Showing papers on "Superexchange published in 1975"
TL;DR: In this article, the relationship between the exchange constants and the bond lengths in the series of insulating antiferromagnetic compounds XMF3 and X2MF4 is investigated.
Abstract: In the series of insulating antiferromagnetic compounds XMF3 and X2MF4 (X=K, Rb, Tl; M=Mn, Co, Ni) the superexchange paths connecting nearest magnetic neighbours are identical collinear (180°) M-F-M bonds. The available experimental data on the exchange constants, J/k, and the bond lengths, R, are reviewed. They enable one to give an estimate of the dependence of J/k on R, as well as the ratio of the J/k values of Mn2+, Co2+, and Ni2+ for a fixed bond length. Assuming a power-law dependence, |J/k|∼R−n, our analysis yields n≃12 for Mn2+, Co2+ as well as Ni2+, in the R range covered experimentally. The ratio of the J/k values of Mn2+, Co2+, and Ni2+ at fixed R=4.074 A is found to be 1:3.6:7.7. The experimental findings are compared with theoretical calculations based upon a recently proposed exchange-perturbation treatment for the cation-anion-cation unit in a three-centre four-electron model. The unpaired electrons on either cation are replaced by one effective electron and the closed-shell electrons of the anion by two spin-paired electrons. Qualitatively, the agreement with the experiment is good in that the model reproduces correctly the observed R dependences as well as the ratio of the J/k's of the three different cations. Also in quantitative respect a close agreement with the experimental J/k values can be obtained within the uncertainties inherent to the effective-electron model.
65 citations
TL;DR: In this article, the authors measured the phosphorescence of betamethylnaphthalene doped into a naphalene−h 8/naphthallene−d 8 mixed crystal and demonstrated that dynamical exciton percolation does occur.
Abstract: The phosphorescence of betamethylnaphthalene doped into a naphthalene−h 8/naphthalene−d 8 mixed crystal has been measured. The results demonstrate that (1) dynamical excitonpercolation does occur (i.e., a transition from an excitoninsulator to an exciton conductor), that (2) it is very useful for the investigation of energy transfer in molecular aggregates, and that (3) it is a critical test of our current knowledge of exciton exchange and superexchange. (AIP)
57 citations
TL;DR: In this paper, the properties of spinels with the formula Ga 0.67 Mo 2 S 4, Al 0.75 Mo 2S 4, Ga 0,67 Cr 2 S4, and Ga 0.,5 V 2S4 have been investigated and the magnetic porperties are explained in terms of B site ferro and antiferromagnetic superexchange interactions.
54 citations
TL;DR: In this paper, the spin-only super-exchange interaction integral for an arbitrary 3D cation, pair is predicted and the magnitude and the sign of the spin only super exchange integral for any 3d cation pair are also calculated.
Abstract: The super-exchange interaction parametersI(Fe, Cr) of Fe3+ and Cr3+ in iron doped rate-earth orthochromitesRCr0.99Fe0.01 O3 (R=La, La0.5Nd0.5, Nd, Sm, Gd, Dy, Y, Er, Yb or Lu) have been obtained from57Fe magnetic hyperfine structure measurementsvia the Mossbauer effect. The dependence of the experimental valuesI(Fe, Cr) on the Fe−O−Cr average superexchange angle θ (depending upon the relative size of the rare-earth (RE) ionR
3+) is well described by the equationI(d5, d
N
) = α
N
+ β
N
cosθ + γ
N
cos2 θ. Within the accessible range of super-exchange angles 142°≲θ≲156°, the Fe3+−O2−−Cr3+ interaction is negative (antiferromagnetic). However, a theoretical analysis predicts a sign reversal forI(Fe, Cr.) at about θ ≈ 162° and thus ferromagnetic character of the interaction between 162° and 180°. The spin-only super-exchange interaction integrals fore
g andt
2g electrons, separately, are also calculated. Their angular dependence is accounted for by the behaviour of the antiferromagnetic kinetic and ferromagnetic potential exchange which are of different character when passing from 180° to 90° super-exchange geometry. The magnitude and the sign of the spin-only super-exchange integral for an arbitrary 3d cation, pair is predicted.
53 citations
TL;DR: The linear chain of classical Heisenberg spins coupled to phonons is studied in this article, where the phonons lead to an effective biquadratic interaction between spins, with important physical consequences.
Abstract: The linear chain of classical Heisenberg spins coupled to phonons is studied. The coupling arises from the phonon-modulation of the nearest-neighbor exchange integral and the cases of both direct exchange and superexchange are treated. The phonons lead to an effective biquadratic interaction between spins, with important physical consequences. Thermodynamic functions at finite temperatures and zero field are evaluated exactly and discussed. It is shown that antiferromagnetically coupled spins in an external field at $T=0\ifmmode^\circ\else\textdegree\fi{}$K can undergo a first-order phase transition; as the field crosses a critical value, the magnetization and lattice spacing change discontinuously.
26 citations
TL;DR: In this article, the tensor operator formalism is used for the superexchange interaction between orbitally degenerate ions in magnetic insulators, and the exchange Hamiltonian is derived for ions described in different representations.
Abstract: The problem of the superexchange interaction between orbitally degenerate ions in magnetic insulators is considered using the tensor operator formalism. The exchange Hamiltonian is derived for ions described in different representations. The number of parameters can be substantially reduced by symmetry considerations and by specifying the exchange mechanism as it is shown on the example of KCoF3.
15 citations
TL;DR: In this article, the anomalous increase in the hyperfine field is attributed to the change in the supertransferred hyperfine interaction caused by the replacement of one Fe 3+ ion with a Cr 3 + ion at the nearest neighbor site of 57 Fe 3+, and the anomaly arises from the concentration dependence of the zero point spin reduction at 57 Fe.
Abstract: Mossbauer experiment has been performed in TbFe 1- x Cr x O 3 . Hyperfine field at 57 Fe nuclei is 550 kOe at 0 K for x =0, and it decreases by 9 kOe when one Fe 3+ ion is replaced by a Cr 3+ ion at the nearest neighbor site of 57 Fe 3+ ion. This decrease is attributed to the change in the supertransferred hyperfine interaction caused by the replacement. Temperature dependence of the hyperfine field is explained by a modified molecular field theory for various concentrations. In the Cr 3+ concentration higher than x =0.8, anomalous increase is observed in the hyperfine field. The anomaly arises from the concentration dependence of the zero-point spin reduction at 57 Fe. The positive superexchange coupling of Fe 3+ –O–Cr 3+ gives a reasonable explanation on the magnitude of the anomaly.
15 citations
TL;DR: In this article, an examination of superexchange paths suggests that Co4(C5H7O2)8 is antiferromagnetic in type and has values in the region of Jnn1 ∼ 5 K, Jnn2 ∼ 10 K, and Jnnn3 ∼ 4 K.
Abstract: According to x‐ray studies, Co4(C5H7O2)8 is an assembly of linear, tetrameric Co2+ ion clusters, which are effectively magnetically isolated. An examination of superexchange paths suggests that this tetramer has two different kinds of nearest‐neighbor exchange constants plus a next‐nearest‐neighbor exchange constant. The only bulk experimental data available so far are the powder susceptibilities from room temperature down to about 0.35 K. This paper demonstrates that this information is apparently sufficient to infer that all three exchange constants are antiferromagnetic in type and have values in the region of Jnn1 ∼ 5 K, Jnn2 ∼ 10 K, and Jnnn3 ∼ 4 K. The interactions therefore compete; and they compete so as to give a paramagnetic character to the low temperature behavior. This is in interesting contrast to the behavior of a sister compound, trimeric nickel (II) acetylacetonate, where the paramagnetic character results from a dominant nearest‐neighbor ferromagnetic exchange constant. The exchange cons...
11 citations
TL;DR: In this paper, the configuration interaction method is applied to the exchange interaction and the orbit-lattice interaction is included in the one-electron Hamiltonian and a contribution is obtained which is antiferromagnetic in the V2+-F--V2+ linear pair in KMgF3.
Abstract: The configuration interaction method is applied to the exchange interaction. The orbit-lattice interaction is included in the one-electron Hamiltonian and, to second order in this Hamiltonian, a contribution is obtained which is antiferromagnetic in the V2+-F--V2+ linear pair in KMgF3. The zero-point vibrational contribution to the superexchange is discussed as well as the temperature-dependent contribution.
8 citations
TL;DR: In this article, the superexchange in V 2+ O 2− ǫV 2+ pair in MgO is calculated including the orbit-lattice interaction into the one-electron.
7 citations
TL;DR: In this paper, the authors analyzed the stepwise magnetization process in an antiferromagnetic Ising Dy 3+ spin system of DyCrO 3, and showed that the existence of magnetic Cr 3+ ion plays an important role in the step-wise magnetisation process.
Abstract: Magnetization process is analysed in an antiferromagnetic Ising Dy 3+ spin system of DyCrO 3 . The existence of magnetic Cr 3+ ion plays an important role in the stepwise magnetization process of DyCrO 3 , compared with the one-step magnetization process of DyAlO 3 which has no magnetic transition-metal ion. The indirect interaction between Dy 3+ spins induced through the Cr 3+ -Dy 3+ exchange interaction, together with the superexchange interaction between Dy 3+ spins, possibly makes the magnetization change stepwise. A future measurement of the magnetoelectric effect would determine the proposed spin-configuration in the intermediate field range.
TL;DR: In this paper, it was shown that close to 50% of the high-spin ions in this compound form isolated spin pairs each of which shows a highly anisotropic superexchange coupling.
Abstract: Magnetic susceptibility, heat capacity, and EPR studies reported here show that close to 50% of the high-spin ${\mathrm{Fe}}^{3+}$ ions in this compound form isolated spin pairs each of which shows a highly anisotropic superexchange coupling. Direct EPR transitions between the superexchange-split energy levels have been observed.
TL;DR: In this article, the unusual EPR spectrum of copper-doped triglycine fluoroberyllate is shown to originate in large complcx molecules of the hypothetical formula [Cu2+·(NH2CH2COO−)2·H2BeF4]4, built into the distorted host lattice.
Abstract: The unusual EPR spectrum of copper-doped triglycine fluoroberyllate is shown to originate in large complcx molecules of the hypothetical formula [Cu2+·(NH2CH2COO−)2·H2BeF4]4, built into the distorted host lattice. Superexchange couplings of Cu2+ ions in the cluster through CuFBeFCu paths are discussed. The isotropic superexchange interactions split the 16-fold degenerate ground crystal state of the four Cu2+ ions into multiplets the further splitting of which, originated by anisotropic interactions, can be described in terms of effective spin Hamiltonians with S = 2 and 1. The observed spectrum is attributed to transitions within the quintet and one of the triplets. By second-order perturbation procedure, the phenomenological parameters of the effective spin Hamiltonians are expressed in terms of parameters directly describing anisotropic as well as isotropic superexchange interactions in the cluster. The magnitudes of these interactions are thus determined from experimental data.
Es wird gezeigt, das das unubliche EPR-Spektrum von kupferdotiertem Triglycinflnoroberyllat von grosen komplexen Molekulen mit der hypothetischen Formel[Cu2+·(NH2CH2COO−)2·H2BeF4]4, herruhrt, die in das gestorte Wirtsgitter eingebaut sind. Superaustauschkopplung der Cu2+-Ionen uber den Weg CuFBeFCu wird diskutiert. Die isotrope Superaustauschwechselwirkung spaltet die 16-fache Entartung des Kristallgrundzustandes der vier Cu2+-Ionen in Multipletts auf, deren weitere, durch anisotrope Wechselwirkungen hervorgerufene Aufspaltungen, durch Terme effcktiver Spin-Hamiltonoperatoren mit S = 2 und 1 beschrieben werden konnen. Das beobachtete Spektrum wird Ubergangen innerhalb des Quintetts und des Tripletts zugeschrieben. Durch Storungsrechnung zweiter Ordnung werden die phanomenologischen Parameter des effektiven Spin-Hamiltonoperators durch Parameter ausgedruckt, die direkt sowohl die isotropen als auch die anisotropen Superaustauschwechselwirkungen im Cluster beschreiben. Die Grosen diescr Wechselwirkungen lassen sich somit aus den experimentellen Daten bestimmen.
TL;DR: In this article, the superexchange interaction parameter for the next-nearest neighbouring pair of magnetic ions in EuO, connected by the 180° exchange interaction has been calculated according to the method of configuration interaction.
Abstract: The superexchange interaction parameter for the next-nearest neighbouring pair of magnetic ions in EuO, connected by the 180° exchange interaction has been calculated according to the method of configuration interaction. The effect of the non-homogeneous potential created by the crystal ions in the region of the chosen complex “metal—ligand-metal” is taken into account. Satisfactory agreement with experimental data is obtained.
[Russian Text Ignored].
TL;DR: The mean magnetic susceptibility anisotropies in three different planes of copper bispyridine dibromide crystal show anomalous behavior around 278K as discussed by the authors. But, this is not the case for all the planes of the bromide complex.
Abstract: The mean magnetic susceptibility anisotropies in three different planes of copper bispyridine dibromide crystal show anomalous behaviour around 278K. The room temperature mean magnetic moment mu 2=1.77 beta (where beta is the Bohr magneton) is not very different from the spin-only value 1.73 beta . The ionic magnetic ellipsoid is found to be on oblate spheroid that is Kperpendicular to >K/sub ///. In this case, theoretically, g/sub ///=2 so that K/sub ///=1249*10-6 at 300K, whereas the measured value is found to be much smaller, namely lambda /sub ///=1084*10-6. The polymeric chain structure of the adjacent Cu2+ ions formed by the sharing of the bromine ions, the ESR reports on isomorphous salt Cu(C5H5N)2Cl2 and the low susceptibility value suggest the presence of an appreciable superexchange interaction in the complex. Following Bleaney and Bowers (1952) expression for the susceptibilities are deduced, including the exchange interaction for this complex having a ground state 2A1g.
TL;DR: In this article, the authors measured the phosphorescence of betamethylnaphthalene doped into a naphalene−h 8/naphthallene−d 8 mixed crystal and demonstrated that dynamical exciton percolation does occur.
Abstract: The phosphorescence of betamethylnaphthalene doped into a naphthalene−h 8/naphthalene−d 8 mixed crystal has been measured. The results demonstrate that (1) dynamical excitonpercolation does occur (i.e., a transition from an excitoninsulator to an exciton conductor), that (2) it is very useful for the investigation of energy transfer in molecular aggregates, and that (3) it is a critical test of our current knowledge of exciton exchange and superexchange. (AIP)
TL;DR: In this article, a calculation procedure is proposed for determining the values of exchange parameters, applicable to spinels which contain three kinds of ions in tetrahedral positions and which have an arbitrary distribution of cations.
Abstract: A calculation procedure is proposed here for determining the values of exchange parameters, applicable to spinels which contain three kinds of ions in tetrahedral positions and which have an arbitrary distribution of cations. The values of exchange integrals obtained here for ferrites of the manganese-zinc system provide a useful basis for a quantitative interpretation of the spread of Curie-temperature readings in experiments which involve ferrites with the same composition.