Mn12 Single-molecule Magnets Research Articles

Exfoliated graphite and ozonated single-wall carbon nanotubes for encapsulation of the single-molecule magnet Mn 12

We outline experiments on single-wall carbon nanotubes (SWNT) and exfoliated graphite (EG), as templates for producing nanostructures of the nanomagnetic manganese acetate cluster (Mn 12[(CH 3COO) 16(H 2O) 4O 12] · 2CH 3COOH · 4H 2O),Mn 12. Mn 12 grown in EG and SWNT exhibit downward shifts in its blocking temperature, ac susceptibility response and magnetic hysteresis loops. Temperature dependence of the magnetization reversal dynamics shows that encapsulation in EG or ozonated SWNT shortens the Arrhenius relaxation time for magnetization reversal, τ o, from 7.8 × 10 −8 to 1.4 × 10 −8s, with a marginal lowering of the activation barrier from 68 ± 5 to 60 ± 5 K. The role of Mn–O modes in the magnetization dynamics together with these data indicate the potential for controlling Mn 12 properties via encapsulation.

Quantum tunneling in a three-dimensional network of exchange-coupled single-molecule magnets

A Mn4 single-molecule magnet (SMM) is used to show that quantum tunneling of magnetization (QTM) is not suppressed by moderate three dimensional exchange coupling between molecules. Instead, it leads to an exchange bias of the quantum resonances which allows precise measurements of the effective exchange coupling that is mainly due to weak intermolecular hydrogen bounds. The magnetization versus applied field was recorded on single crystals of [Mn4]2 using an array of micro-SQUIDs. The step fine structure was studied via minor hysteresis loops.

Examining the thermolysis reactions of nanoscopic Mn12 single molecule magnets

The thermal behavior of three Mn12 single molecule magnets [Mn12O12(O2CC6H5)16(H2O)4]·CH2Cl2·C6H5CO2H (1), [Mn12O12(O2CtBu)16(H2O)4] (2) and [Mn12O12(O2CCHCl2)16(H2O)4] (3) is reported. Aromatic ligands allow the complex 1 to be stable up to 300°C whereas alkyl groups decrease drastically the domain of thermal stability for the complexes 2 and 3. Moreover, the thermal decarboxylation of complexes 2 and 3 generates [Mn6O2(O2CR)10L4] (L=H2O, HO2CR) complexes as characterized by single crystal X-ray diffraction when R=tBu.

A comparative high frequency EPR study of monomeric and dimeric Mn4 single-molecule magnets

We compare high frequency single crystal electron paramagnetic resonance (EPR) data for monomeric and dimeric Mn4 complexes where, in the case of the latter, intra-dimer exchange interactions are known to significantly alter the low temperature quantum behavior. The monomeric system exhibits typical single-molecule S=9/2 EPR spectra; analysis of these data have enabled us to characterize the effective spin Hamiltonian parameters for this material. In the dimeric system, the ground state transition shows a splitting over a narrow range of fields and frequencies. The transfer of spectral weight between these split peaks is consistent with the exchange bias picture which has been developed to explain the absence of a zero-field tunneling resonance in low temperature hysteresis experiments. Indeed, these measurements provide the first independent confirmation for this exchange bias model. The absence of a significant intensity in excited state EPR transitions for the dimer contrasts the results for the monomer. This likely suggests important differences concerning the coherence of excited states of the dimer versus the monomer. No clear indications for intermolecular exchange interactions were observed in the EPR for the monomer.

Definitive spectroscopic determination of the transverse interactions responsible for the magnetic quantum tunneling in Mn(12)-acetate.

We present detailed angle-dependent single crystal electron paramagnetic resonance (EPR) data for field rotations in the hard plane of the S=10 single molecule magnet Mn(12)-acetate. A clear fourfold variation in the resonance positions may be attributed to an intrinsic fourth-order transverse anisotropy (O(4)/(4)). Meanwhile, a fourfold variation of the EPR line shapes confirms a recently proposed model wherein disorder associated with the acetic acid of crystallization induces a locally varying quadratic (rhombic) transverse anisotropy [O (2)/(2) identical with E(S (2)/(x)-S(2)/(y))]. These findings explain most aspects of the magnetic quantum tunneling observed in Mn(12)-acetate.

A new family of Mn12 single-molecule magnets: replacement of carboxylate ligands with diphenylphosphinates

The synthesis, structural characterization and magnetic properties of a new mixed ligand member of the Mn12 family are reported. The reaction of [Mn12O12(O2CPh)16(H2O)4] (4) with diphenylphosphinic acid produces [Mn12O12(O2CPh)7(O2PPh2)9)(H2O)4] (5). The crystal structure of 5 shows the [Mn12O12]16+ core is retained and exhibits two abnormally oriented Jahn-Teller elongation axes. The ligand distribution alternates about the core with the ninth Ph2PO2− group in an equatorial position. Magnetic studies indicate an S=10 ground state and retention of the single-molecule magnetism behavior.

Synthesis, crystal structure and magnetic properties of novel Mn12 single-molecule magnets with thiophenecarboxylate, [Mn12O12(O2CC4H3S)16(H2O)4], and its tetraphenylphosphonium salt

The preparation and physical characterization are reported for novel Mn12 single-molecule magnets having thiophenecarboxylate bridges, [Mn12O12(O2CC4H3S)16(H2O)4] (1), and its PPh4 salt (2). The reaction of the excess amount of 2-thiophenecarboxylic acid (tpcH) and [Mn12O12(OAc)16(H2O)4] in CH2Cl2 gave black crystals of 1, which could be reduced by PPh4I to give 2. From the crystal structure analysis of 2, it is revealed that there are two five-coordinated Mn ions, one of which is assigned to a MnII ion being in the vicinity of the tetraphenylphosphonium cation. Both complexes exhibit out-of-phase a.c. magnetic susceptibility (χ″M) signals in the 5.0–6.5 K range at 997 Hz a.c. frequency, which indicate that they are single-molecule magnets. From Arrhenius plots of the frequency dependence of the temperature of the χ″M peaks, the effective energy barriers Ueff were estimated to be 69 and 57 K for high-temperature phases of 1 and 2, respectively, and 40 K for the low-temperature phase of 1. The reduced magnetization measurement and its analysis indicate that 2 has S=19/2 ground state with g=1.99 and D=−0.61 K.

Mn4 single-molecule magnets with a planar diamond core and S=9

The syntheses and magnetic properties are reported for three Mn4 single-molecule magnets (SMMs): [Mn4(hmp)6(NO3)2(MeCN)2](ClO4)2·2MeCN (3), [Mn4(hmp)6(NO3)4]·(MeCN) (4), and [Mn4(hmp)4(acac)2(MeO)2](ClO4)2·2MeOH (5). In each complex there is a planar diamond core of MnIII2MnII2 ions. An analysis of the variable-temperature and variable-field magnetization data indicate that all three molecules have intramolecular ferromagnetic coupling and a S=9 ground state. The presence of a frequency-dependent alternating current susceptibility signal indicates a significant energy barrier between the spin-up and spin-down states for each of these three MnIII2MnII2 complexes. The fact that these complexes are SMMs has been confirmed by the observation of hysteresis in the plot of magnetization versus magnetic field measured for single crystals of complexes 3 and 4. The hysteresis loops for both of these complexes exhibit steps characteristic of quantum tunneling of magnetization. Complex 4 shows its first step at zero field, whereas the first step for complex 3 is shifted to −0.10 T. This shift is attributable to weak intermolecular antiferromagnetic exchange interactions present for complex 3.

Isolated Single‐Molecule Magnets on the Surface of a Polymeric Thin Film

Individual Mn12 single‐molecule magnets are arranged onto a polymeric film surface using a new, soft, reliable and simple chemical methodology. Polymeric thin films, prepared from a polycarbonate matrix and Mn12 molecules, are subsequently treated with an organic solvent vapor, resulting in nanocomposite thin films that are flexible and optically transparent and could be used for information storage or in quantum computing.

Single-molecule magnets. A Mn12 complex with mixed carboxylate-sulfonate ligation: [Mn12O12(O2CMe)8(O3SPh)8(H2O)4

The synthesis and magnetic properties of a new member of the Mn12 single-molecule magnet family, [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2), are reported. The compound was prepared by treatment of [Mn12O12(O2CMe)16(H2O)4] (1) with eight equivalents of PhSO3H in MeCN. Complex 2·4CH2Cl2 crystallizes in the triclinic space group P. The complex consists of a central [MnIV4O4] cubane held within a nonplanar ring of eight MnIII ions by eight μ3-O2− ions. Eight bridging acetate groups occupying equatorial sites, eight bridging benzenesulfonate groups in axial sites, and four terminal water molecules complete the peripheral ligation of the molecule. The spin of the ground state was established by magnetization measurements in the 2.00–7.00 T field range and 1.80–4.00 K temperature range. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating only axial anisotropy, gave S = 10, g = 1.96 and D = −0.34 cm−1. The cluster exhibits out-of-phase ac susceptibility signals and temperature-dependent hysteresis loops at temperatures below 4.00 K, establishing 2 as a new single-molecule magnet.

Effect of defects on the line shape of electron paramagnetic resonance signals from the single-molecule magnet Mn12: A theoretical study

We estimate the effect of lattice defects on the line shape of electron paramagnetic resonance (EPR) signals from a single crystal of the S=10 single-molecule magnet Mn12, measured with the external magnetic field along the crystal c axis. A second-order perturbation treatment of an effective single-spin Hamiltonian indicates that a small, random and static misorientation of the magnetic symmetry axes in a crystalline lattice can lead to asymmetric EPR peaks. Full spectra are simulated by calculating probability-distribution functions for the resonant fields, employing distributions in the tilt angle of the easy axis from the c axis, in the uniaxial anisotropy parameter, and in the g-factor. We discuss conditions under which the asymmetry in the EPR spectra becomes prominent. The direction and magnitude of the asymmetry provide information on the specific energy levels involved with the EPR transition, the EPR frequency, and the distribution in the tilt angle.

Spin-spin cross relaxation in single-molecule magnets.

The one-body tunnel picture of single-molecule magnets (SMMs) is not always sufficient to explain the measured tunnel transitions. An improvement to the picture is proposed by including also two-body tunnel transitions such as spin-spin cross relaxation (SSCR) which are mediated by dipolar and weak superexchange interactions between molecules. A Mn4 SMM is used as a model system. At certain external fields, SSCRs lead to additional quantum resonances which show up in hysteresis loop measurements as well-defined steps. A simple model is used to explain quantitatively all observed transitions.

D-STRAIN, g-STRAIN, AND DIPOLAR INTERACTIONS IN THE Fe8 AND Mn12 SINGLE MOLECULE MAGNETS: AN EPR LINESHAPE ANALYSIS

We report high sensitivity, high field/frequency (up to 9 tesla/210 GHz) EPR measurements on the Fe8 and Mn12 single molecule magnets. Oriented single crystal studies, carried out in well defined experimental geometries, enable very accurate determinations of EPR lineshapes; we note that this is not possible when studying aligned powders or polycrystals. Analysis of individual resonances, which we can assign to known transitions, reveal complex nonlinearities in the linewidths as functions of energy eigenstate, frequency, and temperature. Our findings compare favorably with recent calculations which introduce distributions in the uniaxial anisotropy parameter D, the Landé g-factor, and dipolar interactions.

A Chemical Modification of a Mn12 Single-Molecule Magnet by Replacing Carboxylate Anions with Diphenylphosphate Anions

Abstract Diphenylphosphate anions can replace with benzoate anions of [Mn12O12(O2CPh)16(H2O)4] to afford a novel Mn12 single-molecule magnet (SMM) with mixed bridging ligands.

A Raman study of the single molecule magnet Mn 12-acetate and analogs

Recent theoretical studies suggest that low-lying Mn–O vibrations play a significant role in the mechanism of quantum tunneling of magnetization (QTM) exhibited by single molecule magnets like Mn 12-acetate (Mn 12). We report a Raman study of Mn 12, deuterated Mn 12, both with S=10, and of Mn 8Fe 4 isostructural, with Mn 12 but with S=2 ground state. Comparison of the detected mode frequencies with theoretical predictions and earlier infrared data supports their role in the spin–vibron interactions and the mechanism of anisotropy and QTM in Mn 12.

Micro-Hall magnetometry studies of thermally assisted and pure quantum tunneling in single molecule magnet Mn 12-acetate

We have studied the crossover between thermally assisted and pure quantum tunneling in single crystals of high spin ( S=10) uniaxial single molecule magnet Mn 12 using micro-Hall effect magnetometry. Magnetic hysteresis experiments have been used to investigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow (∼0.1 K) or broad (∼1 K) temperature interval depending on the magnitude and direction of the applied field. For low external fields applied parallel to the easy axis, the energy levels that dominate the tunneling shift abruptly with temperature. In the presence of a transverse field and/or large longitudinal field these energy levels change with temperature more gradually. A comparison of our experimental results with model calculations of this crossover suggests that there are additional mechanisms that enhance the tunneling rate of low-lying energy levels and broaden the crossover for small transverse fields.