“Molecular Magnets are single molecules consisting of a magnetic core and shielding organic ligands. “Molecular Magnets with versatile dimensionality, whether they are zero-dimensional single-molecule magnets (SMMs) or one-dimensional single-chain magnets (SCMs) have wide potential applications as they permit investigation of the fundamental aspects lying at the interphase of quantum and classical physics at the nanoscale level. These magnets are ultimate in small size magnetic memory,possess sharply defined size, amenable to ligand variation, easily soluble and sub-nanoscale dimensions.
Molecular Nanomagnets (MNMs) have huge potential application in high-density information storage, quantum computing, q-bits, molecular spintronics, molecular refrigerants and miniaturisation of information storage devices (hard disk drives). This arises due to their intrinsic slow relaxation of magnetization and large anisotropic barrier height (Ueff) for magnetization reorientation. This is essentially due to the high spin ground state(S) and large negative zero-field splitting (D).
Molecular Nanomagnets (MNMs) open a new way towards high density magnetic information storage; it indeed becomes possible to dream of anisotropic molecular systems with high spin, assembled from the bottom or “bottom up” from small molecular precursors,on which it would be possible to store information on a single molecule.The challenge is formidable but gives a remarkable field for synthetic chemists, quantum physicists and engineers to work together in synergy. To finally use Molecular Magnets as Bits in a binary System an implementation to a medium is required.

Single molecule magnets (SMMs) could retain magnetisation in the absence of a magnetic field. They Represent the smallest possible magnetic storage device, retaining information in a single molecule rather than in a magnetic particle or an array of particles. SMMs undergo quantum tunnelling and are possible q-bits in quantum computers. “A single molecule that behaves as a nanoscale magnet below a critical temperature. i.e. displays hysteresis of molecular origin”.
A SMM can be regarded as a molecule which exhibits slow relaxation of magnetization of purely molecular origin. It is molecule which will be magnetized in presence of magnetic field and even will retain its magnetization on removal of the field. This feature is solely molecular characteristics and no intermolecular interactions are needed to spur such phenomenon. This differentiates SMMs from its traditional bulk magnet congeners.). The large ground spin state (S) of a molecule can be determined by exact diagonalization using irreducible tensor methodology. This S ground state possesses 2S+1 spin microstates which will correspond to the mS state in the absence of transverse anisotropy. Zero-field splitting can also be expressed with spin Hamiltonian which includes D(axial zfs; changes the energies of the MS states, but not the characters) and E(rhombic/transverse zfs; mixes the MS states and varies the characters of the microstates).
The prerequisite to exhibit SMM characteristics are to have large ground spin state (S) and appreciable amount of magnetic anisotropy (zero-field splitting). To be an active SMM, a molecule has to show relaxation of magnetization below c characteristic blocking temperature (TB) which originates from large spin ground state (S, large number of unpaired electrons) in combination with the presence of large,negative Ising (easy axis) type of magnetic anisotropy(zfs,D) whose cut-off values are being provided by S2|D| or (S2-1/4) |D| for integer and half-integer spins respectively.

The first SMM was discovered in a coordination complex made of 12 oxide and acetate bridged manganese ions, with formula [Mn12O12(O2CMe)16(H2O)4](Mn12Ac) have received appreciable attention due to its unusually large magnetic moment and magnetic bistability. This complex shows large S=10 ground state, and associated with large negative magnetocrystalline anisotropy barrier of 70K which has resulted a characteristic relaxation time and magnetic hysteresis below a blocking temperature (TB) of 3K.Mn12Ac molecule shows axial zero-field splitting parameter D of -0.50 cm-1.
To-date,considerable efforts have been devoted to the development of the fascinating area aiming at the enhancement of both the TB and Ueff. Recent breakthrough in this field occurred with the discovery of two coordinate linear Fe(I) complex [K(crypt-222)][Fe(C(SiMe3)3)2], which leads to large barrier height of 226 cm-1. This is essentially due to the suitable high symmteric ligand field resulted due to the low coordination symmetry around the metal ion.

Single-Chain Magnets (SCMs) are 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. Despite the dependence on D and S energy barrier in SCMS also depend on magnetic exchange parameter. Due to the hysteresis, 0-D systems have been called single-molecule-magnets (SMMs) and 1D systems have analogously been called single-chain-magnets (SCMs), stressing the fact that the hysteresis arises from single-molecule or single-chain properties. This immediately implies that SMMs and SCMs can be made as small as the single constituent units without losing their magnetic properties.
Single Molecule Magnets are a promising path to increase data density. The hurdle which still has to be overcome is to find a way of connecting the molecules to the macroscopic world. If this finally becomes possible, the benefit will be a thousand fold increase in density.