Unquenched Orbital Angular Momentum Research Articles

Study of Magnetic Behavior of Salts of Cationic Dinitrosyl Iron Complexes with Thiocarbamide and its Derivatives

We report magnetic characterization and theoretical modeling of a series of four salts of cationic dinitrosyl iron complexes with thiocarbamide and its derivatives. To explain unusual magnetic behavior of these complexes we have assumed that constituent nitrosyl groups possess not only spins but also unquenched orbital angular momenta. With the aid of this assumption we were able to explain the observed temperature dependences of the effective magnetic moment for all members of the series and to establish magneto-structural correlations.

Measurement of extreme hyperfine fields in two-coordinate high-spin Fe2+ complexes by Mössbauer spectroscopy: essentially free-ion magnetism in the solid state.

Mössbauer studies of three two-coordinate linear high-spin Fe(2+) compounds, namely, Fe{N(SiMe3)(Dipp)}2 (1) (Dipp = C6H3-2,6-(i)Pr2), Fe(OAr')2 (2) [Ar' = C6H3-2,6-(C6H3-2,6-(i)Pr2)2], and Fe{C(SiMe3)3}2 (3), are presented. The complexes were characterized by zero- and applied-field Mössbauer spectroscopy (1-3), as well as zero- and applied-field heat-capacity measurements (3). As 1-3 are rigorously linear, the distortion(s) that might normally be expected in view of the Jahn-Teller theorem need not necessarily apply. We find that the resulting very large unquenched orbital angular momentum leads to what we believe to be the largest observed internal magnetic field to date in a high-spin iron(II) compound, specifically +162 T in 1. The latter field is strongly polarized along the directions of the external field for both longitudinal and transverse field applications. For the longitudinal case, the applied field increases the overall hyperfine splitting consistent with a dominant orbital contribution to the effective internal field. By contrast, 2 has an internal field that is not as strongly polarized along a longitudinally applied field and is smaller in magnitude at ca. 116 T. Complex 3 behaves similarly to complex 1. They are sufficiently self-dilute (e.g., Fe···Fe distances of ca. 9-10 Å) to exhibit varying degrees of slow paramagnetic relaxation in zero field for the neat solid form. In the absence of EPR signals for 1-3, we show that heat-capacity measurements for one of the complexes (3) establish a geff value near 12, in agreement with the principal component of the ligand electric field gradient being coincident with the z axis.

Low-Spin Hexacoordinate Mn(III): Synthesis and Spectroscopic Investigation of Homoleptic Tris(pyrazolyl)borate and Tris(carbene)borate Complexes

Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong σ-donor with little or no π-bonding.

Simple Ligand-Field Theory of d4 and d6 Transition Metal Complexes with a C3 Symmetry Axis

There have been a number of recent studies reporting high-spin d(4,6) complexes with three- and four-coordinate geometry, which exhibit roughly trigonal symmetry. These include complexes of Fe(II) with general formula L(3)FeX, where L = thioether or dialkylphosphine donors of a tripodal chelating ligand and X is a monodentate ligand on the C(3) axis. In these systems, there is unquenched orbital angular momentum, which has significant consequences on the electronic/magnetic properties of the complexes, including magnetic susceptibility, EPR spectra, and magnetic Mössbauer spectra. We describe here a simple model using a description of the d orbitals with trigonal symmetry that along with the application of the spin-orbit interaction successfully explains the magnetic properties of such systems. These d orbitals with 3-fold symmetry are complex orbitals with a parameter, a, that is determined by the bond angle, α, of LFeX. We demonstrate that the E symmetry states in such systems with S > 1/2 cannot be properly "simulated by" or be "represented by" the Zeeman and second-order zero-field spin Hamiltonian alone because by definition the parameters D and E are second-order terms. One must include the first-order spin-orbit interaction. We also find these systems to be very anisotropic in all their magnetic properties. For example, the perpendicular values of g and the hyperfine interaction parameter are essentially zero for the ground-state doublet. For illustrative purposes, the discussion focuses primarily on two specific Fe(II) complexes: one with the bond angle α greater than tetrahedral and another with the bond angle α less than tetrahedral. The nature of the EPR spectra and hyperfine interaction of (57)Fe are discussed.

Spectral origins of high Faraday rotation at 1.5-μm wavelength from Fe and Co in SrTiO3 films

Measurements of Faraday rotation (FR) in [001]-oriented epitaxial films of SrTi1−xFexO3 and SrTi1−xCoxO3 with x = 0.4 and 0.3, respectively, indicate that FR constants (∼103 deg/cm) of these perovskite compounds can approach those of (Bi,Y)3Fe5O12 garnet used in fiber-optical isolators at a wavelength of 1.5 μm. Because the magnetic site in a stoichiometric host is occupied by Ti4+, substituted Fe or Co must also be 4+. A reduction to Fe3+, Fe2+, and Co3+, Co2+ is also expected from O2– loss during the deposition process. The orbital energy-level structures in an octahedral site are examined for the existence of transitions that could account for the similar high FR effects. It was determined that only Fe4+ and Co4+ in low-spin states can satisfy the selection rules. The observation that the respective coercive fields of the FR(H) hysteresis loops exceed those of the magnetic moment M(H) lends credence to the argument that the unquenched orbital angular momentum acting with spin-orbit coupling can become the dominating influence on collective magnetic alignment.

Beyond the spin model: exchange coupling in molecular magnets with unquenched orbital angular momenta

In this critical review we review the problem of exchange interactions in polynuclear metal complexes involving orbitally degenerate metal ions. The key feature of these systems is that, in general, they carry an unquenched orbital angular momentum that manifests itself in all their magnetic properties. Thus, interest in degenerate systems involves fundamental problems related to basic models in magnetism. In particular, the conventional Heisenberg-Dirac-Van Vleck model becomes inapplicable even as an approximation. In the first part we attempt to answer two key questions, namely which theoretical tools are to be used in the case of degeneracy, and how these tools can be employed. We demonstrate that the exchange interaction between orbitally degenerate metal ions can be described by the so-called orbitally-dependent exchange Hamiltonian. This approach has shown to reveal an anomalously strong magnetic anisotropy that can be considered as the main physical manifestation of the unquenched orbital angular momentum in magnetic systems. Along with the exchange coupling, a set of other interactions (such as crystal field effects, spin-orbit and Zeeman coupling), which are specific for the degenerate systems, need to be considered. All these features will be discussed in detail using a pseudo-spin-1/2 Hamiltonian approach. In the second part, the described theoretical background will be used to account for the magnetic properties of several magnetic metal clusters and low-dimensional systems: (i) the dinuclear face-sharing unit [Ti(2)Cl(9)](3-), which exhibits a large magnetic anisotropy; (ii) the rare-earth compounds Cs(3)Yb(2)Cl(9) and Cs(3)Yb(2)Br(9), which, surprisingly, exhibit a full magnetic isotropy; (iii) a zig-zag Co(II) chain exhibiting unusual combination of single-chain magnet behavior and antiferromagnetic exchange coupling; (iv) a trigonal bipyramidal Ni(3)Os(2) complex; (v) various Co(II) clusters encapsulated by polyoxometalate ligands. In the two last examples a pseudospin-1/2 Hamiltonian approach is applied to account for the presence of exchange anisotropy (150 references).

Magnetic spectrum of the two-dimensional antiferromagnetLa2CoO4studied by inelastic neutron scattering

We report measurements of the magnetic excitation spectrum of the layered antiferromagnet ${\text{La}}_{2}{\text{CoO}}_{4}$ by time-of-flight neutron inelastic scattering. In the energy range probed in our experiments (0--250 meV) the magnetic spectrum consists of spin-wave modes with strong in-plane dispersion extending up to 60 meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a small $(\ensuremath{\sim}1\text{ }\text{meV})$ dispersion along the magnetic zone boundary. We show that the magnetic spectrum can be described very well by a model of a Heisenberg antiferromagnet that includes the full spin and orbital degrees of freedom of ${\text{Co}}^{2+}$ in an axially distorted crystal field. The collective magnetic dynamics are found to be controlled by dominant nearest-neighbor exchange interactions, strong XY-like single-ion anisotropy and a substantial unquenched orbital angular momentum.

Magnetic excitations of spin and orbital moments in cobalt oxideSpecial issue on Neutron Scattering in Canada.

Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched orbital angular momentum are studied by neutron scattering. Results of energy scans in several Brillouin zones in the (HHL) plane for energy transfers up to 16 THz are presented. The measurements were performed in the antiferromagnetic ordered state at 6 K (well below TN ∼290 K) as well as in the paramagnetic state at 450 K. Several magnetic excitation modes are identified from the dependence of their intensity on wavevector and temperature. Within a Hund’s rule model, the excitations correspond to fluctuations of coupled orbital and spin degrees of freedom, whose bandwidth is controlled by interionic superexchange. The different <111> ordering domains give rise to several magnetic peaks at each wavevector transfer.

Role of Orbital Degeneracy in the Single Molecule Magnet Behavior of a Mononuclear High-Spin Fe(II) Complex

To explain the single-molecule magnet behavior of the mononuclear complex [(tpaMes)Fe](-) we have developed a model that takes into account the trigonal ligand field splitting of the atomic (5)D term of the Fe(II) ion, and the spin-orbital splitting and mixing of the ligand field terms. The ground ligand field term is shown to be the orbital doublet (5)E possessing an unquenched orbital angular momentum. We demonstrate that the splitting of this term cannot be described by the conventional zero-field splitting Hamiltonian proving thus the irrelevance of the spin-Hamiltonian formalism in the present case. The first-order orbital angular momentum is shown to lead to the strong magnetic anisotropy with the trigonal axis being the easy axis of the magnetization.

Orbital fluctuations in theS=12Mott insulatorSr2VO4

We report a high-energy inelastic neutron-scattering study on ${\text{Sr}}_{2}{\text{VO}}_{4}$ that shows a transition from an orbital liquid phase at high temperature to an orbital-order phase at low temperature with an unquenched orbital angular momentum. Susceptibility under pressure shows a collapse of the magnetic phase transition due to enhanced orbital fluctuations. One possible explanation for our data is the recently proposed magnetically hidden order of Kramers doublets in ${d}^{1}$ electron systems.

Magnetic Field Effect in the Reaction of Recombination of Nitric Oxide and Superoxide Anion

Using a static magnetic field, we observed an important biological radical, nitric oxide, in solution by a spin-sensitive technique. Decomposition of 3-morpholinosydnonimine (SIN-1) in aqueous solution produces a radical pair consisting of nitric oxide and the superoxide anion which recombine to form peroxynitrite. The creation of this radical pair provides a favorable setting for the observation of magnetic field effects. Magnetic field effect of (1.8 ± 0.5)% on the yield of recombination was observed in a relatively high field of 4.7 T over the sampling of 96 samples. The effect is limited by extremely fast relaxation of nitric oxide in liquids due to unquenched orbital angular momentum, and develops in f-pairs via the Δg mechanism. Magnetic field effect due to a radical pair involving nitric oxide in a biological system would require either rather strong magnetic fields in the tesla range or an internal enhancer of magnetic field.

A High-Spin Organometallic Fe−S Compound: Structural and Mössbauer Spectroscopic Studies of [Phenyltris((tert-butylthio)methyl)borate]Fe(Me)

The synthesis and structure of the pseudotetrahedral, sulfur-rich, high-spin organoiron(II) [phenyltris((tert-butylthio)methyl)borate]Fe(Me), [PhTt(tBu)]Fe(Me), 1, are reported. Low-temperature Mössbauer spectroscopic studies reveal an isomer shift of delta = 0.60(3) mm/s and DeltaE(Q) = 0.00(1) mm/s and an S = 2 ground multiplet with a negative zero-field splitting, D = -33(3) cm(-1), E/D approximately = 0.01. The small separation of the ground doublet, Delta approximately = 0.01 cm(-1), allows for observation of X-band EPR signals at g(eff) approximately = 10 (g(z) = 2.6, g(x,y) = 2.00). The relatively large negative zero-field splitting and a highly anisotropic magnetic hyperfine tensor, containing a large orbital z component, {-10(4), -10(4), +33.8(2) MHz}, are concordant with the presence of unquenched orbital angular momentum. Density functional theory (DFT) calculations predict that the lowest-lying orbitals have predominantly d(xy)- and d(x(2)-y(2))-like character, separated by an energy gap small enough to allow mixing through spin-orbit coupling, to generate a negative zero-field splitting, consistent with the experimental observations. The experimental and DFT-calculated isomer shifts are in good agreement (delta(calcd) = 0.5 mm/s). The unusual (for a high-spin ferrous site) null electric field gradients can be qualitatively explained in the frame of the spin-orbit coupling mixing. The very small Fermi contact component of the magnetic hyperfine tensor (A(FC)(exp) = -9 MHz) is not well described by the DFT approach (A(FC)(calcd) = +2 MHz). To our knowledge, this is the first study of a sulfur-coordinated high-spin organoiron(II) complex.

Anisotropic Fermi Couplings Due to Large Unquenched Orbital Angular Momentum: Q-Band 1H, 14N, and 11B ENDOR of Bis(trispyrazolylborate) Cobalt(II)

We report Q-band ENDOR of (1)H, (14)N, and (11)B at the g( parallel) extreme of the EPR spectrum of bis(trispyrazolylborate) cobalt(II) [Co(Tp)(2)] and two structural analogs. This trigonally symmetric, high-spin (hs) S = 3/2 Co(II) complex shows large unquenched ground-state orbital angular momentum, which leads to highly anisotropic electronic g-values (g( parallel) = 8.48, g( perpendicular) = 1.02). The large g-anisotropy is shown to result in large dipolar couplings near g( parallel) and uniquely anisotropic (14)N Fermi couplings, which arise from spin transferred to the nitrogen 2s orbital (2.2%) via antibonding interactions with singly occupied metal d(x(2)-y(2)) and d(z(2)) orbitals. Large, well-resolved (1)H and (11)B dipolar couplings were also observed. Taken in concert with our previous X-band ENDOR measurements at g( perpendicular) ( Myers, W. K.; et al. Inorg. Chem. 2008, 47, 6701-6710 ), the present data allow a detailed analysis of the dipolar hyperfine tensors of two of the four symmetry distinct protons in the parent molecule. In the substituted analogs, changes in hyperfine coupling due to altered metal-proton distances give further evidence of an anisotropic Fermi contact interaction. For the pyrazolyl 3H proton, the data indicate a 0.2 MHz anisotropic contact interaction and approximately 4% transfer of spin away from Co(II). Dipolar coupling also dominates for the axial boron atoms, consistent with their distance from the Co(II) ion, and resolved (11)B quadrupolar coupling showed approximately 30% electronic inequivalence between the B-H and B-C sp(3) bonds. This is the first comprehensive ENDOR study of any hs Co(II) species and lays the foundation for future development.

Orbital‐dependent magnetic properties of molecular cluster containing high‐spin Co(II) ions

AbstractTo shed light on magnetic properties of the magnetic exchange molecular cluster containing the constituent metal ion with unquenched orbital angular momentum, as representative examples two octahedrally coordinated high‐spin Co(II)‐cluster complexes were synthesized and characterized: complex (1) [Co3(Hbzp)6][ClO4]2· 2CH3OH·H2O (bzp = 2‐benzoylpyridine) with a linear core Co(III)‐Co(II)‐Co(III) and complex (2) [Co4L(N3)4(CH3CN)4](ClO4)4·2H2O (L = Macrocyclic schiff base of tetra‐2,6‐pyridinedicarboxaldehyde‐tetra‐ethane‐1,2‐diamine) comprised by two nearly independent Co(II)‐dimers. Following Lines' isotropic exchange approximation the magnetic interaction effective Hamiltonian was introduced, which includes the exchange interactions, the spin‐orbit coupling, the low‐symmetry crystal field, and the intercluster exchange interaction. On the basis of the irreducible tensor operator approach to solve Hamiltonian we have coded the software, BJMAG package, and performed the best‐fit procedure for experimentally temperature variable magnetic susceptibilities for the complexes (1) and (2). The best‐fit parameters were obtained as follows: λ = −126 cm−1, Δ = 432 cm−1, κ = 0.96, mJ′ = 2.7 cm−1 for the complex (1) and λ = −165 cm−1, Δ = 960 cm−1, κ = 0.95, J′ = 9 cm−1, mJ′ = −0.1 cm−1 for the complex (2). To further elucidate the individual role of each interaction in the employed model, the effects of the key factors governing the magnetic properties on temperature variable magnetic susceptibilities were examined in detail. The significance of the intercluster exchange interactions at low temperatures was briefly discussed and a self‐consistent molecular field to calculate the thermally average spin of the nearest neighbor in the intercluster interaction was proposed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Single-Ion Anisotropy and Exchange Interactions in the Cyano-Bridged Trimers MnIII2MIII(CN)6 (MIII = Co, Cr, Fe) Species Incorporating [Mn(5-Brsalen)]+ Units: An Inelastic Neutron Scattering and Magnetic Susceptibility Study

The electronic structures of the compounds K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)M(III)(CN)(6)].2H(2)O (M(III) = Co(III), Cr(III), Fe(III)) have been determined by inelastic neutron scattering (INS) and magnetic susceptibility studies, revealing the manganese(III) single-ion anisotropy and exchange interactions that define the low-lying states of the Mn-M(III)-Mn trimeric units. Despite the presence of an antiferromagnetic intertrimer interaction, the experimental evidence supports the classification of both the Cr(III) and Fe(III) compounds as single-molecule magnets. The value of 17(2) cm(-1) established from AC susceptibility measurements for a spin-reversal barrier of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Cr(CN)(6)].2H(2)O may be readily rationalized in terms of the energy level diagram determined directly by INS. AC susceptibility measurements on samples of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Fe(CN)(6)].2H(2)O are contrary to those previously reported, exhibiting but the onset of peaks below temperatures of 1.8 K at oscillating frequencies in the range of 100-800 Hz. INS measurements reveal an anisotropic ferromagnetic manganese(III)-iron(III) exchange interaction, in accordance with theoretical expectations based on the unquenched orbital angular momentum of the [Fe(CN)(6)](3-) anion, giving rise to an M(s) approximately +/-9/2 ground state, isolated by approximately 11.5 cm(-1) from the higher-lying levels. The reported INS and magnetic data should now serve as a benchmark against which theoretical models that aim to inter-relate the electronic and molecular structure of molecular magnets should be tested.

A Highly Anisotropic Cobalt(II)-Based Single-Chain Magnet: Exploration of Spin Canting in an Antiferromagnetic Array

In this article we report for the first time experimental details concerning the synthesis and full characterization (including the single-crystal X-ray structure) of the spin-canted zigzag-chain compound [Co(H2L)(H2O)]infinity [L = 4-Me-C6H4-CH2N(CPO3H2)2], which contains antiferromagnetically coupled, highly magnetically anisotropic Co(II) ions with unquenched orbital angular momenta, and we also propose a new model to explain the single-chain magnet behavior of this compound. The model takes into account (1) the tetragonal crystal field and the spin-orbit interaction acting on each Co(II) ion, (2) the antiferromagnetic Heisenberg exchange between neighboring Co(II) ions, and (3) the tilting of the tetragonal axes of the neighboring Co units in the zigzag structure. We show that the tilting of the anisotropy axes gives rise to spin canting and consequently to a nonvanishing magnetization for the compound. In the case of a strong tetragonal field that stabilizes the orbital doublet of Co(II), the effective pseudo-spin-1/2 Hamiltonian describing the interaction between the Co ions in their ground Kramers doublet states is shown to be of the Ising type. An analytical expression for the static magnetic susceptibility of the infinite spin-canted chain is obtained. The model provides an excellent fit to the experimental data on both the static and dynamic magnetic properties of the chain.

Irreducible tensor operator method study on molecular magnetism: A practical approach and its use for high-nuclearity magnetic cluster

A new practical approach to molecular magnetism was suggested. First, for the complicated magnetic exchange system with unquenched orbital angular momentum a general form of the effective Hamiltonian including an intercluster exchange interaction of the self-consistent field was introduced. On the basis of the irreducible tensor operator method the practical approach was established, and accomplished by the BJMAG package, where a new strategy to automatically search more good quantum numbers was adopted to solve further block diagonalization in the eigenvalue problem. Finally, potentiality of the approach to isotropic Heisenberg spin clusters was tested through two high-nuclearity clusters, the nonamer cluster and the Mn 7-cluster compound as representative examples.

Electronic structure of the antiferromagnetic phase of [formula omitted

Ab initio calculations analyze the properties of the G-type antiferromagnetic Sr 2 Co 2 O 5 as a high-spin system. The description of the electronic structure is given; all the Co atoms are in a high-spin state with a small unquenched orbital angular momentum. Thermal analysis on a polycrystalline sample shows a magnetic phase transition at high temperatures.

Magnetic relaxation in cyanide based single molecule magnets

The present contribution is aimed at the elaboration of the model of magnetic relaxation in the cyano-bridged pentanuclear Mn(III) 2Mn(II) 3 cluster that belongs to a new family of single molecule magnets (SMM) containing ions with unquenched orbital angular momenta. We proceed from the energy pattern of the cluster formed by the trigonal component of the crystal field acting on the ground-state cubic terms 4 T 1 ( t 2 4 ) of the Mn(III)-ions, spin–orbital interaction and Heisenberg exchange between Mn(II) and Mn(III) ions. The ground state of the cluster possesses the total angular momentum projection ∣ M J ∣ = 15/2, while the energies of the excited states increase with decreasing ∣ M J ∣ values, thus giving rise to a barrier for the reversal of magnetization. The monophonon transitions between the states ∣ M J 〉 and ∣ M J ± 1〉, ∣ M J ± 2〉 induced by electron–vibrational interaction are shown to be allowed. The rates of all possible transitions between the states with 1/2 < ∣ M J ∣ < 15/2 are calculated in the temperature range 0.1 K < T < 3 K. With the purpose of calculation of the temperature dependence of the relaxation time of magnetization we solve the set of master equations for the populations n M J ( t ) of the ∣ M J 〉 states of the Mn(III) 2Mn(II) 3 clusters. The relaxation time is shown to diminish from 10 12s to 10 s with decrease in temperature from 1 K to 3 K for the cluster [Mn(III)(CN) 6] 2[Mn(II)(tmphen) 2] 3 (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) with the trigonal crystal field parameter Δ = −251 cm −1. The obtained values of the relaxation time are in qualitative agreement with the temperature dependence of the ac susceptibilities observed for this SMM. In order to reveal the possibility of enhancing the relaxation time of magnetization in the family of clusters Mn(III) 2Mn(II) 3 we vary the trigonal crystal field parameter ∣ Δ∣( Δ < 0) and demonstrate that increase in ∣ Δ∣ leads to a considerable growth of the relaxation time.

Ising limit for a pair of transition metal ions with unquenched orbital angular momenta and its relevance to single molecule magnets

We discuss the magnetic anisotropy of exchange-coupled pair of transition metal ions with unquenched orbital angular momenta in axially distorted octahedral surroundings. The case of ground orbital doublets of the metal ions is shown to give rise to a strong magnetic anisotropy and Ising type effective exchange Hamiltonian. Providing ferromagnetic exchange such Ising interaction is shown to create a magnetization reversal barrier.