Fast magnetization tunneling in tetranickel(II) single-molecule magnets

A series of Ni(4) cubane complexes with the composition [Ni(hmp)(ROH)Cl](4) complexes 1-4 where R= -CH(3) (complex 1), -CH(2)CH(3) (complex 2), -CH(2)CH(2)(C(4)H(9)) (complex 3), -CH(2)CH(2)CH(2)(C(6)H(11)) (complex 4), hmp(-) is the anion of 2-hydroxymethylpyridine, t-Buhmp(-) is the anion of 4-tert-butyl-2-hydroxymethylpyridine, and dmb is 3,3-dimethyl-1-butanol] and [Ni(hmp)(dmb)Br](4) (complex 5) and [Ni(t-Buhmp)(dmb)Cl](4) (complex 6) were prepared. All six complexes were characterized by dc magnetic susceptibility data to be ferromagnetically coupled to give an S = 4 ground state with significant magnetoanisotropy (D approximately equal to -0.6 cm(-1)). Magnetization hysteresis measurements carried out on single crystals of complexes 1-6 establish the single-molecule magnet (SMM) behavior of these complexes. The exchange bias observed in the magnetization hysteresis loops of complexes 1 and 2 is dramatically decreased to zero in complex 3, where the bulky dmb ligand is employed. Fast tunneling of magnetization is observed for the high-symmetry (S(4) site symmetry) Ni(4) complexes in the crystal of complex 3, and the tunneling rate can even be enhanced by destroying the S(4) site symmetry, as is the case for complex 4, where there are two crystallographically different Ni(4) molecules, one with C(2) and the other with C(1) site symmetry. Magnetic ordering temperatures due to intermolecular dipolar and magnetic exchange interactions were determined by means of very low-temperature ac susceptibility measurements; complex 1 orders at 1100 mK, complex 3 at 290 mK, complex 4 at approximately 80 mK, and complex 6 at <50 mK. This confirms that bulkier ligands correspond to more isolated molecules, and therefore, magnetic ordering occurs at lower temperatures for those complexes with the bulkiest ligands.     

Origin of the fast magnetization tunneling in tetranuclear nickel single-molecule magnets

Several tetranuclear nickel(II) single-molecule magnets (SMMs) have been prepared with the general composition of [Ni(hmp)(ROH)X]₄ · S, where hmp⁻ is the monoanion of 2-hydroxymethylpyridine, X⁻ is either Cl⁻ or Br⁻ and S is the solvate molecule. Magnetization versus magnetic field hysteresis loops for these Ni₄ SMMs show that there is a relatively fast rate of magnetization tunneling (small coercive field) and, in certain cases, an exchange bias present. Detailed measurements have been carried out in order to determine the origin of the fast magnetization tunneling. High-field electron paramagnetic resonance (HFEPR) data were collected on a single crystal of [Zn(hmp)(dmb)Cl]₄ doped with a small amount of Ni(II), where, dmb is 3,3-dimethyl-1-butanol. These variable-frequency/temperature data give values of the single-ion zero-field splitting parameters D_i and E_i, and the orientations of these interactions, for the single Ni^II ions in a Zn₃Ni complex doped into a Zn₄ crystal. HFEPR data were also obtained at many frequencies and temperatures for a single crystal of isostructural [Ni(hmp)(dmb)Cl]₄. Rotation of the single crystal such that the external field is positioned in the hard plane clearly establishes that the transverse zero-field interaction $B_4^4$ is the cause of the fast magnetization tunneling in the S = 4 ground state of this SMM. The magnitude of $B_4^4$ and the Ni₄ D value can be related to the directionality and magnitude of the D_i and E_i interactions at the individual Ni^II ions, determined for the doped crystal. The microenvironments and ligand dynamics were probed by means of a single-crystal X-ray structure at 12 K and by heat capacity data.     

A cubane-type nickel single-molecule magnet with exchange-biased quantum tunneling of magnetization

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Single-molecule magnets: a large Mn30 molecular nanomagnet exhibiting quantum tunneling of magnetization

The largest single-molecule magnet (SMM) to date has been prepared and studied. Recrystallization of known [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(H(2)O)(4)] (1; 8Mn(III), 4Mn(IV)) from CH(2)Cl(2)/MeNO(2) causes its conversion to [Mn(30)O(24)(OH)(8)(O(2)CCH(2)Bu(t))(32)(H(2)O)(2)(MeNO(2))(4)] (2; 3Mn(II), 26Mn(III), Mn(IV)). The structure of 2 consists of a central, near-linear [Mn(4)O(6)] backbone, to either side of which are attached two [Mn(13)O(9)(OH)(4)] units. Peripheral ligation around the resulting [Mn(30)O(24)(OH)(8)] core is by 32 Bu(t)CH(2)CO(2)(-), 2 H(2)O, and 4 MeNO(2) groups. The molecule has crystallographically imposed C(2) symmetry. Variable-temperature and -field magnetization (M) data were collected in the 1.8-4.0 K and 0.1-0.4 T ranges and fit by matrix diagonalization assuming only the ground state is occupied at these temperatures. The fit parameters were S = 5, D = -0.51 cm(-1) = -0.73 K, and g = 2.00, where D is the axial zero-field splitting parameter. AC susceptibility measurements in the 1.8-7.0 K range in a zero DC field and a 3.5 G AC field oscillating at frequencies in the 50-997 Hz range revealed a frequency-dependent out-of-phase (chi(M)') signal below 3 K, indicating 2 to be a single-molecule magnet (SMM), the largest yet obtained. Magnetization versus DC field sweeps show hysteresis loops but no clear steps characteristic of quantum tunneling of magnetization (QTM). However, magnetization decay data below 1 K were collected and used to construct an Arrhenius plot that revealed temperature-independent relaxation below 0.3 K. The fit of the thermally activated region above approximately 0.5 K gave U(eff)/k = 15 K, where U(eff) is the effective relaxation barrier. Resonant QTM was confirmed from the appearance of a "quantum hole" when the recent quantum hole digging method was employed. The combined results demonstrate that SMMs can be prepared that are significantly larger than any known to date and that this new, large Mn(30) complex still demonstrates quantum behavior.     

A new family of 1D exchange biased heterometal single-molecule magnets: observation of pronounced quantum tunneling steps in the hysteresis loops of quasi-linear {Mn2Ni3} clusters

First members of a new family of heterometallic Mn/Ni complexes [Mn(2)Ni(3)X(2)L(4)(LH)(2)(H(2)O)(2)] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H(2)L) have been synthesized, and single crystals obtained from CH(2)Cl(2) solutions have been characterized crystallographically. The molecular structures feature a quasi-linear Mn(III)-Ni(II)-Ni(II)-Ni(II)-Mn(III) core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J(Mn-Ni) ≈ 40 K (1) or 42 K (2); J(Ni-Ni) ≈ 22 K (1) or 18 K (2)) that lead to an S(tot) = 7 ground state, and a sizable uniaxial magnetoanisotropy with D(mol) values -0.55 K (1) and -0.45 K (2). These values are directly derived also from frequency- and temperature-dependent high-field EPR spectra. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal U(eff) = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops of the temperature- and scan rate-dependent magnetization data, but with the first relaxation step shifted above (1) or below (2) the zero crossing of the magnetic field, despite the very similar molecular structures. The different behavior of 1 and 2 is interpreted in terms of antiferromagnetic (1) or ferromagnetic (2) intermolecular interactions, which are discussed in view of the subtle differences of intermolecular contacts within the crystal lattice.     

Nuclear spin driven quantum tunneling of magnetization in a new lanthanide single-molecule magnet: bis(phthalocyaninato)holmium anion

The first measurements of magnetization hysteresis loops on a diluted single crystal of [(Pc)2Ho]-.TBA+ (Pc = phthalocyaninato, TBA = tetrabutylammonium) in the subkelvin temperature range are reported. Characteristic staircase-like structure was observed, indicating the occurrence of the quantum tunneling of magnetization (QTM), which is a characteristic feature of SMMs. The quantum process in the new lanthanide SMMs is due to resonant quantum tunneling between entangled states of the electronic and nuclear spin systems, which is an essentially different mechanism from those of the known transition-metal-cluster SMMs. Evidence of the two-body quantum process was also observed for the first time in lanthanide complex systems.     

A wheel-shaped single-molecule magnet of [MnII 3MnIII 4]: quantum tunneling of magnetization under static and pulse magnetic fields

The reaction of N-(2-hydroxy-5-nitrobenzyl)iminodiethanol (=H3(5-NO2-hbide)) with Mn(OAc)2* 4 H2O in methanol, followed by recrystallization from 1,2-dichloroethane, yielded a wheel-shaped single-molecule magnet (SMM) of [MnII 3MnIII 4(5-NO2-hbide)6].5 C2H4Cl2 (1). In 1, seven manganese ions are linked by six tri-anionic ligands and form the wheel in which the two manganese ions on the rim and the one in the center are MnII and the other four manganese ions are MnIII ions. Powder magnetic susceptibility measurements showed a gradual increase with chimT values as the temperature was lowered, reaching a maximum value of 53.9 emu mol(-1) K. Analyses of magnetic susceptibility data suggested a spin ground state of S=19/2. The zero-field splitting parameters of D and B 0 4 were estimated to be -0.283(1) K and -1.64(1)x10(-5) K, respectively, by high-field EPR measurements (HF-EPR). The anisotropic parameters agreed with those estimated from magnetization and inelastic neutron scattering experiments. AC magnetic susceptibility measurements showed frequency-dependent in- and out-of-phase signals, characteristic data for an SMM, and an Arrhenius plot of the relaxation time gave a re-orientation energy barrier (DeltaE) of 18.1 K and a pre-exponential factor of 1.63x10(-7) s. Magnetization experiments on aligned single crystals below 0.7 K showed a stepped hysteresis loop, confirming the occurrence of quantum tunneling of the on magnetization (QTM). QTM was, on the other hand, suppressed by rapid sweeps of the magnetic field even at 0.5 K. The sweep-rate dependence of the spin flips can be understood by considering the Landau-Zener-Stückelberg (LZS) model.     

One-dimensional supramolecular organization of single-molecule magnets

An out-of-plane dimeric MnIII quadridentate Schiff-base compound, [Mn2(salpn)2(H2O)2](ClO4)2 (salpn(2-) = N,N'-(propane)bis(salicylideneiminate)), has been synthesized and structurally characterized. The crystal structure reveals that the [Mn2(salpn)2(H2O)2](2+) units are linked through weak H-bonds (OHwater...OPh) in one dimension along the c-axis, forming supramolecular chains. The exchange interaction between MnIII ions via the biphenolate bridge is ferromagnetic (J/kB = +1.8 K), inducing an ST = 4 ground state. This dinuclear unit possesses uni-axial anisotropy observed in the out-of-plane direction with DMn2/kB = -1.65 K. At low temperatures, this complex exhibits slow relaxation of its magnetization in agreement with a single-molecule magnet (SMM) behavior. Interestingly, the intermolecular magnetic interactions along the one-dimensional organization, albeit weak (J'/kB = -0.03 K), influence significantly the thermally activated and quantum dynamics of this complex. Thus, unique features such as M vs H data with multiple steps, hysteresis effects, and peculiar relaxation time have been explained considering SMMs in small exchange-field perturbations and finite-size effects intrinsic to the chain arrangement. The magnetic properties of this new complex can be regarded as an intermediate behavior between SMM and single-chain magnet (SCM) properties.     

Dinuclear single-molecule magnets with porphyrin-phthalocyanine mixed triple-decker ligand systems giving SAP and SP coordination polyhedra

Two terbium ions in a triple-decker complex (Pc)Tb(Pc)Tb(T(p-OMe)PP) (Pc = phthalocyaninato, T(p-OMe)PP = tetra-p-methoxyphenylporphyrinato) have shown sharply different magnetic behaviours depending on symmetry of the coordination polyhedron. The fast quantum tunnelling relaxation process in a square-prismatic site has been revealed to be hindered by magnetic-dipolar coupling between the f-electronic systems.     

Exchange bias in Ni4 single-molecule magnets

The syntheses and physical properties are reported for three single-molecule magnets (SMMs) with the composition [Ni(hmp)(ROH)Cl]₄, where R is CH₃ (complex 1), CH₂CH₃ (complex 2) or CH₂CH₂C(CH₃)₃ (complex 3) and hmp⁻ is the monoanion of 2-hydroxymethylpyridine. The core of each complex is a distorted cube formed by four Ni^II ions and four alkoxide hmp⁻ oxygen atoms at alternating corners. Ferromagnetic exchange interactions give a S=4 ground state. Single crystal high-frequency EPR spectra clearly indicate that each of the complexes has a S=4 ground state and that there is negative magnetoanisotropy, where D is negative for the axial zero-field splitting DŜ_z². Magnetization versus magnetic field measurements made on single crystals with a micro-SQUID magnetometer indicate these Ni₄ complexes are SMMs. Exchange bias is seen in the magnetization hysteresis loops for complexes 1 and 2.     

Metalloligands for designing single-molecule and single-chain magnets

2-Hydroxy-N-{2-[2-hydroxyethyl)amino]ethyl}benzamide (L1H?) and 2-hydroxy-N-{2-[2-hydroxybutyl)amino]ethyl}benzamide (L2H?) ligands coordinate to copper ions to give anionic metalloligands, L1Cu? and L2Cu?, after deprotonation of their amide, alcohol and phenol functions. In presence of ancillary ligands as diketones, these metalloligands react with lanthanide salts to yield tetranuclear [Cu-Ln]? complexes while in presence of lanthanide salts alone, L1Cu? gives 1D chains of tetranuclear entities linked through the Ln ions. The micro-SQUID data evidence the SMM behavior of the tetranuclear Tb complexes and the SCM behavior of the Tb chain derivative.     

Strong axiality and Ising exchange interaction suppress zero-field tunneling of magnetization of an asymmetric Dy2 single-molecule magnet

The high axiality and Ising exchange interaction efficiently suppress quantum tunneling of magnetization of an asymmetric dinuclear Dy(III) complex, as revealed by combined experimental and theoretical investigations. Two distinct regimes of blockage of magnetization, one originating from the blockage at individual Dy sites and the other due to the exchange interaction between the sites, are separated for the first time. The latter contribution is found to be crucial, allowing an increase of the relaxation time by 3 orders of magnitude.     

One-dimensional chain of tetranuclear manganese single-molecule magnets

A one-dimensional chain of interconnected single-molecule magnets (SMMs) is obtained that consists of [Mn(4)(hmp)(6)](4+) units bridged by chloride ions. Slow magnetization relaxation is evident in the AC susceptibility data and in magnetization hysteresis measurements for [Mn(4)(hmp)(6)Cl(2)](n)(ClO(4))(2)(n). The magnetization hysteresis loops for this complex are similar to those for an SMM and show significant coercive field and steps at regular magnetic intervals. Spin-canted antiferromagnetic coupling due to misalignment of easy axes of neighboring Mn(4) units is also observed for this complex.     

Quantum tunneling of magnetization in a new [Mn18]2+ single-molecule magnet with s = 13

The reaction between 2-(hydroxyethyl)pyridine (hepH) and a 2:1 molar mixture of [Mn3O(O2CMe)6(py)3](ClO4) and [Mn3O(O2CMe)6(py)3](py) in MeCN leads to isolation of [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (1) in 10% yield. The complex is 2MnII,16MnIII and consists of a Mn4O6 central unit to either side of which is attached a Mn7O9 unit. Magnetization data collected in the 2.0-4.0 K and 20-50 kG ranges were fit to yield S = 13, g = 1.86, and D = -0.13 cm-1 = -0.19 K, where D is the axial zero-field splitting parameter. AC susceptibility studies in the 0.04-4.0 K range at frequencies up to 996 Hz display out-of-phase (chiM' ') signals, indicative of a single-molecule magnet (SMM). Magnetization vs applied DC field scans exhibit hysteresis at <1.0 K, confirming 1 to be a SMM. DC magnetization decay data were collected on both a microcrystalline sample and a single crystal, and the combined data were used to construct an Arrhenius plot. Between 3.50 and 0.50 K, the relaxation rate is temperature-dependent with an effective barrier to relaxation (Ueff) of 14.8 cm-1 = 21.3 K. Below ca. 0.25 K, the relaxation rate is temperature-independent at 1.3 x 10-8 s-1, indicative of quantum tunneling of magnetization (QTM) between the lowest energy Ms = +/-13 levels of the S = 13 state. Complex 1 is both the largest spin and highest nuclearity SMM to exhibit QTM.     

A novel Ni(4) complex exhibiting microsecond quantum tunneling of the magnetization

A highly asymmetric Ni(II) cluster [Ni(4)(OH)(OMe)(3)(Hphpz)(4)(MeOH)(3)](MeOH) (1) (H(2)phpz=3-methyl-5-(2-hydroxyphenyl)pyrazole) has been prepared and its structure determined by means of single-crystal X-ray diffraction by using synchrotron radiation. Variable-temperature bulk-magnetization measurements show that the complex exhibits intramolecular-ferromagnetic interactions leading to a spin ground state S=4 with close-lying excited states. Magnetization and high-frequency EPR measurements suggest the presence of sizable Ising-type magnetic anisotropy, with zero-field splitting parameters D=-0.263 cm(-1) and E=0.04 cm(-1) for the spin ground state, and an isotropic g value of 2.25. The presence of both axial and transverse anisotropy was confirmed through low-temperature specific heat determinations down to 300 mK, but no slow relaxation of the magnetization was observed by AC measurements down to 1.8 K. Interestingly, AC susceptibility measurements down to temperatures as low as 23 mK showed no indication of slow relaxation of the magnetization in 1. Thus, despite the presence of an anisotropy barrier (U approximately 4.21 cm(-1) for the purely axial limit), the magnetization relaxation remains extremely fast down to the lowest temperatures. The estimated quantum tunneling rate, Gamma>0.667 MHz, makes this complex a prime candidate for observation of coherent tunneling of the magnetization.     

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...