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Modification of crystal anisotropy and enhancement of magnetic moment of Co-doped SnO2 thin films annealed under magnetic field.

Loya-Mancilla SM, Poddar P, Das R, Ponce HE, Templeton-Olivares IL, Solis-Canto OO, Ornelas-Gutierrez CE, Espinosa-Magaña F, Olive-Méndez SF - Nanoscale Res Lett (2014)

Bottom Line: 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 [101] direction of Co-doped SnO2 nanocrystals, which is proposed to be the easy magnetization axis.

View Article: PubMed Central - HTML - PubMed

Affiliation: Centro de Investigación en Materiales Avanzados, S.C., Cimav, Av. Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31109 Chihuahua, Chihuahua, Mexico.

ABSTRACT
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 [101] direction of Co-doped SnO2 nanocrystals, which is proposed to be the easy magnetization axis.

No MeSH data available.


Proposed model indicating the location and orientation of the spin on the orbital dz2. Red continuous arrows represent the location and orientation of the spin, and black dashed arrows represent the spin components considered in the obtained FM moments. Red dashed arrows give a qualitative indication of the magnitude of the ferromagnetic moment obtained for each studied case. (a) For sample PP, as the spin-orbit interaction is very strong, the vertical component of the spin will align with the applied magnetic field during the TT. (b) For sample PL, the horizontal component will align with the direction of the field during TT and the total component is smaller than that of (a).
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Figure 6: Proposed model indicating the location and orientation of the spin on the orbital dz2. Red continuous arrows represent the location and orientation of the spin, and black dashed arrows represent the spin components considered in the obtained FM moments. Red dashed arrows give a qualitative indication of the magnitude of the ferromagnetic moment obtained for each studied case. (a) For sample PP, as the spin-orbit interaction is very strong, the vertical component of the spin will align with the applied magnetic field during the TT. (b) For sample PL, the horizontal component will align with the direction of the field during TT and the total component is smaller than that of (a).

Mentions: Crystal field theory (CFT) was used to explain these results in agreement with the anisotropy feature in DMO thin films. For schematic purposes, in Figure 6, a Co2+ ion is placed at the center of the octahedron formed by O atoms on the rutile cell. The 3d orbitals of the Co atom according with the CFT are dz2 and dx2-y2, which are degenerated, in a higher energy level than orbitals dxy, dxz, and dzx. Mimaki et al. [27] studied the bond character of rutile type on SiO2, GeO2, and SnO2, and they observed that the ratio of the electron density of M-O equatorial distances/M-O axial distances (where M is Si, Ge, or Sn) decreases when increases the atomic number of the cation. Then, for SnO2, the electron density is higher on the dz2 orbital and only one unpaired electron will be occupying in this orbital. The FM moments per Co at. are very small, suggesting that the Co atoms have a low spin configuration. This fact is in agreement with the measurements that a higher magnetic moment was obtained for sample PP, this is because dz2 orbital axis forms an angle of 34.1° with SnO2(101) plane (which remains parallel to the surface), and we assume a very strong spin - orbit coupling. With the TT applied in the PP configuration, the vertical component of the spin is aligned in the same direction than that of the magnetic field. The contribution of the possible random orientations of the SnO2 cells is represented in Figure 6a. For PL configuration, the alignment of the spins is in a manner that the horizontal components are parallel to the applied field of the TT, as shown in Figure 6b, and the addition of all these components leads to a total magnetic moment lower than that of the PP configuration. These results can be compared with monocrystalline thin films (e.g., epitaxial films on r-cut sapphire substrate), where the spin components can be better appreciated, and different magnetizations are obtained for PL and PP configurations.


Modification of crystal anisotropy and enhancement of magnetic moment of Co-doped SnO2 thin films annealed under magnetic field.

Loya-Mancilla SM, Poddar P, Das R, Ponce HE, Templeton-Olivares IL, Solis-Canto OO, Ornelas-Gutierrez CE, Espinosa-Magaña F, Olive-Méndez SF - Nanoscale Res Lett (2014)

Proposed model indicating the location and orientation of the spin on the orbital dz2. Red continuous arrows represent the location and orientation of the spin, and black dashed arrows represent the spin components considered in the obtained FM moments. Red dashed arrows give a qualitative indication of the magnitude of the ferromagnetic moment obtained for each studied case. (a) For sample PP, as the spin-orbit interaction is very strong, the vertical component of the spin will align with the applied magnetic field during the TT. (b) For sample PL, the horizontal component will align with the direction of the field during TT and the total component is smaller than that of (a).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4256971&req=5

Figure 6: Proposed model indicating the location and orientation of the spin on the orbital dz2. Red continuous arrows represent the location and orientation of the spin, and black dashed arrows represent the spin components considered in the obtained FM moments. Red dashed arrows give a qualitative indication of the magnitude of the ferromagnetic moment obtained for each studied case. (a) For sample PP, as the spin-orbit interaction is very strong, the vertical component of the spin will align with the applied magnetic field during the TT. (b) For sample PL, the horizontal component will align with the direction of the field during TT and the total component is smaller than that of (a).
Mentions: Crystal field theory (CFT) was used to explain these results in agreement with the anisotropy feature in DMO thin films. For schematic purposes, in Figure 6, a Co2+ ion is placed at the center of the octahedron formed by O atoms on the rutile cell. The 3d orbitals of the Co atom according with the CFT are dz2 and dx2-y2, which are degenerated, in a higher energy level than orbitals dxy, dxz, and dzx. Mimaki et al. [27] studied the bond character of rutile type on SiO2, GeO2, and SnO2, and they observed that the ratio of the electron density of M-O equatorial distances/M-O axial distances (where M is Si, Ge, or Sn) decreases when increases the atomic number of the cation. Then, for SnO2, the electron density is higher on the dz2 orbital and only one unpaired electron will be occupying in this orbital. The FM moments per Co at. are very small, suggesting that the Co atoms have a low spin configuration. This fact is in agreement with the measurements that a higher magnetic moment was obtained for sample PP, this is because dz2 orbital axis forms an angle of 34.1° with SnO2(101) plane (which remains parallel to the surface), and we assume a very strong spin - orbit coupling. With the TT applied in the PP configuration, the vertical component of the spin is aligned in the same direction than that of the magnetic field. The contribution of the possible random orientations of the SnO2 cells is represented in Figure 6a. For PL configuration, the alignment of the spins is in a manner that the horizontal components are parallel to the applied field of the TT, as shown in Figure 6b, and the addition of all these components leads to a total magnetic moment lower than that of the PP configuration. These results can be compared with monocrystalline thin films (e.g., epitaxial films on r-cut sapphire substrate), where the spin components can be better appreciated, and different magnetizations are obtained for PL and PP configurations.

Bottom Line: 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 [101] direction of Co-doped SnO2 nanocrystals, which is proposed to be the easy magnetization axis.

View Article: PubMed Central - HTML - PubMed

Affiliation: Centro de Investigación en Materiales Avanzados, S.C., Cimav, Av. Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31109 Chihuahua, Chihuahua, Mexico.

ABSTRACT
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 [101] direction of Co-doped SnO2 nanocrystals, which is proposed to be the easy magnetization axis.

No MeSH data available.