Exchange-coupled Clusters Research Articles

Exchange bias identifies lamellar magnetism as the origin of the natural remanent magnetization in titanohematite with ilmenite exsolution from Modum, Norway

Large and stable negative magnetic anomalies in southwestern Sweden, southern Norway, the Adirondacks, USA, and Quebec, Canada, are related to rock units with a magnetic fraction consisting primarily of ilmeno-hematite or hemo-ilmenite. It has been suggested that the unusual magnetic stability of these rocks results from lamellar magnetism. This is a type of magnetic remanence, carried by uncompensated magnetic layers at interfaces between nanoscale exsolution structures of antiferromagnetic (AFM) hematite and paramagnetic ilmenite. Here we present the first direct proof that this lamellar magnetism indeed is responsible for the natural remanent magnetization (NRM) of a rock from Modum, Norway. Our argument expands a previous observation, that, in mineral grains from this rock, the cooling of a positive-induced remanence from room temperature to 5 K – which is well below the ordering temperature of ilmenite (57 K) – leads to a large negative shift of the low-temperature (LT) hysteresis loop. This can only be explained by exchange bias due to exchange coupling across the hematite–ilmenite interfaces. In a different experiment, we now have cooled the original NRM of untreated grains to 5 K, and then measured the hysteresis loop. Again, in several separate grains we observed large shifts of the hysteresis curves. This shows that exchange bias develops also from the untreated NRM. This observation proves that the moments, which carry the NRM, also participate in the exchange coupling at the hematite–ilmenite interfaces. Therefore, the NRM is not carried by defect moments or stress-induced moments, which occur in normal bulk hematite. A closer look at the NRM-induced LT loops shows that exchange bias acts in both field directions, though one direction is clearly predominant. This observation can be interpreted as a frozen equilibrium of different proportions of oppositely directed lamellar moments, a key feature of the original lamellar magnetism hypothesis. We discuss lamellar aggregation, and the formation of exchange-coupled clusters to explain the observed high efficiency of lamellar NRM. We also conclude that remanence carried by lamellar moments should not be used for paleointensity estimates of terrestrial or extraterrestrial material.

High-Field EPR Reveals the Strongly Temperature-Dependent Exchange Interaction in “Breathing” Crystals Cu(hfac)2LR

In the overwhelming majority of the exchange-coupled clusters investigated in field of molecular magnetism, the exchange interaction is constant on temperature. “Breathing” crystals of composition Cu(hfac)2LR undergo temperature-induced reversible structural rearrangements accompanied by significant changes of the effective magnetic moment. Using high-field (W-band) EPR, we provide a solid proof of drastic temperature dependence of exchange interaction J(T) in these compounds that originates from temperature dependence of inter-spin distances. Strong dependence J(T) revealed by EPR makes Cu(hfac)2LR breathing crystals interesting and promising systems in the research toward creation of molecular-magnetic switches and related spin devices.

Observation of multiple quantum transitions in magnetic nanoparticles

Absorption at multiple resonance frequencies is observed in magnetic nanoparticles in strong similarity with forbidden multiple quantum transitions known for paramagnetic ions. Temperature and concentration dependences have been studied to reveal the origin of the signals. The results are discussed using a ``quantization'' approach, considering resonance transitions between energy levels of a giant spin corresponding to the total magnetic moment of a nanoparticle. The multiple quantum transitions are ascribed to both the effects of anisotropy and dipole-dipole interactions between nanoparticles.

Nonstoichiometry and low-temperature magnetic properties of FeSi crystals

The results of experimental and theoretical studies of the low-temperature properties of FeSi crystals are presented. The specific features of the magnetic susceptibility are shown to be related to the superparamagnetic behavior of impurity clusters. The thermomagnetic hysteresis phenomena observed are explained using the model of exchange-coupled clusters.

Many-spin effects in inelastic neutron scattering and electron paramagnetic resonance of molecular nanomagnets

Many molecular magnetic clusters, such as single-molecule magnets, are characterized by spin ground states with defined total spin S exhibiting zero-field-splittings. In this work, the spectroscopic intensities of the transitions within the ground-state multiplet are analyzed. In particular, the effects of a mixing with higher-lying spin multiplets, which is produced by anisotropic interactions and is neglected in the standard single-spin description, are investigated systematically for the two experimental techniques of inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR), with emphasis on the former technique. The spectroscopic transition intensities are calculated analytically by constructing corresponding effective spin operators perturbationally up to second order and consequently using irreducible tensor operator techniques. Three main effects of spin mixing are observed. Firstly, a pronounced dependence of the INS intensities on the momentum transfer Q, with a typical oscillatory behavior, emerges in first order, signaling the many-spin nature of the wave functions in exchange-coupled clusters. Secondly, as compared to the results of a first-order calculation, the intensities of the transitions within the spin multiplet are affected differently by spin mixing. This allows one, thirdly, to differentiate the higher-order contributions to the cluster magnetic anisotropy which come from the single-ion ligand-field terms and spin mixing, respectively. The analytical results are illustrated by means of the three examples of an antiferromagnetic heteronuclear dimer, the Mn-[3 x 3] grid molecule, and the single-molecule magnet Mn12.

Electron Spin-Lattice Relaxation in Polymers and Crystals Related to Disorder and Structure Defects

Temperature dependences (4-300 K) of the electron spin-lattice relaxation have been determined by electron spin echo technique for free radicals in two polymeric systems: phenol-formaldehyde resin and polyhydrazodisulphide. The dependences are described in terms of dynamics specific for amorphous systems involving two-level tunnelling states at low temperatures, exchange-coupled clusters of paramagnetic centres and local oscillators. Some universal temperature behaviour of the spin-lattice relaxation of amorphous systems is suggested, with a strong increase in relaxation rate with temperature at low temperatures and much weaker increase above 50 K with characteristic linear temperature dependence in a broad temperature range and cosech(Δ/kT)-type behaviour. It is also shown that the amorphous-type behaviour appears in low temperatures relaxation studies of single crystals but it is due to a non-uniform distribution of doped paramagnetic ions. Such behaviour we have found in Tutton salt crystals doped with Cu 2 + , as well as for free radical centres produced by ionising irradiation in (NH 4 ) 3 H(SeO 4 ) 2 and Li(N 2 H 5 )SO 4 single crystals, where extended phonons are suppressed around radiation damage centres suggesting a local amorphisation of the crystal structure.

Intramolecular excimer formation in a naphthalene-appended dinuclear iron-oxo complex.

The synthesis, structure, and physical properties of a Heisenberg exchange-coupled cluster containing naphthalene groups are described. [Fe2(O)(O2CCH2C10H7)2(TACN-Me3)2]2+ (3) crystallizes in space group P1 with unit cell parameters a = 12.94(2) A, b = 14.84(2) A, c = 15.23(2) A, alpha = 101.12(7) degrees, beta = 90.8(1) degrees, gamma = 114.14(7) degrees, V = 2605(6) A3, and Z = 2 with R = 0.0425 and wR2 = 0.1182. Variable-temperature magnetic susceptibility data indicate that the two high-spin FeIII centers are antiferromagnetically coupled with J = -105 cm-1 (H = -2 JS1.S2), which is typical for this class of compounds. The room-temperature static emission spectrum of the compound in deoxygenated CH3CN solution is centered near 335 nm and has features reminiscent of both methyl-2-naphthylacetate (1) and [Zn2(OH)(O2CCH2C10H7)2(TACN-Me3)2]+ (2) with the following two caveats: (1) the overall emission intensity is roughly a factor of 10 less than that of the free ester (1, phi = 0.13) or the ZnII analogue (2, phi = 0.14), and (2) there is significant broadening of the low-energy shoulder of the emission envelope. Time-correlated single photon counting data revealed biphasic emission for 3 with tau 1 = 4.6 +/- 1 ns and tau 2 = 47 +/- 1 ns. The latter compares favorably with that found for 2 (tau = 47 +/- 1 ns) and is assigned as the S0-S1 fluorescence of naphthalene. Emission anisotropy, time-gated emission spectra, and nanosecond time-resolved absorption measurements all support the assignment of the 4.6 ns component as being due to a singlet excimer that forms between the two naphthylacetate groups of 3, a process that is likely mediated by the structural constraints of the oxo-bis-carboxylato diiron core. No direct evidence for intramolecular electron and/or energy transfer from the photoexcited naphthyl group to the iron-oxo core was obtained, suggesting that the short-lived excimer may contribute to circumventing such pathways in this type of system.

ＣＧＣ（Ｃｏｕｐｌｅｄ Ｇｒａｎｕｌａｒ ａｎｄ Ｃｏｎｔｉｎｕｏｕｓ）垂直二層膜媒体のノイズ低減効果

Reduction of medium noise is the key in perpendicular magnetic recording. The CGC medium that has a layered continuous and granular films is a candidate for obtaining a high SNR. We investigated noise characteristics of the CGC double-layered medium by focusing on transition jitter and magnetization transition analysis with an MFM. Results show that the transition smoothing by depositing the continuous layer was confirmed by both waveform analysis and MFM image analysis. The effect is due to magnetic interaction between the continuous layer and granular layer that resolves exchange-coupled clusters existing in a sole granular layer. Less transition noise, and therefor better SNR, resulted.

Synthesis, crystal structure and physical properties of ZrNiSn semiconductor doped with Mn

The resistivity, thermopower, dc magnetic susceptibility and electron paramagnetic resonance (EPR) spectra of ZrNi 1− x Mn x Sn solid solution with low (0.001≤ x≤0.05) Mn concentration were investigated in the 78–400 K temperature range. Polycrystalline samples of the ZrNi 1− x Mn x Sn compounds were prepared by arc-melting of the pure metals. Quantitative atomic emission spectroscopy confirmed the appropriate Mn contents in all alloys. The lattice parameters of the ZrNi 1− x Mn x Sn compounds with different Mn concentration are almost unchanged. Magnetic susceptibility shows the linear dependence versus inverse magnetic field for all samples, what is evidence for the magnetic interactions in the ZrNi 1− x Mn x Sn compounds. The temperature dependence of magnetic susceptibility for the compounds with 0.005≤ x≤0.05 can be described approximately by a Curie–Weiss law. The results of static magnetic measurements were confirmed by EPR technique. The broad line of I type belongs to Mn 2+ ions in the Ni-sites in crystal lattice coupled by magnetic dipolar interaction. The narrow line of II type is interpreted as an EPR spectrum of exchange-coupled pairs or clusters of more than two Mn 2+ ions. The unusual magnetic properties and the features of dipolar and exchange interaction of the Mn 2+ ions in the lattice of ZrNi 1− x Mn x Sn solid solution are discussed.

Magnetic resonance of exchange-coupled copper complexes in perovskite-structure crystals: The potassium tantalate and cuprate superconductors

This paper reports on parallel EPR studies of high-temperature superconductors based on the cuprate perovskites RBa2Cu3O6+x (R=Y, Gd, Nd) and of KTaO3: Cu, which also has a perovskite structure. EPR measurements performed on copper-doped KTaO3 crystals revealed Cu2+-Cu2+ copper pair centers. The copper ions making up pairs are assumed to occupy adjacent tantalum sites. The pair centers are chains consisting of two equivalent Cu2+ ions and three oxygen vacancies aligned in the 〈100〉 direction. The crucial point in the model proposed is the presence of an oxygen vacancy sandwiched between two Cu2+ ions, whereas the outer vacancies do not necessarily occupy neighboring sites. In this structure, complete charge compensation is achieved. Ferromagnetic exchange coupling takes place between the two copper ions. An investigation of the exchange and superhyperfine interactions of copper centers in crystalline potassium tantalate has permitted the estimation of the respective interactions in crystals of the cuprate superconductors which exhibit magnetic resonance signals due to exchange-coupled copper clusters in the case of oxygen deficiency.

An anomaly in the magnetic susceptibility of the Mg1−x CuxO solid solutions with 0.01≤x≤0.20

The problem of localized superconductivity has motivated the preparation of Mg1−xCuxO solid solutions with NaCl structure and 0.01≤x≤0.20, as well as a study of the magnetization and magnetic susceptibility χ in the 2–400 K temperature range and in magnetic fields of up to 5 T. The temperature dependence of χ is described for all compositions by the Curie-Weiss law, χ = C/(T − θ), where the constant C is close to the value calculated for each composition for μeff = 1.7–1.9µB, and θ is close to zero. For T < 30 K, χ(T) deviates for all compositions toward lower χ, which can be attributed to magnetic ordering of exchange-coupled clusters in the solid solution. At T∼320–330 K, an anomaly of a diamagnetic type, i.e., a decrease of χ by 6–30% of its paramagnetic value, has been observed for all compositions against the background of the generally paramagnetic χ(T). A discussion is presented of alternative reasons for this anomaly and of its possible connection with localized superconductivity.

Analysis of Exchange Interaction and Electron Delocalization as Intramolecular Determinants of Intermolecular Electron-Transfer Kinetics.

During the past decades, spectroscopic characterization of exchange interactions and electron delocalization has developed into a powerful tool for the recognition of metal clusters in metalloproteins. By contrast, the biological relevance of these interactions has received little attention thus far. This paper presents a theoretical study in which this problem is addressed. The rate constant for intermolecular electron-transfer reactions which are essential in many biological processes is investigated. An expression is derived for the dependence of the rate constant for self-exchange on the delocalization degree of the mixed-valence species. This result allows us to rationalize published kinetic data. In the simplest case of electron transfer from an exchange-coupled binuclear mixed-valence donor to a diamagnetic acceptor, the rate constant is evaluated, taking into account spin factors and exchange energies in the initial and final state. The theoretical analysis indicates that intramolecular spin-dependent electron delocalization (double exchange) and Heisenberg-Dirac-van Vleck (HDvV) exchange have an important impact on the rate constant for intermolecular electron transfer. This correlation reveals a novel relationship between magnetochemistry and electrochemistry. Contributions to the electron transfer from the ground and excited states of the exchange-coupled dimer have been evaluated. For clusters in which these states have different degrees of delocalization, the excited-state contributions to electron transfer may become dominant at potentials which are less reductive than the potential at which the rate constant for the transfer from the ground state is maximum. The rate constant shows a steep dependence on HDvV exchange, which suggests that an exchange-coupled cluster can act as a molecular switch for exchange-controlled electron gating. The relevance of this result is discussed in the context of substrate specificity of electron-transfer reactions in biology. Our theoretical analysis points toward a possible biological role of the spin-state variability in iron-sulfur clusters depending on cluster environment.

The Active Site of Hydroxylamine Oxidoreductase from Nitrosomonas: Evidence for a New Metal Cluster in Enzymes

Hydroxylamine oxidoreductase (HAO) from Nitrosomonas europaea catalyzes the oxidation of NH2OH to NOz-. The enzyme contains eight hemes per subunit (63 ma) which participate in catalytic function and electron transport. In the resting ferric state of the enzyme, we find that these hemes can be categorized on the basis of quantitative integer-spin and multifrequency EPR data as follows: (a) Four hemes are noninteracting, ferric C-type hemes. (b) Two hemes are low-spin and weakly spin-coupled through an interaction which is best described as dipolar on the basis of frequency-dependent g values. (c) The final two hemes are exchange-coupled; this cluster is the origin of a new integer-spin EPR signal which can be quantitatively associated with the reactive Pm heme of HAO. Accordingly, treatments that affect the optical properties of Pm and catalysis of the enzyme, such as substrates, CN-, or H202, all caused the integer-spin resonance to vanish. For reduced HAO, a second integer-spin EPR signal is observed which derives from one high-spin ferrous heme; previous Mossbauer studies have shown that the only such heme is Pm. An analysis of potential electronic models for the exchange-coupled cluster indicated that the interacting spins are both S = l/2. It thus appears that the active site of HA0 contains a pair of low-spin hemes which are coupled in a manner not previously observed in proteins. The structure of the diheme cluster and in the presence of substrates or inhibitors, and its bearing on catalysis by HA0 is discussed.

Moessbauer, EPR, and ENDOR studies of the hydroxylase and reductase components of methane monooxygenase from Methylosinus trichosporium OB3b

Soluble methane monooxygenase (MMO) isolated from Methylosinus trichosporium OB3b consists of three components: hydroxylase, reductase, and component B. The active-site diiron cluster of the hydroxylase has been studied with Mossbauer, ANDOR, and APR spectroscopies. Mossbauer spectra of the oxidized cluster show that the two high-spin irons are antiferromagnetically coupled in accord with our preliminary study (Fox et al. J. Biol. Chem. 1988, 263, 10553-10556). Mossbauer studies also reveal the presence of two cluster conformations at pH 9. The excited-state S=2 multiplet of the exchange-coupled cluster (Fe 3+ -Fe 3+ ) gives rise to an integer-spin EPR signal near g=8; this is the first quantitative study of such a signal from any system

Magnetic excitations in an exchange-coupled tetramer cluster of cobalt (II): a study by inelastic neutron scattering

The polyoxometalate K 10[Co 4(H 2O) 2(PW 9O 34) 2].20H 2O contains a ferromagnetically exchange-coupled tetramer of Co 11 encapsulated in between two diamagnetic molecules (PW 9O 34 -9. At 2.5K several inelastic peaks are observed in the energy range 1.5–7 meV, which are assigned to magnetic excitations in the cluster. A tentative interpretation of these data from an anisotropic exchange model yields a cobalt-cobalt interaction of 3meV (24cm -1) and an amount of anisotropy J xy / J z ≌0.6. These values are consistent with the magnetic susceptibility measurements.

Magnetic characterization of tetranuclear copper(II) and cobalt(II) exchange-coupled clusters encapsulated in heteropolyoxotungstate complexes. Study of the nature of the ground states

This paper presents a magnetic characterization of the heteropolyanions [M 4 (H 2 O) 2 (PW 9 P 34 ) 2 ] 10- and [M 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16- (M=Cu(II), Co(II)), with emphasis on the exchange interactions. Their individual heteropolyoxometalate molecules encapsulate a rhomblike arrangement formed by four coplanar MO 6 octahedra sharing edges

The manganese site of the photosynthetic oxygen-evolving complex probed by EPR spectroscopy of oriented photosystem II membranes: the g = 4 and g = 2 multiline signals.

The g = 4 and g = 2 multiline EPR signals arising from the Mn cluster of the photosynthetic oxygen-evolving complex (OEC) in the S2 state were studied in preparations of oriented photosystem II (PSII) membranes. The ammonia-modified forms of these two signals were also examined. The g = 4 signal obtained in oriented PSII membranes treated with NH4Cl at pH 7.5 displays at least 16 partially resolved Mn hyperfine transitions with a regular spacing of 36 G [Kim, D.H., Britt, R.D., Klein, M.P., & Sauer, K. (1990) J. Am. Chem. Soc. 112, 9389-9391]. The observation of this g = 4 "multiline signal" provides strong spectral evidence for a tetranuclear Mn origin for the g = 4 signal and is strongly suggestive of a model in which different spin state configurations of a single exchange-coupled Mn cluster give rise to the g = 4 and g = 2 multiline signals. A simulation shows the observed spectrum to be consistent with an S = 3/2 or S = 5/2 state of a tetranuclear Mn complex. The resolution of hyperfine structure on the NH3-modified g = 4 signal is strongly dependent on sample orientation, with no resolved hyperfine structure when the membrane normal is oriented perpendicular to the applied magnetic field. The dramatic NH3-induced changes in the g = 4 signal resolved in the spectra of oriented samples are suggestive that NH3 binding at the Cl- site of the OEC may represent direct coordination of NH3 to the Mn cluster.(ABSTRACT TRUNCATED AT 250 WORDS)

Re-entrant magnetic behaviour in amorphous Fe-rich Fe100−x Zr x alloys

We report results of57Fe Mossbauer measurements on the magnetically concentrated re-entrant amorphous Fe100−xZr x alloys (8≤x≤12) which give convincing evidence for a microscopic origin of the low-temperature magnetization anomaly atT f<T c. This is shown by an anomalous increase in the magnetic hyperfine field and in the relative intensities of the (†m=0)-Mossbauer lines belowT f upon cooling. No evidence was found for an antiferromagnetic phase. The shape of the hyperfine-field distribution atT=4.2 K in zero external field and in a field of 3 T is unchanged, indicating homogeneous behaviour of all Fe moments near saturation. Ferromagnetic order is not long-ranged, but determined by exchange-coupled magnetic clusters. The cluster moment nearT c is found to decrease strongly with increasing Fe content and extrapolates to zero atx≈96.

High-magnetic-field EPR in Cd1-xMnxTe.

Magnetic resonances have been studied in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te for 0.002\ensuremath{\le}x\ensuremath{\le}0.1 by means of far-infrared magnetotransmission at fields up to B=22 T. EPR of the ${\mathrm{Mn}}^{2+}$ ions is observed, accompanied by many orientation-dependent satellite resonances. These satellites are explained in terms of excitations of exchange-coupled ${\mathrm{Mn}}^{2+}$ pairs, and the presence of anisotropic exchange is strongly suggested. At x\ensuremath{\ge}0.02 a very strong magnetic-field-induced line broadening is observed for B&gt;10 T. This broadening is explained in terms of a cross-correlation mechanism using exchange-coupled clusters as fast-relaxation centers.

Local magnetic properties of iron-rich amorphous Fe100−xZrx alloys

We report results of 57Fe Mössbauer measurements on the magnetically concentrated re-entrant spin-glass amorphous Fe100−xZrx alloys (8⪕x⪕12) which give convincing evidence for a microscopic origin of the low-temperature magnetization anomaly at Tf<TC. This is shown by an anomalous increase in the magnetic hyperfine field and in the relative intensities of the (Δm=0)-Mössbauer lines below Tf upon cooling. No evidence was found for an antiferromagnetic phase. The shape of the hyperfine-field distribution at T=4.2 K in zero external field and in a field of 3 T is unchanged, indicating homogeneous behaviour of all Fe moments near saturation. Ferromagnetic order is not long-ranged, but determined by exchange-coupled magnetic clusters. The cluster moment near TC is found to decrease strongly with increasing Fe content and extrapolates to zero at x≈96.