Systematic study of the electronic structure and the magnetic properties of a few-nm-thick epitaxial (Ni1-xCox)Fe2O4 (x = 0 - 1) layers grown on Al2O3(111)/Si(111) using soft X-ray magnetic circular dichroism: effects of cation distribution

TL;DR

This study investigates the electronic and magnetic properties of ultra-thin epitaxial (Ni1-xCox)Fe2O4 layers grown on Al2O3/Si substrates, revealing cation site occupancy, cation valences, and high inversion parameters conducive to spin filtering.

Contribution

It provides a detailed analysis of cation distributions and magnetic properties in thin (Ni1-xCox)Fe2O4 layers, demonstrating high inversion parameters and potential for spin filter applications, using soft X-ray spectroscopy and modeling.

Findings

Achieved high inversion parameter y of 0.91 in 3.5 nm NiFe2O4 layer.

Identified specific cation site occupancies and valences.

Observed suppressed dead layer effect compared to previous CoFe2O4 studies.

Abstract

We study the electronic structure and the magnetic properties of epitaxial (Ni1-xCox)Fe2O4(111) layers (x = 0 - 1) with thicknesses d = 1.7 - 5.2 nm grown on Al2O3(111)/Si(111) structures, to achieve a high value of inversion parameter y, which is the inverse-to-normal spinel-structure ratio, and hence to obtain good magnetic properties even when the thickness is thin enough for electron tunneling as a spin filter. We revealed the crystallographic (octahedral Oh or tetrahedral Td) sites and the valences of the Fe, Co, and Ni cations using experimental soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra and configuration-interaction cluster-model calculation. In all the (Ni1-xCox)Fe2O4 layers with d = about 4 nm, all Ni cations occupy the Ni2+ (Oh) site, whereas Co cations occupy the three different Co2+ (Oh), Co2+ (Td), and Co3+ (Oh) sites with constant occupancies. According to these features, the occupancy of the Fe3+ (Oh) cations decreases and that of the Fe3+ (Td) cations increases with decreasing x. Consequently, we obtained a systematic increase of y with decreasing x and achieved the highest y value of 0.91 for the NiFe2O4 layer with d = 3.5 nm. From the d dependences of y and magnetization in the d range of 1.7 - 5.2 nm, a magnetically dead layer is present near the NiFe2O4/Al2O3 interface, but its influence on the magnetization was significantly suppressed compared with the case of CoFe2O4 layers reported previously [Y. K. Wakabayasi et al., Phys. Rev. B 96, 104410 (2017)], due to the high site selectivity of the Ni cations. Since our epitaxial NiFe2O4 layer with d = 3.5 nm has a high y values (0.91) and a reasonably large magnetization (180 emu/cc), it is expected to exhibit a strong spin filter effect, which can be used for efficient spin injection into Si.