Occupation sites and valence states of Co dopants in (La, Co)-codoped M-type Sr ferrite: Fe57 and Co59 nuclear magnetic resonance studies

Abstract

To specify preferential occupation sites of Co substituents and to clarify charge and spin states of Co ions in (La, Co)-cosubstituted hexagonal magnetoplumbite-type (M-type) Sr ferrite Sr1−xLaxFe12−yCoyO19 (x, y ≤ 0.4), 57 Fe and 59 Co nuclear magnetic resonance (NMR) spectra are measured under zero and external magnetic fields using powdered and single crystalline specimens. For comparison, NMR investigations of non-doped and La-or Co-doped M-type Sr ferrites are also performed. Ferrimagnetic M-type Sr ferrite contains the following five crystallographic Fe sites: the majority spin sites 12k, 2a, and 2b, and the minority spin sites 4f1 and 4f2. Based on 57 Fe and 59 Co NMR, a plausible model of (La, Co)-codoped Sr ferrite is deduced. To a considerable degree, the charge compensation between La 3+ and Co 2+ works in the equal (La, Co)-codoped case, where more than half of the Co ions are considered to be present in the minority spin 4f1 sites at the center of the oxygen tetrahedra, with the S=3/2 state carrying a small orbital moment owing to spin-orbit interaction. The remaining small number of high-spin Co 2+ (S=3/2, L=1) ions with unquenched orbital moments would be distributed to the other octahedral 12k, 2a, and 4f2 sites.

Co site preference and site-selective substitution in La–Co co-substituted magnetoplumbite-type strontium ferrites probed by 59Co nuclear magnetic resonance

“…Our independent XAS and MCD experiments corroborated by firstprinciple calculations confirmed their conclusion (unpublished). Analyses of 57 Fe NMR and Mössbauer spectra also support the conclusion [9,10]. Thus, we assign the strongest S1 line to 4f 1 -Co and S2/S3 to 2a/12k-Co.…”

“…The fact that none of S1, S2, and S3 was observed in N07 strongly suggests that all of S1, S2, and S3 come from Co 2+ . In fact, for N07, another sharp 59 Co signal, which can be assigned to low-spin Co 3+ , has been observed at ∼50 MHz in a completely different radio-frequency pulse condition [9]. This low-spin Co 3+ signal has not been observed for A15 and O28.…”

“…We assumed that signs of the observed internal fields are all positive for S1, S2, and S3. The applied field dependence of the resonance frequency for S1 showed that the sign of the net internal field is positive for S1 [9]. Positive internal fields can be reasonably assumed also for S2 and S3 because larger orbital moments are expected for octahedrally coordinated 2a/12k-Co.…”

“…In order to improve magnetic performance with limited Co content, site-selective Co substitution is highly required; commercially it is important to reduce the usage of Co because of its unstable price and supply. For the Sr-La-Co system, although it was widely believed that Co mainly occupies the octahedrally coordinated 4f 2 site [8], it is now gradually accepted that Co mainly occupies the tetrahedrally coordinated 4f 1 site and partly octahedrally coordinated 2a and/or 12k sites [5,6,9,10]. Furthermore, it has been revealed that, in the samples synthesized under ambient atmosphere, La 3+ content, x, and Co 2+ content, y, are not necessarily identical but in general x>y, and their difference is compensated by the reduction of Fe 3+ to Fe 2+ [4,[11][12][13].…”

Direct imaging of dopant sites in rare-earth element-doped permanent magnet and correlated magnetism origin

“…These modulations have been recently demonstrated to have an extended application in multiferroic spintronics because of the additional contribution of physical freedom. 8 Diverse state-of-art techniques, including Mössbauer spectroscopy, 9,10 neutron diffraction, 11 X-ray absorption spectroscopy (XAS), 12,13 X-ray spectroscopy, 14 X-ray magnetic circular dichroism (XMCD), 13 nuclear magnetic resonance (NMR) 15 and Raman spectroscopy, 16 have been employed to detect the sites of the dopants and the ordering of cations. However, due to the limitation of the wavelengths of their light sources or instrument resolutions, these techniques can only obtain information on the overall site occupation ratio.…”