Single-molecule magnet
A single-molecule magnet (SMM) is a metal-organic compound that has superparamagnetic behavior below a certain blocking temperature at the molecular scale.
In this temperature range, an SMM exhibits magnetic hysteresis of purely molecular origin.
[4][5][6] This manganese oxide compound features a central Mn(IV)4O4 cube surrounded by a ring of 8 Mn(III) units connected through bridging oxo ligands, and displays slow magnetic relaxation behavior up to temperatures of ca.
Along with raising the blocking temperature, efforts are being made to develop SMMs with high energy barriers to prevent fast spin reorientation.
This definition is the current standard for comparison of single-molecule magnet properties, but otherwise is not technologically significant.
[16] Dy-metallocenium salts are the most recent SMM to achieve the highest temperature of magnetic hysteresis, greater than that of liquid nitrogen.
[9] The magnetic coupling between the spins of the metal ions is mediated by superexchange interactions and can be described by the following isotropic Heisenberg Hamiltonian: where
This barrier depends on the total spin of the molecule's ground state and on its magnetic anisotropy.
It has the remarkable property of showing an extremely slow relaxation of their magnetization below a blocking temperature.
[27] Single-molecule magnets are also based on iron clusters[15] because they potentially have large spin states.
[29] Theoretical computations showed that approximately two magnetic electrons are localized on each Fe atom, with the other atoms being nearly nonmagnetic, and the spin–orbit-coupling potential energy surface has three local energy minima with a magnetic anisotropy barrier just below 3 meV.
[33] A single-molecule magnet is a system of many interacting spins with clearly defined low-lying energy levels.
Anisotropy ensures that a collection of independent spins would be advantageous for quantum computing applications.
Superposition and interference of the independent spins also allows for further simplification of classical computation algorithms and queries.
[34] The quantum search problem typically requests for a specific element to be retrieved from an unordered database.
Single molecular magnets are considered ideal for this function due to their cluster of independent spins.
A study conducted by Leuenberger and Loss, specifically utilized crystals to amplify the moment of the single spin molecule magnets Mn12 and Fe8.
[34] Another approach to information storage with SMM Fe4 involves the application of a gate voltage for a state transition from neutral to anionic.
Using electrically gated molecular magnets offers the advantage of control over the cluster of spins during a shortened time scale.
[16] The specific mode of information transfer includes DVD to another readable medium, as shown with Mn12 patterned molecules on polymers.
A machine learning approach using experimental data has been able to predict novel SMMs that would have large entropy changes, and therefore more suitable for magnetic refrigeration.
[36] The main SMM characteristics that contribute to the entropy properties include dimensionality and the coordinating ligands.
In addition, single-molecule magnets have provided physicists with useful test-beds for the study of quantum mechanics.
Macroscopic quantum tunneling of the magnetization was first observed in Mn12O12, characterized by evenly spaced steps in the hysteresis curve.
[37] The periodic quenching of this tunneling rate in the compound Fe8 has been observed and explained with geometric phases.
