Cooperative Magnetic Phenomena Research Articles

Cooperative magnetic phenomena in artificial spin systems: spin liquids, Coulomb phase and fragmentation of magnetism – a colloquium

Two-dimensional arrays of interacting magnetic nanostructures offer a remarkable playground for simulating, experimentally, lattice spin models. Initially designed to capture the low-energy physics of highly frustrated magnets, they quickly became a lab-on-chip platform to investigate cooperative magnetic phenomena often associated with classical frustrated magnetism. This article reviews the many-body physics which can be visualized, directly in real space, through the magnetic imaging of artificial arrays of magnetic nanostructures. Particular attention is paid to classical spin liquid states, magnetic Coulomb phases and magnetic moment fragmentation. Other phenomena, such as complex magnetic ordering, charge crystallization and monopole-like excitations, are also described in light of the recent advances in the field.

A Rational Route to Coordination Polymers with Condensed Networks and Cooperative Magnetic Properties

AbstractIn this report a rational route to coordination polymers that can show cooperative magnetic phenomena is presented. In this approach compounds based on transition metal cations, small sized terminal N‐bonded anionic ligands and additional neutral N‐donor co‐ligands are heated, which lead to the formation of intermediates, in which the metal cations are linked by the anionic ligands. Predominantly, the use of this method for the synthesis of bridged thio‐ and selenocyanato coordination compounds is described in this article but it can also be extended for the preparation of other compounds. In most cases the intermediates are formed in very pure form and in quantitative yields. Thus, compounds, which are not or at least very difficult to obtain, can be prepared if the synthesis is performed in solution. This is especially valid for thio‐ and selenocyanato coordination compounds, which mostly prefer terminal bonding instead of bridging coordination with less chalcophilic metal cations like e.g. MnII, FeII, CoII, or NiII. It is demonstrated how 1D and 2D networks can selectively be prepared and that this method is predestinated for the synthesis of compounds that show a slow relaxation of the magnetization. A large number of compounds were investigated in which the metal cation, the anionic ligand, or the neutral co‐ligand is exchanged, which allowed the study of structure‐property relationships. Further investigation showed that these reactions can proceed via several different intermediates. At first glance one disadvantage of this approach might be that the intermediates isolated consist of microcrystalline powders, which structurally are difficult to characterize. However, different possibilities are presented to overcome this problem including investigations of analogous compounds based on diamagnetic cadmium cations, which in contrast to the paramagnetic metal cations prefer a bridging coordination of the anionic ligands.

Preparation of New Ligand‐Deficient Thiocyanato Compounds with Cooperative Magnetic Phenomena by Thermal Decomposition of Their Ligand‐Rich Precursors

AbstractThe reaction of nickel thiocyanate with bipy (4,4′‐bipyridine) in water at room temperature leads to the formation of the ligand‐rich 1:2 (1:2 ratio between metal salt and organic ligand) hydrate [{Ni(NCS)2(bipy)(H2O)2}·(bipy)]n (1‐Ni), which is isotypic to its analogues 1‐Mn and 1‐Co reported recently. In their crystal structure, the metal cations are coordinated by two terminal N‐bonded thiocyanato anions, two water molecules, and two bridging bipy ligands in an octahedral coordination mode. These building blocks elongate in linear M–bipy–M chains, which are further connected by hydrogen bonds between H2O and noncoordinated bipy ligands into layers. On heating these precursor compounds, two‐step decomposition is observed: New ligand‐rich 1:2 anhydrous compounds [M(NCS)2(bipy)2]n [M = Mn (2‐Mn), Co (2‐Co), and Ni (2‐Ni)] could be identified as intermediates in the first heating step, and new ligand‐deficient 1:1 compounds [M(NCS)2(bipy)]n [M = Mn (3‐Mn), Co (3‐Co), and Ni (3‐Ni)] could be identified as intermediates in the second heating step. A smooth reaction pathway in their crystal structures is observed: First, water is removed, leading to directly coordinating bipy ligands, and second, half of the bipy ligands are removed, leading to μ‐1,3‐bridging thiocyanato anions. This structural transformation is accompanied by a dramatic change in their magnetic properties: Whereas the ligand‐rich 1:2 hydrates and anhydrous compounds show only Curie–Weiss paramagnetism, the ligand‐deficient 1:1 intermediates show either Curie–Weiss paramagnetism or antiferromagnetic ordering at lower temperatures, mediated by the bridging thiocyanato anions. These results are qualitatively compared with those of the related ligand‐rich and ligand‐deficient compounds with pyrazine and pyrimidine.

Rational design of bridging selenocyanates by thermal decomposition reactions

The powerful use of thermal decomposition reactions as tool for the directed synthesis of a new selenocyanate is described. This method offers a facile access to a large number of new compounds, in which the metal centers are bridged by small anionic linker ligands and therefore, cooperative magnetic phenomena can be expected.

Thermal Decomposition Reactions as Tool for the Synthesis of New Metal Thiocyanate Diazine Coordination Polymers with Cooperative Magnetic Phenomena

Reaction of nickel thiocyanate with pyrimidine at room temperature leads to the formation of the new ligand-rich 1:2 (1:2 = ratio between metal and ligand) compound [Ni(NCS)(2)(pyrimidine)(2)](n) (2) which is isotypic to [Co(NCS)(2)(pyrimidine)(2)](n) (1) reported recently. In the crystal structure, the Ni(2+) ions are coordinated by four N atoms of pyrimidine ligands, which connect the metal centers into layers, and two N atoms of terminal bonded thiocyanato anions within slightly distorted octahedra. If the synthesis is performed under solvothermal conditions and an excess of the metal thiocyanate is used, single crystals of the ligand-deficient 1:1 compound [Ni(NCS)(2)(pyrimidine)](n) (4) are obtained. Investigations on the synthesis of this compound show that it is always contaminated with large amounts of the corresponding ligand-rich 1:2 compound 2. In the crystal structure, the Ni(2+) ions are coordinated by two N atoms of pyrimidine ligands, which connect the metal centers into chains, and two N atoms as well as two S atoms of mu-1,3 bridged thiocyanato anions, which conntect these chains into layers, within a slightly distorted octahedral geometry. On heating compounds 1 and 2 transform quantitatively into the ligand-deficient 1:1 Ni compound 4 and its isotypic Co compound 3. If nickel and cobalt thiocyanate are reacted with an excess of pyrimidine in a solvent-free reaction, discrete ligand-rich 1:4 complexes of composition [M(NCS)(2)(pyrimidine)(4)] (M = Co 5 and Ni 6) are obtained, which could be determined by single crystal structure analysis. In their crystal structure the metal ions are coordinated by four terminal bonded pyrimidine ligands and two terminal N-bonded thiocyanato anions. For the ligand-rich 1:2 and ligand-deficient 1:1 compounds magnetic measurements were performed, which reveal different magnetic properties: The 1:2 compounds show a ferromagnetic and the 1:1 compounds an antiferromagnetic ordering at lower temperatures.

Coordination polymer changing its magnetic properties and colour by thermal decomposition: synthesis, structure and properties of new thiocyanato iron(ii) coordination polymers based on 4,4′-bipyridine as ligand

Thermal decomposition of the ligand-rich 1:2 precursor solvate (1:2 = ratio between metal and ligand) [{Fe(mu2-bipy-N,N')(NCS)2(H2O)2} x bipy]n (1I) (bipy = 4,4-bipyridine) reported recently leads to the stepwise formation of two new intermediates: a ligand-rich solvate-free 1:2 compound of composition [Fe(NCS)2(bipy)2]n (2) and a ligand-deficient 1:1 compound of composition [Fe(NCS)2(bipy)]n (3). The thermal decomposition is accompanied by a colour change from red (1I), via orange (2) into yellow (3). In the crystal structure of 2 the iron(II) cations are coordinated by four N atoms of bridging bipy ligands, which connect the metal centers into layers, and two terminal N-bonded thiocyanato anions within slightly distorted octahedra. In the crystal structure of 3 the iron(II) cations are coordinated by two N atoms of bridging bipy ligands, which are connected by the metal centers into chains, and two N atoms as well as two S atoms of mu-1,3 bridging thiocyanato anions within a slightly distorted octahedral geometry. The thiocyanato anions connect these chains into layers. Due to their connection mode of the metal centers, only for the latter compound can cooperative magnetic phenomena be expected. Magnetic measurements revealed two different magnetic properties: The 1:2 compounds 1I and 2 show Curie-Weiss paramagnetism and the 1:1 compound 3 shows an antiferromagnetic ordering at T(N) = 5.5 K. The reversibility of water reintercalation in the solvent-free compound 2 was investigated and followed by UV-Vis spectroscopy. Herein a new polymorphic metastable modification 1II of the pristine precursor compound 1I was obtained and characterized by single crystal X-ray determination.

Approximations to Brillouin functions for analytic descriptions of ferromagnetism

The statistical theory of magnetism leads to the magnetic field appearing in the two terms of the Brillouin function, both of which are transcendental, making it impossible to solve explicitly for the argument of that function. The replacement of the Brillouin function BJ(x) by a square root function yields simple algebraic expressions for mean field treatments of cooperative magnetic phenomena. This is useful in the teaching of ferromagnetism.

First Examples of Long Range Magnetic Ordering of Structurally Characterized Pentachloroferrate III Di-Anions: FeCl5 2-, and Its Methanol and Aquo Adducts

Abstract A. C. susceptibility and iron-57 Mossbauer spectroscopy measurements are used to investigate the first examples of cooperative magnetic ordering phenomena for FeCl3 2- anions and the related methanol and aquo adducts. Depending on the counter cation and/or solvate identity examples of uncanted antiferromagnetic and canted (weakly ferromagnetic) three dimensional order are observed with critical temperatures - 7K.

Organic and Organometallic Molecular Magnetic Materials—Designer Magnets

AbstractMagnets composed of molecular species or polymers and prepared by relatively low‐temperature organic synthetic methodologies are a focus of contemporary materials science research. The anticipated properties of such molecular‐species‐based magnetic materials, particularly in combination with other properties associated with molecules and polymers, may enable their use in future generations of electronic, magnetic, and/or photonic/photronic devices ranging from information storage and magnetic imaging to static and low‐frequency magnetic shielding. A tutorial of typical magnetic behavior of molecular materials is presented. The three distinct models (intramolecular spin coupling through orthogonal orbitals in the same spatial region within a molecule/ion, intermolecular spin coupling through pairwise “configuration interaction” between spin‐containing moieties, and dipole—dipole, through‐space interactions) which enable the design of new molecular‐based magnetic materials are discussed. To achieve the required spin couplings for bulk ferro‐ or ferrimagnetic behavior it is crucial to prepare materials with the necessary primary, secondary, and tertiary structures akin to proteins. Selected results from the worldwide effort aimed at preparing molecular‐based magnetic materials by these mechanisms are described. Some organometallic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) that is, …︁D•+ A•− D•+ A•−…︁, exhibit cooperative magnetic phenomena. Bulk ferromagnetic behavior was first observed below the critical (Curie) temperature Tc of 4.8 K for [FeIII(C5Me5)2]•+ [TCNE]•− (Me = methyl; TCNE = tetracyanoethylene). Replacement of FeIII with MnIII leads to a ferromagnet with a Tc of 8.8 K in agreement with mean‐field models developed for this class of materials. Replacement with CrIII, however, leads to a ferromagnet with a Tc lowered to 3.65 K which is at variance with this model. Extension to the reaction of a vanadium(o) complex with TCNE leads to the isolation of a magnet with a Tc ≈ 400 K, which exceeds the thermal decomposition temperature of the material. This demonstrates that a magnetic material with a Tc substantially above room temperature is achievable in a molecule/organic/polymeric material. Finally, a new class of one‐dimensional ferrimagnetic materials based on metalloporphins is discussed.

Structure and properties of nonclassical polymers X. Heteroatomic polymers with degenerate half-filled band

It is shown that the group of π-conjugated, nonclassical (non-Kekule) homonuclear, alternative organic polyradicals and polymers with degenerate NBMOs can be essentially extended to a large class of heterocyclic analogues having a set of degenerate MOs. The presence of a set of degenerate MOs (DMOs) results from the molecular topology of the system. The conditions of occurrence of DMOs are determined by the generalized Coulson-Rushbrooke-Longuet-Higgins theorem. The character of spin-exchange interaction of π-electrons in the half-filled band (HFB) of a large group of model polymers, analogues of poly(meta-anilines), has been investigated. It is shown that the main component of the ferromagnetic exchange interaction is the potential (Coulomb) exchange and a smaller contribution of the indirect exchange among the HFB electrons via the delocalized π-electrons in the occupied bands. The theoretical method used, which predicts the existence of a set of DMOs, may serve as a guiding principle in the design of narrow-band, high-spin organic polymers in which cooperative magnetic phenomena can arise.

New Magnetically Ordered Materials with High Tc

Abstract Magnets comprised of molecules, ions, and polymers is a focus of contemporary materials science research. The anticipated attributes of organic/molecular-based magnetic materials may enable their use in future generations of electronic, magnetic and/or photonic/photronic devices. Some organometallic solids comprising linear chains of alternating metallocenium donors, D, and cyanocarbon acceptors, A, i. e., -D-+A D+A, exhibit cooperative magnetic phenomena, i. e., ferro-, antiferro-, ferri-, and metamagnetism. For [FeIII(C5Me5)2]-+[TCNE] - (Me = methyl; TCNE = tetracyanoethylene) bulk ferromagnetic behavior is observed below the critical (Curie) temperature, Tc, of 4.8 K. Replacement of FeIII with MnIII leads to a ferromagnet with a Tc of 8.8 K in agreement with mean-field models developed for this class of materials. Extension to the reaction of a vanadium(0) complex with TCNE lead to the isolation of a magnet with a Tc ∼ 400 K which exceeds the thermal decomposition of the material. A new class ...

Nuclear magnetic order in solid 3He

Over the past 20 years our understanding of cooperative magnetic phenomena in solid 3He has expanded greatly. Starting from the concept of atomic exchange, the author explains how early experiments led to the abandonment of the naive Heisenberg nearest neighbor antiferromagnetic Hamiltonian, in favor of a multiple exchange model. He then describes the detailed experimental picture of the spin-ordered phases which has developed over the past decade, and the successes and failures of the multiple exchange model to explain the experimental results. Today solid 3He is a powerful model magnetic system for extending our general understanding of magnetic order, and should continue to play an important role for many years to come.

Organometallic ferromagnets

Some organometallic solids comprising linear chains of alternating m 8 = 1/2 metallocenium donors, D, and cyanocarbon acceptors, A, i.e. ••• D +*A~*D+*A- * •••, exhibit cooperative magnetic phenomena, i.e. ferro-, antiferro-, ferri-, and meta-magnetism. For [Fe II (C 5 Me 5 ) 2 ]+*[T C N E ]- (Me = methyl; T C N E = tetracyanoethylene) bulk ferromagnetic behaviour is observed below the Curie temperature of 4.8 K. Replacement of Fe III with Fe II , Ni III and Cr III leads to complexes with dia-, antiferro- and ferrimagnetic behaviour, respectively. These results are consistent with a model of configuration mixing of the lowest charge transfer excited state with the ground state developed earlier to understand the magnetic coupling of such systems. The model predicts the magnetic coupling as a function of electron configuration and direction of charge transfer (retro or reverse) and is a useful guide to designing new organic and /or organometallic complexes with cooperative magnetic coupling. To test the model and identify new materials with ferromagnetic coupling new TCNE-based electron transfer salts were prepared.

Molecular/Organic Ferromagnets

Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.

Ferromagnetic Charge Transfer Complexes

Abstract Some ionic molecular solids comprised of linear chains of alternating S = ½ metallocenium donors, D, and S = ½ planar polycyanohydrocarbon acceptors, A, i.e., …D.+A.-D-+A.−…., exhibit coop erative magnetic phenomena, ie., fern, antifem, fem-, and meta- magnetism. The high temperature (T ≥ 50 K) magnetic susceptibility can usually be fit by the Curie-Weiss expression, x ∞ (T - θ)-l, with 8 > 0 for those salts with dominant ferromagnetic interactions and 8 < 0 for those salts with dominant antiferromagnetic interactions.1

ChemInform Abstract: Novel Magnetic Properties of Solid Helium‐3

Nuclear magnetic ordering in solid helium‐3 provides the ultralow‐temperature physicist with a unique opportunity to study cooperative magnetic phenomena in a quantum spin system. There are two known—and very different— magnetic phases of low‐density solid He3, and at higher densities ferromagnetism may exist. with ordering temperatures at low densities near 1 mK, these systems are easily accessible with modern demagnetization cryostats. Over the next few years experimental and theoretical studies of these systems will advance considerably our understanding of nuclear magnetism.

Novel Magnetic Properties of Solid Helium-3

Nuclear magnetic ordering in solid helium-3 provides the ultralow-temperature physicist with a unique opportunity to study cooperative magnetic phenomena in a quantum spin system. There are two known—and very different— magnetic phases of low-density solid He3, and at higher densities ferromagnetism may exist. with ordering temperatures at low densities near 1 mK, these systems are easily accessible with modern demagnetization cryostats. Over the next few years experimental and theoretical studies of these systems will advance considerably our understanding of nuclear magnetism.

Linear Chain Ferromagnetic Charge Transfer Compounds

AbstractCharge transfer complexes possessing a … DADA … structure with both the donor, D, and acceptor, A, being S = 1/2 radicals may exhibit cooperative magnetic phenomena. The complex [Fe(C5Me5)2]+·[TCNQ]−· exhibits metamagnetic behavior. The similarly structured [TCNE]−· and [C4(CN)6]−· complexes are ferromagnets, whereas the [DDQ]−· salt is a paramagnet. The high temperature magnetic susceptibility obeys the Curie‐Weiss expression with θ = + 30, + 30, and + 3 for the [TCNE]−·, [C4(CN)6]−·, and [TCNQ]−· salts, respectively. The ferromagnetic [TCNE]−· salt exhibits zero field Zeeman split 57Fe Mossbauer spectra with an internal field of 425.6 kOe at 4.23 K. After reviewing the current papers discussing ferromagnetism in molecular (organic) compounds, a qualitative model consistent with the necessary bulk spin alignment required for a ferromagnet is presented.

ESR and magnetic susceptibility studies of graphite intercalated with transition metal hexafluorides

ESR signals attributed to intercalated paramagnetic species in HOPG/MF 6 (M=Os, Ir) for stage I compounds were observed for the first time. A single metallic ESR line exhibiting significant field anisotropy was observed. The field dependence could be fitted to g 2 = g 2 Ir,cos 2θ+g 2 tsin 2θ with g Ir = 1.73±0.05 and g t = 3.23 for HOPG/OsF 6 and g Ir = 2.4±0.1 and g t = 3.23 for HOPG/IrF 6 at T = 6K. These g values are temperature independent above 6K. Signal intensities diminish with temperature roughly according to the Curie Law and are practically unobservable above 40K for both systems. No resonance associated with conduction carriers could be observed. A strong anisotropy of the g value can be attributed to (1) cooperative magnetic phenomena, (2) crystal field effects, or (3) exchange with conduction carriers. The first possibility was eliminated by magnetic susceptibility measurements performed over a range of 2–150K using a squid magnetometer. Susceptibility data obey Curie-Weiss behaviour except for deviations below 10K. In this lower temperature range they differ significantly from those reported by Bartlett. The magnetic moments (and Weiss temperatures) are 2.7μ B(θ = −15K) and 2.3μ B(θ = O) for stage I and II HOPG/OsF 6, respectively. For stage I HOPG/IrF 6, μ B ∼ 0.7μ B(θ = 0). Susceptibility data as well as the temperature dependence of the ESR line intensity strongly support crystal field effects as the main mechanism accounting for the g values. Following Bartlett, we assume the intercalated species is OsF − 6(5d 3) in HOPG/OsF 6. The crystal field is mainly due to fluorines in the OsF − 6 anion. For the 5d 3 configuration perturbation theory by Griffith yields g t ∼ 2g Ir in rough agreement with our results. The ratio of spin-orbit coupling to overall crystal field splitting, ζ/Δ, is estimated to be ∼ 0.1. Possibly the most important consequence of the ESR study is the existence of a well-defined symmetry and uniform orientation within the graphite planes leading to well defined crystal field effects.

Exchange Interactions in Magnetic Compounds

The basic feature of the Heisenberg-Dirac-Van Vleck model of cooperative magnetic phenomena is the assumption that the interaction Vij between a pair of atoms i and j in a crystal has the form Vij=−2JijSi·Sj,where Jij is an exchange interaction and Si and Sj are spin operators. Such an assumption seems naturally better suited to insulating compounds than to metals and alloys. In principle, it should be possible to determine the Jij for magnetic compounds by comparing experimental data with theoretical predictions based on Eq. (1). In practice, the matter is complicated by the fact that the H-D-VV model is oversimplified, and that even in its oversimplified form exact solutions cannot be obtained for real three-dimensional crystals. The general problem of evaluating exchange interactions in magnetic compounds from experimental data is reviewed, and specific results are presented for some antiferromagnetic and ferrimagnetic systems where a reasonably good determination of the exchange interactions can be obtained. Some correlations and regularities in the data are noted and discussed. A long review article on this topic is being published elsewhere.