Mn12 Single-molecule Magnets Research Articles

Mn12 single molecule magnets deposited on μ-SQUID sensors: the role of interphases and structural modifications

Direct measurements of the linear ac susceptibility and magnetic relaxation of a few Mn12 monolayers deposited on a μ-SQUID sensor are reported. In order to integrate the molecules into the device, DPN has been the technique of choice. It enabled the structuration of the molecules on the most sensitive areas of the sensor without the need for any previous functionalization of the molecule or the substrate, while controlling the number of molecular units deposited on each array. The measurements reveal that their characteristic SMM behaviour is lost, a fact that is attributed to molecular distortions originated by the strong surface tensions arising at the molecular interphases.

Ultra-low temperature structure determination of a Mn12 single-molecule magnet and the interplay between lattice solvent and structural disorder

We have determined the ultra-low temperature crystal structure of the archetypal single-molecule magnet (SMM) [Mn12O12(O2CMe)16(H2O)4]·4H2O·2MeCO2H (1) at 2 K, by using a combination of single-crystal X-ray and single-crystal neutron diffraction. This is the first structural study of any SMM in the same temperature regime where slow magnetic relaxation occurs. We reveal an additional hydrogen bonding interaction between the {Mn12} cluster and its solvent of crystallisation, which shows how the lattice solvent transmits disorder to the acetate ligands in the {Mn12} complex. Unusual quantum properties observed in 1 have long been attributed to disorder. Hence, we studied the desolvation products of 1, in order to understand precisely the influence of lattice solvent on the structure of the cluster. We present two new axially symmetric structures corresponding to different levels of desolvation of 1, [Mn12O12(O2CMe)16(H2O)4]·4H2O (2) and [Mn12O12(O2CMe)16(H2O)4] (3). In 2, removal of acetic acid of crystallisation largely resolves positional disorder in the affected acetate ligands, whereas removal of lattice water molecules further resolves the acetate ligand disorder in 3. Due to the absence of acetic acid of crystallisation, both 2 and 3 have true, unbroken S4 symmetry, showing for the first time that it is possible to prepare fully axial Mn12–acetate analogues from 1, via single-crystal to single-crystal transformations.

Tight-binding model of Mn12single-molecule magnets: Electronic and magnetic structure and transport properties

We describe and analyze a tight-binding model of single molecule magnets (SMMs) that captures both the spin and spatial aspects of the SMM electronic structure. The model generalizes extended Huckel theory to include the effects of spin polarization and spin-orbit coupling. For neutral and negatively charged Mn12 SMMs with acetate or benzoate ligands the model yields the total SMM spin, the spins of the individual Mn ions, the magnetic easy axis orientation, the size of the magnetic anisotropy barrier and the size of the HOMO-LUMO gap consistent with experiment. For neutral molecules the predicted spins and spatial locations of the HOMO are consistent with the results of density functional calculations. For the total spin and location of the LUMO density functional theory-based calculations yield varied results while the present model yield results consistent with experiments on negatively charged molecules. For Mn12 SMMs with thiolate- and methylsulphide-terminated benzoate ligands (Mn12-Ph-Th) we find the HOMO to be located on the magnetic core of the molecule, but (unlike for the Mn12 SMMs that have previously been studied theoretically), we predict the LUMO and near LUMO orbitals of Mn12-Ph-Th to be located on ligands. Therefore we predict that for these Mn12 SMMs resonant and off-resonant coherent transport via near-LUMO orbitals, not subject to Coulomb blockade, should occur. We propose that this effect can be used to identify specific experimentally realized SMM transistors in which the easy axis and magnetic moment are approximately parallel to the direction of the current flow. We also predict effective spin filtering by these SMMs to occur at low bias whether the transport is mediated by the HOMO that is on the magnetic core of the SMM or by near LUMO orbitals located on the nominally non-magnetic ligands.

Detection of the intrinsic two energy gaps in individual Mn12 single molecule magnets

The deposition of Mn12-ac single molecule magnets on HOPG surfaces was investigated using microRaman spectrograph, scanning tunneling microscopy and spectroscopy (STM/S). The STM and Raman studies indicate that isolated Mn12-ac molecules remain intact upon physically absorbing on HOPG surfaces. Reproducible STS curves reveal that Mn12-ac molecules exhibits two unambiguous energy gaps, 2.34 and 0.87eV, respectively. The former is corresponding to the minority-spin gap of the inner Mn4+, while the later is the majority-spin gap from the outer Mn3+ ions. These findings can be reasonably interpreted according to the former band structure calculations on Mn12O12 molecules.

Transverse field Ising ferromagnetism in Mn12-acetate-MeOH

We report measurements of the magnetic susceptibility of single crystals of Mn$_{12}$-acetate-MeOH, a new high-symmetry variant of the original single molecule magnet Mn$_{12}$-acetate. A comparison of these data to theory and to data for the Mn$_{12}$ acetate material shows that Mn$_{12}$-acetate-MeOH is a realization of a transverse-field Ising ferromagnet in contrast to the original Mn$_{12}$ acetate material, in which solvent disorder leads to effects attributed to random field Ising ferromagnetism.

Frequency-domain terahertz transmission spectra of Mn3 and Mn12 single-molecule magnets

Frequency-domain terahertz transmission spectra of Mn3 and Mn12 single molecule magnets (SMMs) have been measured at different temperatures, and hence the anisotropic parameters D2 and D4 of the spin Hamiltonian \(\hat H = D_2 \hat S_z^2 + D_4 \hat S_z^4\) have been calculated. For Mn12 SMM, D2=−10.9 GHz and D4=−2.59×10−2 GHz, while for Mn3 SMM, D2=−22.0 GHz and D4 can be considered negligible. This suggests Mn3 SMM can be considered as a simpler and more suitable candidate for magnetic quantum tunneling research.

Preparation and magnetic properties of Mn12 clusters with 4-cyanobenzenecarboxylate ligand, [Mn12O12(O2CC6H4-p-CN)16(H2O)4] and its tetraphenylphosphonium salts

The new Mn12 magnetic clusters with 4-cyanobenzenecarboxylate ligand, [Mn12O12(O2CC6H4-p-CN)16(H2O)4] (1) and its singly (2) and doubly (3) reduced analogs as their tetraphenylphosphonium salts, have been synthesized and characterized by elemental analyses, Raman, ESI-MS spectra and magnetic measurements with a SQUID magnetometer. Unlike the known Mn12 oxocarboxylate clusters, which are very soluble in CH3CN or CH2Cl2, the complex 1 is not dissolved in organic solvents providing an indication for strong intermolecular interactions which lead to strong dipole–dipole interactions between clusters and affect the magnetic behavior. The one-electron and two-electron reduced clusters (2, 3) contain the bulk counterion and dissolve in CH3CN. They show magnetic properties characteristic for anion Mn12 single-molecule magnets.

Spin-filtering and switching effects of a single-molecule magnet Mn(dmit)2

We investigated spin-dependent transport properties of a single-molecule magnet Mn(dmit)2 with a coplanar or perpendicular conformation using first-principles density functional theory combined with nonequilibrium Green’s function method. It was found that the current flowing through the junction comprised of two Au leads and a Mn(dmit)2 molecule is high spin-polarized, up to a high efficiency of 82%, if the two ligands of the molecule are orientated in the same plane. The current is strongly suppressed when a ligand is rotated and perpendicular to the other. These results suggest that Mn(dmit)2 is a potential candidate for spin filters or molecular switches.

Magnetic field dependent transport through a Mn4 single-molecule magnet

We present a preliminary study of the single-electron transport response of a Mn4 single-molecule magnet in which pyridyl-alkoxide groups have been added to electrically protect the magnetic core and to increase the stability of the molecule during the experiments. Three-terminal single-electron transistors with nanogapped gold electrodes formed by electromigration and a naturally oxidized aluminum backgate were used to perform experiments at temperatures down to 240 mK in the presence of arbitrarily oriented magnetic fields. Coulomb blockade and electronic excitations that curve with the magnetic field and present zero-field splitting represent evidence of magnetic anisotropy. Level anticrossings and large excitation slopes are associated with the behavior of molecular states with high-spin values (S ∼ 9), as expected from Mn4.

Building of a novel Mn12 Single Molecule Magnet by assembly of anisotropic {Mn3(μ3-O)(salox)3}+ triangles

Assembly of triangular {Mn(III)(3)(O)(salox)(3)}(+) fragments mediated by azido ligands, results in the dodecanuclear [Mn(12)O(4)(salox)(12)(N(3))(4)(MeOH)(4)(H(2)O)(2)] complex with S = 8 ground state and SMM response.

Control of the transition temperatures of metallomesogens by specific interface design: application to Mn12 single molecule magnets

Reaction of the Single Molecule Magnet [Mn(12)O(12)(CH(3)CO(2))(16)(H(2)O)(4)] (Mn(12)) with mesogenic dendritic ligands Li (i = 4, 5) quantitatively yields functional clusters [Mn(12)O(12)(Li-H)(16)(H(2)O)(4)] (i = 4, 5) that self-organize into a thermotropic SmA-type liquid crystalline phase. The perturbation of the molecular interface by methylation of the terminal mesogenic cyanobiphenyl groups induces a significant decrease of the clearing temperature without affecting the magnetic properties and the supramolecular organization of the Mn(12)-based clusters.

Mn12 single-molecule magnet aggregates as magnetic resonance imaging contrast agents

Mn(12) single-molecule magnets have been dispersed in water through an emulsion-assisted self-assembly method with an improved stability in water, in order to investigate the use of Mn(12) as MRI contrast agents.

Effects of bonding type and interface geometry on coherent transport through the single-molecule magnetMn12

We examine theoretically coherent electron transport through the single-molecule magnet Mn$_{12}$, bridged between Au(111) electrodes, using the non-equilibrium Green's function method and the density-functional theory. We analyze the effects of bonding type, molecular orientation, and geometry relaxation on the electronic properties and charge and spin transport across the single-molecule junction. We consider nine interface geometries leading to five bonding mechanisms and two molecular orientations: (i) Au-C bonding, (ii) Au-Au bonding, (iii) Au-S bonding, (iv) Au-H bonding, and (v) physisorption via van der Waals forces. The two molecular orientations of Mn$_{12}$ correspond to the magnetic easy axis of the molecule aligned perpendicular [hereafter denoted as orientation (1)] or parallel [orientation (2)] to the direction of electron transport. We find that the electron transport is carried by the lowest unoccupied molecular orbital (LUMO) level in all the cases that we have simulated. Relaxation of the junction geometries mainly shifts the relevant occupied molecular levels toward the Fermi energy as well as slightly reduces the broadening of the LUMO level. As a result, the current slightly decreases at low bias voltage. Our calculations also show that placing the molecule in the orientation (1) broadens the LUMO level much more than in the orientation (2), due to the internal structure of the Mn$_{12}$. Consequently, junctions with the former orientation yield a higher current than those with the latter. Among all of the bonding types considered, the Au-C bonding gives rise to the highest current (about one order of magnitude higher than the Au-S bonding), for a given distance between the electrodes. The current through the junction with other bonding types decreases in the order of Au-Au, Au-S, and Au-H. Importantly, the spin-filtering effect in all the nine geometries stays robust and their ratios of the majority-spin to the minority-spin transmission coefficients are in the range of 10$^3$ to 10$^8$. The general trend in transport among the different bonding types and molecular orientations obtained from this study may be applied to other single-molecular magnets.

One-dimensional supramolecular assembly of an Mn12 single molecule magnet by ligand interactions

Abstract A new Mn12 complex was synthesized using ligand substitution reaction of Mn12–OAc with 4-(thiophen-3-yl)benzoic acid and complex’s structural and magnetic properties were analysed. [Mn12O12(O2CC6H4C4H3S)16(H2O)3]·14CH2Cl2 (1) crystallized in the P21/c space group. Intermolecular π–π interactions between phenyl and thiophene rings of two adjacent Mn12 molecules result in one-dimensional supramolecular assembly of 1 in the crystal. On the other hand, steric repulsion between the neighbouring molecules causes unusual ligand arrangement and coordination geometry of Mn(III) ion with five coordination. The ac magnetic study of 1 gives Ueff = 69.98 K and 1/τ0 = 1.456 × 108 s−1 and dc reduced magnetization measurement gives S = 10, g = 1.95 and D = −0.425 cm−1 showing that outer ligand distortion has little effect on the magnetic properties.

Anisotropy barrier reduction in fast-relaxingMn12single-molecule magnets

A novel angle-swept high-frequency EPR (HFEPR) technique is described that enables in-situ alignment of single-crystal samples containing low-symmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based method involves recording spectra at fixed frequency and field, while sweeping the field orientation. The method is applied to the study of a low-symmetry Jahn-Teller variant of the spin S = 10 Mn12 SMMs (e.g. Mn12-acetate). The low-symmetry complex is also an SMM, but with a significantly reduced barrier to magnetization reversal (Ueff ~ 43 K) and, hence, faster relaxation at low temperature in comparison with the high-symmetry species. Mn12 complexes that crystallize in lower symmetry structures exhibit a tendency for one or more of the Mn(III) Jahn-Teller axes to be abnormally oriented, which is believed to be the cause of the faster relaxation. An extensive HFEPR study of [Mn12O12(O2CCH2But)16(H2O)4].CH2Cl2.MeNO2 is presented in order to examine the influence of the abnormally oriented Jahn-Teller axis on the effective barrier. The reduction in the axial anisotropy, D, is found to be insufficient to account for the nearly 40% reduction in Ueff. However, the reduced symmetry of the Mn12 core gives rise to a very significant 2nd order transverse anisotropy, E ~ D/6. This, in turn, causes a significant mixing of spin projection states well below the top of the classical barrier. Thus, magnetic quantum tunneling is the dominant factor contributing to the barrier reduction in fast relaxing Mn12 SMMs.

Electronic and Magnetic Study of Polycationic Mn12 Single-Molecule Magnets with a Ground Spin State S = 11

The preparation, magnetic characterization, and X-ray structures of two polycationic Mn(12) single-molecule magnets [Mn(12)O(12)(bet)(16)(EtOH)(4)](PF(6))(14).4CH(3)CN.H(2)O (1) and [Mn(12)O(12)(bet)(16)(EtOH)(3)(H(2)O)](PF(6))(13)(OH).6CH(3)CN.EtOH.H(2)O (2) (bet = betaine = (CH(3))(3)N(+)-CH(2)-CO(2)(-)) are reported. 1 crystallizes in the centrosymmetric P2/c space group and presents a (0:2:0:2) arrangement of the EtOH molecules in its structure. 2 crystallizes in the noncentrosymmetric P4 space group with two distinct Mn(12) polycations, [Mn(12)O(12)(bet)(16)(EtOH)(2)(H(2)O)(2)](14+) (2A) and [Mn(12)O(12)(bet)(16)(EtOH)(4)](14+) (2B) per unit cell. 2A and 2B show a (1:1:1:1) distribution of the coordinated solvent molecules. Interestingly, bond valence sum calculations extracted from X-ray diffraction data indicate the presence of two Mn(2+) ions in the Mn(12) core for both 1 and 2. This finding is confirmed by X-ray absorption spectroscopy (XAS) measurements. A complete magnetic characterization, including subkelvin micro-SQUID magnetometry and inelastic neutron scattering (INS) measurements, permits to extract the parameters of the giant spin Hamiltonian of these polycations. Compared with the archetypal Mn(12) acetate, an increase in the value of the ground spin state from S = 10 to S = 11 together with a decrease in the effective energy barrier, is observed for 1 and 2. Such a result is consistent with the reduction of two Mn(3+) to the less anisotropic Mn(2+) ion in the structures.

Polarized light spectroscopy of azobenzene or disperse red 1 with Mn12 single-molecule magnets in PMMA film hybrid materials

Abstract A series of organic/inorganic hybrid materials containing the single-molecule magnet [Mn12(CH3COO)16(H2O)4O12]·2CH3COOH·4H2O with the photochromic compounds azobenzene or disperse red 1 cast into polymethyl methacrylate films was prepared. To understand the photomodulation of analogous mixtures’ AC susceptibility, we investigated the molecular alignment based on Weigert’s effect accompanying cycles of cis-trans photoisomerization by alternate irradiations with polarized UV and visible light. Polarized electronic spectra of the hybrid materials demonstrated gradually increasing anisotropy. These features suggest interactions between photochromic compounds and the Mn12 complex in polymethyl methacrylate.

Crystal lattice desolvation effects on the magnetic quantum tunneling of single-molecule magnets

High-frequency electron paramagnetic resonance (HFEPR) and AC susceptibility measurements are reported for a new high-symmetry Mn12 complex, [Mn12O12(O2CCH3)16(CH3OH)4].CH3OH. The results are compared with those of other high-symmetry spin S = 10 Mn12 single-molecule magnets (SMMs), including the original acetate, [Mn12(O2CCH3)16(H2O)4].2CH3CO2H.4H2O, and the [Mn12O12(O2CCH2Br)16(H2O)4].4CH2Cl2 & [Mn12O12(O2CCH2But)16(CH3OH)4].CH3OH complexes. These comparisons reveal important insights into the factors that influence the values of the effective barrier to magnetization reversal, Ueff, deduced on the basis of AC susceptibility measurements. In particular, we find that variations in Ueff can be correlated with the degree of disorder in a crystal which can be controlled by desolvating (drying) samples. This highlights the importance of careful sample handling when making measurements on SMM crystals containing volatile lattice solvents. The HFEPR data additionally provide important spectroscopic evidence suggesting that the relatively weak disorder induced by desolvation strongly influences the quantum tunneling interactions, and that it is under-barrier tunneling that is responsible for a consistent reduction in Ueff that is found upon drying samples. Meanwhile, the axial anisotropy deduced from HFEPR is found to be virtually identical for all four Mn12 complexes, with essentially no measurable reduction upon desolvation.

First-Principles Study of Electron Transport through the Single-Molecule MagnetMn12

We examine electron transport through a single-molecule magnet Mn(12) bridged between Au electrodes using the first-principles method. We find crucial features which were inaccessible in model Hamiltonian studies: spin filtering and a strong dependence of charge distribution on local environments. The spin filtering remains robust with different molecular geometries and interfaces, and strong electron correlations, while the charge distribution over the Mn(12) strongly depends on them. We point out a qualitative difference between locally charged and free-electron-charged Mn(12).

Synthesis and Photoinduced Magnetic Properties of a Mn12 Single Molecule Magnet by the cis-trans Isomerism of Azobenzene

[Mn 12 O 12 (azo-L) 16 (H 2 O) 4 ] (1), a new Mn12 single molecule magnet containing a photochromic azobenzene ligand, has been successfully synthesized by substitution of acetate ligand of Mn12 with 6-[4-{4-hexyloxyphenyl(azo)}-phenoxy]hexanoic-1-acid. The reversible photoisomerization of the azobenzene group was confirmed by UV-visible absorption spectroscopy. The temperature and field dependence of dc susceptibility and the temperature and the frequency dependence of ac susceptibility were measured for the cis and the trans isomer of 1. The magnetization value of the cis isomer in dc measurement is higher than that of the trans isomer. The cis isomer of 1 has a slower relaxation because cis-trans photoisomerization of the azobenzene group in peripheral ligands induces changes in its structure and dipole moment.