Antiferromagnetic Perovskites Research Articles

Electronic structure of antiferromagneticLaMnO3and the effects of charge polarization

The electronic structure of $A$-type antiferromagnetic insulator $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ is investigated by the GW approximation. The band gap and spectrum are in a good agreement with experimental observation. The lifetime of electrons in conduction bands is much shorter than that of holes in valence bands. The insulator-to-metal transition with antiferromagnetic-to-ferromagnetic transition with photocarrier injection is attributed to the characteristic properties of excited electron states in $A$-type antiferromagnetic perovskite systems. The onsite $d\text{\ensuremath{-}}d$ Coulomb interaction is strongly screened at the low energy region by mobile ${e}_{g}$ electrons.

Probing structural inhomogeneities induced by exchange striction aboveTNin antiferromagnetic perovskites

Measurements of the temperature dependence of the thermal conductivity $\ensuremath{\kappa}(T)$ of the antiferromagnetic perovskites ${\mathrm{SmNiO}}_{3},$ ${\mathrm{CaMnO}}_{3},$ ${\mathrm{LaMnO}}_{3},$ and ${\mathrm{YCrO}}_{3}$ are compared to that of the nonmagnetic perovskite ${\mathrm{LaGaO}}_{3}.$ Whereas ${\mathrm{LaGaO}}_{3}$ exhibits a classic phonon thermal conductivity, the others all show a remarkable suppression of $\ensuremath{\kappa}(T)$ in the paramagnetic phase that increases with the degree of covalence in the $M--\mathrm{O}$ bond. This suppression is attributed to exchange-strictive fluctuations that are enhanced by semicovalent-exchange interactions.

Control of magnetic ordering by Jahn--Teller distortions in Nd(2)GaMnO(6) and La(2)GaMnO(6).

The substitution of Ga(3+) into the Jahn--Teller distorted, antiferromagnetic perovskites LaMnO(3) and NdMnO(3) strongly affects both the crystal structures and resulting magnetic ordering. In both compounds the Ga(3+) and Mn(3+) cations are disordered over the six coordinate sites. La(2)GaMnO(6) is a ferromagnetic insulator (T(c) = 70 K); a moment per Mn cation of 2.08(5) mu(B) has been determined by neutron powder diffraction at 5 K. Bond length and displacement parameter data suggest Jahn--Teller distortions which are both coherent and incoherent with the Pnma space group symmetry of the perovskite structure (a = 5.51122(4) A, b = 7.80515(6) A, c = 5.52947(4) A) at room temperature. The coherent distortion is strongly suppressed in comparison with the parent LaMnO(3) phase, but the displacement ellipsoids suggest that incoherent distortions are significant and arise from local Jahn--Teller distortions. The preparation of the new phase Nd(2)GaMnO(6) has been found to depend on sample cooling rates, with detailed characterization necessary to ensure phase separation has been avoided. This compound also adopts the GdFeO(3)-type orthorhombically distorted perovskite structure (space group Pnma, a = 5.64876(1) A, b = 7.65212(2) A, c = 5.41943(1) A at room temperature). However, the B site substitution has a totally different effect on the Jahn--Teller distortion at the Mn(3+) centers. This phase exhibits a Q(2) mode Jahn--Teller distortion similar to that observed in LaMnO(3), although reduced in magnitude as a result of the introduction of Ga(3+) onto the B site. There is no evidence of a dynamic Jahn-Teller distortion. At 5 K a ferromagnetically ordered Nd(3+) moment of 1.06(6) mu(B) is aligned along the y-axis and a moment of 2.8(1) mu(B) per Mn(3+) is ordered in the xy plane making an angle of 29(2) degrees with the y-axis. The Mn(3+) moments couple ferromagnetically in the xz plane. However, along the y-axis the moments couple ferromagnetically while the x components are coupled antiferromagnetically. This results in a canted antiferromagnetic arrangement in which the dominant exchange is ferromagnetic. Nd(2)GaMnO(6) is paramagnetic above 40(5) K, with a paramagnetic moment and Weiss constant of 6.70(2) mu(B) and 45.9(4) K, respectively. An ordered moment of 6.08(3) mu(B) per Nd(2)GaMnO(6) formula unit was measured by magnetometry at 5 K in an applied magnetic field of 5 T.

A quantum mechanical investigation of the electronic and magnetic properties of perovskite

The ground-state electronic structures of ferromagnetic and antiferromagnetic perovskite in the ideal cubic phase have been investigated using the ab initio periodic Hartree - Fock approach. The system is a wide-gap insulator. The antiferromagnetic phase is correctly predicted to be the more stable (0.07 eV per Mn atom at the equilibrium geometry), but the superexchange interaction is substantially overestimated. The energy difference between the two phases increases slowly and linearly under compression, at variance with that for , that shows an exponential behaviour when the M - M distance is reduced. As regards the electronic structure, three unpaired electrons occupy very localized -type d orbitals on Mn. About 1.8 electrons (according to a Mulliken partition scheme for the electronic charge) occupy Mn -type states ( and Mn d orbitals), which however overlap significantly with oxygen p orbitals; the degree of spin polarization of these bond states is very low. The electronic structure of the system is discussed in terms of the density of states and charge- and spin-density maps.

Surface and bulk spin waves in antiferromagnetic perovskites and their interaction with defects near surfaces

The effect of a single magnetic impurity on the excitation spectrum of a semi-infinite Heisenberg antiferromagnet is studied by Green-function methods. The influence of bulk and surface spin waves on the impurity-mode energies is determined, and the theory is applied to perovskite-structure antiferromagnets.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pairing forces of magnetic origin in antiferromagnetic perovskites

The antiferromagnetic Heisenberg model is extended to account for the presence of holes. The holes move along a sublattice whose sites are located in between the spin sites. The spin-hole coupling arises from the modification of the exchange interaction between two neighboring spins when the site between them is occupied by a hole. The use of a recently developed method that introduces spin-0 excitations for dealing with the Heisenberg antiferromagnetic model allows to map the model Hamiltonian onto a Fröhlich one, with the spin-0 magnetic excitations substituting phonons. This way the BCS mechanism of attraction is recovered in a context in which the charge carriers move in an antiferromagnetic environment. The case of electrons moving through the spin lattice is discussed as well.

Supertransferred hyperfine interaction: Perturbed angular correlation ofCdm111in antiferromagnetic NiO, CoO, and MnO

The time-differential perturbed angular correlation of $^{111}\mathrm{Cd}^{m}$ substituted as a dilute impurity into antiferromagnetic NiO, CoO, and MnO has been observed. The following magnetic fields are found at the Cd nucleus (4\ifmmode^\circ\else\textdegree\fi{}K): NiO (191.1 \ifmmode\pm\else\textpm\fi{} 2.5 kOe), CoO (170.8 \ifmmode\pm\else\textpm\fi{} 3.0 kOe), MnO (194.7 \ifmmode\pm\else\textpm\fi{} 2.5 kOe). They are compared with the Cd hyperfine fields in the antiferromagnetic perovskites KNi${\mathrm{F}}_{3}$:Cd, KCo${\mathrm{F}}_{3}$:Cd, and RbMn${\mathrm{F}}_{3}$:Cd reported earlier. The oxides are found to be more covalent than the fluorides. From the ratio of the hyperfine fields in MnO:Cd and NiO:Cd a new value of the spin density parameter ${f}_{\ensuremath{\sigma}}(\mathrm{Mn}\ensuremath{-}\mathrm{O})=8.1%$ has been estimated.

Stress induced linear dichroism of antiferromagnetic perovskites : KNif3, KCoF3 and RbCoF3. I. — Magnetic origin and evidence for domain twin-walls movements

Stress induced linear dichroism of antiferromagnetic perovskites : KNif3, KCoF3 and RbCoF3. I. — Magnetic origin and evidence for domain twin-walls movements

Stress induced linear dichroism of antiferromagnetic perovskites : KNiF 3, KCoF3 and RbCoF3. II. —Spectroscopic calculations and assignments

Stress induced linear dichroism of antiferromagnetic perovskites : KNiF 3, KCoF3 and RbCoF3. II. —Spectroscopic calculations and assignments

Extended Theory of Magnon Sideband Shapes. II. Impure Three-dimensional Antiferromagnets

A phenomenological model is given for calculating magnon sideband lineshapes in the absorption spectra of antiferromagnetic crystals. The method allows an exact expression for the lineshape to be obtained when dispersion within the Brillouin zone of both the exciton energy and the exciton-magnon interaction strength is included. The effect of a substitutional spin impurity is calculated exactly, and a simple example is given which approximates the effects of an impurity on a magnon sideband in the antiferromagnetic perovskite RbMnF3 ? It is thought that the ability of the model to predict the existence of local modes and resonances justifies the simple form of the model Hamiltonians chosen. The pure crystal sideband Iineshape can also be well determined from information on the exciton dispersion and the behaviour of the exciton-magnon interaction strength within the Brillouin zone.

Interaction of light with defects in antiferromagnetic perovskites

The theory of defects in antiferromagnetic insulators is applied to the perovskite structure and the Green functions calculated. The conditions imposed by symmetry on the interaction of light with such defects are also investigated. A comparison is made with experiments on Eu3+ and Ni2+ impurities in KMnF3 and RbMnF3.

Theory of Localized Magnons and Magnon-Exciton Interactions in Antiferromagnetic Perovskites

The antiferromagnetic perovskites have a simple cubic-magnetic structure with nearest-neighbor exchange interactions only. The perfect-crystal Green functions can be easily expressed in terms of tabulated integrals, so that they are particularly suitable for making quantitative comparison between theory and experiment. This is done for exciton-magnon interactions in pure crystals, and for magnons associated with impurities. Electric-dipole absorption in the pure crystal can occur when an exciton is created on one magnetic ion and a spin deviation on its nearest neighbor. The resulting magnon sideband reflects the interaction between the exciton and magnon which are in close proximity. If the exchange between a pair, when one is electronically excited is J′, and the normal exchange is J, then satisfactory agreement with the experimental lineshape can be obtained for J′=0.5J in RbMnF3. The positions of localized magnons are calculated as a function of impurity-host exchange, and compared with experimental results on Ni2+ in RbMnF3. The symmetry of the defect system is also considered, showing that additional terms are allowed. The most important of these is probably electric-dipole absorption in the far infrared, creating two localized magnons.

Open-Shell Hartree—Fock Molecular Orbital Theory of Transition Metal Ion Complexes: KNiF3

In order to investigate the magnetic properties of transition metal salts, we have developed the open shell single determinant Hartree—Fock SCF molecular orbital method for transition metal ion complexes. This cluster approach removes many of the shortcomings of earlier treatments1 and, in principle, provides a framework for obtaining fairly accurate and meaningful results within the single determinant approximation. These wave functions thus provide a realistic basis set for configuration interaction calculations of superexchange and other properties. By choosing a special one-center basis set for the cluster, we are able to compute all matrix elements of the molecular Hamiltonian accurately including the effects of the crystal field external to the cluster. Furthermore, the computation is sufficiently fast so that we may include all electrons of the molecular cluster while allowing considerable variational freedom in all representations. Detailed self-consistent field calculations have been made using, as example, KNiF3, the antiferromagnetic Perovskite treated extensively by previous workers.1 By considering a NiF64− cluster in its crystalline environment, the crystal field splitting parameter 10 Dq is found to be in fairly good agreement with experiment, but sensitive to a number of factors. The resulting molecular orbitals are compared with those obtained by earlier LCAO calculations. While the radial distributions are quite reasonable, the limitations imposed on these first calculations with this one-center method are apparent; the restriction to low angular momentum in the basis sets used leads to a poor description of ligand core orbitals. Spin densities throughout the cluster have been computed; neutron magnetic form factors have been calculated for comparison with experiment. In agreement with earlier work2 based on a single parameter LCAO scheme, we find that the cluster spin density gives a large peak in the form factor near zero scattering angles compared with the Ni2+ free ion result. This peak is seen to arise from the eg spin density on the ligand sites; the spin density from all the remaining molecular orbitals, unpaired by exchange polarization, is found to be small (≈2%). The limitations imposed by the restricted basis set used have led us to consider a triatomic system, Ni2F, as the cluster for the calculation of F19 hyperfine interactions. While the calculated isotropic hyperfine term was found to be too large relative to experiment, the computed anisotropic hyperfine interaction agreed (surprisingly) well. A full report will be submitted to the Physical Review.