-methoxy-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) moieties. The polymeric chains behaved as 1D ferrimagnets with the easy axis of magnetization along their trigonal axis and slow relaxation of magnetization. On the other hand, these properties could not be found in the manganese(II) analogue and it was also proven that they could not result from a spin glass behavior or 3D ordering.

Interestingly, SCMs can be constructed from SMMs. A good example is the first strategy applied to organize ferromagnetic SMMs into 1D chains as reported by Clérac and coworkers [5] The applied building blocks were MnIII-NiII-MnIII linear trinuclear complexes, [Mn2(5-Rsaltmen)2Ni(pao)2(phen)](ClO4)2 (where R?=?Cl or Br, Rsaltmen2- = N,N´-(1,1,2,2-tetramethyl-ethylene)bis(5-R-salicylideneiminate). The SMM properties of these compounds, with ground state of S = 3 and uniaxial magnetic anisotropy around ~2.4?K, play a crucial role in the theoretical description of the resulting SCMs.

Currently, a common strategy applied to obtain SCMs is the synthesis of metal - bulky radical chains where large organic substituents help to isolate the chains magnetically. Three options are recommended: ferromagnetic, ferrimagnetic, or spin-canted chains [6].

In recent years, much progress has been noted in SCM science. A review covering the progress until 2014 was published by Brooker and coworkers [7].

Cassaro, Vaz et al. [8] reported on SCMs with a very high blocking temperature of ~13.2?K and a very high coercive field of 49?kOe at 4.0?K for one of them. The compounds are based on metal (MnII or CoII) hexafluoroacetylacetonates and (1-naphthyl nitronylnitroxide) radicals. The remarkably high values of the relevant properties were detected in the CoII complex.

Zhang, Wang, Song, and coworkers [9] synthesized SCMs based on octacyanotungstate with highest energy barriers for cyanide compounds of 252(9) and 224(7) K for (Ph4P)[CoII(3-Mepy)2.7(H2O)0.3WV(CN)8]?·?0.6H2O, and (Ph4As)[CoII(3-Mepy)3WV(CN)8] (2,3-Mepy?=?3-methylpyridine), respectively.

The use of such cyanometalate complexes is very promising due to the possibility to construct coordination polymers and also because it is available for many metals in different oxidation states. Even quite unstable building blocks such as [Mn(CN)6]3- can be used: Dunbar and coworkers reported on an SCM {[(tptz)MnII(H2O)MnIII(CN)6]2MnII(H2O)2} n ?·?4n?MeOH·2n?H2O (tptz?=?2,4,6-tri(2-pyridyl)-1,3,5-triazine) with an energy barrier of 40.5(7) K [10].

1.1.2 Single-Ion Magnets (SIMs)

In the case of single-ion magnets (SIMs or s - single-ion molecular magnets or - mononuclear single-molecule magnets) the magnetic properties are governed by a single ion, without direct coupling to any other magnetic centers. Here, particularly suitable building blocks are Dy3+, Tb3+, or Ho3+due to their high-spin ground state and unquenched orbital angular momentum, resulting in high magnetic anisotropy.

In such simple systems the first significant progress in increasing the energy barrier was achieved with the introduction of Ishikawa's "double-decker" bis(phthalocyaninato)lanthanide ions [11].

Recently, SIMs were found also in 3d metal and actinide complexes. From examples of these materials it became evident how small changes in the central metal ion coordination environment can impact the magnetic properties. The main problem in enhancing the lanthanide-based SIMs turned out to be the quantum tunneling of magnetization.

The first endohedral SMM, DySc2N@C80, was reported by Greber and coworkers [12]. A single Dy3+ ion was encapsulated in a diamagnetic carbon cage along with two Sc3+ ions and nitride counterions. Its ground state could be stabilized by the ligand field. By a combination of element-specific X-ray magnetic circular dichroism () and SQUID magnetometry the SMM behavior of this compound was shown to originate from the single Dy3+ ion. Especially for the magnetically...