Quantum Tunneling Of Magnetization Research Articles

Field-induced oscillation of magnetization blocking barrier in a holmium metallacrown single-molecule magnet

<h2>Summary</h2> Single-molecule magnets (SMMs) exhibit magnetization blocking in the presence of strong magnetic anisotropy. By employing molecular engineering, the blocking barrier can be significantly modified, remaining independent from other external factors, such as magnetic field. Taking advantage of hyperfine coupling of electronic and nuclear spins further highlights their unique functionality. However, a poor understanding of relaxation mechanisms limits the exploitation of nuclear-spin molecular qubits. Here, we report the first observation of field-dependent oscillation of the magnetization blocking barrier in a holmium metallacrown magnet driven by the switch of relaxation mechanisms involving hyperfine interaction. First-principles calculations reveal an activated temperature dependence of magnetic relaxation, dominated either by incoherent quantum tunneling of magnetization at anti-crossing points or by Orbach-like processes at crossing points. This mechanism demonstrates that these relaxation barriers can be consecutively switched by increasing the external field, which paves a way for manipulating magnetization dynamics of SMMs using hyperfine interaction.

Ytterbium-Centered Isotopic Enrichment Leading to a Zero-Field Single-Molecule Magnet.

An unprecedented combination of isotopic enrichment and magnetic dilution approaches for a prolate ytterbium(III)-based complex was performed. It results in the appearance of the first observations of a nuclear spin effect on both quantum tunneling of magnetization and slow magnetic relaxation for an ytterbium complex under a zero applied field.

In Situ Ligand Formation in the Synthetic Processes from Mononuclear Dy(III) Compounds to Binuclear Dy(III) Compounds: Synthesis, Structure, Magnetic Behavior, and Theoretical Analysis.

Guided by the self-assembled process and mechanism, the strategy of in situ Schiff base reaction would be capable of bringing a feasible method to construct and synthesize lanthanide compounds with distinct structures and magnetic properties. A mononuclear Dy(III) compound was synthesized through a multidentate Schiff base ligand and a chelating β-diketonate ligand, which was named as [Dy(L)(bppd)]·CH3OH [1; H2L = N,N'-bis(2-hydroxy-5-methyl-3-formylbenzyl)-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine and bppd = 3-bis(pyridin-2-yl)propane-1,3-dione]. Furthermore, a new binuclear Dy(III) compound, [Dy2(H2Lox)(bppd)3]·8CH3OH [2; H4Lox = N,N'-bis[2-hydroxy-5-methyl-3-(hydroxyiminomethyl)benzyl]-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine], was obtained via an in situ synthetic process. Under similar synthetic conditions, [Dy(L)(ctbd)] [3; ctbd = 1-(4-chlorophenyl)-4,4,4-trifluoro-1,3-butanedione] and [Dy2(H2Lox)(ctbd)3]·CH3OH·C4H10O (4) were synthesized by modifying the β-diketonate ligand and in situ Schiff base reaction. Compound 3 is a mononuclear configuration, while compound 4 exhibits a binuclear Dy(III) unit. Therein, formylbenzyl groups of H2L in 1 and 3 were changed to (hydroxyiminomethyl)benzyl groups in 2 and 4, respectively. In isomorphous 2 and 4, two Dy(III) centers are connected through two phenol O- atoms of the H2Lox2- ligand to form a binuclear structure. Eight-coordinated Dy(III) ions with different distortions can be observed in 1-4. The crystals of 1 and 3 suffered dissolution/precipitation to obtain 2 and 4, respectively. The relationship between the structure and magnetism in compounds 1-4 was discussed through the combination of structural, experimental, and theoretical investigations. Especially, the rates of quantum tunneling of magnetization of 1-4 were theoretically predicted and are consistent with the experimental results. For 2 and 4, the theoretically calculated dipolar parameters Jdip are consistent with the experimental observation of weak ferromagnetic coupling.

The anisotropy of the internal magnetic field on the central ion is capable of imposing great impact on the quantum tunneling of magnetization of Kramers single-ion magnets.

In this work, a theoretical method, taking into account the anisotropy of the internal magnetic field (B[combining right harpoon above]int), is proposed to predict the rate of quantum tunneling of magnetization (QTM), i.e., τQTM-1, for Kramers single-ion magnets (SIMs). Direct comparison to both experimental and previous theoretical results of three typical Kramers SIMs indicates the necessity of the inclusion of the anisotropy of B[combining right harpoon above]int for accurate description of QTM. The predictions of the method here are consistent with the theory proposed by Prokof'ev and Stamp (PS). For Kramers SIMs of high magnetic axiality, the QTM rates, predicted by the method here, are almost linearly proportional to the results by the PS method. The dependence of τQTM-1 on various parameters is analyzed for model systems. The averaged magnitude of B[combining right harpoon above]int (Bave) and principal g value of the axial direction (gZ) are the parameters on which τQTM-1 is linearly dependent. The ones on which τQTM-1 is quadratically dependent are gXY, i.e., the principal g value of the transversal direction, and xaniso characterizing the anisotropy of B[combining right harpoon above]int. Compared to Bave and gZ, gXY and xaniso provide a higher order of dependence for QTM. Therefore regulation of the SMM property via introduction of desired values of gXY and xaniso ought to be a strategy more efficient than the one via Bave and gZ. Being different from the one via gXY, the strategy via xaniso to regulate the QTM has been rarely touched upon according to our best knowledge. However, this strategy could also lead to significant improvement since it is the same as gXY in the aspect of the dependence of τQTM-1.

Exchange-biased quantum tunnelling of magnetization in a [Mn3]2 dimer of single-molecule magnets with rare ferromagnetic inter-Mn3 coupling.

A covalently-linked dimer of two single-molecule magnets (SMMs), [Mn6O(O2CMe)6(1,3-ppmd)3](ClO4)2, has been synthesized from the reaction of [Mn3O(O2CMe)6(py)3](ClO4) with 1,3-phenylene-bis(pyridin-2-ylmethanone) dioxime (1,3-ppmdH2). It contains two [MnIII3O]+7 triangular units linked by three 1,3-ppmd2- groups into an [Mn3]2 dimer with D3 symmetry. Solid-state dc and ac magnetic susceptibility measurements showed that each Mn3 subunit retains its properties as an SMM with an S = 6 ground state. Magnetization vs. dc field sweeps on a single crystal reveal hysteresis loops below 1.3 K exhibiting exchange-biased quantum tunnelling of magnetization (QTM) steps with a bias field of +0.06 T. This is the first example of a dimer of SMMs showing a positive exchange bias of the QTM steps in the hysteresis loops, and it has therefore been subjected to a detailed analysis. Simulation of the loops determines that each Mn3 unit is exchange-coupled with its neighbour primarily through the 1,3-ppmd2- linkers, confirming a weak ferromagnetic inter-Mn3 interaction of J12≈ +6.5 mK (Ĥ = -2Jŝi·ŝj convention). High-frequency EPR studies of a microcrystalline powder sample enable accurate determination of the zero-field splitting parameters of the uncoupled Mn3 SMMs, while also confirming the weak exchange interaction between the two SMMs within each [Mn3]2 dimer. The combined results emphasize the ability of designed covalent linkers to generate inter-SMM coupling of a particular sign and relative magnitude, and thus the ability of such linkers to modulate the quantum physics. As such, this work supports the feasibility of using designed covalent linkers to develop molecular oligomers of SMMs, or other magnetic molecules, as multi-qubit systems and/or other components of new quantum technologies.

Holmium(iii) molecular nanomagnets for optical thermometry exploring the luminescence re-absorption effect.

Coordination complexes of lanthanide(3+) ions can combine Single-Molecule Magnetism (SMM) with thermally modulated luminescence applicable in optical thermometry. We report an innovative approach towards high performance SMM-based optical thermometers which explores tunable anisotropy and the luminescence re-absorption effect of HoIII complexes. Our concept is shown in dinuclear cyanido-bridged molecules, {[HoIII(4-pyridone)4(H2O)2][MIII(CN)6]}·nH2O (M = Co, 1; Rh, 2; Ir, 3) and their magnetically diluted analogues, {[HoIIIxYIII1–x(4-pyridone)4(H2O)2][MIII(CN)6]}·nH2O (M = Co, x = 0.11, 1@Y; Rh, x = 0.12, 2@Y; Ir, x = 0.10, 3@Y). They are built of pentagonal bipyramidal HoIII complexes revealing the zero-dc-field SMM effect. Experimental studies and the ab initio calculations indicate an Orbach magnetic relaxation with energy barriers varying from 89.8 to 86.7 and 78.7 cm−1 K for 1, 2, and 3, respectively. 1–3 also differ in the strength of quantum tunnelling of magnetization which is suppressed by hyperfine interactions, and, further, by the magnetic dilution. The YIII-based dilution governs the optical properties as 1–3 exhibit poor emission due to the dominant re-absorption from HoIII while 1@Y–3@Y show room-temperature blue emission of 4-pyridone. Within ligand emission bands, the sharp re-absorption lines of the HoIII electronic transitions were observed. Their strong thermal variation was used in achieving highly sensitive ratiometric optical thermometers whose good performance ranges, lying between 25 and 205 K, are adjustable by using hexacyanidometallates. This work shows that HoIII complexes are great prerequisites for advanced opto-magnetic systems linking slow magnetic relaxation with unique optical thermometry exploiting a luminescence re-absorption phenomenon.

Hetero-tri-spin systems: an alternative stairway to the single molecule magnet heaven?

The search for molecule-based magnetic materials has stimulated over the years the development of extremely rich coordination chemistry. Various combinations of spin carriers have been investigated and illustrated by a plethora of hetero-spin complexes: 3d-nd, 3d-4f, 2p-3d, and 2p-4f. More recently, two other classes of hetero-spin complexes have grown rapidly: compounds containing three different paramagnetic metal ions, or one radical and two different paramagnetic metal ions (all within the same molecular entity). Such new classes of systems represent a challenge both from a synthetic and theoretical point of view. Indeed, the synthetic control and the understanding of the spin topology effect on the overall magnetic behavior from first-principles is a difficult problem to be solved. The presence of different spin carriers in a single molecule makes such compounds particularly interesting because they offer the possibility of developing new magnetic properties, different from those of hetero-bi-spin or homo-spin systems. A critical overview taking the case of 2p-3d-4f complexes is the focus of this perspective paper. An original organic picture of the state-of-art in this field and new hints about the main directions that should be pursued to achieve hetero-tri-spin systems with large anisotropy barriers, low quantum tunneling of magnetization and, possibly, large blocking temperatures are provided in this article through an analysis based on numerically revisiting already published data and a critical survey of the literature reported so far. The reasons for the limited success obtained for the largely used 3d-2p-4f topology are given along with the ones explaining the failure for the 2p-4f-3d case. The still never synthesized linear 2p-3d-4f spin topology seemed to be the most promising one based on the results obtained for the unique closed hetero-tri-spin closed triangular system synthesized so far.

High-Coordinate Mononuclear Ln(III) Complexes: Synthetic Strategies and Magnetic Properties

Single-molecule magnets involving monometallic 4f complexes have been investigated extensively in last two decades to understand the factors that govern the slow magnetization relaxation behavior in these complexes and to establish a magneto-structural correlation. The prime goal in this direction is to suppress the temperature independent quantum tunneling of magnetization (QTM) effect via fine-tuning the coordination geometry/microenvironment. Among the various coordination geometries that have been pursued, complexes containing high coordination number around Ln(III) are sparse. Herein, we present a summary of the various synthetic strategies that were used for the assembly of 10- and 12-coordinated Ln(III) complexes. The magnetic properties of such complexes are also described.

Extending the family of reduced [Mn12O12(O2CR)16(H2O)x]n− complexes, and their sensitivity to environmental factors

The family of reduced [Mn12O12(O2CR)16(H2O)4]n− (n = 1, 2) single-molecule magnets (SMMs) has been expanded with the synthesis of new members using I− as a stoichiometric one-electron reducing agent of the neutral n = 0 complexes. Electrochemical data, 1H NMR spectra of the [Mn12O12(O2CCH2Cl)16(H2O)3]0,−,2− salts, and the crystal structures of two complexes, (PPh4)[Mn12O12(O2CCHCl2)16(H2O)4] and (PPh4)2[Mn12O12(O2CCH2Cl)16(H2O)3] have been obtained. For (PPh4)2[Mn12O12(O2CCH2Cl)16(H2O)3], the data confirm an S = 10 ground state, and magnetization vs dc field scans on a pristine crystal exhibit hysteresis loops possessing steps due to quantum tunneling of magnetization (QTM). In addition, two forms of the cluster have been identified in the crystal with distinctly different relaxation barriers, with the faster-relaxing form being the majority one in pristine crystals from mother liquor, and the slower-relaxing one becoming the majority one in vacuum-dried samples. Similar behavior is seen for (PPh4)[Mn12O12(O2CCHCl2)16(H2O)4] with an S = 19/2 ground state, which shows three forms to be present in the hysteresis loops of a pristine crystal, all converting to a slow-relaxing form on vacuum-drying. High-frequency EPR spectra of the latter confirm a high axial anisotropy with D = −0.477 cm−1 and rationalize its effective relaxation barrier Ueff = 57 K. The combined work emphasizes a high sensitivity of such ultra-small nanomagnets to environmental influences, affecting their properties significantly, especially their quantum properties that are so important to many potential new 21st century applications.

Manipulation of Molecular Qubits by Isotope Effect on Spin Dynamics

Controlling spin behavior via external stimuli is a key route to develop molecular spintronics devices. Photons, temperature, pressure, chemicals, and electric field are the possible stimuli. Herei...

Synthesis and physical property of GaN:Mn nanoparticles

The GaN:Mn nanostructure was prepared using direct current (DC) arc discharge plasma method. No catalysts or templates are involved in the experiment. The test of energy dispersive x-ray spectroscopy (EDXS) detected the signals of Ga, N, Mn, and the Mn signal strength is slightly stronger with the increasing Mn content. The X-ray diffraction peak (XRD) is sharp, and there is a clear XRD peak shift towards larger angle with the increase of doping concentration. It is indicated that GaN:Mn nanoparticles have good crystallinity, and the introduced Mn has changed the lattice parameter. Photoluminescence (PL) shows that the absorption of visible light is a relatively broad range. The Mn doped induced the energy levels in the band gap, leading to the increasing of yield of electron-hole pair, and the optical property enhanced with the increasing of Mn concentration. Meanwhile the complicated magnetic transition has been found due to the quantum tunneling of magnetization. The magnetism is from the doped Mn, and its concentration impacts the strength of magnetic coupling.

Magnetic Properties and Electronic Structure of the S = 2 Complex [MnIII{(OPPh2)2N}3] Showing Field-Induced Slow Magnetization Relaxation

The high-spin S = 2 Mn(III) complex [Mn{(OPPh2)2N}3] (1Mn) exhibits field-induced slow relaxation of magnetization (Inorg. Chem. 2013, 52, 12869). Magnetic susceptibility and dual-mode X-band electron paramagnetic resonance (EPR) studies revealed a negative value of the zero-field-splitting (zfs) parameter D. In order to explore the magnetic and electronic properties of 1Mn in detail, a combination of experimental and computational studies is presented herein. Alternating-current magnetometry on magnetically diluted samples (1Mn/1Ga) of 1Mn in the diamagnetic gallium analogue, [Ga{(OPPh2)2N}3], indicates that the slow relaxation behavior of 1Mn is due to the intrinsic properties of the individual molecules of 1Mn. Investigation of the single-crystal magnetization of both 1Mn and 1Mn/1Ga by a micro-SQUID device reveals hysteresis loops below 1 K. Closed hysteresis loops at a zero direct-current magnetic field are observed and attributed to fast quantum tunneling of magnetization. High-frequency and -field EPR (HFEPR) spectroscopic studies reveal that, apart from the second-order zfs terms (D and E), fourth-order terms (B4m) are required in order to appropriately describe the magnetic properties of 1Mn. These studies provide accurate spin-Hamiltonian (sH) parameters of 1Mn, i.e., zfs parameters |D| = 3.917(5) cm-1, |E| = 0.018(4) cm-1, B04 = B42 = 0, and B44 = (3.6 ± 1.7) × 10-3 cm-1 and g = [1.994(5), 1.996(4), 1.985(4)], and confirm the negative sign of D. Parallel-mode X-band EPR studies on 1Mn/1Ga and CH2Cl2 solutions of 1Mn probe the electronic-nuclear hyperfine interactions in the solid state and solution. The electronic structure of 1Mn is investigated by quantum-chemical calculations by employing recently developed computational protocols that are grounded on ab initio wave function theory. From computational analysis, the contributions of spin-spin and spin-orbit coupling to the magnitude of D are obtained. The calculations provide also computed values of the fourth-order zfs terms B4m, as well as those of the g and hyperfine interaction tensor components. In all cases, a very good agreement between the computed and experimentally determined sH parameters is observed. The magnetization relaxation properties of 1Mn are rationalized on the basis of the composition of the ground-state wave functions in the absence or presence of an external magnetic field.

Remarkable Energy Barrier for Magnetization Reversal in 3D and 2D Dysprosium‐Chloranilate‐Based Coordination Polymers

Herein, we report two coordination polymers (CPs)[{Dy(Cl2An)1.5(CH3OH)}.4.5H2O]n(CP1) and [Dy(Cl2An)1.5(DMF)2]n(CP2) (where Cl2An2-= chloroanilate (2,5-dihydroxy-1,4-benzoquinone dianion))exhibiting field inducedsingle molecule magnet behavior with moderate barrier of magnetization reversible.Detailed structural and topological analysis disclose that CP1possesses three dimensional network whereas CP2has a two dimensional layered type structure. In both CPs, the magnetic measurements show weak antiferromagnetic exchange interaction between the dysprosium centers and field induced slow magnetic relaxation with the barrier of 175(9) K (CP1) and 145(7) K (CP2) for CPs1and2, respectively.Notably, thevaluesfor CP1and CP2represent remarkableenergy barrier of magnetization reversalin anymetal-chloranilate based 3D (CP1) and 2D (CP2)coordinationpolymers [Anchor] .The temperature and field dependence of relaxation time indicate the presence of multiple relaxation like direct, QTM, Raman, and Orbach processes in both the CPs. The theoreticalab initiocalculations reinforce the experimentally observed higher energy barrier in CP1as compared to CP2due to the presence of large transverse anisotropy on the ground state in the later compound. The average value of the transition magnetic moment between the computed low-lying spin-orbit states also rationalized the possible relaxation process as Orbach and Raman process through the first excited state. The BS-DFT calculations have been carried out to provide more insight into the exchange interaction.

Trigonal Prismatic Cobalt(II) Single-Ion Magnets: Manipulating the Magnetic Relaxation Through Symmetry Control.

Two mononuclear trigonal prismatic Co(II) complexes [Co(tppm*)][BPh4]2 (1) and [Co(hpy)][BPh4]2·3CH2Cl2 (2) (tppm* = 6,6',6″-(methoxymethanetriyl)tris(2-(1H-pyrazol-1-yl)pyridine; hpy = tris(2,2'-bipyrid-6-yl)methanol) were synthesized by incorporating the Co(II) ions in two pocketing tripodal hexadentate ligands. Magnetic studies indicate similar uniaxial magnetic anisotropy while having distinct dynamic magnetic properties for two complexes, of which 1 exhibits clear hysteresis loops and Orbach process governed magnetic relaxation with an effective energy barrier (Ueff) of 192 cm-1, among the best examples in transition metallic SIMs, about 10 times larger than that of 2 (Ueff = 20 cm-1, extracted by fitting the data to an Orbach relaxation process but there is no real state at this energy). Such pronounced difference is ascribed to the dominant Raman process and quantum tunneling of magnetization (QTM) in 2 owing to the structural distortion and symmetry breaking, indicated by a nearly perfect trigonal prismatic geometry (D3 local symmetry) for 1 and a more distorted configuration for 2 (C3 local symmetry). Ab initio calculations predict strong axial anisotropy for 1 with minimal QTM probability, with the transverse component of anisotropy being estimated to be much higher for 2 than 1, leading to a 10-fold lower Ueff value than 1.

Seeking magneto-structural correlations in easily tailored pentagonal bipyramid Dy(III) single-ion magnets

Controlling molecular magnetic anisotropy via structural engineering is delicate and fascinating, especially for single-molecule magnets (SMMs). Herein a family of dysprosium single-ion magnets (SIMs) sitting in pentagonal bipyramid geometry have been synthesized with the variable-size terminal ligands and counter anions, through which the subtle coordination geometry of Dy(III) can be finely tuned based on the size effect. The effective energy barrier (Ueff) successfully increases from 439 to 632 K and the magnetic hysteresis temperature (under a 200 Oe/s sweep rate) raises from 11 to 24 K. Based on the crystal-field theory, a semi-quantitative magneto-structural correlation deduced experimentally for the first time is revealed that the Ueff is linearly proportional to the structural-related value S20 corresponding to the axial coordination bond lengths and the bond angles. Through the evaluation of the remanent magnetization from hysteresis, quantum tunneling of magnetization (QTM) is found to exhibit negative correlation with the structural-related value Stun corresponding to the axial coordination bond angles.

A Study of Magnetic Relaxation in Dysprosium(III) Single‐Molecule Magnets

Although the development of single-molecule magnets (SMMs) is rapid, there are only two families of high energy barrier (Ueff ) dysprosium(III) SMMs known so far: the cyclopentadienyl (Cp) family with a sandwich structure and the pentagonal-bipyramidal (PB) family with D5h symmetry. These high-barrier SMMs, which usually possess Ueff >500 cm-1 allow the separate study of the four magnetic relaxation paths, namely, direct, quantum tunnelling, Raman and Orbach processes, in detail. Whereas the first family is chemically more challenging to modify the Cp rings, it is shown herein that the latter family, with the common formulae [DyX1 X2 (Leq )5 ]+ , such as X1 /X2 =- OCMe3 , - OSiMe3 , - OPh, Cl- or Br- ; Leq =THF/pyridine/4-methylpyridine, can be readily fine-tuned with a range of axial and equatorial ligands by simple substitution reactions. This allows unambiguous confirmation that the Ueff mainly depends on the identity of X1 and X2 , rather than on Leq . More importantly, the fitted parameters are barrier dependent. If X1 is an O donor and X2 is a halide, 500<Ueff <600 cm-1 , log τ0avg (s)=-10.66, log Cavg (s-1 K-n )= -5.05, navg =4.1 and TH =9 K (in which τ0 is the pre-exponential factor for the Orbach relaxation process, C and n are parameters used to describe Raman relaxation, and TH is the highest temperature at which magnetic hysteresis is observed). For cases in which both X1 and X2 are O donors, 900<Ueff <1300 cm-1 , log τ0avg (s)=-11.63, log Cavg (s-1 K-n )= -6.03, navg =4.1 and 18<TH <25 K. Based on these results, it can be further concluded that Ueff not only has a linear correlation to the axial Dy-X bond lengths, but also to TH for these PB SMMs. This represents the first systematic study of a family of lanthanide SMMs and derives the first magneto-structural correlation in Dy SMMs.

Hyperfine and quadrupole interactions for Dy isotopes in DyPc2 molecules

Nuclear spin levels play an important role in understanding magnetization dynamics and implementation and control of quantum bits in lanthanide-based single-molecule magnets. We investigate the hyperfine and nuclear quadrupole interactions for 161Dy and 163Dy nuclei in anionic DyPc2 (Pc = phthalocyanine) single-molecule magnets, using multiconfigurational ab initio methods (beyond density-functional theory) including spin–orbit interaction. The two isotopes of Dy are chosen because the others have zero nuclear spin. Both isotopes have the nuclear spin I = 5/2, although the magnitude and sign of the nuclear magnetic moment differ from each other. The large energy gap between the electronic ground and first-excited Kramers doublets, allows us to map the microscopic hyperfine and quadrupole interaction Hamiltonian onto an effective Hamiltonian with an electronic pseudo-spin that corresponds to the ground Kramers doublet. Our ab initio calculations show that the coupling between the nuclear spin and electronic orbital angular momentum contributes the most to the hyperfine interaction and that both the hyperfine and nuclear quadrupole interactions for 161Dy and 163Dy nuclei are much smaller than those for the 159Tb nucleus in TbPc2 single-molecule magnets. The calculated separations of the electronic-nuclear levels are comparable to experimental data reported for 163DyPc2. We demonstrate that hyperfine interaction for the Dy Kramers ion leads to tunnel splitting (or quantum tunneling of magnetization) at zero field. This effect does not occur for TbPc2 single-molecule magnets. The magnetic field values of the avoided level crossings for 161DyPc2 and 163DyPc2 are found to be noticeably different, which can be observed from the experiment.

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

Low temperature magnetization measurements were performed on single crystals of the pyridyl-alcohol-supported [Ni(μ3-Cl)Cl(HL·S)]4 coordination cluster and they revealed open hysteresis loops up to 30 mK at a sweep rate of 0.07 T s−1. The studied molecule with intramolecular ferromagnetic coupling Jintra of 15.251 K (+10.6 cm−1) is an exchange-biased single-molecule magnet that exhibits intermolecular antiferromagnetic coupling Jinter of −0.004 K (−0.003 cm−1) in the supramolecular environment of four other cubane-type units. Quantum tunneling of magnetization characterized by five steps at −0.1, −0.05, 0, 0.05 and 0.1 T was observed.

Structural Modulation of Fluorescent Rhodamine-Based Dysprosium(III) Single-Molecule Magnets.

Two rhodamine 6G-based mononuclear dysprosium complexes, [Dy(LR)(LA)2](ClO4)3·Et2O·1.5MeOH·0.5H2O (1) and [Dy(LR)(H2O)4(MeCN)](ClO4)3·2H2O·MeCN (2) (LR = salicylaldehyde rhodamine 6G hydrazone, LA = 2-pyridylcarboxaldehyde benzoyl hydrazone), are synthesized, aiming at improving the magnetic behavior by modulating their coordination environment. Both complexes own one exclusive short Dy-Ophenoxy coordination bond as the predominant bond and exhibit single-molecule magnet behavior under zero dc field with the energy barrier (Ueff/kB) of 90 K (1) and 320 K (2) and apparent hysteresis at 1.9 K. The ab initio calculations indicate that the short Dy-Ophenoxy bond determines the direction of magnetic anisotropic axis for 1 and 2. The quantum tunneling of magnetization (QTM) between the ground Kramers doublets (KDs) in 1 cannot be neglected, leading to an experimental Ueff/kB much lower than the calculated energy of the first excited state (318.2 K). For 2, the stronger magnetic anisotropy and negligible QTM between the ground KDs guarantees that the energy barrier is close to the calculated energy of first KDs (320.8 K). On the other hand, the presence of ring-opened xanthene moiety makes complexes 1 and 2 in the solid state emit red light with emission bands of 645 and 658 nm, respectively.

Zero-field slow relaxation of magnetization in cobalt(ii) single-ion magnets: suppression of quantum tunneling of magnetization by tailoring the intermolecular magnetic coupling.

The correlation between magnetic relaxation dynamics and the alignment of single-ion magnets (SIMs) in a crystal was investigated using four analogous cobalt(ii) complexes with unique hydrogen-bond networks. The hydrogen-bonding interactions in the crystals resulted in a relatively short intermolecular Co⋯Co distance, which led to non-zero intermolecular magnetic coupling. All the complexes with a Co⋯Co distance shorter than 6.5 Å exhibited zero-field slow magnetic relaxation as weak magnetic interactions split the ground ±Ms levels and suppressed quantum tunneling of magnetization (QTM). In particular, antiferromagnetically coupled one-dimensional chain SIM networks effectively suppressed QTM when the two intrachain Co⋯Co distances were non-equivalent. However, when the two distances in a chain were equivalent and each molecular symmetry axis aligned parallell within the chain, QTM suppression was insufficient because magnetic coupling from the adjacent molecules was virtually cancelled. Partial substitution of the CoII ion with the diamagnetic ZnII ion up to 33% for this complex resulted in complete QTM suppression in the absence of an external field. These results show that the manipulation of intermolecular distances and alignments is effective for suppressing undesired QTM events in SIMs.