SPring-8, the large synchrotron radiation facility

Date : 16:00-17:00　July 2, 2007

Place : SPring-8 "HOUKOUKAN"

Speaker : Takayuki UOZUMI

Affiliate : Graduate School of Engineering , Osaka Prefecture Univ.

Title : Double-cluster-model approach for magnetic circular dichroism at L-absorption edges of CoMnO3 and microscopic origin of orbital ferromagnetism

Abstract : CoMnO₃, with ilmenite structure, has known to be an orbital-ferromagnetic compound, where the spin moments (S=3/2) of Co²⁺ (3d⁷) and Mn⁴⁺ (3d³) cancel each other in the crystal and thus the orbital magnetic moments are responsible for the ferromagnetism, though the microscopic origin of that is not understood well. Recently, Mizumaki (JASRI) et al., succeeded in the microscopic observations of ferromagnetic arrangements of orbital magnetic moments and almost complete cancellation of spin magnetic moments between Co and Mn at low temperature, by means of magnetic circular dichroism (MCD) measurements at the L-absorption edges and magnetic Compton scattering measurements. In this seminar, we report the MCD analyses by means of a double cluster model, CoMnO₉, and propose a microscopic mechanism for the orbital ferromagnetism.
Conventionally, a single impurity model , such as CoO₆ and MnO₆, including only one transition metal as impurity, has been used in analyses of core-level spectra. However, applying them to the present system, we cannot reproduce the sign of Mn MCD. This situation clearly shows some limitations in the conventional single-cluster-model approach, and we need to include correlation effect between spin moments or orbital moments of Co and Mn. Thus, we employ CoMnO₉ cluster model, including full-multiplet coupling effect. The present model includes Hund coupling due to full-multiplet interaction, crystal field, and direct- or indirect- (via O 2p orbital) hybridization between Co 3d and Mn 3d, and offers a proper framework for the description of the correlation between multiplets of Co and Mn, arising from cooperative or competitive effect of the interaction. Then, if we assume the value of ～0.5eV for (ddσ) between the σ-orbitals of Co and Mn, we obtain a antiferromagnetic correlation for spin moments and ferromagnetic correlation for orbital moments between Co and Mn, and the resulting ground state satisfactorily reproduces the sign of MCD both for Co and Mn. The microscopic scenario for the orbital ferromagnetism is as follows: (1) singlet state is formed between σ orbitals of Co and Mn, due to the relatively large value of (ddσ). (2) the other electrons of Mn are arranged in parallel to the σ electron at Mn site, due to the exchange interaction among Mn 3d electrons, which introduce the antiferromagnetic spin correlation between Co and Mn. (3) the orbital magnetic moments are induced at the Co and Mn site through the spin-orbit interaction. In this case the orbital moments are ferromagnetically aligned, because the sign of spin-orbit coupling constant differs between Co²⁺ (3d⁷) and Mn⁴⁺ (3d³).
In addition to the above topic, we want to present and discuss a new framework for core-level spectroscopy analyses, which is given by a combination of APW band calculation and single-impurity Anderson model approach.

Organizer : Mizumaki (PHS 3870)