Ferromagnetic Exchange Coupling Research Articles

A semiempirical investigation of interelectronic exchange coupling in bisected poly(1,4‐phenylene) polycation model systems

AbstractAM1 SCF‐MO‐CI computations find the bisected biphenyl dication to have nearly degenerate triplet and singlet states, with the lowest‐energy state being a quininoid singlet planar dication. The bisected perchlorobiphenyl dication favors a triplet ground state by a small amount (0.4‐1.9 kcal/mol), in qualitative agreement with recent experimental findings and with theoretical expectations that such an orthogonal open‐shell pi‐system should exhibit ferromagnetic exchange coupling. The higher oligomeric bisected para‐linked phenylenes polycations do not show an appreciable computational preference for a high‐spin multiplicity ground state either with or without perchlorine substitution. Chlorine substitution para to the 1,1′‐linkage may lend a unique stabilization to the biphenyl system, which is not available in higher oligomeric analogs of poly(1,4‐phenylene)s. The small magnitude of ferromagnetic exchange in these systems suggests that small geometric or substituent effects may confound experimental efforts to design polymeric ferromagnetic materials by this strategy. © 1992 John Wiley & Sons, Inc.

Magnetic and EPR Spectroscopic Investigations on a Ferromagnetic Coupled Trinuclear Cu(II) Complex

Magnetization measurements and X-band EPR investigations on the trimeric exchange coupled Copper(II)-complex [(Cu3(C6H5CO2)4{(C)2NC2H4O}2(CH3OH)2] are reported. The field dependence of the magnetization at 3.7 K is consistent with a quartet ground state and proofs that ferromagnetic exchange coupling is operative between adjacent Cu(II) – centres in the trimeric molecule. – A strong temperature dependent variation of the g value from geff ≈ 4.3 at liquid helium temperature to geff ≈ 2.6 at room temperature was observed in the EPR experiment. Different explanations for the g shift are discussed.

Ferromagnetic-bilayer spin-wave resonance: Reassignment of interface-affected lines.

A microscopic theory of standing spin-wave resonance in exchange-coupled bilayer films is presented. The interface-inhomogeneity model (an effective interface exchange coupling and intrinsic uniaxial interface anisotropy parameters are introduced) and the Heisenberg model with spin (exchange and Zeeman) Hamiltonian are used to derive an energy dispersion relation and spin-wave-mode functions for exchange-coupled bilayer ferromagnetic films. The theory is valid for Bravais lattices with arbitrary film-surface or interface orientations in a configuration where the static external field is perpendicular to the film surface and the values of the ferromagnetic and antiferromagnetic interface exchange couplings are arbitrary. The analysis is based on the exact solution of the eigenvalue problem of a bilayer film achieved with the interface rescaling approach. It is shown that the pattern of the resonance spectrum is determined by the nature of the interface exchange-coupling integral ${\mathit{J}}^{\mathit{A}\mathit{B}}$, with the first two (high-field side) lines exhibiting pronounced intensities when the interface coupling is antiferromagnetic. We show moreover that the highest-field line arises by excitation of the mode that is localized on the bilayer interface. We also show that this mode represents a complex bilayer vibration formed by the in-phase combination of sublayer vibrations, and this result corrects the general belief of experimenters that the mode under consideration is due to an out-of-phase combination of sublayer vibrations.

ESR investigation of some copper(II)-dioxime-dichloride compounds

The following copper(II)-dioxime-dichloride compounds: [Cu(G)Cl2]2, [Cu(P)Cl2]2, [Cu(DMG)Cl2]2 and [Cu(O)Cl2]2 (where G=C2H4O2N2, P=C3H6O2N2, DMG=C4H8O2N2, O=C8H14O2N2) were investigated by ESR method. Spectra of powder samples obtained in the g≃2 region suggest the presence of triplet ground state (S=1) realized by a weak ferromagnetic exchange coupling. In liquid and frozen solutions the monomeric species (S=1/2) prevail. Some delicate changes in the coordination polyhedra symmetry in terms of ligand molecules and solvents nature were evidence. A 4p-admixture degree of 2% in the dxy ground state was estimated for pseudotetrahedral (Td) species in frozen solutions.

Spin engineering: Direct determination of the Ruderman-Kittel-Kasuya-Yosida far-field range function in ruthenium.

Oscillations in both ferromagnetic and antiferromagnetic indirect exchange coupling through Ru are directly measured. Ferromagnetic coupling is determined by spin engineering multilayered structures comprised of trilayers of ${\mathrm{Ni}}_{80}$${\mathrm{Co}}_{20}$/Ru/${\mathrm{Ni}}_{80}$${\mathrm{Co}}_{20}$ in which one of the ${\mathrm{Ni}}_{80}$${\mathrm{Co}}_{20}$ layers is strongly antiferromagnetically pinned to a Co layer. Oscillations in antiferromagnetic exchange coupling are measured in companion ${\mathrm{Ni}}_{80}$${\mathrm{Cu}}_{20}$/Ru multilayers. The dependence of the exchange coupling strength on Ru thickness is well described by the Ruderman-Kittel-Kasuya-Yosida far-field range function, providing a direct measurement of this function through a transition metal.

Magnetic properties of the quasi-1D compounds M 2Cu 2O 5 with M = Y, In, Sc or a rare earth

We report on the magnetostructural correlation of the oxocuprate series M 2Cu 2O 5 (M is a trivalent cation). In the case where M = Y 3+, Lu 3+, In 3+ or Sc 3+ the magnetic properties are discussed on basis of a Heisenberg J-alternating chain model. The influence of neighboring chains is introduced as a mean-field effect. The ferromagnetic exchange coupling constants are shown to depend strongly on the magnitude of the Cu-O-Cu angle as well as the Cu-O bond lengths in the superexchange pathways.

Isolated ferromagnetic bonds in the two-dimensional spin-1/2 Heisenberg antiferromagnet (abstract)

Several mechanisms for high-temperature superconductivity invoke the strong antiferromagnetic correlations within the CuO planes. In particular, properties of the high-Tc compounds are believed to be related to defects in the planes, e.g., static vacancies, ferromagnetic bonds, and mobile holes. We consider the effects of isolated ferromagnetic links on an otherwise antiferromagnetic square lattice. The addition of holes in La2CuO4 (by doping with Sr) introduces a local ferromagnetic exchange coupling between Cu spins. We describe the Heisenberg antiferromagnet within the linearized spin-wave theory; the problem of a single ferromagnetic bond embedded in an otherwise antiferromagnetic square lattice can then be solved exactly. The procedure is similar to the one employed in Ref. 1 for a vacancy. The longitudinal terms, involving Sz, tend to enhance the ordered staggered magnetic moment, competing in this way with the transverse terms, involving Sx and Sy, which represent the quantum fluctuations and suppress the sublattice magnetization. The interplay between these two interactions is discussed as a function of the ferromagnetic coupling strength.

Magnetic properties of spin-labelled polyacetylene

Polyacetylene has been chemically modified by the incorporation of spin-labelled moieties. The magnetic properties were characterized by static magnetic susceptibility measurements and ESR spectroscopy. In spite of the very low spin concentrations of about 10 −3 per CH unit employed, magnetic spin-spin interactions were observed. Magnetically correlated units consisting at least of pairs of spins with ferromagnetic exchange coupling ( J/ k B ⩾ 8 K) were found in galvinoxyl-modified polyacetylene.

Amorphous oxides with ferromagnetic exchange coupling and optical transparency

0.5La1−xSrxMnO3⋅0.5B2O3 (0.2≤x≤1.0) oxides were melt quenched using a twin-roll technique. The resultant ribbons were amorphous for thicknesses below 30 μm, which was confirmed by x-ray diffraction, anomalous x-ray scattering, and transmission electron microscopy. They were characterized by high crystallization temperatures above 920 K and optical transparency. Their Curie temperatures are higher than 380 K over a wide range of Sr concentrations (0.3≤x≤1.0), thus differing from crystalline La1−xSrxMnO3 perovskites. Magnetic susceptibility above the Curie temperature follows the Curie–Weiss law with a positive paramagnetic Curie temperature, suggesting ferromagnetic spin coupling in the present amorphous oxides. The ferromagnetic behavior of the amorphous state may be explained by positive double-exchange interactions of Mn3+ and Mn4+ ions.

Magnetic static and scaling properties of the weak random-axis magnet (DyxY1-x)Al2.

The effects of a random component of the magnetocrystalline anisotropy on the magnetic properties and critical behavior of polycrystalline ${\mathrm{DyAl}}_{2}$ have been investigated using dc magnetic measurements. Random magnetic anisotropy (RMA) is produced by site-diluting ferromagnetic ${\mathrm{DyAl}}_{2}$ with the nonmagnetic, isomorphic intermetallic ${\mathrm{YAl}}_{2}$. Dilution distorts the cubic Laves-phase unit cell because of a slight lattice mismatch thereby lowering the local crystal symmetry in a random fashion. Additional contributions to the RMA come from spin-orbit scattering by the conduction electrons. Hysteresis loops display little remanence and very small coercive fields, suggesting a weak RMA. This is consistent with estimates of the RMA strength D obtained using an approach of Chudnovsky et al. The magnetization at high temperatures (T>4${\mathit{T}}_{\mathit{c}}$) is well described by a Curie-Weiss law. The paramagnetic Curie temperatures are positive, implying an average ferromagnetic exchange coupling between Dy ions, and increase with x. The paramagnetic moment shows no evidence of quenching across the series, thus confirming the well-localized nature of the 4f electronic orbitals. Low-field thermal scans of the bulk dc magnetization show no sign of a spontaneous moment for Dy concentrations 0.10\ensuremath{\le}x\ensuremath{\le}0.90, yet a sharp increase in the magnetization occurs at a temperature ${\mathit{T}}_{\mathit{c}}$ that increases with x. A ferromagnetic scaling analysis applied to the line of transitions at ${\mathit{T}}_{\mathit{c}}$ results in a surprisingly good collapse of the magnetization data. By extension of prior theoretical work of Aharony and Pytte, a direct connection can be made between pure and RMA exponents, which gives remarkable agreement with the experimental values.

Chemical Principles of Designing Organometallic Ferromagnets

The ferromagnetic exchange coupling in the mixed-bridged system Cu(II)-O/CO 2 -Cu(II) is investigated using SCF/CI calculations on dimeric model compounds. Starting form the geometry of the Cs-symmetric model molecule [(H 2 O)(CH 3 O)Cu(μ-O-CH 3 O)(μ-O,O'-HCO 2 )Cu(H 2 O)(CH 3 O)], M1 with non-co-planar copper coordination planes, series of model molecules with varied geometries were developed and the singlet-triplet-splitting, 2J calc , calculated. The influences of following parameters were studied : the dihedral angle φ AB between the copper coordination planes, 0°≤φ AB ≤90°, the copper-μ-oxygen distance, 1.79 A≤r Cu-O ≤2.09 A, and the replacement of the strongest ferromagnetic coupling is expected for φ AB ∼ 50 ± 10°, short Cu-O distances, and O-ligands instead of N-ligands. For planar geometries (φ AB ∼ O) antiferromagnetic coupling is possible dependent on the Cu-O distance

Optimization of Ferromagnetic Coupling in O/CO2-Bridged Copper(II) Oligomers

Abstract The ferromagnetic exchange coupling in the mixed- bridged system Cu(II)-O/CO2-Cu(II) is investigated using SCF/CI calculations on dimeric model compounds. Starting from the geometry of the C5-symmetric model molecule [(H2O)(CH3O)Cu(μ-O-CH3O)(μ-O,O′-HCO2)Cu(H2O)(CH3O):, M1, with non-co-planar copper coordination planes, series of model molecules with varied geometries were developed and the singlet-triplet-splitting, 2Jcalc, calculated. The influences of following parameters were studied: the dihedral angle ϕAB between the copper coordination planes, O°≤ABϕ 90°, the copper-μ-oxygen distance, 1.79 A ≤ rCu-O ≤ 2.09 A, and the replacement of the H2O ligands vs. NH3 ligands. The optimized geometry for the strongest ferromagnetic coupling is expected for ϕAB∼50±10°, short Cu-O distances, and O-ligands instead of N-ligands. For planar geometries (ϕAB∼O) antiferromagnetic coupling is possible dependent on the Cu-O distance.

A binuclear Ni(II) complex with 2,2′:6′,2″-terpyridine exhibiting ferromagnetic exchange coupling

A binuclear Ni(II) complex with 2,2′:6′,2″-terpyridine exhibiting ferromagnetic exchange coupling

Spin glass behaviour of [formula omitted]Fe alloys

Low-field (2–100 Am−1) ac magnetic susceptibility measurements (T = 4.2–70 K) on a set of Cu99−xAuxFe1 alloys (x=12, 37, 50.7 at%) show cusp-like peaks in susceptibility indicating spin glass behaviour. The freezing temperature are in the range Tf≈6−8 K. The susceptibility data are found to deviate from extrapolated high temperature (T ≥ 3Tf) Curie-Weiss-like behaviour in a manner consistent with the existence of couplings of a predominantly antiferromagnetic nature in Cu62Au37Fe1 and of a ferromagnetic nature in the expanded lattice of Cu48.3Au50.7Fe1. The Cu87Au12Fe1 sample exhibits behaviour consistent with a balance of antiferromagnetic and ferromagnetic exchange couplings.

Influence of coercivity squareness on media noise in thin-film recording media

Signal and media noise measurements have been made for a wide variety of Co-alloy longitudinal magnetic recording media. A strong correlation between media noise and the coercivity squareness, S*, is observed at high transition density. The optimum ratio of isolated pulse amplitude to media noise is found for S*≂0.75. This behavior arises from reduced ferromagnetic exchange coupling between neighboring grains of the film which influence both the coercivity squareness and also the uniformity of the domain boundary at the recorded transitions. The low-noise characteristics of sputtered γ-Fe2O3 also appear to correlate with low values of coercivity squareness. This work confirms that for systems in which media noise is a limiting factor, optimum performance may require considerably lower values of coercivity squareness than commonly used, determined by the relative contribution of media noise to the overall system noise.

MAGNETISM AND SUPERCONDUCTIVITY IN DOPED PLANAR CuO2 SYSTEMS

We present a model for the magnetic phases and superconductivity in doped planar CuO 2 systems. Electronic holes on the oxygens introduce local ferromagnetic exchange coupling between the Cu spins. The resulting frustration destroys the antiferromagnetic state characterizing the undoped planes, and generates a new spin glass phase. This frustration also yields an attractive interaction between the holes, whose range decreases with increasing doping. Arguing that transport is via the non-bonding in-plane oxygen orbitals, we use the BCS approximation to obtain an excellent estimate of the superconducting transition temperature T c (x) , for La 2−x Sr x CuO 4.

Magnetic phase diagram and magnetic pairing in doped La2CuO

We discuss the temperature-concentration phase diagram of doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. The addition of holes introduces a local ferromagnetic exchange coupling between Cu spins. The resulting frustration destroys the 3D N\'eel state characterizing pure ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, and generates a new 3D spin-glass phase. In the paramagnetic phase, strongly correlated Cu spins in the planes are canted by the holes, yielding an oscillating dipole-dipole attraction between holes. The possible relevance to superconductivity is discussed.

Magnetic properties of (C6H5CH2C4H11N2Cl)2CuCl4 and (CH2OHCH2NH3)2CuCl4

The two title compounds are two-dimensional magnetic systems with ferromagnetic interlayer exchange coupling. Magnetic susceptibility measurements ferromagnetic interlayer exchange coupling. Magnetic susceptibility measurements yield J/k=4.59 and 13.0 K for the two compounds, respectively. A smooth correlation between J and the intralayer Cu-Cu distance is observed. The latter compound orders at TN∼8 K, while no evidence for long-range order in the former is observed down to 1.4 K. The electron paramagnetic resonance linewidths of (C6H5CH2C4H11N2Cl)2CuCl4 are abnormally large with a dominant cos2 θ angular dependence. The assumption that the linewidths are dominated by spin anisotropies leads to an estimate of ‖dz/J‖≂0.14.

Deviations from Curie-Weiss behavior in spin-glasses

The magnetic susceptibility in the spin-glass concentration regime of amorphous alloys ${({\mathrm{Fe}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x})}_{75}{\mathrm{P}}_{16}{\mathrm{B}}_{6}{\mathrm{Al}}_{3}$ with $x=0.04, 0.06, \mathrm{and} 0.08$ has been measured using a superconducting quantum interference detector magnetometer with an external field of 5 Oe for temperatures $5<~T<~60.5$ K, and a Faraday balance with an external field of 5 kOe for $4.3<~T<~300$ K. We observe a high-temperature ($T>20{T}_{f}$) Curie-Weiss law with positive values for the paramagnetic Curie temperature, indicating predominantly ferromagnetic exchange couplings. At temperatures approaching the spin-glass freezing temperature, however, there are strong deviations from Curie-Weiss behavior. A simple theory is developed which allows a determination of the angular spin correlations in the spin-glass ground state from these deviations. The number of such correlated spins (1.5 to 4.4) determined from our data compares well with computer-simulation studies by Walker and Walstedt as well as data obtained by other authors for Au-Fe, Cu-Mn, and Co- and Mn-aluminosilicate spin-glasses.

Magnetic properties of a random anisotropy model with both ferromagnetic and antiferromagnetic exchange coupling

The model of Harris et al. for random magnetic anisotropy is examined for the case of nearest neighbour antiferromagnetic and next-nearest neighbour ferromagnetic exchange coupling. Initial magnetization curves, critical fields for the metamagnetic transition, and hysteresis loops were obtained for a variety of parameters using the molecular field approximation and were compared with previous work. It is shown that the metamagnetic transition becomes a discontinuous transition and that the hysteresis loops change their shape considerably in the presence of ferromagnetic exchange. The results are discussed in the light of experimental work.