Research

Molecular Magnets (分子磁体)

 

 

Why molecular magnets?  The pace of miniaturizing electronic devices and the desire for higher information storage capability have pushed modern hard-drive technology to the edge of the superparamagnetic limit.  Beyond this limit, we have to resort to a new class of information storage material or fundamentally improve the current technology. Molecular magnets, possessing well-defined nano-sized inorganic cores encapsulated by organic shells, have been heralded as an important step in the miniaturization process of future memory devices.  Furthermore, these molecule-based materials can be potentially used in quantum information processing and low-temperature cooling applications.  The following endeavors are guided by these overarching research themes:

·           Single-Molecule Magnets (单分子磁体)and Single-Chain  Magnets(单链磁体)

Single-Molecule Magnets (SMMs) are usually high-nuclearity metal-cluster complexes with high ground-spin states and large axial magnetic anisotropy, which generate the energy barrier between different spin states.  With decreasing temperature, this energy barrier becomes increasingly significant, and the temperature below which the spins are “blocked” in one direction and cannot be easily reversed (that is, slow relaxation of magnetization) by thermal energy is termed “blocking temperature”.    The relaxation time increases exponentially as temperature decreases. This makes the molecules behave as a “magnet”, exhibiting large hysteresis loops possibly useful for magnetic information storage at the molecular level.  More interestingly, the hysteresis loops of SMMs are usually characterized with “steps”, caused by the quantum-tunnelling magnetism.  This unique feature suggests the potential use of SMMs for quantum computation.  The molecular structure, energy diagram, and magnetic hysteresis of the first SMM, the seminal dodecamanganese cluster, {Mn12}, are shown in Figure 1.  

 

Figure 1 Structure of {Mn12} (left). Color codes: brown, Mn(III), S = 2; green, Mn(IV), S = 3/2; orange, oxygen and grey, carbon. {Mn12} has a ground spin-state ST = 10 and an axial molecular magnetic anisotropy D = –0.5 cm-1, leading to an energy barrier of U = 71 K (see the figure, middle), which gives Mn12 “magnet-type” behavior only below the boiling point of helium (see the figure, right).

Single-Chain Magnets (SCMs) are usually anisotropic single-spins or SMMs that are “stringed” into one-dimensional chains. They can be regarded as a special class of SMMs that are composed of magnetically isolated and individually magnetizable chains. Although it is well established that one-dimensional magnetic systems with short-range interactions do not experience long-range order at a finite temperature, the bulk material may remain in a paramagnetic state just as SMMs, and can also display long relaxation times of the magnetization promoted by the combination of a large uniaxial anisotropy and large magnetic interactions between the high-spin magnetic units of the chain.  This type of one-dimensional system can thus behave as a magnet. Despite all the envisioned better-than-SMM properties of SCMs, successful examples of SCMs are still much fewer than SMMs, thus making the study of SCMs a field not only fundamentally stimulating but potentially rewarding with possibility of developing cutting-edge molecule-based magnetic materials and corresponding technologies.  

 

·           Geometrically Frustrated Magnets (几何阻措磁体)

Spin-frustration is defined as a system’s inability to satisfy simultaneously antiferromagnetic interactions between spins within the same magnetic entity.  This phenomenon is significant in the context of molecular magnetism as it can lead to macroscopic degeneracies and qualitatively new states of matter.  Spin-frustrated systems are ubiquitous, with geometrically frustrated magnets (GFMs) being one extensively studied family. As shown in Figure 2, the remaining spin orientations in the triangular and square plaquetts are “frustrated” if the magnetic interactions are to be antiferromagnetic between adjacent spins.

Figure 2 Basic frustrated plaquettes: a) a equilateral triangle, b) a tetrahedron, c) a square plane.

 

     By the sharing of corners, edges, or faces, molecular assemblies featuring the triangular and square plaquetts, structurally more sophisticated spin-frustrated lattices may be realized.   The 11 uniform Archimedean tilings (lattices) shown in Figure 3 are of particular interest as they are characterized by the basic connections of the triangles and squares in two-dimension.  The magnetic properties of these higher-dimensional systems are appealing as suggested by theoretical studies that revealed the interplay of geometric frustration and quantum fluctuations, leading to an exotic quatum paramagnetic spin-liquid ground state.

 Figure 3. The 11 Archimedean tilings. The T1, T2, T3, and T8 tilings are well-known as the triangular, square, honeycomb, and Kagomé lattices, respectively. In the vertex notation, nm, n is the number of sides in the m polygons joined at a vertex and if there are different types of polygons the nm notations are separated by periods.

 

       Except for the famous Kagomé lattice (T8) most of the other lattices remain largely unexplored. To realize these potentially intriguing targets, the practice of crystal engineering and the principles of coordination chemistry are fundamentally important. The recently reported ‘Maple-leaf’ (T4a and b in Figure 3) and the ‘star’ (T9 in Figure 3) featuring the trianglar [Fe3O(O2CR)6]+ sub-building units bridged bycarboxylate linkers clearly suggest the power of crystal engineering, a practice firmly rooted in coordination chemistry.  By employing judicioulsy designed spin-frustrated units, assemblies that have not yet been realized may be discovered, together with interesting magnetic properties.

 

·           Molecular Magnetic Refrigerants(分子磁致冷剂)

There is a tremendous desire to develop magnetic cooling systems as a replacement of the conventional vapor compression system that produces ozone-depleting chlorofluorocarbons.     Magnetic refrigeration works on the principle of magnetocaloric effect (MCE), originated from the coupling of a magnetic field with magnetic moments carried by itinerant or localized electrons and quantified in terms of temperature and/or entropy changes. Although the MCE has been known for more than 120 years, its applications for magnetic cooling have been hindered by the unavailability of inexepensive materials possessing large MCE

Equipped with the fundamental knowledge of coordination chemistry and facilitated by modern instrumentation for structural analysis and magnetic property investigation, we now have the ability to design molecules and supramolecules that are shown to display intriguing magnetic properties including large MCE.  Early examples include highly anisotropic single-molecule magnets such as cluster complexes of Mn and Fe.  More recent work suggests that heterometallic clusters containing both transition metal and lanthanide elements can exhibit huge MCE.  We will be pursuing such complexes and explore their properties in the context of making materials potentially useful for magnetic cooling.

 

·           Molecular Spintronics (分子自旋电子器件)

The idea of spin electronics (spintronics) has been built on the discovery of the giant magnetoresistance (GMR), tunneling effect, and tunneling magnetoresistance (TMR) effect, which has made a major impact on today’s daily life, for instance, by improving hard drive information storage. With the current pace of device miniaturization, the size limit of conventional bulk spintronic materials, down to a few nanometers and in the size regime of molecules, will soon be reached.  The concept of molecular spintronics naturally arises, the essence of which being molecules displaying similar or even better TMR than bulk materials.  Attractive applications of molecular spintronics range from the use of molecular magnetism to incorporate non-volatile memory into electronics, to the use of individual electron spins acting as a quantum bit in a quantum computer.  

Current investigations of magnetic molecule based spintronic devices have led to some exciting results such as the observation of a Coulomb blockade diamond (the phenomenon of voltage dependent electron tunneling currents) and the Kondo effect (increasing resistance as temperature decreases due to the scattering effect of magnetic ions).  More exciting results are expected to occur in some magnetic molecules with long spin coherence and spin relaxation times ascribed to a weak spin-orbit and hyperfine coupling to the environment. In addition, their spin states might be tuned at a single-molecule level by using local electric fields, but this has not yet been realized. This area is obviously still in its infancy and there is huge room for further development.

 

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