Photoemission and x-ray absorption studies of valence states in (Ni,Zn,Fe,Ti)3O4 thin films exhibiting photoinduced magnetization

Abstract

By means of photoemission and x-ray absorption spectroscopy, we have studied the electronic structure of (Ni,Zn,Fe,Ti)3O4 thin films, which exhibits a cluster glass behavior with a spin-freezing temperature Tf of ∼230K and photoinduced magnetization (PIM) below Tf. The Ni and Zn ions were found to be in the divalent states. Most of the Fe and Ti ions in the thin films were trivalent (Fe3+) and tetravalent (Ti4+), respectively. While Ti doping did not affect the valence states of the Ni and Zn ions, a small amount of Fe2+ ions increased with Ti concentration, consistent with the proposed charge-transfer mechanism of PIM.

Estimate of the cation distribution for Ti-doped Ni0.68Fe2.32O4spinel ferrites by fitting magnetic moments at 10 K

“…It can be seen that the lattice parameter a increases from 8.35 Å when x = 0.00 to 8.40 Å when x = 0.312, which cannot be explained if Ti ions are assumed to be in the Ti 4+ form , due to the effective radius of Ti 4+ (0.605 Å) being smaller than that of Fe 2+ (0.78 Å) and Ni 2+ (0.69 Å), with the coordination number 6 .…”

“…Many reports have assumed that Ti cations appear in the form Ti 4+ in ferrites according to the traditional views . Kale et al prepared Ni 1+ x Ti x Fe 2−2 x O 4 (0 ≤ x ≤ 0.7) polycrystalline samples.…”

“…Srivastava et al prepared Ni 0.7+ x Zn 0.3 Ti x Fe 2−2 x O 4 (0 ≤ x ≤ 0.08), and using Mössbauer spectra analysis, concluded that all Ti 4+ cations entered the lattice at the [B] sites, and result in the canted spin structure. Kobayashi et al prepared Zn 0.6− x Ni 0.4+ x Ti x Fe 2− x O 4 ( x = 0.0, 0.2, 0.3) films. They claimed that most Ti ions in the samples were Ti 4+ cations.…”

Magnetic moments of Ti cations in Ti-doped Ni0.68Fe2.32O4 spinel ferrites

“…Srivastava et al [7] studied Ni 0.7+x Zn 0.3 Ti x Fe 2À2x O 4 (0 6 x 6 0.08) samples, and considered that all Ti 4+ cations entered [B] sites, with the result that a canted spin structure arose. Kobayashi et al [8] produced Ni 0.4+x Zn 0.6Àx Ti x Fe 2Àx O 4 (0 6 x 6 0.3) films, and concluded that most Ti ions in the samples were Ti 4+ cations, with a small amount of Ti 3+ . Dwivedi et al [9] prepared CoTi 2x Fe 2À2x O 4 (x = 0.0, 0.05, and 0.1) powder samples, and found that the saturation magnetization of the samples decreased with increasing Ti doping level.…”

“…To date, it has been assumed that Ti cations in ferrites are in the form of Ti 4+ ions and that they have no magnetic moment [6][7][8][9][10][11][12]. Jin et al [6] prepared Fe 3Àx Ti x O 4 (0 6 x 6 0.09) films on (0 0 1) oriented MgO substrates.…”