Paramagnetic Units Research Articles

Long-range magnon transfer across a bridging ferromagnetic chain via sequential and tunnel routes

A theoretical study of the mechanism of magnon transfer through a ferromagnetic chain (F) has been carried out from magnetically ordered contact A to similar contact B (AFB system). The regime of spin excitation transport is considered, when the inner section of the chain with identical paramagnetic units acts as a bridge for magnon transfer and thereby is poorly populated by magnons. In this case, the magnon transfer can be carried out by sequential hopping the localized magnon across all units of the chain or/and tunneling the magnon between the terminal units of the chain by a “superexchange” mechanism. The latter involves in the tunneling route the virtual delocalized magnons. The analytical dependence of the corresponding transfer rates on the number of paramagnetic bridge units is found and the magnon analog of the Seebeck and Peltier effects is predicted.

Modeling of the Charge-Transfer-Induced Spin Transition in the Tetranuclear Cyanide-Bridged [Co2Fe2] Complex.

The course of the charge-transfer-induced spin transition demonstrated by the cyanide-bridged tetranuclear [Co2Fe2(bpy*)4(CN)6(tp*)2](PF6)2·2CP·8BN complex has been followed by DFT calculations of the single-point energies for different total spin values of the complex in a wide temperature range. With the aid of these calculations, the picture of spin conversion, that the compound undergoes, has been restored. It has been demonstrated that at 100 K the two crystallographically unique tetranuclear Fe2Co2 subunits A and B present in the structure contain diamagnetic low-spin FeII and low-spin CoIII ions. From the three subunits A, A', and B detected crystallographically in the compound at 200 K, only the A ones contain paramagnetic low-spin FeIII and high-spin CoII ions, while at 260 K, a half of all clusters contained in the crystal are in this state. From the DFT calculations, it also follows that at 320 K in the crystal only paramagnetic units are present. The results obtained are in accordance with the experimental data on the magnetic susceptibility. A possibility to predict new materials exhibiting spin transitions on the basis of DFT calculations of single-point energies as functions of temperature is discussed.

Electronic and magnetic properties of stoichiometric CeAuBi2.

We report the electronic and magnetic properties of stoichiometric CeAuBi2 single crystals. At ambient pressure, CeAuBi2 orders antiferromagnetically below a Néel temperature (TN) of 19 K. Neutron diffraction experiments revealed an antiferromagnetic propagation vector , which doubles the paramagnetic unit cell along the c axis. At low temperatures several metamagnetic transitions are induced by the application of fields parallel to the c axis, suggesting that the magnetic structure of CeAuBi2 changes as a function of field. At low temperatures, a linear positive magnetoresistance may indicate the presence of band crossings near the Fermi level. Finally, the application of external pressure favors the antiferromagnetic state, indicating that the 4f electrons become more localized.

Photoconductive bent-core liquid crystalline radicals with a paramagnetic polar switchable phase

Planarization of the central paramagnetic unit facilitates the formation of an antiferroelectric B2 phase, exhibiting tri-stable electro-optical switching and photoconductivity.

Magnetic properties and magnetic structures ofTbBaMn2O5.75: Possible observation of unconventional polaron trimers

The magnetism of the oxygen deficient, $A$-site layered perovskite $\mathrm{TbBaM}{\mathrm{n}}_{2}{\mathrm{O}}_{5.75}$ has been studied by magnetic susceptibility vs temperature measurements and neutron powder diffraction. Four magnetic phases have been identified. Above 457 K a charge-melted state exists where ferromagnetic double-exchange interactions between the Mn ions dominate. Below 457 K charge ordering occurs and the exchange interactions become primarily antiferromagnetic. The magnetic moment in this phase is too large to be consistent with a charge ordering of $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ but could be explained by having two ${e}_{g}$ electrons being shared among three Mn centers, causing them to act as a single paramagnetic unit. This suggests that $S=11/2\phantom{\rule{0.16em}{0ex}}(\mathrm{M}{\mathrm{n}}^{3.33+}{)}_{3}$ polaron trimers may be present along with $\mathrm{M}{\mathrm{n}}^{3+}$. Below 203 K long-range ordering of the moments on the Mn ions occurs. Neutron powder diffraction data at 52 K reveal a magnetic structure containing an \ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow} pattern of spins which is consistent with the formation of tripolarons. Below 11 K the $\mathrm{T}{\mathrm{b}}^{3+}$ ions also undergo long-range magnetic ordering.

Magnetic fabrics in deformed metaperidotites of the Outokumpu Deep Drill Core, Finland: Implications for a major crustal shear zone

The Outokumpu (OKU) assemblage was studied in drill cores from the 2516m deep OKU Deep Drill Hole, Finland, and we observed that the strongest magnetic anomalies found in borehole measurements are related to three ferrimagnetic metaperidotite units with magnetic susceptibilities of up to 93×10−3SI separated by paramagnetic rock units. The main ferrimagnetic minerals are magnetite and minor pyrrhotite. Magnetic fabric studies were done in order to examine deformation within the OKU assemblage during the collisional deformation event 1.9Ga ago. Well-defined magnetic axes (kmax, kmin) and nearly horizontal magnetic foliation subparallel to the macroscopic foliation occur at the top and the bottom of the OKU formation. Low- and high-field AMS measurements showed that ferri- and paramagnetic subfabrics are coaxial. Paramagnetic units intercalated between the ferrimagnetic units show mostly triaxial magnetic fabrics, and low degrees of magnetic anisotropy values (P′<1.3) with a high variation of the shape factor (T) from prolate to oblate shapes. P′ of the ferrimagnetic metaperidotite is high and range between 1.3 and 3.6 and T tends to more oblate shapes (T>0). Ferrimagnetic unit 2 shows the highest magnetic susceptibility and P′ up to 5.6, which is related to a high amount of magnetite and pyrrhotite mostly concentrated in huge aggregates and veins.Alignment of magnetic minerals parallel with the structural foliation and the high P′ values in the ferrimagnetic metaperidotite indicate that magnetic fabric was acquired in a high strain zone. The magnetic fabrics represent shear zone (SC) fabrics, which have been formed during early obduction-related deformation of the Svecofennian orogeny. The stacked sequence of the ferri- and paramagnetic metaperidotite bodies can be interpreted as a thrust system with an imbricate fan, in which three individual listric thrust sheets occur. This interpretation is in accordance with previous tectonic models of the Outokumpu area.

Cs2NiO2 Revisited. Crystal Structure and Magnetic Properties

Single crystals as well as microcrystalline powders of Cs2NiO2 were obtained via the azide/nitrate route from appropriate mixtures of CsN3, CsNO3 and NiO. The single-crystal structure analysis confirmed that Cs2NiO2 crystallizes in the tetragonal space group I4/mmm (Z = 2, a = 4.4090(3), c = 13.576(3) Å , R1 = 0.036, wR2 = 0.093). Above 45 K, Cs2NiO2 is paramagnetic, and an analysis based on the Curie-Weiss law has resulted in μ = 2.89 μB paramagnetic units, θ = −30.8 K and TN ∼ 20 K.

Effects of Solvents, Ligand Aromaticity, and Coordination Sphere on the g Tensor of Anionic o-Semiquinone Radicals Complexed by Mg2+ Ions: DFT Studies

Density functional theory (DFT) was employed to study the impact of Mg(2+) ions on the o-semiquinone radical anions of different aromaticity in protic and aprotic solvents. After the geometry optimization of ligands and complexes, their g tensors were computed at the UBP86/TZVP and UB3LYP/TZVP theory levels. The suitability of various model systems, assuming continuum dielectric approaches, different Mg(2+) coordination spheres (completed by solvent molecules), and inclusion of additional solvent molecules H-bonded to the ligands, was tested in terms of correlation between the experimental and calculated g-shifts. The effects of complexation, ligands aromaticity, and solvents on the electron spin density for o-semiquinones are discussed. To recognize clearly the changes in the nature of the g tensor components, the contributions from particular excited states were analyzed. A structural characterization of the tested complexes is expected to be helpful in investigations on the complicated biosystems in which the similar paramagnetic units are present.

A new chiral, poly-imidazole N8-ligand and the related di- and tri-copper(ii) complexes: synthesis, theoretical modelling, spectroscopic properties, and biomimetic stereoselective oxidations

The new poly-imidazole N(8) ligand (S)-2-piperazinemethanamine-1,4-bis[2-((N-(1-acetoxy-3-(1-methyl-1H-imidazol-4-yl))-2-(S)-propyl)-(N-(1-methyl-1H-imidazol-2-ylmethyl)))ethyl]-N-(phenylmethyl)-N-(acetoxy), also named (S)-Pz-(C2-(HisIm))(2) (L), containing three chiral (S) centers, was obtained by a multi-step synthesis and used to prepare dinuclear [Cu(2)(L)](4+) and trinuclear [Cu(3)(L)](6+) copper(II) complexes. Low-temperature EPR experiments performed on [Cu(2)(L)](4+) demonstrated that the two S = ½ centers behaved as independent paramagnetic units, while the EPR spectra used to study the trinuclear copper complex, [Cu(3)(L)](6+), were consistent with a weakly coupled three-spin ½ system. Theoretical models for the two complexes were obtained by DFT/RI-BP86/TZVP geometry optimization, where the structural and electronic characteristics nicely supported the EPR experimental findings. In addition, the theoretical analysis unveiled that the conformational flexibility encoded in both [Cu(2)(L)](4+) and [Cu(3)(L)](6+) arises not only from the presence of several σ-bonds and the bulky residues attached to the (S)-Pz-(C2-(HisIm))(2) ligand scaffold, but also from the poor coordination ability of the tertiary amino groups located in the ligand side-chains containing the imidazole units towards the copper(II) ions. Both the dinuclear and trinuclear complexes are efficient catalysts in the stereoselective oxidation of several catechols and flavonoid compounds, yielding the corresponding quinones. The structural features of the substrate-catalyst adduct intermediates were assessed by searching the conformational space of the molecule through MMFF94/Monte Carlo (MMFF94/MC) methods. The conformational flexibility of the bound ligand in the complexes proves to be beneficial for substrate binding and recognition. For the dinuclear complex, chiral recognition of the optically active substrates derives from weak electrostatic interactions between bound substrates and folded regions of the ligand scaffold. For the trinuclear complex, in the case of L/D-Dopa, the chiral recognition has a remarkable stereoselectivity index of 75%, the highest so far reported for this type of reaction. Here the dominant contribution to stereoselectivity arises from the direct interaction between a donor group (the Dopa carboxylate) far from the substrate reaction site (the catechol ring) with the additional (third) copper center not involved in the oxidative catalysis. On the other hand, in the case of bulky substrates, such as L/D-catechin, the observed poor substrate recognition is associated with much weaker interactions between the chiral regions of the complex and the chiral part of the substrate.

Use of a cubane-type Mo 3CoS 4 molecular cluster as paramagnetic unit in the synthesis of hybrid charge-transfer salts

Selective substitution of the chlorine atom coordinated to cobalt in the paramagnetic Mo 3(CoCl)S 4(dmpe) 3Cl 3 (dmpe = 1,2-bis(dimethylphosphanyl) ethane) complex with a S = 1/2 ground state has been achieved by iodine oxidation to afford the also paramagnetic [Mo 3(CoI)S 4(dmpe) 3Cl 3]I ([ 1]I) salt with a S = 1 ground state in almost quantitative yield. Replacement of chorine by iodine has no significant effect on the structural and electrochemical properties of the Mo 3CoS 4 system. Metathesis of the [ 1]I salt with the paramagnetic nickel anionic dithiolate [Ni(mnt) 2] − (mnt = maleonitrilodithiolate) affords [ 1] 2[Ni(mnt) 2]. The stoichiometry evidenced by X-ray analysis reveals that reduction of the [Ni(mnt) 2] − radical to the corresponding diamagnetic closed shell [Ni(mnt) 2] 2− dianion, presumably via dismutation, has occurred during the metathesis process. The crystal structure of [ 1] 2[Ni(mnt) 2] consists of [Ni(mnt) 2] 2− dianions sandwiched by two cluster 1 + cations which yield { 1 +·[Ni(mnt) 2] 2−· 1 +} subunits arranged along the crystallographic c axis. Magnetic susceptibility measurements for [ 1] 2[Ni(mnt) 2] show a χ T product of 0.99 emu K/mol largely unchanged in the 10–300 K range. This behavior agrees with the presence of an S = 1 cluster 1 + cation while the Ni(mnt) 2 moiety does not contribute to the paramagnetism of the sample.

Copper(II) complexes containing tetrathiafulvalene substituted by a bipyridine ligand: Synthesis, X-ray diffraction and EPR characterizations

We report a preliminary study of two novel complexes combining a paramagnetic transition metal (Cu(II)) and a cyclophane-type TTF-bipyridine ligand TTF-bipy = ( cis)2,7-bis(pentylsulfanyl)-3,6-[2,2′-bipyridine-bis(4,4′-methanediyl)bis-sulfanyl]tetrathiafulvalene. The syntheses of the complexes Cu(hfac) 2(TTF-bipy) (hfac = hexafluoroacetylacetonate) and Cu(NO 3) 2(TTF-bipy)·H 2O are reported. The results of single-crystal EPR measurements are discussed on the basis of structural features deduced from preliminary X-ray diffraction data and indicate that these neutral molecular complexes behave as isolated paramagnetic units.

Zener Polaron Ordering Variants Induced byA-Site Ordering in Half-Doped Manganites

We have studied the magnetism of the half-doped charge ordered manganite YBaMn2O6. A formation of ferromagnetic plaquettes of four Mn atoms in the charge ordered phase below T_{CO} approximately 480 K is inferred from high temperature magnetic susceptibility data and the magnetic structure, as determined by neutron powder diffraction at T=1.5 K. The results indicate that new fourfold Mn paramagnetic units form between T_{N}<T<T_{CO}, and that they order in a noncollinear mode below T_{N}=190 K. The magnetism of YBaMn2O6 is consistent with what is expected from the Zener polarons ordering picture of the charge ordering phenomenon in manganites [A. Daoud-Aladine, Phys. Rev. Lett. 89, 097205 (2002)10.1103/PhysRevLett.89.097205].

Self-assembly of carboxylic substituted PTM radicals: From weak ferromagnetic interactions to robust porous magnets

An overview of the work that have been developed over the last six years in our group on the use of polychlorotriphenylmetyl radicals (PTM) functionalized by carboxylic groups to access to purely organic/molecular magnetic materials is reported. From the seminal work on the monocarboxylic PTM (Section 2), of great importance to determine both the ability of these molecules to form intermolecular H-bonds and the nature of the intermolecular interactions mediated through the resulting supramolecular motifs, we will move to the self-assembly of PTM radicals functionalized with two and three carboxylic groups (Section 3). In those cases, the self-assembly of the paramagnetic units yield robust and porous magnetic structures, associating in some cases magnetic ordering to the latest remarkable characteristics. The last part of the review will present the latest results obtained with the idea to increase both the structural and magnetic dimensionality in purely organic PTM-based materials using a PTM radical functionalized by six carboxylic groups (Section 4). New trends and challenges for this research line, concerning the design and synthesis of new PTM radicals, as well as the obtaining of PTM based sensors or multifunctional materials will be presented in the concluding section (Section 5).

The Magnetic Structure and Electronic Ground States of Mott Insulators GeV4S8 and GaV4S8

The ground states of magnetic Mott insulators GaV4S8 and GeV4S8 were calculated with full-potential LAPW methods using the LDA + U approach. Both compounds undergo structural distortions from cubic to rhombohedral (GaV4S8) or orthorhombic (GeV4S8) symmetry at low temperatures. GaV4S8 is ferromagnetic below TC = 10 K, whereas GeV4S8 shows antiferromagnetic order with TN = 13 K. The spin structure of GeV4S8 was determined by neutron diffraction and described in the magnetic space group Pbmn21. The magnetic propagation vector is [1/2, 1/2, 0] relating to the cubic paramagnetic unit cell. The LDA + U calculations (U = 2 eV) confirm the magnetic insulating ground states for the first time with magnetic moments and energy gaps in very good agreement with the experimental data. This method also reproduces the antiferromagnetic spin ordering of GeV4S8. The Jahn−Teller instability of the degenerated levels in the V4 cluster MO drives the structural distortions, depending on the cluster electron count. Our results ...

Charge and Zener polaron order in Bi 0.75Sr 0.25MnO 3: a comparison with Bi 0.75Ca 0.25MnO 3

We present a detailed magnetic and structural study of Bi 0.75Sr 0.25MnO 3. With the aid of electron diffraction, neutron and synchrotron powder diffraction measurements we have found that at ∼600 K the structure presents a considerable distortion of the cell parameters accompanied by a change in the space group and by the appearance of a modulation of the average structure. This modulation doubles the a and c lattice parameters (in the P bnm setting) of the high-temperature structure. Magnetic data reveal below this temperature that the paramagnetic units are not single Mn ions but a coexistence of Mn 3+ and Mn–Mn pairs. This behavior strongly contrasts with that of Bi 0.75Ca 0.25MnO 3 evidencing the strong differences between the Bi 1− x Sr x MnO 3 and Bi 1− x Ca x MnO 3 families.

Charge and Zener polaron order in Bi0.75Sr0.25MnO3: a comparison with Bi0.75Ca0.25MnO3

We present a detailed magnetic and structural study of Bi 0.75 Sr 0.25 MnO 3 . With the aid of electron diffraction, neutron and synchrotron powder diffraction measurements we have found that at ∼600 K the structure presents a considerable distortion of the cell parameters accompanied by a change in the space group and by the appearance of a modulation of the average structure. This modulation doubles the a and c lattice parameters (in the P bnm setting) of the high-temperature structure. Magnetic data reveal below this temperature that the paramagnetic units are not single Mn ions but a coexistence of Mn 3+ and Mn–Mn pairs. This behavior strongly contrasts with that of Bi 0.75 Ca 0.25 MnO 3 evidencing the strong differences between the Bi 1− x Sr x MnO 3 and Bi 1− x Ca x MnO 3 families.

A three-dimensional ferrimagnet composed of mixed-valence Mn4 clusters linked by an [Mn[N(CN)2]6]4- unit.

A ferrimagnet with a 3D molecular network (depicted) has been synthesized and characterized. The compound is composed of units of mixed-valence Mn4 clusters, a unit that is well known for its single-molecule magnet properties, and MnII paramagnetic units. Both building blocks are linked by a dicyanamide bridging ligand to form a covalently bonded three-dimensional network. The compound becomes a ferrimagnet at 4.1 K that exhibits a spin-flip phenomenon under the influence of a magnetic field.

Effects of quenching and annealing on the high-temperature magnetic properties of rapidly quenched Au80Fe20 alloy

Rapidly quenched Au 80 Fe 20 ribbons and platelets exhibit a dominant paramagnetic response above room temperature; the ordering paramagnetic units are clusters of highly correlated spins. Annealing enhances spin clusterization and brings about the precipitation of a small fraction (0.5%) of interacting Fe nanoparticles, with a radius of 3 nm and an average interparticle distance of 50 nm.

Hybrid Molecular Magnets Obtained by Insertion of Decamethylmetallocenium Cations into Layered, Bimetallic Oxalate Complexes: [ZIIICp*2][MIIMIII(ox)3] (ZIII=Co, Fe; MIII=Cr, Fe; MII=Mn, Fe, Co, Cu, Zn; ox=oxalate; Cp*=pentamethylcyclopentadienyl)

A new series of hybrid organometallic - inorganic layered magnets with the formula [Z(III)Cp*2][M(II)M(III)(ox)3] (Z(III) = Co, Fe; M(III) = Cr, Fe; M(II) = Mn, Fe, Co, Cu, Zn; ox = oxalate; Cp* = pentamethylcyclopentadienyl) has been prepared. All of these compounds are isostructural and crystallize in the monoclinic space group C2/m, as found by X-ray structure analysis. Their structure consists of an eclipsed stacking of the bimetallic oxalate-based extended layers separated by layers of organometallic cations. These salts show spontaneous magnetization below To, which corresponds to the presence of ferro-, ferri-, or canted antiferromagnetism. Compounds in which the paramagnetic deca-methylferrocenium is used instead of the diamagnetic decamethylcobaltocenium are good examples of chemically constructed magnetic multilayers with alternating ferromagnetic and paramagnetic layers. The physical properties of this series have been thoroughly studied by means of magnetic measurements and ESR and Mossbauer spectroscopy. We have found that the two layers are electronically quasiindependent. As a consequence, the bulk properties of these magnets have not been significantly affected by the insertion of a paramagnetic layer of S = 1/2 spins in between the extended layers. In fact, the critical temperatures remain unchanged even when comparing [MCp*2]+ derivatives with [XR4]+ compounds (X = N, P; R = Ph, nPr, nBu). Nevertheless, the presence of the paramagnetic layer has been shown to have some influence on the hysteresis loops of these compounds. In the same context, the spin polarization of the paramagnetic units (which arises from the internal magnetic field created by the bimetallic layers in the ordered state) has been observed by Mossbauer and ESR spectroscopy.

THE INTERACTIONS BETWEEN PERIPHERAL AND CENTRAL UNITS IN SUPRAMOLECULAR AND METALLOSUPRAMOLECULAR STRUCTURES: THE CHARACTERIZATION OF SOME MONO-, DI- AND TRINUCLEAR COMPLEXES OF A VICINAL OXIME-IMINE LIGAND

The 1:1 molar ratio reaction of p-phenylenediamine with isonitrosoacetylacetone in chloroform led to the formation of the half unit ligand (HL); (1). Two types of the trans octahedral (L)2Ni 2H2O complex were characterized; the green molecular complex (2) and the associated supramolecular dark brown complex (3). Molecular association in (3) took place via inter-molecular hydrogen bonding between the amino group of a molecule and the oxygen sites of an adjacent molecule. The 1:1 molar ratio reactions of (2) with the metal acetates M(OAc)2 (M = divalent nickel, copper or cobalt) produced the self-assembly structure (4) whereby the metal acetate is coordinated to the amino groups of the nickel(II) complex. Reaction of the dinuclear (4) with another metal acetate (1:1 molar ratio) gave the trinuclear terminated structure (5). Similar reactions of (3) with nickel acetate (1:1 or 1:2) led to formation of complexes with metallosupramolecular structures. An antiferromagnetic interaction between the peripheral and central paramagnetic units was observed from those complexes with copper(II) at the peripheral location. In all these cases the metal ions are bridged via the aromatic Schiff-base moiety. The suggested structures of the mono-, di- and trinuclear coordination compounds are in accordance with the analytical, spectral and magnetic moment data.