Crucial Role of the Co Cations on the Destabilization of the Ferrimagnetic Alignment in Co-Ferrite Nanoparticles with Tunable Structural Defects

Abstract

The key role of the structural defects on the magnetic properties of cobalt ferrite nanoparticles (NPs) is investigated by complementary local probes: element-and site-specific X-ray magnetic circular dichroism (XMCD) combined with high-resolution transmission electron microscopy of individual NPs. A series of monodisperse samples of 8 nm NPs with a tunable amount of structural defects were prepared by thermal decomposition of Fe(III) and Co(II) acetylacetonates in the presence of a variable concentration of 1,2-hexadecanediol. The particles show a partial inverse spinel structure, and their stoichiometry and cation distribution are comparable along the series. Element-specific XMCD hysteresis loops at all the cationic sites show a decrease in squareness and an increase in both the closure field and the high-field susceptibility as the NPs become more structurally defective, suggesting the progressive loss of the collinear ferrimagnetism. However, the Co 2+ cations in octahedral sites are significantly more affected by the structural defects than the rest of the cations. This is because structural defects cause local distortions of the crystal field acting on the orbital component of the cations, yielding effective local anisotropy axes that cause a prevalent Co 2+ spin canting through the spin−orbit coupling, owing to the relatively large value of the partially unquenched moment of these cations, as found by XMCD. All in all, our results emphasize the crucial role of the Co 2+ cations on the destabilization of the collinear ferrimagnetism with the inclusion of structural defects in cobalt ferrite NPs.

Magnetic Nanoparticles: from the Nanostructure to the Physical Properties

“…This series of samples was also an excellent model to study by element-and site-specific XMCD the cation moments and site distribution since samples showed similar size distribution and stoichiometry. 71,93 The Fe L2,3 edge XAS and XMCD spectra showed the typical features previously described for CoXFe3-xO4. 148,149 In particular, XMCD spectra of Fe showed the three characteristic peaks corresponding to Fe L2,3 edges: the lowest negative energy peak corresponded to octahedral Fe 2+ (Oh), the positive peak to tetrahedral Fe 3+ (Td), and the highest negative peak to Fe 3+ (Oh).…”

“…So far, an overview of the parameters that affect the synthesis of Fe3-xO4 NP and the importance of achieving strict control over them has been discussed. In this section, we are going to focus on the special case of cobalt ferrite CoxFe3-xO4 NP where, although some of the previous strategies can also be applied, 62,71,93,94 an additional level of complexity is added associated with the requirement of two different metal precursors.…”

Reduced magnetocrystalline anisotropy of CoFe$_2$O$_4$ thin films studied by angle-dependent x-ray magnetic circular dichroism

“…Therefore, the distribution of Co 2+ (O h ) is expected have dominant effects on the MCA. Such modifications of magnetic anisotropy induced by the distribution of Co 2+ (O h ) are also reported for the CoFe 2 O 4 nanoparticles [21][22][23]. The cation distribution in these films has been studied using XMCD by Wakabayashi et al From the comparison of y and the normalized MCA (the magnitude of MCA divided by M ) as functions of the film thickness d plotted in Fig.…”

Influence of the cation distribution, atomic substitution, and atomic vacancies on the physical properties of CoFe2O4 and Ni<

“…Recently, Moya et al have shown that different values of the inversion parameter lead to different values of the total spin magnetic moment, due to a lowering of the magnetic coupling and to a canting of the cation magnetic moments, in particular for Co 2+ cations. Two reasons were given by these authors to explain this effect: the first one is a direct consequence of the noninverse structure, with Co 2+ (Td)-O-Fe 3+ (Oh) antiferromagnetic superexchange interactions lower than the dominant Fe 3+ (Td)-O-Fe 3+ (Oh) ones; the second reason potentially comes from structural relaxations induced by other defects [95]. From our calculations (see Supplemental Material [43]), we indeed found that the antiferromagnetic coupling constant between cations in Oh and in Td atomic sites is reduced by 2-3 meV, for both CFO and NFO, when the inversion degree λ decreases from 1 to 0.875.…”