Ac Magnetic Susceptibility Data Research Articles

Tau-borides (Mnx{Ru,Os,Ir}1-x)23B6: X-ray single crystal and TEM data, physical properties

Investigation (electron microprobe and X-ray powder and single crystal diffraction analyses) of the phase relations in the Mn-rich corner (> 45 at% Mn) of the systems Mn-{Ru,Os,Ir}-B, prompted in all three systems a ternary compound with formula (Mnx{Ru,Os,Ir}1-x)23B6. For the systems Mn-Ru-B, Mn-Ir-B phase equilibria have been determined at 950°C (Ru), 900°C (Ir) revealing in both cases a small homogeneity region at constant B-content (Mn1-xRux)23B6 (0.29 < x < 0.37); (Mn1-xOsx)23B6 (0.33 < x < 0.37), as well as (Mn1-xIrx)23B6 (0.29 < x < 0.34). The crystal structure of the three compounds was determined from single crystal and X-ray powder intensity data analyses to be isotypic with the Cr23B6-type (so-called tau-phase, space group Fm3¯m, No. 225). In all cases Mn atoms fully occupy the 4a site (0,0,0) at the origin of the unit cell. For the 8c site (¼,¼,¼) we observed a random distribution of Mn1-x(PM)x with decreasing PM content (PM stands for a platinum group metal atom) going from Ru to Os and Ir, where Mn atoms fully occupy 8c. Whereas the remaining sites (48h, 32f) show various ratios of the two metal species, boron atoms fully occupy the centers (24e site) of the Archimedean metal atom antiprisms. A transmission electron microscopic study confirms the absence of superstructures related to these metal atom disorder, thus Mn and PM atoms randomly share their sites. The latter is well reflected in the behavior of the electrical resistivity of these compounds, which is dominated by disorder scattering. For (Mn0.6{Ru,Os}0.4)23B6, temperature dependent dc magnetization studies reveal distinct antiferromagnetic-like anomalies at TN ≅ 72 K and 114 K, respectively. Low field dc and ac magnetic susceptibility data of (Mn0.7Ir0.3)23B6 display a distinct ferromagnetic-like transition at TC = 280 K, consistent with a pronounced specific heat anomaly and a rather continuous temperature dependent evolution of magnetic anisotropy effects. Mechanical properties (hardness) characterize the tau phases among rather hard and brittle intermetallics (about 5–6 GPa).

Optimized analysis of the AC magnetic susceptibility data in several spin-glass systems using the Vogel–Fulcher and Power laws

In spin-glasses (SG), the relaxation time τ (=1/2πf) vs Tf data at the peak position Tf in the temperature variation of the ac magnetic susceptibilities at different frequencies f is often fit to the Vogel–Fulcher Law (VFL): τ = τ0 exp[Ea/kB(Tf − T0)] and to the Power Law (PL): τ = τ0* [(Tf−TSG)/TSG]−zυ. Both of these laws have three fitting parameters each, leaving a degree of uncertainty since the magnitudes of the evaluated parameters τ0, Ea/kB, τ0*, and zυ depend strongly on the choice of T0 and TSG. Here, we report an optimized procedure for the analysis of τ vs Tf data on seventeen SG systems for which we could extract such data from published sources. In this optimized method, the data of τ vs Tf are fit by varying T0 in the linear plots of Ln τ vs 1/(Tf − T0) for the VFL and by varying TSG in the linear plot of Ln τ vs Ln (Tf − TSG)/TSG for the PL until optimum fits are obtained. The analysis of the associated magnitudes of τ0, Ea/kB,τ0*, and zυ for these optimum values of T0 and TSG shows that the magnitudes of τ0*, τ0, and zυ fail to provide a clear distinction between canonical and cluster SG. However, new results emerge showing Ea/(kBT0) &amp;lt; 1 in canonical SG, whereas Ea/(kBT0) &amp;gt;1 for cluster SG systems, and the optimized T0 &amp;lt; optimized TSG in all cases. Although some interpretation of these new results is presented, a more rigorous theoretical justification of the boundary near Ea/(kBT0) ∼ 1 is desired along with testing of these criteria in other SG systems.

Synergy of Magnetic Anisotropy and Ferromagnetic Interaction Triggering a Dimeric Cr(II) Zero-Field Single-Molecule Magnet

A novel CrII-dimeric complex, [CrIIN(SiiPr3)2(μ-Cl)(THF)]2 (1), has been successfully constructed using a bulky silyl-amide ligand. Single-crystal structure analysis reveals that complex 1 exhibits a binuclear motif, with a Cr2Cl2 rhombus core, where two equivalent tetra-coordinate CrII centers in the centrosymmetric unit display quasi-square planar geometry. The crystal structure has been well simulated and explored by density functional theory calculations. The axial zero-field splitting parameter (D < 0) with a small rhombic (E) value is unambiguously determined by systematic investigations of magnetic measurements, high-frequency electron paramagnetic resonance spectroscopy, and ab initio calculations. Remarkably, ac magnetic susceptibility data unveil that 1 features slow dynamic magnetic relaxation typical of single-molecule magnet behavior with Ueff = 22 K in the absence of a dc field. This increases up to 35 K under a corresponding static field. Moreover, magnetic studies and theoretical calculations point out that a non-negligible ferromagnetic coupling (FMC) exists in the dimeric Cr-Cr units of 1. The coexistence of magnetic anisotropy and FMC contributes to the first case of CrII-based single-molecule magnets (SMMs) under zero dc field.

Radical‐Bridged Heterometallic Single‐Molecule Magnets Incorporating Four Lanthanoceniums

AbstractThe syntheses and magnetic properties of organometallic heterometallic compounds [K(THF)6]{CoI[(μ3‐HAN)RE2Cp*4]2} (1‐RE) and [K(Crypt)]2{CoI[(μ3‐HAN)RE2Cp*4]2} (2‐RE) containing hexaazatrinaphthylene radicals (HAN⋅3−) and four rare earth (RE) ions are reported. 1‐RE shows isolable species with ligand‐based mixed valency as revealed by cyclic voltammetry (CV) thus leading to the isolation of 2‐RE via one‐electron chemical reduction. Strong electronic communication in mixed‐valency supports stronger overall ferromagnetic behaviors in 2‐RE than 1‐RE containing Gd and Dy ions. Ac magnetic susceptibility data reveal 1‐Dy and 2‐Dy both exhibit slow magnetic relaxation. Importantly, larger coercive field was observed in the hysteresis of 2‐Dy at 2.0 K, indicating the enhanced SMM behavior compared with 1‐Dy. Ligand‐based mixed‐valency strategy has been used for the first time to improve the magnetic coupling in lanthanide (Ln) SMMs, thus opening up new ways to construct strongly coupled Ln‐SMMs.

Radical-Bridged Heterometallic Single-Molecule Magnets Incorporating Four Lanthanoceniums.

The syntheses and magnetic properties of organometallic heterometallic compounds [K(THF)6 ]{CoI [(μ3 -HAN)RE2 Cp*4 ]2 } (1-RE) and [K(Crypt)]2 {CoI [(μ3 -HAN)RE2 Cp*4 ]2 } (2-RE) containing hexaazatrinaphthylene radicals (HAN⋅3- ) and four rare earth (RE) ions are reported. 1-RE shows isolable species with ligand-based mixed valency as revealed by cyclic voltammetry (CV) thus leading to the isolation of 2-RE via one-electron chemical reduction. Strong electronic communication in mixed-valency supports stronger overall ferromagnetic behaviors in 2-RE than 1-RE containing Gd and Dy ions. Ac magnetic susceptibility data reveal 1-Dy and 2-Dy both exhibit slow magnetic relaxation. Importantly, larger coercive field was observed in the hysteresis of 2-Dy at 2.0 K, indicating the enhanced SMM behavior compared with 1-Dy. Ligand-based mixed-valency strategy has been used for the first time to improve the magnetic coupling in lanthanide (Ln) SMMs, thus opening up new ways to construct strongly coupled Ln-SMMs.

Strong Axiality in a Dysprosium(III) Bis(borolide) Complex Leads to Magnetic Blocking at 65 K.

Substituted dysprosocenium complexes of the type [Dy(CpR)2]+ exhibit slow magnetic relaxation at cryogenic temperatures and have emerged as top-performing single-molecule magnets. The remarkable properties of these compounds derive in part from the strong axial ligand field afforded by the cyclopentadiene anions, and the design of analogous compounds with even stronger ligand fields is one promising route toward identifying new single-molecule magnets that retain a magnetic memory at even higher temperatures. Here, we report the synthesis and characterization of a dysprosium bis(borolide) compound, [K(18-crown-6)][Dy(BC4Ph5)2] (1), featuring the dysprosocenate anion [Dy(BC4Ph5)2]- with a pseudoaxial coordination environment afforded by two dianionic pentaphenyl borolide ligands. Variable-field magnetization data reveal open magnetic hysteresis up to 66 K, establishing 1 as a top-performing single-molecule magnet among its dysprosocenium analogues. Ac magnetic susceptibility data indicate that 1 relaxes via an Orbach mechanism above ∼80 K with Ueff = 1500(100) cm-1 and τ0 = 10-12.0(9) s, whereas Raman relaxation and quantum tunneling of the magnetization dominate at lower temperatures. Compound 1 exhibits a 100 s blocking temperature of 65 K, among the highest reported for dysprosium-based single-molecule magnets. Ab initio spin dynamics calculations support the experimental Ueff and τ0 values and enable a quantitative comparison of the relaxation dynamics of 1 and two representative dysprosocenium cations, yielding additional insights into the impact of the crystal field splitting and vibronic coupling on the observed relaxation behavior. Importantly, compound 1 represents a step toward the development of alternatives to substituted dysprosocenium single-molecule magnets with increased axiality.

Ferromagnetic Coupling and Single-Ion Magnet Phenomenon in Mononuclear Ruthenium(III) Complexes Based on Guanine Nucleobase

Two mononuclear RuIII complexes of formula trans-[RuCl4(Hgua)(dmso)]·2H2O (1) and trans-[RuCl4(Hgua)(gua)]·3H2O (2) [Hgua = protonated guanine (gua), dmso = dimethyl sulfoxide] have been synthesized and characterized magnetostructurally. 1 and 2 crystallize in the monoclinic system with space groups P21/n and Pc, respectively. Each RuIII ion in 1 and 2 is bonded to four chloride ions and one (1) or two (2) nitrogen atoms from guanine molecules and one sulfur atom (1) of a dmso solvent molecule, generating axially compressed octahedral geometries in both cases. In their crystal packing, the RuIII complexes are connected through an extended network of N-H⋯Cl hydrogen bonds and π⋯Cl intermolecular interactions, forming novel supramolecular structures of this paramagnetic 4d ion. Variable-temperature dc magnetic susceptibility data were obtained from polycrystalline samples of 1 and 2 and their plots show a different magnetic behavior. While 1 is a ferromagnetic compound at low temperature, 2 exhibits a behavior of magnetically isolated mononuclear RuIII complexes with S = 1/2. The study on ac magnetic susceptibility data reveal slow relaxation of the magnetization, when external dc fields are applied, only for 2. These results highlight the presence of field-induced single-ion magnet (SIM) behavior for this mononuclear guanine-based RuIII complex.

New family of Ln9Mn4 (Ln = Gd, Tb, Dy) and Y9Mn4 clusters from the use of methyl-2-pyridyl-ketone oxime in heterometallic Mn chemistry

A new family of [Ln9Mn4O8(OH)4(O2CPh)17(mpko)4] (LnIII = Gd (2), Tb (3), Dy (4); 2MnIII, 2MnIV) clusters and their diamagnetic YIII analog (1) have been obtained from the reaction of Mn(O2CPh)2, LnCl3 or YCl3, methyl-2-pyridyl-ketone oxime (mpkoH), and NMe4OH in a 2:2:2:4 M ratio in MeCN/CH2Cl2 (22:3 mL). The crystal structure of 4·CH2Cl2·8MeCN reveals a very low-symmetry core comprising a [Dy2Mn2O4] cubane sandwiched between a {Dy7MnO4(OH)4} unit and an external Mn atom; alternatively, it can be described as a zig-zag 1-D {MnIVOMnIVO2MnIIIOMnIII}6+ unit sandwiched between Dy5 and Dy4 units. Fitting of the variable-temperature, solid-state dc and ac magnetic susceptibility data on the Mn4 unit of 1 revealed dominant antiferromagnetic (AF) Mn⋯Mn interactions: J1 = −22.5(9) cm−1, J2 = −7.6(5) cm−1, and J3 = +0.30(3) cm−1, with a constant g = 1.97, where J1, J2, and J3 are the J(MnIVMnIV), J(MnIVMnIII), and J(MnIIIMnIII) exchange parameters, respectively. These parameters indicate an S = 4 ground state with ST = 3 and ST = 2 excited states at 2.4 and 4.2 cm−1, respectively, above it. The many non-equivalent Mn∙∙∙Gd interactions in Gd9Mn4 complex 2 were deduced to include both F and AF interactions leading to an S = 18 or 19 ground state. Complexes 3 and 4 exhibited frequency-dependent ac out-of-phase (χ“M) signals below 4 K indicating them to be weak SMMs.

Long-Range Ferromagnetic Exchange Interactions Mediated by Mn-CeIV-Mn Superexchange Involving Empty 4f Orbitals.

Reactions involving reductive aggregation of MnO4- in methanol in the presence of CeIV and an excess of carboxylic acid have led to the synthesis of structurally related Ce/Mn clusters, [Ce3Mn5O8(OMe)(O2CBut)13(MeOH)] (1) and [Ce2Mn3O5(O2CPh)9(MeOH)3] (2), containing at least one {Mn2Ce2O4} cubane unit. The cores of both clusters contain Mnx units separated by three (1) or two (2) CeIV ions. Fits of variable-temperature, solid-state dc and ac magnetic susceptibility data reveal dominant ferromagnetic interactions within 1 and 2, resulting in the maximum S = 17/2 and S = 5 ground state spins, respectively, and thus suggesting significant ferromagnetic (F) interactions between the Mnx units that are ≥6 Å apart and separated by four intervening bonds through diamagnetic CeIV. Fits of magnetic susceptibility data also revealed unusual long-range F interactions, and this finding was further supported by high-field EPR measurements and simulations. Density functional theory calculations and a Wannier function analysis confirm long-range interactions and indicate a Mn-Ce-Mn superexchange pathway via Mn-d/Ce-f orbital overlap/hybridization.

Synthesis, Crystal Structure, and Cooperative 3d–5d Magnetism in Rock Salt Type Li4NiOsO6 and Li3Ni2OsO6

Two new transition metal oxides with the nominal chemical compositions of Li4NiOsO6 and Li3Ni2OsO6 were successfully synthesized. Both compounds crystallize in an ordered rock salt structure type in the monoclinic C2/m space group. The crystal structures were determined using both synchrotron X-ray and time-of-flight neutron, powder diffraction data. In both phases, Ni2+ ions are present while oxidation states of osmium are +6 and +5 in Li4NiOsO6 and Li3Ni2OsO6, respectively. Ni2+ ions in the hypothetical fully ordered phase form a honeycomb arrangement in the ab crystallographic plane and these hexagons are centered by osmium ions. The magnetic layers are separated along the c axis by the octahedra, which are centered by Li+ (or Li+/Ni2+, depending on the chemical compositions). Crystal structure refinements reveal that there is some degree of mixed occupancy in cationic positions. Temperature dependent magnetic susceptibility data for both phases show ferrimagnetic transitions with predominant antiferromagnetic (AFM) interactions among 3d electrons of nickel and 5d electrons of osmium. Iso-thermal magnetization loops as a function of the applied magnetic field below the transition temperatures confirm the ferrimagnetic nature in magnetic transitions. Temperature dependent heat capacity data, however, did not exhibit any anomaly in either phase, indicating the absence of long-range magnetic ordering. The lack of long-range order for both Os5+ and Os6+-based compounds was also confirmed by low temperature neutron diffraction data down to 10 K. Temperature dependent AC magnetic susceptibility data in various frequencies for both samples indicate that Li4NiOsO6 exhibits spin-glass-like behavior, while the transition temperature for Li3Ni2OsO6 is nearly frequency independent.

Observation of skyrmion-like magnetism in magnetic Weyl semimetal Co3Sn2S2

Polycrystalline samples Co3Sn2S2+x (0 ≤ x ≤ 0.34), (Co1-yNiy)3Sn2S2.26 (0 ≤ y ≤ 0.05), and (Co1-zFez)3Sn2S2.26 (0 ≤ z ≤ 0.05) were synthesized using a solid-state-reaction method and characterized by X-ray diffraction, electron probe microanalyzer, and magnetic measurements. Single crystal Co3Sn2S2 was also grown by self-flux method and analyzed by detailed magnetic measurements to study its isotropic nature. In this study, we reproduced the low magnetic field anomalous magnetism, as previously reported, with a negligible sample-dependent effect. Furthermore, based on ac magnetic susceptibility data, we established a H-T phase diagram including the skyrmion-like A phase of Co3Sn2S2. Both application of external high-pressure and chemical substitutions revealed anomalous magnetisms in low magnetic fields. The possible origin of the field-induced A phase can be the non-collinear skyrmion-like phenomenon. These interesting findings in Weyl semimetal Co3Sn2S2 provide a new candidate in spin-frustrated systems to explore the complexity of magnetic field–induced magnetism.

Feasibility of ground state spin switching in a molecular analogue of the mixed-metal oxides with the perovskite structure

The carboxylate substitution reaction between the reported all-benzoate complex [Ce3Mn8O8(PhCO2)18(PhCO2H)2] (1) and 2-Cl-benzoic acid has led to isolation of [Ce3Mn8O8(2-Cl-PhCO2)15.3(PhCO2)2.7(H2O)2] (2). The structures of 1 and 2 are very similar, both containing a tetragonally-elongated MnIII8 cuboid with a CeIV at its center and two CeIII attached to opposite faces, but 2 shows extensive disorder in the ligands and the Cl atom positions and occupancies. Variable-temperature (T) solid-state dc and ac magnetic susceptibility data for 2 are significantly different from those for 1 and are assigned as due to small ligand-induced structural perturbation of the core that has brought an A-type antiferromagnetic (AF) excited state much closer to the C-type AF ground state. Fits of the dc χMT vs T data support this interpretation but due to the ligand disorder in 2, a more definitive description of the lowest energy spin states of 2 is not possible. Nevertheless, these results provide important support for the feasibility of ground state spin switching by ligand-induced core perturbation (‘chemical pressure’) in this family, and they encourage further studies.

AC Magnetic Susceptibility of Y3Ba5Cu8O18 Substituted by Nd3+ and Ca2+ Ions

Effect of Nd3+ and Ca2+ ion substitutions in Y3Ba5Cu8O18 (Y-358) was studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical resistivity, and ac magnetic susceptibility measurements. Superconducting samples of type Y3 − xNdxBa5 − yCayCu8O18 (with 0 ≤ x, y ≤ 0.4) were synthesized by the standard solid-state reaction technique. XRD patterns revealed that all the samples were mainly composed of Y-358 phase with orthorhombic structure. SEM micrographs showed that Nd3+ and Ca2+ ion substitutions had the ability to alter the grain morphology of Y-358 to irregular randomly oriented grains with smaller average grain sizes. The temperature-dependent electrical resistivity curves indicated that the superconducting transition temperature Tc was decreased from 96.51 to 87.41 K as the substitution contents were increased from 0 to 0.4. Detailed AC magnetic susceptibility measurements were done using a quantum design physical property measurement system (PPMS) at different dc fields (B = 0.5–20 kG) and frequency of 1 kHz. A comparison between electrical resistivity and AC magnetic susceptibility data was performed. It was found that Tcs obtained from AC magnetic susceptibility were much lower than those obtained from the resistivity measurements, for all studied samples. This was discussed in terms of filamentary paths that could produce sharp pronounced drops in diamagnetism. Generally, the depression in Tc was attributed to the decrease in relative volume fraction of Y-358 or due to trapping of mobile holes or other mechanism connected with oxygen vacancy disorder. Werthamer–Helfand–Hohenberg (WHH) theory was employed to study the temperature dependence of the upper critical magnetic field Bc2(T) for the samples. The relative importance of both orbital and paramagnetic effects in the suppression of superconductivity was described with the aid of Maki parameter (κM). The estimated values of the upper critical magnetic field at 0 K, Bc2(0), were found to decrease from 37.3 to 13.5 T as Nd3+ and Ca2+ contents were increased from 0 to 0.4. This decrement was attributed to the weak electron–phonon coupling and to the orbital as well as spin pair breaking mechanisms. Furthermore, Ginzburg–Landau coherence length ξGL of Y3 − xNdxBa5 − yCayCu8O18 was found to increase with the increase of the Nd3+ and Ca2+ contents. Therefore, Nd3+ and Ca2+ ions play an important role in the pair-breaking mechanism and in suppressing superconductivity of the Y-358 phase.

Coercive Field Model for Description of Mono- and Multidomain Magnetic Granular Systems

The study of nanoscopic magnetic systems of granular materials is an interesting subject for fundamental research in the field of condensed matter physics that can be modeled through convenient theory. Here, we evaluate the attempt time τ0 and magnetic anisotropy energy barrier Ean of magnesium ferrite (MgFe2O4) samples annealed at different temperatures using scaling plots from ac magnetic susceptibility data. The scaling of ln(τ) versus 1/Tm″ is valid for a range of annealing temperatures. Particularly, we identify MgFe2O4 nanoparticles with an increase in size and the emerging of agglomerates, which are also corroborated by data from XRD, TEM, and magnetic measurements. By analyzing ZFC-FCW curves and working out ⟨HC⟩T, we show that the model describing noninteracting monodomain nanoparticles remains valid for samples without agglomerates, as long as the received thermal energy does not surpass a given threshold value. For increasing annealing energy, the magnetic anisotropy energy barrier is substanti...

Synthesis of a Neutral Mononuclear Four-Coordinate Co(II) Complex Having Two Halved Phthalocyanine Ligands That Shows Slow Magnetic Relaxations under Zero Static Magnetic Field.

Syntheses of a novel pseudotetrahedral four-coordinate mononuclear Co(II) complex that has two halved phthalocyanine moieties as the ligands, [Co(half-Pc)2] (1), and its magnetic properties as a single molecule magnet (SMM) are reported. A one pot reaction of phthalonitrile and lithium methoxide followed by the coordination to a Co(II) ion gave 1 as an orange solid in a moderate yield. X-ray crystallography on 1 reveals tetragonally distorted coordination geometry around the Co(II) ion. The M- HT-1 plots suggest that 1 has large axial magnetic anisotropy. The ac magnetic susceptibility data of the magnetically diluted 1 (dil.1) clearly show that the complex acts as an SMM even in the absence of the external static magnetic field ( Hdc). The influence of intermolecular and intramolecular interactions for the magnetic relaxation behaviors has been discussed by comparison of the magnetic data of 1 and dil.1. The Orbach process is suggested as the predominant mechanism of magnetic relaxations in the high-temperature range, and the Arrhenius plots provide the effective relaxation energy barrier and pre-exponential factor of Ueff = 54.0 cm-1 and τ0 = 3.17 × 10-10 s, respectively. The direct estimation of the axial anisotropic parameter of 1 was successfully performed by high-field, multifrequency ESR measurements up to 55 T and 2.5 THz. The evaluated axial zero-field splitting (ZFS) energy of 57.0 cm-1 is comparable to the Ueff energy, confirming that the magnetic relaxations are initiated by the thermal excitation from the ground | M S⟩ = |±3/2⟩ states to the |±1/2⟩ states in the high-temperature range. The results of the ab initio calculations based on the CAS(7,5) SCF wave functions indicate that the ground states of 1 consist mainly of | M S⟩ = |±3/2⟩ states, while the first excited states are the mixture of | M S⟩ = |+1/2⟩ and |-1/2⟩.

Lanthanide chain assembled in metal–organic frameworks: Slow relaxation of the magnetization in Dy(III) and Er(III) complexes

Three new LnIII-MOFs, {[Dy1.5(TAPB)1.5(DMF)]·9DMF}n (Ln = Dy (1), Er (2), Ho (3); TAPB = 4, 4′, 4″-(triazine‑2, 4, 6‑triyl‑tris‑(benzene‑4, 1-diyl)) tribenzoate), have been synthesized via a solvothermal method by using a trigonal N-containing tricarboxylate as the ligand. The coordination geometries around the central metal ions in the MOFs exhibit different deviations from ideal square antiprism (D4d symmetry) because of the coordinated solvent molecules. The ac magnetic susceptibility data is consistent with single chain magnet behavior for Dy(III) and Er(III) complexes, which have been studied by the noncritical scaling theory.

Mechanism of magnetisation relaxation in {MIII2DyIII2} (M = Cr, Mn, Fe, Al) "Butterfly" complexes: how important are the transition metal ions here?

We describe the synthesis, characterisation and magnetic studies of four tetranuclear, isostructural "butterfly" heterometallic complexes: [MIII2LnIII2(μ3-OH)2(p-Me-PhCO2)6(L)2] (H2L = 2,2'-((pyridin-2-ylmethyl)azanediyl)bis(ethan-1-ol), M = Cr, Ln = Dy (1), Y (2), M = Mn, Ln = Dy (3), Y (4)), which extend our previous study on the analogous 5 {Fe2Dy2}, 6 {Fe2Y2} and 7 {Al2Dy2} compounds. We also present data on the yttrium diluted 7 {Al2Dy2}: 8 {Al2Dy0.18Y1.82}. Compounds dc and ac magnetic susceptibility data reveal single-molecule magnet (SMM) behaviour for complex 3 {Mn2Dy2}, in the absence of an external magnetic field, with an anisotropy barrier U eff of 19.2 K, while complex 1 {Cr2Dy2}, shows no ac signals even under applied dc field, indicating absence of SMM behaviour. The diluted sample 8 {Al2Dy0.18Y1.82} shows field induced SMM behaviour with an anisotropy barrier U eff of 69.3 K. Furthermore, the theoretical magnetic properties of [MIII2LnIII2(μ3-OH)2(p-Me-PhCO2)6(L)2] (M = Cr, 1 or Mn, 3) and their isostructural complexes: [MIII2DyIII2(μ3-OH)2(p-Me-PhCO2)6(L)2] (M = Fe, 5 or Al, 7) are discussed and compared. To understand the experimental observations for this family, DFT and ab initio CASSCF + RASSI-SO calculations were performed. The experimental and theoretical calculations suggest that altering the 3dIII ions can affect the single-ion properties and the nature and the magnitude of the 3dIII-3dIII, 3dIII-DyIII and DyIII-DyIII magnetic coupling, thus quenching the quantum tunneling of magnetisation (QTM) significantly, thereby improving the SMM properties within this motif. This is the first systematic study looking at variation and therefore role of trivalent transition metal ions, as well as the diamgnetic AlIII ion, on slow relaxation of magnetisation within a series of isostructural 3d-4f butterfly compounds.

Large Anisotropy Barrier in a Tetranuclear Single-Molecule Magnet Featuring Low-Coordinate Cobalt Centers.

The tetranuclear cobalt cluster compound [Co4(μ-NPtBu3)4][B(C6F5)4] (tBu = tert-butyl) was synthesized by chemical oxidation of Co4(NPtBu3)4 with [FeCp2][B(C6F5)4] and magnetically characterized to study the effect of electronic communication between low-coordinate metal centers on slow magnetic relaxation in a transition metal cluster. The dc magnetic susceptibility data reveal that the complex exhibits a well-isolated S = 9/2 ground state, which persists even to 300 K and is attributed to the existence of direct metal-metal orbital overlap. The ac magnetic susceptibility data further reveals that the complex exhibits slow magnetic relaxation in the absence of an applied field, and that the relaxation dynamics can be fit with a combination of Orbach, quantum tunneling, and Raman relaxation processes. The effective spin reversal barrier for this molecule is 87 cm-1, the largest reported to date for a transition metal cluster, and arises due to the presence of a large easy-axis magnetic anisotropy. The complex additionally exhibits waist-restricted magnetic hysteresis and magnetic blocking below 3.6 K. Taken together, these results indicate that coupling of low-coordinate metal centers is a promising strategy to enhance magnetic anisotropy and slow magnetic relaxation in transition metal cluster compounds.

Quenching the Quantum Tunneling of Magnetization in Heterometallic Octanuclear {TMIII4 DyIII4 } (TM=Co and Cr) Single-Molecule Magnets by Modification of the Bridging Ligands and Enhancing the Magnetic Exchange Coupling.

We report the synthesis, structural characterisation, magnetic properties and provide an ab initio analysis of the magnetic behaviour of two new heterometallic octanuclear coordination complexes containing CoIII and DyIII ions. Single-crystal X-ray diffraction studies revealed molecular formulae of [CoIII4 DyIII4 (μ-OH)4 (μ3 -OMe)4 {O2 CC(CH3 )3 }4 (tea)4 (H2 O)4 ]⋅4 H2 O (1) and [CoIII4 DyIII4 (μ-F)4 (μ3 -OH)4 (o-tol)8 (mdea)4 ]⋅ 3 H2 O⋅EtOH⋅MeOH (2; tea3- =triply deprotonated triethanolamine; mdea2- =doubly deprotonated N-methyldiethanolamine; o-tol=o-toluate), and both complexes display an identical metallic core topology. Furthermore, the theoretical, magnetic and SMM properties of the isostructural complex, [CrIII4 DyIII4 (μ-F4 )(μ3 -OMe)1.25 (μ3 -OH)2.75 (O2 CPh)8 (mdea)4 ] (3), are discussed and compared with a structurally similar complex, [CrIII4 DyIII4 (μ3 -OH)4 (μ-N3 )4 (mdea)4 (O2 CC(CH3 )3 )4 ] (4). DC and AC magnetic susceptibility data revealed single-molecule magnet (SMM) behaviour for 1-4. Each complex displays dynamic behaviour, highlighting the effect of ligand and transition metal ion replacement on SMM properties. Complexes 2, 3 and 4 exhibited slow magnetic relaxation with barrier heights (Ueff ) of 39.0, 55.0 and 10.4 cm-1 respectively. Complex 1, conversely, did not exhibit slow relaxation of magnetisation above 2 K. To probe the variance in the observed Ueff values, calculations by using CASSCF, RASSI-SO and POLY_ANISO routine were performed on these complexes to estimate the nature of the magnetic coupling and elucidate the mechanism of magnetic relaxation. Calculations gave values of JDy-Dy as -1.6, 1.6 and 2.8 cm-1 for complexes 1, 2 and 3, respectively, whereas the JDy-Cr interaction was estimated to be -1.8 cm-1 for complex 3. The developed mechanism for magnetic relaxation revealed that replacement of the hydroxide ion by fluoride quenched the quantum tunnelling of magnetisation (QTM) significantly, and led to improved SMM properties for complex 2 compared with 1. However, the tunnelling of magnetisation at low-lying excited states was still operational for 2, which led to low-temperature QTM relaxation. Replacement of the diamagnetic CoIII ions with paramagnetic CrIII led to CrIII ⋅⋅⋅DyIII coupling, which resulted in quenching of QTM at low temperatures for complexes 3 and 4. The best example was found if both CrIII and fluoride were present, as seen for complex 3, for which both factors additively quenched QTM and led to the observation of highly coercive magnetic hysteresis loops above 2 K. Herein, we propose a synthetic strategy to quench the QTM effects in lanthanide-based SMMs. Our strategy differs from existing methods, in which parameters such as magnetic coupling are difficult to control, and it is likely to have implications beyond the DyIII SMMs studied herein.

Comparative Magnetic Studies in the Solid State and Solution of Two Isostructural 1D Coordination Polymers Containing CoII/NiII-Curcuminoid Moieties

Two novel 1D coordination chains containing the curcuminoid (CCMoid) ligand 9Accm have been characterized: [CoII(9Accm)2(4,4´-bpy)]n (1) and [NiII(9Accm)2(4,4´-bpy)]n (2). The two compounds were synthesized by solvothermal and microwave (MW) assisted techniques, respectively, and crystals of both systems were directly obtained from the mother solutions. Crystal structures of 1 and 2 prove that both systems are isostructural, with the ligands in a trans configuration. The two chains have been magnetically characterized in solution by paramagnetic 1H NMR, where 1 displayed typical features from CoII systems, with spread out signals; meanwhile, 2 showed diamagnetic behaviour. The dissociation of the latest in solution and the stability of the “[Ni(9Accm)2]” unit were proved by further experiments in C5D5N. Additional UV-Vis absorption and fluorescence studies in solution were performed using exclusively 1. In the solid state χMT vs. T and M/NµB vs. H/T data were collected and fitted for 1 and 2; both systems display Ising plane anisotropy, with significant D values. System 1 presented slow relaxation of the magnetization, displaying frequency dependence in the in-phase/out-phase ac magnetic susceptibility data, when an external dc field of 0.2 T was applied. Finally, 1 was deposited on a HOPG (highly oriented pyrolytic graphite) substrate by spin-coating and analysed by AFM.