High-spin Phase Research Articles

Excitation spectra ofLaCoO3

We study the excitation spectra of ${\mathrm{LaCoO}}_{3}$ using a multiorbital tight-binding model. Making the self-energy correction due to local three-body scattering to the Hartree-Fock (HF) approximation, we calculate the spectral density for the low-spin phase. It is shown that the band gap is strongly reduced from the HF value, and that the intensity is transferred from the lower part to the upper part of the valence band and to the satellite in the spectral density projected onto the Co 3d states. The results are in good agreement with the spectra of photoemission experiments. We also calculate the spectral densities for the intermediate-spin phase and the high-spin phase in relation to the nonmagnetic-to-paramagnetic transition. The peak intensities around the top of the valence band are smaller than that for the low-spin phase. In ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CoO}}_{3}$, the energies for the intermediate-spin and high-spin phases are found to become lower than for the low-spin phase with increasing x, indicating the instability of the low-spin phase. This is consistent with the susceptibility data and the photoemission spectra.

Magnetic x-ray dichroism in 2p absorption spectra of Fe/Cu(001)

Large magnetic circular dichroism using circularly polarized synchrotron radiation has been observed at the L2,3 absorption edges of thin (1–12 monolayers) fcc Fe films grown on Cu(001). Dramatic changes in the 2p branching ratio are observed when the orientation of photon helicity and sample magnetization are varied from parallel to antiparallel. The temperature and film thickness dependence of the perpendicular anisotropy in these films could be monitored by variations in the 2p branching ratio. Finally, our results are described in a simple theoretical framework that allows a determination between the predicted low- and high-spin phases for fcc Fe. Our data suggest a high-spin phase with a moment of 2.0–2.5 μB/atom. Interestingly, thicker films with remanant magnetic moments in the film plane present smaller branching ratio variations consistent with either a reduced moment or with domain closure for these films. The surface sensitivity and elemental specificity of this technique make it particularly attractive for the study of surface and thin film magnetism.

High-spin α low-spin transition in [Fe(NCS) 2(4,4′-bis-1,2,4-triazole) 2](H 2O). X-ray crystal structure and magnetic, mössbauer and EPR properties

The Fe II ion in [Fe(NCS) 2(4,4′-bis-1,2,4-triazole) 2](H 2O) has distorted tetragonal symmetry with two trans-oriented NCS − ligands. It shows a very abrupt high-spin α low-spin transition at 123.5 K on cooling and 144.5 K on warming. The compound is rather stable in vacuo at room temperature; however, samples which have passed the spin transition once lose their water of hydration above 240 K in vacuo. The dehydrated substance does not show a spin transition; it is high spin in the whole temperature range. Mössbauer ligand-field spectra and magnetic behaviour of both the hydrated and non-hydrated compounds are discussed. The spin transition has been followed by EPR measurements with the aid of traces of Cu 2+ ions which could be substituted for Fe 2+ in the tetragonal structure. In the high-spin phase the EPR signal is very broad and featureless; in the diamagnetic low-spin phase it is very sharp and resolved in hyperfine and superhyperfine structures. This unusual method to follow the spin transition was shown to be quite generally applicable. In the crystal structure of Fe(NCS) 2(C 4H 4N 6) 2(H 2O) the distances FENCS are 2.125(3) Å, and FeN(ligand) 2.180(3) and 2.188(2) Å. The water molecule is connected to the non-coordinating ligand N-atom by hydrogen bonding.

Spectroscopic and bonding characteristics of high- and low-spin phases of fcc iron

Self-consistent electronic band structures and densities of states are presented for two different ferromagnetic states of fcc iron. The calculations carried out at the lattice constant a = 3.628 Å yield two different spin-polarized states with magnetic moments 1.35μ B (low-spin phase) and 2.50μ B (high-spin phase). Bonding mechanism and the electronic pressure are discussed in detail. The calculated densities of states are used to analyze the issue of a high- to low-spin moment transition in PtFe 3 observed recently in photoemission [E. Kisker, E.F. Wassermann and C. Carbone, Phys. Rev. Lett. 58 (1987) 1784].

Ferromagnetic phases of bcc and fcc Fe, Co, and Ni

The different magnetic phases of the bcc and fcc forms of Fe, Co, and Ni are studied by analyzing total-energy surfaces in moment-volume parameter space obtained from energy-band calculations using a local-spin-density approximation. The surfaces, found by calculating total energies while holding both the magnetic moment and the volume fixed, offer a method for studying phases that are inaccessible to traditional self-consistent-field methods. We find that magnetic moments can change discontinuously with volume and that there are ranges of coexistence for different magnetic phases. In the multiphase ranges, these elemental magnetic systems exhibit metamagnetic behavior. Our results show that bcc Co is ferromagnetic for all volumes studied, that fcc Co can exist in either a nonmagnetic or a ferromagnetic phase, and that there is a range of volumes where the two phases can coexist. For Fe, the bcc form exhibits a stable ferromagnetic phase for all volumes considered, but the fcc form can exist in any of three phases---a nonmagnetic, a low-spin, and a high-spin phase---all of which can coexist in limited volume ranges. For Ni, the fcc form exhibits a stable ferromagnetic phase, but the bcc form can exist in both a nonmagnetic and, at expanded volumes, a ferromagnetic phase. The volume ranges for all magnetic phases are clearly identified for the bcc and fcc forms of Fe, Co, and Ni.

Structural phase transitions involving changes in spin and valency. A theory of the three phase transitions of LaCoO3. II

For pt.I see ibid., vol.9, p.3731 (1976). The thermodynamics of the two-sublattice model of LaCoO3 is discussed. The model is described by a Hamiltonian including two trivalent and two charge-transfer states of cobalt ions. The ions interact effectively through coupling to fully symmetrical vibrations of octahedral ion-ligand complexes. In the molecular-field approximation a single order parameter is introduced, simultaneously related to the antidistortive order, the high-spin-low-spin order, as well as the charge order. Five zero-temperature phases are possible: the low-spin phase, the high-spin phase, the two-sublattice high-spin-low-spin phase, the mixed-valency-low-spin phase and, finally, the charge-order phase. A three-dimensional phase diagram for finite temperatures is obtained numerically.