Single-chain Magnet Behavior Research Articles

Design of bimetallic magnetic chains based on oxalate complexes: towards single chain magnets

We describe the synthesis, structure and magnetic characterization of several oxalate-based bimetallic 1D systems. We will exemplify how by suitable choice of the molecular building blocks and strict control of their arrangement in the solid state, the magnetic properties of these low-dimensional materials can be tuned to finally obtain bimetallic oxalate chains behaving as single-chain magnets (SCM). First, we will focus on compounds [K(18-crown-6)][MII(bpy)Cr(ox)3] (1, 2; MII = Mn, Co; bpy = C10N2H8). The MnCr derivative behaves as a 1D ferromagnet down to 2 K, the lowest investigated temperature. The lack of magnetic ordering in this chain prompted us to prepare the more anisotropic MnCo derivative with the aim of checking whether the single-chain magnetic behaviour was observed. However, this compound exhibits long-range ferromagnetic ordering at Tc = 5.5 K due to the poor chain insulation. Next, in order to slightly change the supramolecular interactions between neighbouring chains we substituted the bpy ligand with dpy, in view of its ability to establish H bonds, yielding the bimetallic compounds [K(18-crown-6)][Co(bpy)MIII(ox)3] (3, 4; MIII = Cr, Fe; dpy = C10N3H9). These compounds exhibit long-range magnetic ordering at 4.6 and 11 K, respectively, and a frequency dependence of the ac susceptibility at lower temperatures. However, such behaviour cannot be attributed to a SCM relaxation, but to the movement of domain walls in the ordered state. Finally, by introducing an additional cation, which increases the interlayer separation, we obtained the compound [K(18-crown-6)]1/2[(18-crown-6)(FC6H4NH3)]1/2[Co(H2O)2Cr(ox)3] (5) exhibiting a SCM-like behaviour.

Ferrimagnetic [CoII3(μ3-OH)2(RCO2)4] chains embedded in a laminar hybrid material exhibiting single-chain magnet behaviour

The ligand, benzophenone-2,4'-dicarboxylate, was used as a polytopic linker and magnetic separator to bridge the inorganic hydroxide-bridged cobat(ii) chain motifs into a laminar inorganic-organic hybrid materials with nearest inter-chain Co(ii)Co(ii) distance of ca. 8.14 A, which exhibits single-chain magnet behaviour.

Probing Single-Chain Magnets in a Family of Linear Chain Compounds Constructed by Magnetically Anisotropic Metal-Ions and Cyclohexane-1,2-Dicarboxylate Analogues

Five new metal-carboxylate chain-based laminated compounds, namely, infinity2[FeII(e,e-trans-1,2-chdc)] (3) (1,2-chdc = cyclohexane-1,2-dicarboxylate), infinity2[NiII(mu-OH2)(e,a-cis-1,2-chdc)] (4), infinity2[CoII(mu-OH2)(1,2-chedc)] (5) (1,2-chedc = cyclohex-1-ene-1,2-dicarboxylate), infinity2[Co5II(mu3-OH)2(OH2)2(1,2-chedc)4] (6), and infinity2[CoII(4-Me-1,2-chdc)] (7) (4-Me-1,2-chdc = trans-4-methylcyclohexane-1,2-dicarboxylate) have been hydrothermally synthesized. In these series of magnetic chain-based compounds, 3 and 7 have the same dimeric paddle-wheel M(II)-carboxylate chain as the previously reported compound, infinity2[CoII(trans-1,2-chdc)] (2). However, compound 3 does not behave as a single-chain magnet (SCM) but simply an alternating ferro-antiferro magnetic chain. Compound 4 has the cis conformation of 1,2-chdc ligand, which leads to a uniform aqua-carboxylate-bridged Ni(II) chain. Such a Ni-O chain exhibits strong antiferromagnetic interactions, leading to a diamagnetic ground state. Compound 5 features a corner-sharing triangular chain, or delta-chain, which is part of a Kagom lattice. However, 5 does not exhibit a spin-frustrated effect but simply spin competition. Compound 6 has a unique pentanuclear CoII cluster, which is further connected by the syn-anti carboxylate into a chain structure. Compound 6 exhibits antiferromagnetic interactions among the Co(II) ions, and no SCM behavior is observed. These results might indicate that the dimeric paddle-wheel Co(II)-carboxylate chain is essential in obtaining SCM behavior in this family of compounds. Although 2 and 7 have very similar SCM behavior, alternating current magnetic studies show that 7 has a higher energy barrier than that of 2. Such behavior is probably caused by the larger anisotropic energy barrier in 7.

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.

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.

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.

Ligand Design for Heterobimetallic Single‐Chain Magnets: Synthesis, Crystal Structures, and Magnetic Properties of MIICuII (M=Mn, Co) Chains with Sterically Hindered Methyl‐Substituted Phenyloxamate Bridging Ligands

Two new series of neutral oxamato-bridged heterobimetallic chains of general formula [MCu(L(x))(2)] m DMSO (m=0-4) (L(1)=N-2-methylphenyloxamate, M=Mn (1 a) and Co (1 b); L(2)=N-2,6-dimethylphenyloxamate, M=Mn (2 a) and Co (2 b); L(3)=N-2,4,6-trimethylphenyloxamate, M=Mn (3 a) and Co (3 b)) have been prepared by reaction between the corresponding anionic oxamatocopper(II) complexes [Cu(Lx)2](2-) with Mn(2+) or Co(2+) cations in DMSO. The crystal structures of [CoCu(L2)2(H2O)2] (2 b') and [CoCu(L3)2(H2O)2]4 H2O (3 b') have been solved by single-crystal X-ray diffraction methods. Compounds 2 b' and 3 b' adopt zigzag and linear chain structures, respectively. The intrachain Cu...Co distance through the oxamate bridge is 5.296(1) A in 2 b' and 5.301(2) A in 3 b', while the shortest interchain Co...Co distance is 5.995(5) A in 2 b' and 8.702(3) A in 3 b', that is, the chains are well isolated in the crystal lattice due to the presence of the bulky methyl-substituted phenyl groups. Although both Mn(II)Cu(II) and Co(II)Cu(II) chains exhibit ferrimagnetic behaviour with moderately strong intrachain antiferromagnetic coupling (-J(Mn,Cu)=24.7-27.9 cm(-1) and -J(Co,Cu)=35.0-45.8 cm(-1); H= Sigma -J(M,Cu)S(M,i)S(Cu,i)), only the Co(II)Cu(II) chains show slow magnetic relaxation at low temperatures (T(B)<3.5 K), which is characteristic of single-chain magnets (SCMs) because of the high magnetic anisotropy of the Co(II) ion. The blocking temperatures T(B) along this series of chains vary according to the steric hindrance of the aromatic substituent of the oxamate ligand in the series L(1)<L(2)<L(3). Analysis of the SCM behaviour for 3 b and 3 b' on the basis of Glauber's theory for a one-dimensional Ising system showed a thermally activated mechanism for the magnetic relaxation (Arrhenius law dependence). The activation energies E(a) to reverse the magnetisation direction are 38.0 (3 b) and 16.3 cm(-1) (3 b'), while the preexponential factors tau(0) are 2.3x10(-11) (3 b) and 4.0x10(-9) s (3 b').

Direct evidence of exchange interaction dependence of magnetization relaxation in a family of ferromagnetic-type single-chain magnets

Three Mn2Ni chains having the same repeating motif,[–MnIII–ON–NiII–NO–MnIII–(OPh)2–]n, possessing a structural C2 axis on the NiII ion and an inversion center at the midpoint of MnIII– (OPh)2–MnIII have beensynthesized:[Mn2(saltmen)2Ni(L)2(py)2](A)(saltmen22=N,N9–(1,1,2,2-tetramethylethylene)bis(salicylideneiminate); L = 1-methylimidazole-2-aldoximate (miao2), A = ClO4 2, 1; PF6 2, 2; L = 1-ethylimidazole-2-aldoximate (eiao2), A = ClO4 2, 3). Due to the strong antiferromagnetic interaction (J/kB # 219 K) between the MnIII and NiII ions via the –ON– linkage and the weak ferromagnetic interaction (J9/kB = +0.43 to +0.49 K) between the [Mn–ON– Ni–NO–Mn] trimer units via the –(OPh)2– linkage in addition to the strong anisotropy of the trimer unit (D/kB = 22.6 to 23.0 K), all three compounds can be viewed at low temperatures as chains of ferromagnetically coupled Ising-like anisotropic ST = 3 units. The combination of DC and AC susceptibility measurements revealed their single-chain magnet behavior (t0 # 1 6 10210 s, D/kB # 59 K). The observed energy barrier (D) is smaller than D/kB # 70 K found in the original series of MnIII 2Ni single-chain magnets possessing D/kB#22.5 K and J9/kB # +0.7 K, and is significantly larger than D/kB # 21 K of the MnIII 2Ni single-molecule magnets possessing D/kB = 22.4 K reported previously. The present report is the first systematic investigation of the linear exchange coupling (J9) dependence of the magnetization relaxation in a family of compounds including related Mn2Ni single-chain magnets and single-molecule magnets.

Origin of the Single Chain Magnet Behavior of the Co(H2L)(H2O) Compound with a 1D Structure

The paper is aimed at the elucidation of the main factors responsible for the single-chain magnet behavior of the cobalt(II) disphosphonate compound Co(H2L)(H2O) with a 1D structure. The model takes into account the spin-orbit interaction, the axial component of the octahedral crystal field acting on the ground-state cubic 4T1 terms of the Co(II) ions, the antiferromagnetic exchange interaction between Co(II) ions as well as the difference in the crystallographic positions of these ions. The conditions that favor the single-chain magnet behavior based on spin canting in a 1D chain containing inequivalent Co(II) centers are discussed. The peculiarities of this behavior in chains containing orbitally degenerate ions are revealed. The qualitative explanation of the experimental data is given.

Solvent induced reversible change of magnetic properties in a Fe(II)–Fe(III) single chain magnet

A single chain magnet (SCM), catena-[Fe II(ClO 4) 2{Fe III(bpca) 2}]ClO 4 ( 1) includes three nitromethane molecules per a Fe(II)–Fe(III) unit as crystallized solvent which occupy the pores surrounded by four chains. Compound 1 can release or reabsorb these MeNO 2 molecules like “sponges” possessing the chain structure of 1, accompanied by a reversible changing of SCM behavior during this process. Compound 1 is the first nano-magnet which shows the solvent induced reversible change of magnetic properties.

Synthesis, Crystal Structures, and Magnetic Properties of Cyano-Bridged Heterobimetallic Chains Based on [(Tp)Fe(CN)3]-

With the use of the tailored cyanometalate precursor, (Bu4N)[(Tp)Fe(CN)3] (Tp = Tris(pyrazolyl)hydroborate) as the building block to react with fully solvated Cu(II), Co(II), and Ni(II) cations, four one-dimensional (1D) heterobimetallic cyano-bridged chain complexes of squares, [(Tp)2Fe(III)2(CN)6Cu(CH3OH).2CH3OH]n (1), [(Tp)2Fe(III)2(CN)6Cu(DMF).DMF]n (2), [(Tp)2Fe(III)2(CN)6M(CH3OH)2.2CH3OH]n (M = Co (3) and Ni (4)), have been prepared. In complexes 1 and 2, the Cu(II) ions are pentacoordinated in the form of a slightly distorted square-based pyramid, and they are linked by distorted octahedrons of [(Tp)Fe(CN)3]- to form 1D chains of squares. In complexes 3 and 4, both the central Co(II) and Ni(II) ions have a slightly distorted octahedral coordination geometry, and they are bridged by [(Tp)Fe(CN)3]- to form similar 1D chains of squares. There are weak interchain pi-pi stacking interactions through the pyrazolyl groups of the Tp ligands for complexes 3 and 4. The crystal structures and magnetic studies demonstrate that complexes 1 and 2 exhibit intrachain ferromagnetic coupling and single-chain magnets behavior, and the blocking temperature is ca. 6 K for complex 1 and ca. 3 K for complex 2. Complexes 3 and 4 show significant metamagnetic behavior, where the cyanides mediate the intrachain ferromagnetic coupling between Fe(III) and Co(II) or Ni(II) ions and the interchain pi-pi stacking interactions lead to antiferromagnetic couplings. The field dependence of the magnetization measurements shows that the critical field is around 1 kOe for complex 3 and 0.8 kOe for complex 4 at 1.8 K.

Single‐Chain Magnet Behavior in an Alternated One‐Dimensional Assembly of a MnIII Schiff‐Base Complex and a TCNQ Radical

An alternated 1:1 chain compound of a Mn(III) salen derivative and the TCNQ monoradical was synthesized: [Mn(5-TMAMsaltmen)(TCNQ)](ClO(4))(2) (1) (TCNQ=tetracyano-p-quinodimethane; 5-TMAMsaltmen=N,N'-(1,1,2,2-tetramethylethylene) bis(5-trimethylammoniomethylsalicylideneiminato)). Compound 1 has a zigzag chain structure packed with adjacent chains with an interchain MnMn distance of over 8 Angatrom. As compound 1 contains no crystallization solvent, the void spaces between chains are occupied only by ClO(4) (-) counter ions. Compound 1 has a structure reminiscent of what has been observed in the family of Mn(III)(porphyrin)-TCNE or -TCNQ compounds reported previously by Miller and co-workers and we demonstrate herein its unique single-chain magnet behavior among this family of compounds. The direct current (dc) magnetic measurements established the one-dimensional nature of compound 1 with an antiferromagnetic exchange coupling, J/k(B) approximately -96 K, between the Mn(III) ion and TCNQ radical and with an activated correlation length (Delta(xi)=26.5 K) at low temperatures (50-15 K). The slow relaxation of the magnetization was shown in compound 1 by the field hysteresis of the magnetization observed below 3.5 K (with a coercive field up to 14 kOe at 1.8 K). Single-crystal magnetization measurements demonstrated the uniaxial symmetry of this compound and allowed an estimation of the anisotropy field, H(a) approximately 97 kOe. The absence of magnetic ordered phase or spin-glass behavior was established by heat-capacity calorimetry measurements that exhibit no abnormality of C(p) between 0.5 K and 10 K. The study of the magnetization relaxation by combined ac (alternating current) and dc techniques showed that compound 1 possesses a single relaxation time (tau). As the consequence of the finite size of the chain, the temperature dependence of tau presents two activated regimes above and below 4.5 K with tau(01)=2.1 x 10(-10) s, Delta(tau1)=94.1 K and tau(02)=6.8 x 10(-8) s and Delta(tau2)=67.7 K, respectively. The detailed analysis of these dynamics properties together with the correlation length, allows an unambiguous demonstration of the single-chain magnet behavior in 1.

Synthetic Strategy for Rational Design of Single-Chain Magnets

Abstract A magnetically-isolated one-dimensional chain can behave as if “magnet”— this sentence may be opposed to our common sense. In this review, we will show that a “magnet-like” behavior can effectively be found in only a magnetic spin chain. In 2001, which is almost forty years after the pioneering theoretical work of R. J. Glauber [J. Math. Phys., 4, 294 (1963)] on the dynamics of ferromagnetically-coupled Ising spin chains, the first experimental evidence of such behavior has been discovered in a real one-dimensional compound [A. Caneschi et al. Angew. Chem., Int. Ed., 40, 1760 (2001)]. In 2002, we have reported on a one-dimensional compound [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 (saltmen2− = N,N′-(1,1,2,2-tetramethylethylene)bis(salicylideneiminato), pao− = pyridine-2-aldoximato, and py = pyridine) which can be considered as a chain of ferromagnetically-coupled anisotropic ST = 3 units [J. Am. Chem. Soc., 124, 12837 (2002)]. Magnetic measurements on this compound revealed the presence of slow relaxation and large field-dependent hysteresis of the magnetization at low temperatures. The topology of this chain is very close to the “ideal” chain imagined by Glauber and hence, still to date, the simplest system to probe the Glauber dynamics. By analogy to the Single-Molecule Magnets described in the introduction of this paper, we called this type of material: Single-Chain Magnet (SCM). To obtain simple SCM systems, we have developed a step-by-step synthetic strategy using primary and secondary building blocks to control the chemistry of the targeting materials and the physics of their characteristic magnetic properties. In this review, we will describe this synthetic strategy from the elementary building blocks to the final series of Single-Chain Magnets obtained since 2002. The magnetic properties of all these materials will be discussed in detail. In particular, the experimental SCM behavior will be described and analyzed in relation with the generalization of the Glauber model for real systems.

Mn${{{{III}\hfill \atop 2\hfill}}}$(5‐Rsaltmen)2NiII(pao)2(L)]2+: An ST=3 Building Block for a Single‐Chain Magnet That Behaves as a Single‐Molecule Magnet

AbstractMnIII‐NiII‐MnIII linear‐type trinuclear complexes bridged by oximate groups were selectively synthesized by the assembly reaction of [Mn2(5‐Rsaltmen)2(H2O)2](ClO4)2 (5‐Rsaltmen2−=N,N′‐(1,1,2,2‐tetramethylethylene) bis(5‐R‐salicylideneiminate); R=Cl, Br) with [Ni(pao)2(phen)] (pao−=pyridine‐2‐aldoximate; phen=1,10‐phenanthroline) in methanol/water: [Mn2(5‐Rsaltmen)2Ni(pao)2(phen)](ClO4)2 (R=Cl, 1; R=Br, 2). Structural analysis revealed that the [MnIII‐ON‐NiII‐NO‐MnIII] skeleton of these trimers is in every respect similar to the repeating unit found in the previously reported series of 1D materials [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 (L1=pyridine, 4‐picoline, 4‐tert‐butylpyridine, N‐methylimidazole; A=ClO4−, BF4−, PF6−, ReO4−). Recently, these 1D compounds have attracted a great deal of attention for their magnetic properties, since they exhibit slow relaxation of the magnetization (also called single‐chain magnet (SCM) behavior). This unique magnetic behavior was explained in the framework of Glauber's theory, generalized for chains of ferromagnetically coupled anisotropic spins. Thus, in these 1D compounds, the [MnIII‐ON‐NiII‐NO‐MnIII] unit was considered as an ST=3 anisotropic spin. Direct‐current magnetic measurements on 1 and 2 confirm their ST=3 ground state and strong uniaxial anisotropy (D/kB≈−2.4 K), in excellent agreement with the magnetic characteristic deduced in the study on the SCM series. The ac magnetic susceptibility of these trimers is strongly frequency‐dependent and characteristic of single‐molecule magnet (SMM) behavior. The relaxation time τ shows a thermally activated (Arrhenius) behavior with τ0≈1×10−7 s and Δeff/kB≈18 K. The effective energy barrier for reversal of the magnetization Δeff is consistent with the theoretical value (21 K) estimated from |D|${S{{2\hfill \atop {\rm T}\hfill}}}$. The present results reinforce consistently the interpretation of the SCM behavior observed in the [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 series and opens new perspectives to design single‐chain magnets.

Single-Chain Magnet (NEt4)[Mn2(5-MeOsalen)2Fe(CN)6] Made of MnIII−FeIII−MnIII Trinuclear Single-Molecule Magnet with an ST = 9/2 Spin Ground State

The cyano-bridged trinuclear compound, (NEt(4))[Mn(2)(salmen)(2)(MeOH)(2)Fe(CN)(6)] (1) (salmen(2)(-) = rac-N,N'-(1-methylethylene)bis(salicylideneiminate)), reported previously by Miyasaka et al. (ref 19d) has been reinvestigated using combined ac and dc susceptibility measurements. The strong frequency dependence of the ac susceptibility and the slow relaxation of the magnetization show that 1 behaves as a single-molecule magnet with an S(T) = (9)/(2) spin ground state. Its relaxation time (tau) follows an Arrhenius law with tau(0) = 2.5 x 10(-)(7) s and Delta(eff)/k(B) = 14 K. Moreover, below 0.3 K, tau saturates around 470 s, indicating that quantum tunneling of the magnetization becomes the dominant process of relaxation. (NEt(4))[Mn(2) (5-MeOsalen)(2)Fe(CN)(6)] (2) (5-MeOsalen(2)(-) = N,N'-ethylenebis(5-methoxysalicylideneiminate)) is a heterometallic one-dimensional assembly made of the trinuclear [Mn(III)(SB)-NC-Fe(III)-CN-Mn(III)(SB)] (SB is a salen-type Schiff-base ligand) motif similar to 1. Compound 2 has two types of bridges, a cyano bridge (-NC-) and a biphenolate bridge (-(O)(2)-), connecting Mn(III) and Fe(III) ions and the two Mn(III) ions, respectively. Both bridges mediate ferromagnetic interactions, as shown by modeling the magnetic susceptibility above 10 K with g(av) = 2.03, J(Mn)(-)(Fe)/k(B) = +6.5 K, and J'/k(B) = +0.07 K, where J' is the exchange coupling between the trimer units. The dc magnetic measurements of a single crystal using micro-SQUID and Hall-probe magnetometers revealed a uniaxial anisotropy (D(T)/k(B) = -0.94 K) with an easy axis lying along the chain direction. Frequency dependence of the ac susceptibility and time dependence of the dc magnetization have been performed to study the slow relaxation of the magnetization. A mean relaxation time has been found, and its temperature dependence has been studied. Above 1.4 K, both magnetic susceptibility and relaxation time are in agreement with the dynamics described in the 1960s by R. J. Glauber for one-dimensional systems with ferromagnetically coupled Ising spins (tau(0) = 3.7 x 10(-)(10) s and Delta(1)/k(B) = 31 K). As expected, at lower temperatures below 1.4 K, the relaxation process is dominated by the finite-size chain effects (tau'(0) = 3 x 10(-)(8) s and Delta(2)/k(B) = 25 K). The detailed analysis of this single-chain magnet behavior and its two regimes is consistent with magnetic parameters independently estimated (J'and D(T)) and allows the determination of the average chain length of 60 nm (or 44 trimer units). This work illustrates nicely a new strategy to design single-chain magnets by coupling ferromagnetically single-molecule magnets in one dimension.

An azide-bridged homospin single-chain magnet: [Co(2,2'-bithiazoline)(N3)2]n.

A novel end-on azide-bridged homospin 1D chain, Co(bt)(N3)2 (1) (bt = 2,2'-bithiazoline), is constructed by sharing edges of Co(II) distorted octahedrons to form a helix, which shows magnetic hysteresis with steps and slow relaxation below 5-6 K, typical of single-chain magnet behavior.

Evidence for single-chain magnet behavior in a Mn(III)-Ni(II) chain designed with high spin magnetic units: a route to high temperature metastable magnets.

We herein present the synthesis, crystal structure, and magnetic properties of a new heterometallic chain of MnIII and NiII ions, [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 (1) (saltmen2- = N,N'-(1,1,2,2-tetramethylethylene) bis(salicylideneiminate) and pao- = pyridine-2-aldoximate). The crystal structure of 1 was investigated by X-ray crystallographic analysis: compound 1 crystallized in monoclinic, space group C2/c (No. 15) with a = 21.140(3) A, b = 15.975(1) A, c = 18.6212(4) A, beta = 98.0586(4) degrees , V = 6226.5(7) A3, and Z = 4. This compound consists of two fragments, the out-of-plane dimer [Mn2(saltmen)2]2+ as a coordination acceptor building block and the neutral mononuclear unit [Ni(pao)2(py)2] as a coordination donor building block, forming an alternating chain having the repeating unit [-Mn-(O)2-Mn-ON-Ni-NO-]n. In the crystal structure, each chain is well separated with a minimum intermetallic distance between Mn and Ni ions of 10.39 A and with the absence of interchain pi overlaps between organic ligands. These features ensure a good magnetic isolation of the chains. The dc and ac magnetic measurements were performed on both the polycrystalline sample and the aligned single crystals of 1. Above 30 K, the magnetic susceptibility of this one-dimensional compound was successfully described in a mean field approximation as an assembly of trimers (Mn...Ni...Mn) with a NiII...MnIII antiferromagnetic interaction (J = -21 K) connected through a ferromagnetic MnIII...MnIII interaction (J'). However, the mean field theory fails to describe the magnetic behavior below 30 K emphasizing the one-dimensional magnetic character of the title compound. Between 5 and 15 K, the susceptibility in the chain direction was fitted to a one-dimensional Ising model leading to the same value of J'. Hysteresis loops are observed below 3.5 K, indicating a magnet-type behavior. In the same range of temperature, combined ac and dc measurements show a slow relaxation of the magnetization. This result indicates the presence of a metastable state without magnetic long-range order. This material is the first experimental design of a heterometallic chain with ST = 3 magnetic units showing a "single-chain magnet" behavior predicted in 1963 by R. J. Glauber for an Ising one-dimensional system. This work opens new perspectives for one-dimensional systems to obtain high temperature metastable magnets by combining high spin magnetic units, strong interunit interactions, and uniaxial anisotropy.