Role of Ti3dCarriers in Mediating the Ferromagnetism ofCo∶TiO2Anatase Thin Films

Abstract

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co:TiO2 anatase films using soft and hard x-ray photoemission spectroscopy with probe sensitivities of ∼1 nm and ∼10 nm, respectively. We obtain direct evidence of metallic Ti 3+ states in the bulk, which get suppressed to give a surface semiconductor, thus indicating a surface-bulk dichotomy. X-ray absorption and high-sensitivity resonant photoemission spectroscopy reveal Ti 3+ electrons at the Fermi level (EF ) and high-spin Co 2+ electrons occurring away from EF . The results show the importance of the charge neutrality condition: Co3+ (VO is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d -O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.PACS numbers: 79.60.-i, 73.20.-r, 75.50.-y Dilute ferromagnetic oxides have been at the forefront of a paradigm shift in the search for spintronics and magneto-optic device materials [1][2][3][4][5][6]. However, for achieving reliable thin-film device performance, it is necessary to ensure the surface and bulk electronic properties of candidate materials. This is particularly true for high Curie-temperature oxide ferromagnets for spintronics which can be synthesized only in thin film form, such as Co:The main question that still remains enigmatic: What changes drive the long-range coupling of local moments in an insulating parent? The pre-runners, namely, dilute magnetic semiconductors (DMSs) which consisted of magnetic 3d transition metal (TM) ions doped in III-V and II-VI compounds, showed a Curie-temperature (T c ) well-below room temperature e.g. InMnAs, GaMnAs, ZnCrTe, etc [7][8][9]. Theoretical studies of carrier-mediated ferromagnetism played a pivotal role in predicting high-T c ferromagnets in the dilute substitution limit [10]. The spin-charge degrees of freedom and the anisotropic character of the dopant TM d -orbitals results in a directional dependence of exchange energies and T c 's of upto 170 K [11,12]. The limitation of T c 's significantly lower than room temperature motivated studies on alternative parents, like oxides and nitrides. The discovery of ferromagnetism in Co-doped TiO 2 (Co:TiO 2 ), with a T c exceeding 300 K [2], was crucial in expanding the field to oxides, leading to a rapid increase of new materials and phenomena arising from a synergetic marriage of semiconductor physics and strongly correlated systems.From extensive magnetic and transport studies, it is now well-accepted that Co:TiO 2 films grown in a reducing environment or annealed in vacuum result in Co segregation [13,14], while films grown in an oxidizing condition are intrinsically ferromagnetic with high spin Co 2+ [15,16]. While initial x-ray magnetic circular dichroism (XMCD) measurements and annealing in vacuum concluded that the ferromagnetism was extrinsic due to segregated Co metal clusters [14], subsequent XMCD studies showed intrinsic ferromagnetism in Co:TiO 2 grown in oxidizing conditions [17]. A soft xray surface...

Ferromagnetism of cobalt-doped anatase TiO2 studied by bulk- and surface-sensitive soft x-ray magnetic circular dichroism

“…Some studies that also claim the ferromagnetism of Co-doped TiO 2 is due to Co metal clusters [19][20][21][22]. The recent theoretical study by Calderon et al [23], electric fieldinduced anomalous Hall effect (AHE) study by Yamada et al [24] and x-ray photoemission spectroscopy study by Ohtsuki et al [25] suggested the ferromagnetism of Co-doped TiO 2 is due to carrier mediated. However, direct information about the magnetization as a function of carrier density has been lacking.…”

First-Principles Calculations of Angle-Resolved and Spin-Resolved Photoemission Spectra of Cr(110) Surfaces at the2p−3dCr Resonance

“…However, second order processes, like resonant inelastic X-ray scattering (RIXS) and resonant photoemission (RPES), remain a major challenge for theory. For RPES, existing approaches are semiempirical [7][8][9][10], based on a well defined two-holes final state and on small clusters, and thus do not take into account the delocalization of intermediate states, the bandstructure of the system and multiple scattering effects in the propagation of photoelectrons.The huge experimental output from RPES on correlated materials [7,[11][12][13][14][15][16] and the intriguing quest for a determination of local magnetic properties put forward by pioneering experiments [14][15][16] call for advancements in the theoretical description of this spectroscopy. In experiments on CuO and Ni, it was shown that the RPES photocurrent with circular polarized light is spin polarized in antiferromagnets [14,15] and Curie paramagnets [16].…”

“…The huge experimental output from RPES on correlated materials [7,[11][12][13][14][15][16] and the intriguing quest for a determination of local magnetic properties put forward by pioneering experiments [14][15][16] call for advancements in the theoretical description of this spectroscopy. In experiments on CuO and Ni, it was shown that the RPES photocurrent with circular polarized light is spin polarized in antiferromagnets [14,15] and Curie paramagnets [16].…”

The Recovery of a Magnetically Dead Layer on the Surface of an Anatase (Ti,Co)O2 Thin Film via an Ultrathin TiO2 Capping Layer

“…(La,Sr)MnO 3 showed a magnetic disorder on a surface, which was probably associated with a structural disorder [11]. In the case of (Ti,Co)O 2 , the magnetically dead layer was at least~5 nm thick [9] and could be attributed to a surface oxidation and/or a depletion layer due to the different surfaces and bulk electronic structures observed via soft and hard X-ray photoemission spectroscopy [12]. X-ray photoemission spectroscopy under ultraviolet illumination suggested that photoinduced carriers at a depleted layer intermediate exchange coupling between Co spins [13].…”