Density functional studies of single molecule magnets

A method for the calculation of the second-order anisotropy parameters of single molecular magnets from the single particle orbitals is reviewed. We combine this method with density functional calculations to predict the magnetic anisotropy parameters of several single molecule magnets: Mn₁₂-acetate, Mn₁₀, Co₄, Fe₄, Cr₁ and V₁₅. Comparison with available experimental data shows that it is possible to predict these values quite accurately from density functional wavefunctions.     

A family of rare-earth-based single chain magnets: playing with anisotropy

The first family of rare-earth-based single chain magnets is presented. Compounds of general formula [M(hfac)3(NITPhOPh)], where M = Eu, Gd, Tb, Dy, Ho, Er, or Yb, and PhOPh is the nitronyl-nitroxide radical (2,4'-benzoxo-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), have been structurally characterized and found to be isostructural. The characterization of both static and dynamic magnetic properties of the whole family is reported. Dy, Tb, and Ho compounds display slow relaxation of the magnetization, and ac susceptibility shows a thermally activated regime with energy barriers of 69, 45, and 34 K for Dy, Tb, and Ho compounds, respectively, while only a frequency-dependent susceptibility is observed for Er below 2.0 K. In Gd and Yb derivatives, antiferromagnetic interactions dominate. The pre-exponential factors differ by about 4 orders of magnitude. Finite size effects, due to naturally occurring defects, affect the static and dynamic properties of the compounds differently.     

Relaxation dynamics of dysprosium(III) single molecule magnets

Over the past decade, lanthanide compounds have become of increasing interest in the field of Single Molecule Magnets (SMMs) due to the large inherent anisotropy of the metal ions. Heavy lanthanide metal systems, in particular those containing the dysprosium(III) ion, have been extensively employed to direct the formation of a series of SMMs. Although remarkable progress is being made regarding the synthesis and characterization of lanthanide-based SMMs, the understanding and control of the relaxation dynamics of strongly anisotropic systems represents a formidable challenge, since the dynamic behaviour of lanthanide-based SMMs is significantly more complex than that of transition metal systems. This perspective paper describes illustrative examples of pure dysprosium(III)-based SMMs, published during the past three years, showing new and fascinating phenomena in terms of magnetic relaxation, aiming at shedding light on the features relevant to modulating relaxation dynamics of polynuclear lanthanide SMMs.     

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(O₂C^tBu)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.     

One pot grafting of tetrairon(III) single molecule magnets on silicon

Herein we report on the Si grafting of two Fe₄ derivatives, [Fe₄(Lⁱ)₂(tmhd)₆], in which tmhd is 2,2,6,6-tetramethylheptane-3,5-dionate and H₃Lⁱ = R–C(CH₂OH)₃ is a tripodal ligand with R = CH₂CH–CH₂–O–CH₂ (H₃L¹) and CH₂CH–(CH₂)₉–O–CH₂ (H₃L²). These complexes were specifically designed to be directly anchored on the H-terminated silicon surface via the hydrosilylation reaction. The complexes were grafted by a one pot route based on the photoinduced hydrosilylation followed by a ligand exchange step in the same reaction solution. The resulting decorated surfaces were characterized using X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared spectroscopy (ATR-IR) and atomic force microscopy (AFM).     

Chemical strategies and characterization tools for the organization of single molecule magnets on surfaces

Addressing individual bistable magnetic molecules, known as Single Molecule Magnets (SMMs), is a fascinating goal at the borderline between molecular magnetism and spin electronics. This tutorial review focuses on the first step towards single-molecule experiments, namely the organization of SMMs on surfaces. Both preparation and characterization of surface-supported SMMs prove to be quite demanding and a multidisciplinary approach is necessary, which is described here using selected examples. We first illustrate the chemical strategies devised to assemble SMMs and to control their orientation on surfaces. Then, we present characterization tools, which have been selected on the basis of their relevance to address specific points, i.e. the chemical composition of the deposited SMM films, the organization of the molecules on the surface, the intramolecular arrangement of the spins, the magnetic anisotropy of SMMs, and eventually the dynamics of their magnetization on surfaces. Particular attention is devoted to techniques exploiting synchrotron light.     

Approaching the uniaxiality of magnetic anisotropy in single-molecule magnets

Single-molecule magnets(SMMs), which can exhibit slow magnetization relaxation and bulk-magnet-like hysteresis of purely molecular origin, are promising candidates for high-density information storage, molecular spintronics, and quantum computing.To realize their applications, it is crucial to improve the blocking temperature(T_B) and the effective relaxation barrier(U_eff).Three decades of multidisciplinary research have yielded distinct SMMs with a state-of-the-art U_ef...     

Influence of the ligand shell on the surface orientation of Mn12 single molecule magnets

Here we report the synthesis, investigation as well as surface deposition of a truly axial symmetry Mn₁₂-diphenylphosphinate (Mn₁₂-phn) single molecule magnet. Out of 16 acetate ligands encapsulating the Mn₁₂O₁₂ core, 12 ligands were exchanged by diphenylphosphinate in this compound. Mn₁₂-phn shows well-defined magnetic hysteresis curves indicating a very high crystal quality. A monolayer of Mn₁₂-phn was chemically grafted on a functionalized Au(1 1 1) surface via ligand exchange reaction and studied by means of scanning tunneling microscopy and spectroscopy. Via distance–voltage spectroscopy we determine the real-space height of the Mn₁₂-phn molecules with high accuracy. A large spread in the measured molecular heights obtained from the distance–voltage spectra indicates the absence of preferential orientation of Mn₁₂-phn molecules with respect to the surface which we attribute to the equal anchoring probability of all diphenylphosphinate ligands in Mn₁₂-phn while none of the four acetate ligands are exchanged. These results are compared with the experimental data obtained from a different Mn₁₂ derivative containing 16 thiophenecarboxylate ligands. In general, we show that the substitution of the ligand shell may have a major impact on the surface orientation of the Mn₁₂ clusters deposited on Au, i.e. on the orientation of the easy magnetization axis.     

Magnetic and optical bistability in tetrairon(III) single molecule magnets functionalized with azobenzene groups

Tetrairon(III) complexes known as "ferric stars" have been functionalized with azobenzene groups to investigate the effect of light-induced trans-cis isomerization on single-molecule magnet (SMM) behaviour. According to DC magnetic data and EPR spectroscopy, clusters dispersed in polystyrene (4% w/w) exhibit the same spin (S = 5) and magnetic anisotropy as bulk samples. Ligand photoisomerization, achieved by irradiation at 365 nm, has no detectable influence on static magnetic properties. However, it induces a small but significant acceleration of magnetic relaxation as probed by AC susceptometry. The pristine behaviour can be almost quantitatively recovered by irradiation with white light. Our studies demonstrate that magnetic and optical bistability can be made to coexist in SMM materials, which are of current interest in molecular spintronics.     

A criterion for the anisotropy barrier in single-molecule magnets

A criterion for the height of the anisotropy barrier Delta=|D|S2 in single-molecule magnets, and therefore the blocking temperature TB proportional to Delta, is presented. In particular, it is found that the anisotropy barrier does not increase with S as S2 but as S0. Consequences concerning the strategies to enhance Delta or TB, respectively, are discussed.     

Ordered patterning of nanometric rings of single molecule magnets on polymers by lithographic control of demixing

We report a new patterning process which takes place as a result of demixing of a binary polymer/solute mixture. An efficient, sustainable approach for ordering nanosized rings of so-called single molecule magnets (SMMs) is thus provided. It exploits the self-organization process in which SMM patterned film evolves to a spatially correlated pattern of nanosized rings. At long time, the anisotropic patterning of the film drives the ring to coalesce, into parallel lines of nanometric width.     

Tetranuclear [Cu-Ln]2 single molecule magnets: synthesis, structural and magnetic studies

A trianionic ligand H3L (2-hydroxy-N-(2-[[(2-hydroxyphenyl)methylene]amino]-2-methylpropyl)benzamide) with an inner N2O2 coordination site and an oxygen atom coming from an amide function not involved in this site yields monoanionic LCu- complexes that react with Ln(hfa)3 x 2H2O (hfa = hexafluoroacetylacetonato ligand) to give dinuclear Cu-Ln complexes that self-assemble into tetranuclear species, as demonstrated by the structural determination of the [LCuDy(hfa)2(dmf)2]2 complex. High-spin species are then isolated for two ferromagnetic interactions are active in the [Cu-Gd]2 entities, through the double phenoxo bridge (J = 3.2 cm(-1)) and through the single amide bridge (j = 0.54 cm(-1)). These interactions are still present in the [Cu-Tb]2 and [Cu-Dy]2 complexes which behave as single molecule magnets (SMMs), due to the introduction of anisotropic Ln ions in place of Gd ions.     

Defect-dicubane Ni2Ln2 (Ln = Dy, Tb) single molecule magnets

Two pairs of Ni(2)Dy(2) and Ni(2)Tb(2) complexes, [Ni(2)Ln(2)(L)(4)(NO(3))(2)(DMF)(2)] {Ln = Dy (1), Tb (2)} and [Ni(2)Ln(2)(L)(4)(NO(3))(2)(MeOH)(2)]·3MeOH {Ln = Dy (3), Tb (4)} (H(2)L is the Schiff base resulting from the condensation of o-vanillin and 2-aminophenol) possessing a defect-dicubane core topology were synthesized and characterized. All four complexes are ferromagnetically coupled, and the two Dy-analogues are found to be Single Molecule Magnets (SMMs) with energy barriers in the range 18-28 K. Compound 1 displays step-like hysteresis loops, confirming the SMM behavior. Although 1 and 3 show very similar structural topologies, the dynamic properties of 1 and 3 are different with blocking temperatures (3.2 and 4.2 K at a frequency of 1500 Hz) differing by 1 K. This appears to result from a change in orientation of the nitrate ligands on the Dy(III) ions, induced by changes in ligands on Ni(II).     

Dimers and chains of {3d-4f} single molecule magnets constructed from heterobimetallic tectons

A tetranuclear complex and a 1-D coordination polymer with a ladder-like topology have been obtained by connecting [Ni(II)Dy(III)] nodes with dicarboxylato ligands: [Ni?(valpn)?Dy?(III)(pdca)?(NO?)(H?O)?](NO?)·4H?O 1, and (∞)1[Ni?(H?O)?(valpn)?Dy?(tfa)?]·4CH?CN 2 (valpn2? = the dianion of the Schiff base resulting from reacting o-vanillin with 1,3-propanediamine; pdca2? = the dianion of 2,6-pyridinedicarboxylic acid; tfa2? = the dianion of the terephthalic acid). The magnetic measurements show a ferromagnetic interaction between Ni(II) and Dy(III), and that both compounds behave like SMM with strong tunnelling. The barrier of 2 (17.4 K) is higher than that of 1 (13.6 K).     

Independent linear combinations of spectroscopic constants for axially symmetric molecules

The problems of indeterminacy and correlation of spectroscopic constants of axially symmetric molecules are discussed in the dependence on the selection rules of the data set used in analysis of high resolution infrared spectra. Derivation of independent linear combinations of the correlated spectroscopic parameters, for several cases of the allowed and forbidden bands, is carried out for the most important molecular point groups, up to the third order theory. The significance of the forbidden, overtone and hot band transitions in the separate determination of individual constants is also evaluated.     

Single-molecule magnets of ferrous cubes: structurally controlled magnetic anisotropy

Tetranuclear Fe(II) cubic complexes were synthesized with Schiff base ligands bridging the Fe(II) centers. X-ray structural analyses of six ferrous cubes, [Fe4(sap)4(MeOH)4].2H2O (1), [Fe4(5-Br-sap)4(MeOH)4] (2), [Fe4(3-MeO-sap)4(MeOH)4].2MeOH (3), [Fe4(sae)4(MeOH)4] (4), [Fe4(5-Br-sae)4(MeOH)4].MeOH (5), and [Fe4(3,5-Cl2-sae)4(MeOH)4] (6) (R-sap and R-sae were prepared by condensation of salicylaldehyde derivatives with aminopropyl alcohol and aminoethyl alcohol, respectively) were performed, and their magnetic properties were studied. In 1-6, the alkoxo groups of the Schiff base ligands bridge four Fe(II) ions in a mu3-mode forming [Fe4O4] cubic cores. The Fe(II) ions in the cubes have tetragonally elongated octahedral coordination geometries, and the equatorial coordination bond lengths in 4-6 are shorter than those in 1-3. Dc magnetic susceptibility measurements for 1-6 revealed that intramolecular ferromagnetic interactions are operative to lead an S = 8 spin ground state. Analyses of the magnetization data at 1.8 K gave the axial zero-field splitting parameters (D) of +0.81, +0.80, +1.15, -0.64, -0.66, and -0.67 cm(-1) for 1-6, respectively. Ac magnetic susceptibility measurements for 4-6 showed both frequency dependent in- and out-of-phase signals, while 1-3 did not show out-of-phase signals down to 1.8 K, meaning 4-6 are single-molecule magnets (SMMs). The energy barriers to flip the spin between up- and down-spin were estimated to 28.4, 30.5, and 26.2 K, respectively, for 4-6. The bridging ligands R-sap2- in 1-3 and R-sae2- in 4-6 form six- and five-membered chelate rings, respectively, which cause different steric strain and Jahn-Teller distortions at Fe(II) centers. The sign of the D value was discussed by using angular overlap model (AOM) calculations for irons with different coordination geometry.     

An EPR method for probing surface magnetic fields, dipolar distances, and magnetization fluctuations in single molecule magnets

We outline a spectroscopic method for probing the effective magnetic field B on the surface of crystals of the single molecule magnet (SMM) [(C6H15N3)6Fe8(mu3-O)2(mu2-OH)12]Br7(H2O)Br.H2O, (Fe8Br8). This technique utilizes the line shape changes in the EPR spectra of the organic radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) (g = 2.0036, single peak) adsorbed onto the sample. The temperature dependence of the EPR line shifts scale with the sample's magnetization as measured by a SQUID magnetometer. Analysis of the line shape in terms of dipolar coupling between the DPPH and the SMM molecules on the surface, yields their average dipolar distance. The method's potential for measuring the magnetization fluctuation dynamics is briefly pointed out using the SMM [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate).     

The origin of reactivity bands in atom—molecule collisions

Classical trajectories are calculated on model designed to reproduce the reactive—unreactive bands found with the SSMK and Yates—Lester surfaces for the H + H₂ system and other A + BC systems. The model surfaces are based on a rectilinear reaction path, with a constant period of vibration, and have an exit region corresponding to reaction. A simple surface obtained by taking a cut at constant R_BC though a collinear potential-energy function is not as satisfactory as one to which a platform has been added to allow for the zero-point energy normal to the minimum reaction path. General principles are derived for obtaining reaction probabilities and reactive—unreactive boundaries for such surfaces. The model surfaces predict the bands in a general way, and throw light on the reason they occur, but do not lead to quantitative predictions.