Modification of crystal anisotropy and enhancement of magnetic moment of Co-doped SnO2 thin films annealed under magnetic field
© Loya-Mancilla et al.; licensee Springer. 2014
Received: 27 June 2014
Accepted: 18 October 2014
Published: 25 November 2014
Co-doped SnO2 thin films were grown by sputtering technique on SiO2/Si(001) substrates at room temperature, and then, thermal treatments with and without an applied magnetic field (HTT) were performed in vacuum at 600°C for 20 min. HTT was applied parallel and perpendicular to the substrate surface. Magnetic M(H) measurements reveal the coexistence of a strong antiferromagnetic (AFM) signal and a ferromagnetic (FM) component. The AFM component has a Néel temperature higher than room temperature, the spin axis lies parallel to the substrate surface, and the highest magnetic moment m =7 μB/Co at. is obtained when HTT is applied parallel to the substrate surface. Our results show an enhancement of FM moment per Co+2 from 0.06 to 0.42 μB/Co at. for the sample on which HTT was applied perpendicular to the surface. The FM order is attributed to the coupling of Co+2 ions through electrons trapped at the site of oxygen vacancies, as described by the bound magnetic polaron model. Our results suggest that FM order is aligned along  direction of Co-doped SnO2 nanocrystals, which is proposed to be the easy magnetization axis.
Research in semiconducting oxide thin films has fundamental importance due to its wide range of applications in optoelectronics , photoluminescence , sensing devices , etc. Furthermore, experiments with nanomaterials where the electron charge and its spin orientation, regarded as an additional degree of freedom, are together considered to produce new physical effects situated on the field of spintronics, which is a wide research area that offers options to fabricate faster and lower energy consuming solid state devices. In this field, one of the most surprising and potentially claims is that nonmagnetic semiconductors as Ge and GaAs become ferromagnetic by doping with a few percent of 3d transition metals (TM) as Mn [4–6]. One of the most important issues that need to be solved is how to enhance their Curie temperature (T c ), which remains far below room temperature (RT). Diluted magnetic oxides (DMO) are wide bandgap oxides as SnO2, TiO2, ZnO, etc., that can be used as host semiconducting oxides, and doping with most of the 3d transition metals will produce RT ferromagnetism (FM) [7–9]. However, this is not always successful, as nonmagnetic materials can be obtained depending on the synthesis process and crystal characteristics [7–9]. Research on the physical mechanism governing the ferromagnetic order on DMOs has focused mainly on the following aspects: (a) oxygen vacancy (VO) defects [10, 11], (b) interstitial cations defects , and (c) 3d TM doping [13, 14]. The concentration of the doping element remains below the percolation limit established by the relation x p ~2/Z0, where Z0 is the coordination number of the cation , for SnO2x p ~32 at.% , and this concentration is only limited by the solubility limit of the dopant. However, the concentration of dopant cations is chosen, in most of the cases, below 10 at.% to avoid antiferromagnetic (AFM) coupling between neighboring 3d cations. A particular feature observed on DMO thin films is that different saturation magnetizations (Ms) are obtained depending on the crystallographic direction on which the magnetic M(H) measurement is performed. In other words, Ms obtained in different crystallographic directions does not converge at large magnetic fields as conventional ferromagnetic films do . Normally, in DMO out-of-plane Ms⟂ is 2 to 3 times higher than in-plane Ms//[18, 19], suggesting that spin-orbit interaction in these materials is very strong and that measured Ms is in fact the spin component along a given direction. Furthermore, M(H) loops in DMO are anhysteretic, and maximum values of coercivity reach some teens of oersted (Oe). Research on 3d TM-doped SnO2 thin films was highly motivated after Ogale et al. , reported a giant magnetic moment of 7.5 μB/Co with a high T C =650 K on 5 at.% Co-doped SnO2 epitaxial thin films. In this paper, we present a detailed study of the evolution of the intensity of the magnetic moment per Co atom (either FM or AFM) on Co-doped SnO2 thin films sputtered on SiO2/Si(001) substrates at RT followed by thermal treatments in vacuum under magnetic field applied out-of-plane and in-plane configurations.
Results and discussion
where I(hkl)E° and I(h ' k ' l ')T correspond to experimental and theoretical intensities of a given plane, and I(hkl)E and I(h ' k ' l ')T correspond to experimental and theoretical intensities for those families of lattice planes which show a particular change. This equation was used to establish the effect of HTT on the morphology of the Co-doped SnO2 grains in the film. A pure SnO2 crystal has a specific shape in order to minimize the total surface-free energy corresponding to S order parameter equal to one, depicted on the inset of Figure 2c. The change on the intensity of certain peaks provides a number to describe morphological changes. Applying Equation 2 to the intensities of (002) and (110) peaks compared to the intensity of (101) peak, which is normally parallel to the substrate surface for SnO2 thin films (Figure 2b), it is possible to calculate the order parameter S and determine the relative grain shape. This grain shape evolution is attributed to the growth along a component of the spin direction, which is parallel to the direction of HTT during TT. This assumption can explain how grain growth is favored along some defined directions by the direction of HTT. In Figure 2c, the relative intensities of the S order parameter and a representation of the grain shape for each sample compared to that of pure SnO2 grains are shown.
Magnetic moments m in μ B per Co at. obtained from all measurements at 2 and 300 K
In Figure 5, the ZFC-FC measurement performed on sample PP is shown and the increasing magnetization with decreasing temperature has been observed in other DMO systems [25, 26], similar to that of a superparamagnetic system with the difference that there is no any observed blocking temperature. The peak observed at 41 to 47 K can be attributed to a minority of Co3O4 precipitates undetectable by XRD experiments. The 1/X graph presented on the insert shows that the extrapolation of the straight dotted line intercepts the temperature axis at a negative value of T, corresponding to the paramagnetic Curie temperature θ P .
Polycrystalline Co-doped SnO2 thin films were grown by RF sputtering at room temperature. Crystallinity of the films was improved by thermal treatment at 600°C with and without an external magnetic field. The relative intensities of (002) and (110) peaks of the XRD patterns were compared with the intensity of the (101) peak, through the order parameter S related to the shape of the nanocrystals. The thermal treatment under magnetic field changed the shape of the crystals, as growth is favored along the direction where the spin is aligned. The analysis of magnetic properties resulted in the observation of two magnetic phases: ferromagnetic (FM) and antiferromagnetic (AFM), where AFM component has a Néel temperature barely higher than RT arising from the coupling between Co atoms in the region near to the interface with the substrate, where Co concentration is ≈ 13 at.%, which is higher than the solubility limit of Co on SnO2. Moreover, a modification of crystal anisotropy due to the thermal treatment under magnetic field was observed, enhancing the FM moment for films where the magnetic field during thermal treatment was applied in a direction perpendicular to the substrate surface. The FM moment produced by Co ions arises from the interaction of these ions through the spherical orbit of the electron on the polaron produced by the oxygen vacancy. As the intensity of the magnetization depends on the direction in which the measurement is done and as dz2 orbital has the highest probability to contain the unpaired electron, then, we suggest that the spin is perpendicular to the axis of this orbital and parallel to the  direction. Proposing that this direction SnO2 is the easy axis of magnetization and that magnetization measured along any direction corresponds to the spin component.
SMLM is a Ph.D. student in materials sciences at CIMAV. PP is a senior researcher at the National Chemical Laboratory (NCL) in India, and RD is a Ph.D. student at NCL. The rest of the authors work at Cimav Chihuahua. ILTO works as a technical engineer for general laboratory support. OOSC is in charged for the TEM sample preparations using FIB. CEOG performs the TEM observations. FEM is a professor researcher working with the theoretical calculations, and HEEP is a researcher working on the thin films by sputtering. SFOM is a experimental researcher and head of the group.
The authors thank Enrique Torres Moye for his assistance in the XRD measurements and Denisse López for her contribution on thin films growth.
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