Site-Specified Magnetic States in Ferrites Probed by Magnetic Circular X-Ray Dichroism

To study magnetic states in ferrimagnetic Fe-oxides, we have measured magnetic circular x-ray dichroism at K-edge of Id-transition metal. The observed dichroic signals are separately assigned to the contribution come from 3d-cation in tetrahedral or octahedral sites. We discuss the magnetic states of 3d-metal ion in terms of local enviroment. Magnetic circular x-ray dichroism (MCXD is a powerful technique for studying magnetic states in magnetic materials because of an element-specificity, a shell-se 1 ectivity, and an angular momentum sensitivity. To clarify the difference in magnetic states of Sdtransition metal (M) ions depending on local environment, e.g., tetrahedral (Td) or octahedral (Oh) symmetry, we have applied this technique to various Fe-oxide ferrimagnets with the spinel or garnet structure. In general, K-edge absorption spectrum in M-oxides is characterized by a pre-peak structure and an enhanced main-peak one around the edge. The pre-peak has been assigned to the 1s -+ 3d dipole-allowed transitions in Td-sites whereas the main-peak is properly related to the Is -+ 4p transitions [I]. In this report we distinguish between the contributions of T d and Oh sites to the spectrum through the preferred substitution of M-ion in spinel and the difference in site occupation between spinel and garnet. Hence, the magnetic states of M-ion are discussed in terms of crystal field and local symmetry. The Fe-oxides studied in this work include MFe20r (M=Mn,Fe,Co,and Ni), 7-FeaOa, and LiFeSOa as spinel-type, R3Fe5012 (R=Y and Ho) as garnet-type, and BaFe120ts as magnetoplumbite-type. MCXD spectrum was recorded in transmission mode for powder sample using the left-circularly polarized x-rays on BL-28B at Photon Factory 2.5 GeV storage-ring in KEK. The MCXD spectrum (Apt) is defined as the difference in absorption with reversing the direction of magnetic field: Apt = \pi$(?) pt(j)], and x-ray absorption near-edge structure (XANES : pt) is shown as the average: pt = kt(?) +pt(L)]/2, where T (I) represents the antiparallel (parallel) configuration of magnetic field with respect to the wave vector of incident x-ray. The MCXD spectrum was systematically measured at the K-edge of Mn, Fe, Co, and Ni. 3.RESULTS AND DISCUSSION Fe K-edge XANES and MCXD spectra are shown in Figures l(a) and l(b), respectively. In the case of inverse spinel, the substitution of co2+, Ni2+, Li+ ion, or defect for Feaf in Oh-sites causes scarcely any change in the XANES spectrum; on the other hand, in the case of Mn-ferrite with the normal spinel structure, the substitution of Mna+ ion for Fe3+ in Td-sites induces a significant reduction of pre-peak intensity. On the contrary, in the garnet-type ferrite including more Td-sites, the pre-peak intensity clearly increases in comparison with that in spinel. These observations are in good agreement with the assignment that the pre-peak is attributed to the cations in Td-sites. It is not clear, however, whether the main-peak can be associated with the Oh-sites or not, because its feature is spread over the large continuum background resulting from the Is -+ 4p dipole transitions. To clarify this, magnetic effect in XANES spectrum, i.e., dichroic signal, will be effectual, because it provides the information on magnetic polarization in the interesting electronic states. Figure l(b) shows the Fe K-edge MCXD in this series. The observed spectrum is formed a remarkable dispersion-type profile and located in both the pre-peak (--0 eV) and the main-peak (15 eV) regions. In Mn-ferrite, the dichroism almost disappears in the pre-peak region but yields prominent spectrum at the main-peak position. In the other ferrites, the intensity ratio of MCXD at main-peak to that at pre-peak is consistent with the substitution for the Fe ion in Oh-sites. In garnet ferrite, the dispersion-type dichroic signal is clealy observed only in the pre-peak region and shows an opposite sign to that in spinel, which is interpreted as the magnetic site dominantly contributed to bulk magnetization in ferrimagnet. Harada and Kotani [2] have theoretically reproduced the dispersion-type dichroic spectrum of Fe3+ ion in the Tdor Oh-symmetry by taking pd hybridization with 0'ligands and spin-orbit coupling in tag orbitals into account. We Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19971105 C1-270 JOURNAL DE PHYSIQUE IV can associate the dichroism in the main-peak region with Fe ion in Oh-sites. However, a subtle MCXD signal in the main-peak region in garnet is still an open question. To verify our assignment, moreover, we have measured XANES and MCXD spectra concerning the substituted M-ion in spinel. Figures 2(a) and 2(b) show the Co and Ni K-edge spectra, respectively. In XANES, the spectra exhibit an enhanced white-line like absorption and a substantial reduction of pre-peak intensity. In MCXD, a positive-doublepeak appears at the position corresponding to the white-line like absorption, and no dichroic signal at the pre-peak is observed. These experimental facts demonstrate that the magnetic Co2+ or Ni2f ion is preferably substituted for the Fez+ ion in Oh-sites. The similarity between the Co and Ni MCXD spectra may be related to a resemblance of local magnetic states in such divalent ion in spite of the difference in the ground-state 3belectronic configurations. Although there is no interpretation of such positivedouble-peak dichroic spectrum, it is associated with crystal-field splitting, exchange interaction, charge-transfer, p d hybridization, etc. For the Mn K-edge, the characteristic dispersion-type spectrum is expected because of the same 3ds electronic configuration as the Fe3+ ion. Actually, such dichroic signal is observed in a wide region of the edge, so that the structure is very complicated and possibly related to the inverse spinel structure. Consequently, we conclude that the dichroic signal in the main-peak region is reasonably attributed to Fe ion in Oh-sites. Let us briefly discuss the magnetic states of Fe3+ ion in terms of crystal field and local symmetry. The dichroism can be interpreted as the mechanism [2] composed of the dipole-allowed 1s -t 3d transitions, which is due to the hybridization between Fe 3d and ligand 2p orbitals, and the spin-orbit coupling in the Sbstates, which leads to a splitting in the degenerated t2, and e, multiplets. Hence, the MCXD is very sensitive to the local enviroment of polyhedron. The ground state electronic configurations of the cation in Oh-symmetry studied in this work are in a high-spin state, so that the dichroism for the substituted cations range from Mn(3ds) to Ni(3ds) is essentially related to the electron-filling of the 3d t2,-states. 1 MnFeflI