Intercluster Exchange Interactions Research Articles

Quasimolecular J_{tet}=3/2 Moments in the Cluster Mott Insulator GaTa_{4}Se_{8}.

Quasimolecular orbitals in cluster Mott insulators provide a route to tailor exchange interactions, which may yield novel quantum phases of matter. We demonstrate the cluster Mott character of the lacunar spinel GaTa_{4}Se_{8} using resonant inelastic x-ray scattering (RIXS) at the Ta L_{3} edge. Electrons are fully delocalized over Ta_{4} tetrahedra, forming quasimolecular J_{tet}=3/2 moments. The modulation of the RIXS intensity as function of the transferred momentum q allows us to determine the cluster wave function, which depends on competing intracluster hopping terms that mix states with different character. This mixed wave function is decisive for the macroscopic properties since it affects intercluster hopping and exchange interactions and furthermore renormalizes the effective spin-orbit coupling constant. The versatile wave function, tunable via intracluster hopping, opens a new perspective on the large family of lacunar spinels and cluster Mott insulators in general.

Peripheral chiral spin textures and topological Hall effect in CoSi nanomagnets

The spin structure and transport behavior of B20-ordered CoSi nanomagnets are investigated experimentally and by theoretical calculations. B20 materials are of interest in spin electronics because their noncentrosymmetric crystal structure favors noncoplanar spin structures that yield a contribution to the Hall effect. However, stoichiometric bulk CoSi is nonmagnetic, and combining magnetic order at and above room temperature with small feature sizes has remained a general challenge. Our CoSi nanoclusters have an average size of 11.6 nm and a magnetic ordering temperature of 330 K. First-principle calculations and x-ray circular dichroism experiments show that the magnetic moment is predominantly confined to the shells of the clusters. The CoSi nanocluster ensemble exhibits a topological Hall effect, which is explained by an analytical model and by micromagnetic simulations on the basis of competing Dzyaloshinskii-Moriya and intra- and intercluster exchange interactions. The topological Hall effect is caused by formation of chiral spin textures in the shells of the clusters, which exhibit fractional skyrmion number and are therefore termed as paraskyrmions (closely related to skyrmion spin structures). This research shows how nanostructuring of a chiral atomic structure can create a spin-textured material with a topological Hall effect and a magnetic ordering temperature above room temperature.

Interpenetrated (8,3)-c and (10,3)-b Metal–Organic Frameworks Based on {FeIII3} and {FeIII2CoII} Pivalate Spin Clusters

Two new magnetic cluster-based 3D coordination polymers consisting of μ3-oxo-centered cationic homometallic [FeIII3O(O2CCMe3)6]+ or neutral heterometallic [FeIII2CoIIO(O2CCMe3)6] coordination clusters bridged by different N,N′-donor ligands into three-dimensional networks of {[Fe3O(O2CCMe3)6(4,4′-bpy)1.5](OH)·0.75(CH2Cl2)·x(H2O)}n (1, x ≈ 8) and {[Fe2CoO(O2CCMe3)6(bpe)0.5(pyz)]}n (2) (where 4,4′-bpy = 4,4′-bipyridine; bpe = 1,2′-bis(4-pyridyl)ethylene; pyz = pyrazine) have been prepared under solvothermal conditions. Single-crystal X-ray diffraction studies reveal the existence of a 6-fold interpenetrated network with rare (8,3)-c (etc) topology for 1 and a 3-fold interpenetrated network with (10,3)-b (ths) topology for 2. The interpenetration effectively results in very low BET surface areas, and the compounds have to be regarded as nonporous. Magnetic studies of 1 and 2 point to both ferro- and antiferromagnetic intra- and intercluster exchange interactions between the isotropic FeIII and the strongly anisotropic CoII spin centers. 57Fe Mössbauer spectroscopy confirms the uniform ferric (+III) valence state in both 1 and 2, and low-temperature data for 1 point toward distinct hyperfine fields for the Fe sites.

Spin-glass-like dynamics of ferromagnetic clusters in La0.75Ba0.25CoO3

We report a magnetization study of the compound La0.75Ba0.25CoO3 where the Ba2+ doping is just above the critical limit for percolation of ferromagnetic clusters. The field cooled and zero-field cooled (ZFC) magnetization exhibit thermomagnetic irreversibility and the ac susceptibility shows a frequency dependent peak at the ferromagnetic ordering temperature (TC ≈ 203 K) of the clusters. These features indicate the presence of a non-equilibrium state below TC. For the non-equilibrium state, the dynamic scaling of the imaginary part of the ac susceptibility and the static scaling of the nonlinear susceptibility clearly establish a spin-glass-like cooperative freezing of ferromagnetic clusters at 200.9(2) K. The assertion of the occurrence of spin-glass-like freezing of ferromagnetic clusters is further substantiated by ZFC ageing and memory experiments. We also observe certain dynamical features which are not present in a typical spin glass, such as: the initial magnetization after ZFC ageing first increases and then decreases with the waiting time; and there is an imperfect recovery of relaxation in negative temperature cycling experiments. This imperfect recovery transforms to perfect recovery for concurrent field cycling. Our analysis suggests that these additional dynamical features have their origin in the inter-cluster exchange interaction and cluster size distribution. The inter-cluster exchange interaction above the magnetic percolation level gives a superferromagnetic state in some granular thin films, but our results show the absence of a typical superferromagnetic-like state in La0.75Ba0.25CoO3.

Topology of exchange interactions in copper-nitroxide based molecular magnets studied by EPR

Switchable copper-nitroxide based molecular magnets Cu(hfac)2LR (‘breathing crystals’) have one-dimensional (1D) polymer-chain structure and 1D magnetic motif; however, the directions of polymer and magnetic chains in the crystal do not coincide. In this work we report the detailed electron paramagnetic resonance (EPR) study of the topology of intercluster exchange interactions forming the magnetic chains in a series of breathing crystals. The obtained results provide unambiguous manifestation of 1D magnetic behaviour, which degree correlates with the magnitude of intercluster exchange interaction. Analysis of experimental angular-dependent EPR data allowed us to determine relative orientations of magnetic chains with respect to polymer chains. The obtained angle between them is ≈40–46° for three studied compounds. The comparison of EPR and x-ray diffraction data confirms that 1D magnetic chains are formed by interacting adjacent spin triads of neighbouring polymer chains.

EPR of Spin Transitions in Complexes of Cu(hfac)2 with tert-Butylpyrazolylnitroxides

Polymer chain complexes [Cu(hfac)2LR]n exhibit thermally and light-induced magnetic anomalies in many aspects similar to a spin crossover. These compounds attracted significant attention in the field of molecular magnetism and have been extensively studied by electron paramagnetic resonance (EPR) during the last several years. All compounds studied so far were based on copper(II) ions bridged by pyrazolyl-substituted nitronylnitroxides. The present work reports the first EPR study of complexes of Cu(hfac)2 with tert-butylpyrazolylnitroxides—a new type of nitroxide ligand expected to modify exchange interaction pathways and physical properties of the crystals. The Q-band EPR spectra of three representative novel compounds are principally different from those studied previously, supporting the assumption that the magnetic motif of the compound has changed. Dominant intercluster exchange interactions are now found along the structural polymer chains. This complicates the EPR detection of phase transitions to some extent; however, theoretical modeling of the observed spectral changes allows for unambiguous assignment of different spin states and transitions between them. The magnitudes of intercluster exchange interaction were estimated to be ca. 0.1–1.5 cm−1 for the studied compounds.

Orbital‐dependent magnetic properties of molecular cluster containing high‐spin Co(II) ions

AbstractTo shed light on magnetic properties of the magnetic exchange molecular cluster containing the constituent metal ion with unquenched orbital angular momentum, as representative examples two octahedrally coordinated high‐spin Co(II)‐cluster complexes were synthesized and characterized: complex (1) [Co3(Hbzp)6][ClO4]2· 2CH3OH·H2O (bzp = 2‐benzoylpyridine) with a linear core Co(III)‐Co(II)‐Co(III) and complex (2) [Co4L(N3)4(CH3CN)4](ClO4)4·2H2O (L = Macrocyclic schiff base of tetra‐2,6‐pyridinedicarboxaldehyde‐tetra‐ethane‐1,2‐diamine) comprised by two nearly independent Co(II)‐dimers. Following Lines' isotropic exchange approximation the magnetic interaction effective Hamiltonian was introduced, which includes the exchange interactions, the spin‐orbit coupling, the low‐symmetry crystal field, and the intercluster exchange interaction. On the basis of the irreducible tensor operator approach to solve Hamiltonian we have coded the software, BJMAG package, and performed the best‐fit procedure for experimentally temperature variable magnetic susceptibilities for the complexes (1) and (2). The best‐fit parameters were obtained as follows: λ = −126 cm−1, Δ = 432 cm−1, κ = 0.96, mJ′ = 2.7 cm−1 for the complex (1) and λ = −165 cm−1, Δ = 960 cm−1, κ = 0.95, J′ = 9 cm−1, mJ′ = −0.1 cm−1 for the complex (2). To further elucidate the individual role of each interaction in the employed model, the effects of the key factors governing the magnetic properties on temperature variable magnetic susceptibilities were examined in detail. The significance of the intercluster exchange interactions at low temperatures was briefly discussed and a self‐consistent molecular field to calculate the thermally average spin of the nearest neighbor in the intercluster interaction was proposed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Spin dynamics in the manganese tetramer compoundα-MnMoO4

α-MnMoO4 is atetrameric magnetic cluster system which undergoes a transition to three-dimensional antiferromagnetic orderat TN = 10.7 K. In theordered state the Mn2+ spins (sν = 5/2) are ferromagnetically aligned within the tetramer, resulting in a total cluster spinS = 10. The magnetic excitation spectrum consists of eight excitation modes of tetramerorigin propagating in the whole reciprocal space. From single-crystal inelasticneutron scattering investigations the three-dimensional coupling scheme isrationalized in the framework of a tetramer-based dispersion model. The tetramerstates and the energy dispersion of all the magnetic excitations are described byHeisenberg-like intra- and intercluster exchange interactions, respectively, thusα-MnMoO4 is a suitable tetrameric model system to study the interplay of these interactions.

Low-dimensional exchange interactions, structural deformation, and antiferromagnetic ordering in lithium manganite LiMnO2

Structure deformation in lithium manganite LiMnO2 has been analyzed. It is shown that suppression of the Jahn-Teller effect with a decrease in temperature is related to striction effects, which are caused by intercluster exchange interactions. It is concluded that low-dimensional interactions play an important role in the formation of magnetic properties of lithium manganite.

Irreducible tensor operator method study on molecular magnetism: A practical approach and its use for high-nuclearity magnetic cluster

A new practical approach to molecular magnetism was suggested. First, for the complicated magnetic exchange system with unquenched orbital angular momentum a general form of the effective Hamiltonian including an intercluster exchange interaction of the self-consistent field was introduced. On the basis of the irreducible tensor operator method the practical approach was established, and accomplished by the BJMAG package, where a new strategy to automatically search more good quantum numbers was adopted to solve further block diagonalization in the eigenvalue problem. Finally, potentiality of the approach to isotropic Heisenberg spin clusters was tested through two high-nuclearity clusters, the nonamer cluster and the Mn 7-cluster compound as representative examples.

Ferromagnetic properties of deformation-induced martensite transformation in AISI 304 stainless steel

Ferromagnetic properties of plastically deformed AISI 304ss have been studied using magnetic hysteresis and Barkhausen emissions methods. The present study has been concentrated on low volume fraction of martensite, i.e., below 58 pct, as compared to the available literature for a higher percentage of martensite. In measured materials, the coercivity increased with deformation and had a tendency to go toward saturation value. A linear increase in remanence with the deformation was observed. A large number of small amplitude of Barkhausen emissions were found at low percentage of martensite, indicating that magnetization rotation took place within a small region. However, large amplitude Barkhausen emissions were observed with the increase of deformations. Angular variation of Barkhausen emissions indicated the formation of rolling texture within the materials. A model has been proposed to explain the results. At the initial stage, small martensite clusters are formed, which grow with the deformation, and the intracluster exchange interaction becomes predominant. With the increase of deformation, martensite volume fraction increases. In this process, existing clusters grow and new clusters are formed. As a result, martensite clusters come closer and intercluster exchange interaction becomes important.

Magnetism of Co nanocluster films

A cluster-beam technique is used to produce nearly monodispersed nanoclusters of Co while independently varying their size and concentration. The Co clusters are embedded in Cu and ${\mathrm{SiO}}_{2}$ to form films with cluster size varying from 300 to 9000 atoms and Co concentration varying from 10 to 50 vol %. The Co magnetization ${(M}_{s})$ increases with increasing cluster size and decreases with increasing Co concentration for a given cluster size but is always less than the bulk value. First-principles calculations are used to analyze the experimental data in terms of the local environment effects at the cluster-matrix interface and spin-glass-like behavior due to the strong ferro- and antiferromagnetic intercluster exchange interactions for small separations.

Single-crystal EPR studies of a spin-frustrated imidazolate-bridged trinuclear copper(II) complex

The single-crystal and polycrystalline powder EPR spectra of the trinuclear compound [L 3Cu 3(Im) 3](ClO 4) 3, L = 1,4,7-trimethyl-1,4,7-triazacyclononane, and Im = imidazolate, have been measured in the temperature range 4.2–300 K. The spectra are explained based on the spin-frustration, the low symmetry, and the intercluster exchange interactions.

Magnetismus von Europium-Clustern in intermetallischen Verbindungen mit Magnesium Teil 2 Über den Einfluβ von Aluminium auf die Intra- und Intercluster-Wechselwirkungen in Eu(Mg 1− xAl x) 5,2

The magnetic susceptibilities of the phases Eu(Mg 1− x Al x ) 5.2 with x=0.05 and 0.10 (which crystallize with a EuMg 5.2-type structure containing triangular Eu(4f 7) clusters) have been determined between 3.2 and 295 K. In the temperature range 40–295 K where the compounds obey the Curie-Weiss law with μ≈8.1 μ B , in contrast with EuMg 5.2, the paramagnetic Curie temperatures Θ p decrease from 7 K (EuMg 5.2) to 4.5 K ( x=0.05) or 0.5 K ( x=0.10). Below 40 K characteristic deviations from the Curie-Weiss straight line occur which on the basis of the Heisenberg and molecular field models allow the separation of intra- and intercluster exchange interactions. Within the triangular clusters the interaction is ferromagnetic whereas the intercluster effect is antiferromagnetic. The change in Θ p values with increasing aluminium content, tending towards an increase in antiferromagnetic contributions, results from a reduction in the ferromagnetic intracluster coupling and a strengthening of the antiferromagnetic intercluster effect.

Magnetism of europium clusters in the intermetallic compounds EuxSr1−xMg5.2 and (EuxSr1−x)3Mg13

Eu x Sr 1− x Mg 5.2 (I) ( x = 1, 0.33) with triangular heavy atom clusters and (Eu x Sr 1− x ) 3 Mg 13 (II) ( x = 0.11, 0.22 and 0.46) with triangular and octahedral clusters have been investigated magnetochemically (method, Faraday-Curie; temperature, 3.6–294 K; magnetic field, 0.07–1.4 T). In addition the specific heat C was measured for EuMg 5.2 between 0.1 and 25 K. In analogy to the determination of metal-metal interactions in polynuclear complexes the intracluster and intercluster exchange interactions between the paramagnetic Eu[4f 7 ] centres have been analysed on the basis of the Heisenberg model and the molecular field approximation respectively. The magnetism of I and II can be explained by ferromagnetic intracluster exchange interactions of similar magnitude ( J = 0.46 cm −1 and 0.55 cm −1 respectively, independent of the europium content) and intercluster exchange interactions of an antiferromagnetic nature (I) or of greater diversity depending on europium concentration (II). The parameter values obtained are discussed in a structural context.