Single-chain Magnet Research Articles

Intermolecular Contact-Tuned Magnetic Nature in One-Dimensional 3d−5d Bimetallic Systems: From a Metamagnet to a Single-Chain Magnet

Assembling [W(CN) 6(bpy)] (-) and magnetic anisotropic Mn Schiff bases produced two Mn (III)(3d)-W (V)(5d) bimetallic chains. Modulation of the types and degrees of interchain pi-pi interactions in the one-dimensional coordination polymers leads to the variation of the magnetic behavior from a metamagnetic character to a single-chain magnet property.

Single Chain Magnets Based on the Oxalate Ligand

The anionic oxalate-bridged bimetallic chain [Co(H2O)2Cr(ox)3]- shows slow relaxation of the magnetization, typical of the so-called single-chain magnets, when crystallized in segregated layers in a mixed salt with the supramolecular cations [C12H24O6K]+ and [(C12H24O6)(FC6H4NH3)]+. This is the first time that such phenomenon has been observed in an oxalate-bridge material. In view of the wide synthetic versatility exhibited by the oxalate ligand, it opens the door for the realization of a complete family of SCM materials whose physical properties might be tuned by the suitable replacement of M3+ ions within the chain. The information extracted from the systematic study of these compounds should provide important information concerning the main parameters that affect the slow-relaxation phenomena in this sort of 1D nanomagnets.

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.

Rational Assembly of High‐Spin Polynuclear Magnetic Complexes into Coordination Networks: the Case of a [Mn4] Single‐Molecule Magnet Building Block

AbstractThe synthesis of transition‐metal high‐spin complexes and of infinite coordination networks of these high‐spin carriers now represents a recognized subdiscipline of coordination chemistry. This new research trend has been particularly encouraged by the discoveries that molecular and one‐dimensional coordination aggregates may behave as nanomagnets, as illustrated by the so‐called single‐molecule magnets (SMMs) and single‐chain magnets (SCMs). While the synthesis of isolated polynuclear high‐spin complexes still rely mostly on serendipitous assembly with appropriate ligands, the synthesis of SCMs or extended networks of high‐spin complexes demands a more designed and controlled bottom‐up assembly of precursor complexes and bridging species selected for their coordination abilities. In this context, the case of a [Mn4] SMM is unique in the literature, as one‐, two‐, and three‐dimensional networks have been rationally designed by using this SMM unit as a building block, giving rise to original magnetic properties. This review gathers all reported systems that we know of, having the corresponding [Mn4O6] core, either in isolated complexes or in frameworks. Their structures and, when relevant, the synthetic strategy and magnetic properties are described. The demonstrated and potential outcomes, in terms of physical properties, of such coordination assemblies of high‐spin complexes are then discussed. These are highlighted through examples with other building blocks, to broaden the scope of possible strategies and building blocks, and thus provide a basis for the further development of this promising area. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Slow relaxation of the magnetization in rationally designed single-chain magnets

Slow relaxation of the magnetization in rationally designed single-chain magnets

Slow Magnetic Relaxation in CoIICuII Coordination Oligomer Built into Mesoporous Material

AbstractThe ferrimagnetic system CoCu(opba) [opba = ortho‐phenylenebis(oxamato)] was employed to prepare a traditional chain [CoCu(opba)]·4H2O (1) and a nanocomposite by incorporation in porous Vycor glass (PVG). This nanocomposite was made by first anchoring [Bu4N]2[Cu(opba)] on PVG [PVG‐Cu (2)] and then treating it “in situ” with cobalt(II) acetate to obtain the nanomagnet PVG‐CuCo (3). Magnetic measurements show that 1 consists of a one‐dimensional ferrimagnet with strong intrachain antiferromagnetic coupling and weak interchain interactions that result in spin‐glass behavior below 3.5 K. Nanocomposite 3 presents ferrimagnetic chains limited by the nanopore size, which leads to a slow relaxation of the magnetization following Arrhenius' law, frequency dependence for in‐phase and out‐of‐phase susceptibility, and hysteresis below the blocked regime temperature (< 6 K). These features are characteristics of single‐chain magnets (SCM).(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A Two-Dimensional Iron(II) Carboxylate Linear Chain Polymer that Exhibits a Metamagnetic Spin-Canted Antiferromagnetic to Single-Chain Magnetic Transition

A two-dimensional iron(II) carboxylate coordination polymer, [Fe(pyoa)2]infinity, where pyoa is 2-(pyridin-3-yloxy)acetate, has been prepared by hydrothermal synthesis. Its crystal structure reveals a single iron(II) site with an elongated octahedral coordination environment containing four equatorial carboxylate oxygens and two axial pyridyl nitrogens; the iron(II) sites are linked by syn-anti micro-carboxylates to form chains along the b axis that have an Fe...Fe separation of 4.910 A. The shortest interchain and interlayer Fe...Fe distances are 6.453 and 11.125 A, respectively. The 4.2-295 K Mössbauer spectra of [Fe(pyoa) 2] infinity consist of a single paramagnetic high-spin iron(II) quadrupole doublet. The axial Fe-N bond direction defines the Jahn-Teller axis at an iron(II) site and, consequently, the orientation of the single-ion magnetic anisotropy. Thus, along the b axis in a given chain, the spins are collinear and parallel to the Jahn-Teller axis. The Jahn-Teller axes of adjacent intralayer chains have different orientations with an angle of 79.2 degrees between the axes in adjacent chains in a bc layer. [Fe(pyoa)2]infinity exhibits field-induced metamagnetic behavior such that, in an applied field smaller than the critical field, the iron(II) spin-canted moments experience intrachain ferromagnetic interactions and weak interchain antiferromagnetic interactions; the spin canting yields weak ferromagnetism. In an applied field larger than the critical field, the weak antiferromagnetic interchain interactions are overwhelmed to yield superparamagnetic-like slow-magnetic relaxation with an energy barrier of 23(3) K. Single-crystal magnetic studies reveal a quasi-uniaxial magnetic anisotropy with the a axis as the easy-magnetic axis and the b axis as the hard-magnetic axis; the susceptibility measured along the easy a axis may be fit with the Glauber model to yield an effective intrachain exchange coupling constant of 2.06(8) K. A dynamic analysis of the susceptibility yields a 6.3(1) K energy barrier for intrachain domain wall creation. The observed field-assisted superparamagnet-like behavior is consistent with the dynamics of a single-chain magnet. Thus, [Fe(pyoa)2]infinity is best considered as a "metamagnetic-like" single-chain magnet.

Local spin dynamics in doped cobalt(II)-radical magnetic chains studied by 1H NMR

We investigated the spin dynamics of Zn(II)-doped “single-chain magnet” (SCM) Co(hfac) 2NITPhOMe (in short CoPhOMe) by means of 1H NMR measurements. CoPhOMe was the first magnetic chain that showed a slowly relaxing magnetization (commonly called spins’ or magnetization “slowing down” mechanism), which follows the predictions of the Glauber model: the electronic spin freezing sets in before any transition to long-range magnetic order. The slowing down of the magnetization is also evidenced in the Zn-doped samples, which present chains that reach an average length of some tenth of spins for the highest doping, together with growing finite-size effects as the doping concentration is increased. The nuclear spin–lattice relaxation rate T 1 - 1 ( T ) at different applied magnetic fields, displayed a high-temperature peak and a low-temperature shoulder that become lower and shifts toward higher temperatures with increasing the field. This behaviour is qualitatively in agreement with the recent AC susceptibility data that revealed the Glauber dynamics and finite-size effects, like the collective reversal of the spins of each chain segment.

Molecular magnetism, status and perspectives

A short review is made of molecular magnetism, trying to discuss what is alive and well, with perspectives for the future. All the main fields of activity are mentioned, ranging from the so-called spin cross-over systems to the quest for organic (molecular) ferromagnets. Particular attention is devoted to some of the recent advances in these fields, highlighting also the opportunities for the development of applications. Low dimensional magnets are perhaps the best opportunity to use molecules, and the status of single-molecule and single chain magnets is discussed. The last part is devoted to the organization of magnetic molecules and to the development of techniques which allow to measure the magnetic properties of thin layers and, in perspective, of single molecules.

Synthesis, crystal structure and magnetic properties of a new cyanide-bridged iron(III)–nickel(II) ferromagnetic chain

The reaction of the iron(III) unit fac-[Fe{HB(pz) 3}(CN) 3] − [ HB ( pz ) 3 - = hydrotris ( 1 - pyrazolyl ) borate ] as the lithium salt ( 1) with the nickel(II) complex mer-[Ni(dpt)(H 2O) 3](ClO 4) 2 [dpt = dipropylenetriamine] in water affords the heterometallic compound of formula {[Fe III{HB(pz) 3}(CN) 3] 2[Ni II(dpt)]} n · 3 nH 2O ( 2). The structure of 2 has been determined by X-ray diffraction on single crystals and their magnetic properties have been investigated in the temperature range 1.9–300 K. Compound 2 is a zigzag chain compound with regular alternating bis-monodentate [Fe(1){HB(pz) 3}(CN) 3] − units and [Ni(dpt)] 2+ cations, the six coordination around the nickel atom being achieved by the coordination of another [Fe(2){HB(pz) 3}(CN) 3] − group acting as a monodentate end-cap ligand. Magnetic data of 2 show the occurrence of intrachain ferromagnetic coupling ( J = +11.4 cm −1 across bridging cyanide) and weak interchain antiferromagnetic interactions ( J′ ca. −0.07 cm −1). These last ones are responsible for the observed metamagnetic behaviour of 2, the value of the critical field being H c = 700 G. The incipient frequency-dependence of the out-of-phase signal of 2 under applied dc field H > H c that is observed at T < 4.5 K supports the occurrence a slow relaxation of the magnetization, a feature which is reminiscent of the behaviour of the single chain magnets (SCMs). The use of the heteroleptic species [Fe{HB(pz) 3}(CN) 3] − as a ligand toward fully solvated metal ions and coordinatively unsaturated preformed complexes to prepare heterometallic complexes with new spin topologies is analyzed and discussed in the light of the available magneto-structural data.

Sterically-induced synthesis of 3d–4f one-dimensional compounds: A new route towards 3d–4f single chain magnets

Hexanuclear lanthanide complexes have been used as molecular precursors to built 3d–4f molecular chains. These complexes were originally targeted as building blocks for the synthesis of lanthanides-containing coordination polymers but reacting them with the 3d molecular precursor [Cu(opba)] 2− lead to Ln(III)–Cu(II) hetero-bimetallic chains with general formula [Ln(NO 3)(DMSO) 2Cu(opba)(DMSO) 2] ∞ with Ln = Gd–Er. The reaction mechanism can be explained by a sterically-induced reaction where the attack of the [Cu(opba)] 2− moiety is driven by the hexanuclear lanthanide clusters geometry. Static magnetic properties of the Gd- and Dy-based chains have been investigated as well as the dynamic magnetic properties of the Dy-containing compound. These studies confirmed that this chemical strategy can possibly yield to 3d–4f single chain magnets.

Molecular nanomagnetism in Florence: Advancements and perspectives

Magnetic materials exhibiting magnetic hysteresis in the absence of magnetic order are fascinating systems for fundamental science and possible applications but the temperature at which the magnetic memory is observed remains rather low. The intrinsic limits of the single molecule magnet and single chain magnet approaches are here briefly discussed as well as possible perspectives in molecular nanomagnetism.

Delicate Crystal Structure Changes Govern the Magnetic Properties of 1D Coordination Polymers Based on 3d Metal Carboxylates

Homo- and heterometallic 1D coordination polymers of transition metals (Co II, Mn II, Zn II) have been synthesized by an in-situ ligand generation route. Carboxylato-based complexes [Co(PhCOO)2]n (1 a, 1 b), [Co(p-MePhCOO)2]n (2), [ZnMn(PhCOO)4]n (3), and [CoZn(PhCOO)4]n (4) (PhCOOH=benzoic acid, p-MePhCOOH=p-methylbenzoic acid) have been characterized by chemical analysis, single-crystal X-ray diffraction, and magnetization measurements. The new complexes 2 and 3 crystallize in orthorhombic space groups Pnab and Pcab respectively. Their crystal structures consist of zigzag chains, with alternating M(II) centers in octahedral and tetrahedral positions, which are similar to those of 1 a and 1 b. Compound 4 crystallizes in monoclinic space group P2 1/c and comprises zigzag chains of M II ions in a tetrahedral coordination environment. Magnetic investigations reveal the existence of antiferromagnetic interactions between magnetic centers in the heterometallic complexes 3 and 4, while ferromagnetic interactions operate in homometallic compounds (1 a, 1 b, and 2). Compound 1 b orders ferromagnetically at TC=3.7 K whereas 1 a does not show any magnetic ordering down to 330 mK and displays typical single-chain magnet (SCM) behavior with slowing down of magnetization relaxation below 0.6 K. Single-crystal measurements reveal that the system is easily magnetized in the chain direction for 1 a whereas the chain direction coincides with the hard magnetic axis in 1 b. Despite important similarities, small differences in the molecular and crystal structures of these two compounds lead to this dramatic change in properties.

The Canted Antiferromagnetic Approach to Single-Chain Magnets

The reaction of manganese(III) acetate meso-tetraphenylporphyrin with phenylphosphinic acid provides the one-dimensional compound of formula [Mn(TPP)O2PHPh] x H2O, which crystallizes in the monoclinic C2/c space group. The chain structure is generated by a glide plane resulting in Jahn-Teller elongation axes of the MnIII octahedra that alternate along the chain. The phenylphosphinate anion transmits a sizable antiferromagnetic exchange interaction that, combined with the easy axis magnetic anisotropy of the MnIII sites, gives rise to a canted antiferromagnetic arrangement of the spins. The static single-crystal magnetic properties have been analyzed with a classical Monte Carlo approach, and the best fit parameters for the exchange and single ion anisotropy are J = -0.68(4) K and D = -4.7(2) K, respectively (using the -2JS(i)S(j) formalism for the exchange). Below 5 K the single-crystal dynamics susceptibility reveals a frequency-dependent out-of-phase signal typical of single-chain magnets. The extracted relaxation time follows the Arrhenius law with delta = 36.8 K. The dynamic behavior has been rationalized and correlated to geometrical parameters of the structure. The contribution of the correlation length to the energy barrier has been investigated, and it has been found that the characteristic length that dominates the dynamics strongly exceeds the correlation length estimated from magnetic susceptibility.

Single chain magnets: where to from here?

Single chain magnets (SCMs) are an interesting class of molecular polymeric materials displaying slow relaxation of the magnetization. They provide, at low temperatures, a magnetic hysteretic behaviour for a single polymeric chain. Although their behaviour evokes better-known magnetic nanoparticles and single-molecule magnets (SMMs), the similarity is mainly apparent. The fundamental differences in the physical origin of the magnetic behaviour offer perspectives that are still largely unexplored. Here we review the progress made in the synthesis, characterization and theoretical understanding of SCMs, highlighting differences and similarities with SMMs. For each of the points we then present a perspective of the advantages offered by this class of materials and we point out the main aspects that remain to be developed in the field.

Single-chain magnets constructed with a twisting arrangement of the easy-plane of iron(II) ions

Abstract A novel class of single-chain magnets (SCMs), catena-[FeII(ClO4)2{FeIII(bpca)2}]ClO4 and its derivative, were synthesized using the spin-carrier components possessing hard-axis anisotropy (or easy-plane anisotropy, D &gt; 0). The easy-axis-type anisotropy of whole molecules of these compounds, which is essential for the formation of SCMs, arises from the twisted arrangement of easy-planes of Fe(II) along the chain axis. Alternating high-spin Fe(II) and low-spin Fe(III) chain complexes behave as an SCM with a typical frequency-dependent ac susceptibility which obeys Arrhenius law. Below 7 K, catena-[FeII(ClO4)2{FeIII(bpca)2}]ClO4 showed a short-range spin-ordering even in zero external field in a time range of Mössbauer spectroscopy as well as the muon-spin-relaxation (μSR) spectroscopy. Since the easy-axis-type magnetic anisotropy originated from the structural motif of the twisting arrangement of Fe(II) ions, the overall magnetic property was very sensitive to the small structural changes arising from adsorption/desorption of the crystal solvents, and catena-[FeII(ClO4)2{FeIII(bpca)2}]ClO4 showed a reversible change in magnetism that has been referred to as "a magnetic sponge". In its derivative, controls of the molecular structure, the arrangement of chains in the crystal, and magnetic properties both in dc and ac susceptibility have been achieved by the introduction of methyl group on a bpca- ligand, which bridges and mediates the magnetic interaction of the adjoining Fe(II)/Fe(III) ions.

A homospin iron(ii) single chain magnet

Synthetic, structural and magnetic studies of a new Fe(II) single chain magnet are reported.

Ligand design for multidimensional magnetic materials: a metallosupramolecular perspective

The aim and scope of this review is to show the general validity of the 'complex-as-ligand' approach for the rational design of metallosupramolecular assemblies of increasing structural and magnetic complexity. This is illustrated herein on the basis of our recent studies on oxamato complexes with transition metal ions looking for the limits of the research avenue opened by Kahn's pioneering research twenty years ago. The use as building blocks of mono-, di- and trinuclear metal complexes with a novel family of aromatic polyoxamato ligands allowed us to move further in the coordination chemistry-based approach to high-nuclearity coordination compounds and high-dimensionality coordination polymers. In order to do so, we have taken advantage of the new developments of metallosupramolecular chemistry and in particular, of the molecular-programmed self-assembly methods that exploit the coordination preferences of metal ions and specifically tailored ligands. The judicious choice of the oxamato metal building block (substitution pattern and steric requirements of the bridging ligand, as well as the electronic configuration and magnetic anisotropy of the metal ion) allowed us to control the overall structure and magnetic properties of the final multidimensional nD products (n = 0-3). These species exhibit interesting magnetic properties which are brand-new targets in the field of molecular magnetism, such as single-molecule or single-chain magnets, and the well-known class of molecule-based magnets. This unique family of molecule-based magnetic materials expands on the reported examples of nD species with cyanide and related oxalato and dithiooxalato analogues. Moreover, the development of new oxamato metal building blocks with potential photo or redox activity at the aromatic ligand counterpart will provide us with addressable, multifunctional molecular materials for future applications in molecular electronics and nanotechnology.

A MnIII2NiII single-chain magnet separated by a thick isolating network of BPh4− anions

The assembly reaction of a Mn(III) salen-type dimeric complex, [MnIII2(saltmen)2(H2O)2](ClO4)2 (saltmen2- = N,N'-(1,1,2,2-tetramethylethylene)bis(salycylideneiminate)), and a Ni(II) oximato complex, [Ni(II)(pao)2(phen)] (pao- = pyridine-2-aldoximate; phen = 1,10-phenanthroline), in the presence of NaBPh4 yielded a heterometallic chain of [MnIII2(saltmen)2NiI)(pao)2(phen)](BPh4)2 (Mn2Ni-BPh4) having a [-Mn(III)-ON-Ni(II)-NO-Mn(III)-(OPh)2-] repeating unit surrounded by a "zeolite-like" network of BPh4- anions. Thanks to such bulky ion-walls, each chain is not only structurally separated with a nearest inter-chain metalmetal distance of 14.4 A (MnNi) but also magnetically extremely well isolated. This magnetic isolation and the ferromagnetic interactions between S = 3 anisotropic units constituting the chain play key roles that induce single-chain magnet behavior in this system.

Constructing magnetic molecular solids by employing three-atom ligands as bridges

The combination of some three-atom bridges with paramagnetic 3d transition metal ions results in the systematic isolation of molecular magnetic materials, ranging from single-molecule and single-chain magnets to layered weak ferromagnets and three-dimensional porous magnets. The design strategy and role of secondary components, such as co-ligands, templates and other mixed short ligands are discussed.