Lakshmi Krishnaa Suresh, Simi M., Dr. Gopal K
Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri 690525, Kerala, India

As we continue to advance in science and technology, the demand for more efficient and powerful data storage devices keeps on increasing. One of the potential solutions that scientists are exploring that the use of single molecule magnets (SMMs) as the information storage material with a nanometric scale. SMMs are a class of metallo-organic compounds containing high-spin transition or lanthanidemetal ions exhibiting retention of magnetic information (magnetization with hysteresis effects) even under zero or weak applied magnetic field1.Thus, these molecules can retain their magnetic orientation even in the absence of an external magnetic field, making them ideal candidates for information storage applications. Recent advances in synthetic chemistry have led to the development of SMMs with various shapes, sizes, and magnetization properties. These molecules have attracted significant attention lately since they can perform logical operations on their magnetic state, potentially leading to the development of quantum computing2. However, the use of SMMs in practical applications is limited by their low magnetic anisotropy as well as very low operating temperatures called “blocking temperatures, TB” (near to liquid Helium temperature, 0-10 K). Researchers are exploring diverse strategies, such as doping of SMMs with various combination of metal ions (transition and lanthanide metal ions) with ligands and surface coating functionalization, to enhance their magnetic properties together with increasing their TB to room-temperature3.
Two criticalparameters of a magnetic molecule, i.e., large spin ground state (S≠ 0) and large negative magnetic anisotropy (D ˂ 0; uniaxial anisotropy), making it a tiny magnet called SMM. The non-zero and large spin ground state (S≠ 0) of a molecule accomplished by the assembly of mono- or multi-nuclear paramagnetic metal ions with appropriate organic ligands4. The magnetic anisotropy characterized by the parameter D, generates the energy barrier U= │D│S2(Figure 1)thatresiststhe spin reversal leads to the highest spin state of the molecule pointing along with a preferred axis (easy axis) even in the absence of external magnetic field, making it a nanoscale magnet.With an appropriate field and temperature conditions, a stable magnetization in either of the–S or +S state can be preserved for a long period of time (in years), due to separation of those two spin states by an energy barrierU (parabolic anisotropy barrier)5. Thus, the existence of such energy barrierUpreventing magnetization from thermally relaxing. These effects are primarily an intra-molecular phenomenon, and such magnetic molecules thus literally behave as nanoscale magnets, referred as SMMs⁶.

The discovery of SMM behaviour with a [Mn12O12(O2CMe)16(H2O)4]complex, [Mn12Ac] (Figure 2) in 1980s led to the rise of the concept “molecular magnetism” which accelerated the research towards the various types SMMs alongside with their functional applications.Complex[Mn12Ac] with the ground state spin of S = 10,exhibitedthe delayed relaxation of magnetization (with hysteresis property) at very low temperatures (TB = 3 K),it was characterized with the magnetic anisotropyparameter D=–0.46cm-1andanisotropybarrier U = 62 K⁷.

Figure 2.(a) Structure of [Mn₁₂Ac] complex⁷. (b)Magnetization versus magnetic field hysteresis loop for Mn12-acetate Complex.Figures are reproduced with permission¹

The concept of using molecular magnetic materials as information carriers and processors, was introduced in the late 1990s, and since then, researchers have been successful in making several SMM-based devices. However, one of the main challengesthat the distinctive magnetic properties of SMMs were only observed at extremely low temperatures, typically below –250°C (blocking temperatures, TB; SMMs operating temperature). Nevertheless, recent advances in the field have made it possible to create SMMs that exhibit characteristic magnetic properties at higher temperatures (closer to room-temperature), thereby increasing their potential as information storage devices8. For instance, in 2018, a group of researchers from the University of Manchester reported a new type of SMM that retained its magnetic properties up to –213°C, which is the highest ever recorded for an SMM.Apart from its potential application in information storage, a single molecule magnet’s versatility, robustness, and smaller size have also made them promising candidates for medicinal applications. The magnetic properties of SMMs could be useful in developing targeted drug delivery systems, for example, by using an external magnetic field to control the molecule’s position and release the drug at a specific location with a desired time intervals⁹.

Compared to “traditional” metal ion – ligand coordination chemistry, organometallic chemistry has had little influence on the development of SMMs because first organometallic SMM was identified just in 2010¹⁰

However, organometallic chemistry and chemistry using ligands classified as extended organometallic family members have seen a lot of interesting advancements in recent years. Several of these advances in magnetic hysteresis and anisotropy barriers in organometallics have also resulted in striking”world records”. Separate sections cover organolanthanide SMMs, organoactinide SMMs, and low-coordinate transition-metal SMMs.The final category is a crucial emerging area within the SMM industry, and organometallic chemistry is expected to play a larger role¹⁰

Taking everything into account, single molecule magnets are a promising technology that has the potential to revolutionize the field of quantum computing, information storage and medical industries. Although significant progress has been made in the field, there are still several challenges to overcome before SMMs become a commercially viable technology. The researchers are working towards enhancing their magnetic properties and increasing their operating temperatures. Despite the challenges faced, SMMs represent a new paradigm in the field of materials science, offering exciting possibilities for future technological developments¹⁰

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