Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase

Abstract

We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1−xO2 anatase on SrTiO3(001) for x=∼0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1−xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1−xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material.

Strong Room-Temperature Ferromagnetism in Co2+-Doped TiO2 Made from Colloidal Nanocrystals

“…19 000 cm -1 (ε ≈ 6 M -1 cm -1 ) that can be tentatively associated with the low symmetry split 4 T 1 (F) → 4 T 1 (P) ligand-field transition of pseudo-octahedral Co 2+ . The analogous transition in Co(H 2 O) 6 2+ is observed at 19 400 cm -1 (ε = 4.8 M -1 cm -1 ), 23 and although a similar ligand field strength is anticipated for Co 2+ in anatase TiO 2 , it is expected to be substantially broadened in the latter by the reduction in site symmetry and by possible charge compensation inhomogeneities. The presence of this transition is confirmed by MCD spectroscopy ( Figure 3B), which reveals a broad negative pseudo-A term feature in the same energy region (-19 000 cm -1 , + 21 500 cm -1 ).…”

“…6 The origin of this ferromagnetism continues to be debated in the literature, however. Some thin films have been shown to contain cobalt metal nanoparticles, 7,8 fostering the widespread belief that metallic nanoparticles are responsible for the ferromagnetism in other samples as well.…”

Theory of dopants and defects in Co-dopedTiO2anatase

“…For example, the location of Co dopants in this material (substitutional versus interstitial), their oxidation state, and their magnetic properties have been the subject of intense scrutiny. 2,3,4,6,8 Furthermore, it has been suggested that O vacancies, which are believed to give rise to the observed n-type behavior in pure TiO 2 anatase, 10 may provide free electrons which mediate the exchange interaction between the Co dopants. 3,4 This possibility is very different from the hole mediated exchange interactions which are believed to describe ferromagnetic order in a wide variety of other dilute magnetic semiconductors, including InMnAs, GaMnAs, and MnGe, 11,12,13 and may have a direct bearing on the origin of the anomalously high Curie temperature observed in Co-doped TiO 2 anatase.…”

“…2,3,4,5,6,7,8,9 One promising line of inquiry is to understand first the role of dopants and native defects in the TiO 2 host. For example, the location of Co dopants in this material (substitutional versus interstitial), their oxidation state, and their magnetic properties have been the subject of intense scrutiny.…”

Structure and magnetism of V-doped SnO2thin films: effect of the substrate

“…Motivated by theoretical predictions [1,2], much work has been done on the transition-metal doped oxides in which room-temperature ferromagnetism has been observed, including ZnO [3], TiO 2 [4,5] and SnO 2 [6][7][8]. However, compared with the booming materials research on the magnetic oxide semiconductors, the understanding of their magnetism is still in its infancy, with an ongoing debate about intrinsic ferromagnetism and extrinsic magnetic properties.…”