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Magnetic properties of Ni-implanted ITO thin films
Journal of Magnetism and Magnetic Materials 375 (2015) 129–135
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of Ni-implanted ITO thin films F. Ay a,n, B. Aktaş a, R.I. Khaibullin b, V.I. Nuzhdin b, B.Z. Rameev a,b a b
Gebze Institute of Technology, Department of Physics, 41400 Kocaeli, Turkey Kazan Physical-Technical Institute, 420029 Kazan, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2014 Received in revised form 22 September 2014 Accepted 22 September 2014 Available online 8 October 2014
The magnetic properties of Ni-implanted ITO thin films have been investigated by ferromagnetic resonance (FMR) technique and vibrating sample magnetometry (VSM) techniques. Commercially available ITO thin films on fused silica substrates have been implanted with different fluences of Ni þ ions with energy of 40 keV and ion current density of 8 mA/cm2 at room temperature. The samples with three doses of 0.5 1017, 1.0 1017 and 1.5 1017 ions/cm2 have been studied. Room temperature ferromagnetism has been observed in the nickel-implanted ITO samples with fluences of 1.0 1017 and 1.5 1017 ions/cm2. The magnetic properties of the samples have been explained by the formation of Ninanoparticles in the implanted surface layer of the ITO films. Although the formation of a diluted magnetic oxide phase cannot be ruled out entirely, the analysis of our FMR and VSM data reveals that the metallic Ni nanoparticles, formed during high-dose implantation process, have major contribution to the magnetic properties of the Ni-implanted ITO thin films. The sizes of the Ni-nanoparticles have been calculated from the blocking temperatures obtained by the VSM measurements. The filling factor of the Ni ferromagnetic phase in the granular magnetic layer has also been estimated by the effective magnetization approach applied to the FMR results. & Elsevier B.V. All rights reserved.
1. Introduction Room temperature ferromagnetism in the oxide semiconductors, such as ZnO, SnO2 or TiO2 doped by transition-metal (TM) ions (Fe, Co, Ni etc.), is very intriguing topic of solid state research due to their potential applications in the spintronics [1–3]. However, the studies of diluted magnetic semiconductors showing ferromagnetic features have revealed a huge number of controversies in both theoretical and experimental understanding of their properties [4]. One of the possible candidates for realization of the oxide-based ferromagnetic semiconductor is Indium Tin Oxide (ITO, or Sn-doped In2O3) [5], which is widely used in the industry as a transparent conductor [6–8]. A number of publications on the magnetic properties of the transition metal doped ITO have been published. It has been shown that doping of ITO by transition-metal (TM) ions could be a route to make this semiconductor oxide magnetic [9–20]. In this respect it would be interesting to apply the ion implantation technique in attempt to prepare diluted magnetic oxide material. In this paper, ITO films implanted with 40 keV Ni þ ions to the fluences of 0.5 1017, 1.0 1017 and 1.5 1017 ions/cm2 have been studied. The Ni-implanted ITO films on quartz substrates have been characterized by scanning electron microscope (SEM) and the energyn
Corresponding author. Tel.: þ 90 262 6051351; fax: þ90 262 6051305. E-mail address: ayfi[email protected] (F. Ay).
http://dx.doi.org/10.1016/j.jmmm.2014.09.075 0304-8853/& Elsevier B.V. All rights reserved.
dispersive X-ray (EDX) analysis. The thickness of the virgin and implanted films has been controlled by X-Ray Reflectivity (XRR) technique. The magnetic properties of the Ni-implanted ITO films have been studied by ferromagnetic resonance (FMR) and Vibrating Sample Magnetometer (VSM) techniques. Room-temperature ferromagnetism in the Ni-implanted ITO films has been observed with the magnetization increasing with the implanted ion fluence. It has been shown that the ferromagnetism in the ITO system implanted by Ni ions to high fluences is related to appearance of nano-sized clusters of nickel dispersed in the ITO matrix.
2. Sample preparation and experimental techniques Ni:ITO thin flms have been prepared by ion implantation technique at room temperature using ion-beam accelerator ILU-3 under vacuum at a residual pressure of 10 5 Torr. Commercially available ITO thin films on fused silica substrates have been implanted with 40 keV Ni þ ions to fluences of 0.5 1017, 1.0 1017 and 1.5 1017 ions/cm2 and ion current density J¼ 8 mA/cm2 at room temperature. The ITO films have the nominal thickness of about 30 nm. Surface morphology has been investigated by the scanning electron microscope Philips XL 30 SFEG and the chemical composition of the samples has been characterized by the energy-dispersiveX-ray (EDX) spectrometer. The thicknesses of the deposited
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F. Ay et al. / Journal of Magnetism and Magnetic Materials 375 (2015) 129–135
layers have been determined by X-Ray Reflectivity (XRR) technique using Rigaku Smartlab X-ray Diffractometer. Magnetic properties of the thin films have been investigated by ferromagnetic resonance (FMR) and Vibrating Sample Magnetometer (VSM) techniques. Bruker EMX X-band electrons spin resonance spectrometer has been used in FMR studies. The measurements have been performed at room temperature with static magnetic field up to 22 kOe. For VSM magnetometry measurements Quantum Design Physical Property Measurement System (PPMS) with the 9 T superconducting solenoid equipped with EverCool system has been used.
3. Experimental results 3.1. SEM and EDX measurements SEM image of the ITO film on the quartz substrate implanted with Ni þ to a dose (fluence) of 1.5 1017 ions/cm2 is presented in Fig. 1. As seen in Fig. 1, the high-dose implantation results in formation of the bright spots. Energy dispersive X-ray (EDX)
results at 15 kV, for various areas shown in Fig. 1, are given in Fig. 2a and b, for rectangular area and the spot, respectively. The results of the EDX analysis (Table 1) shows that Ni/ITO ratio for the spot is more than two times higher than for the larger rectangular area. 3.2. XRR Measurements and results The thicknesses of all ITO thin films have been determined by X-Ray Reflectivity (XRR) technique to be about 28.5 nm, as shown in Figs. 3–5. Surprisingly, the XRR data does not reveal a appreciable thinning of the ITO films with the fluence as it has been observed previously in other implanted oxides (see e.g. [21]). In such system as ZnO wurtzite or TiO2 rutile the high-dose implantation results in the noticeable surface sputtering of the implanted host material. On the other hand, the XRR study shows an essential increase of the film surface roughness with the fluence. The surface roughness of the virgin film has been estimated with use of Rigaku-GlobalFit to be about of 0.4 nm. Unfortunately it turned out hardly possible to obtain an accurate value of the surface roughness for the implanted films. However, our rough estimation shows that it increases to 2 nm for the ITO film implanted to the highest fluence (1.5 1017 ions/cm2). 3.3. VSM measurements
Fig. 1. SEM image of the Ni:ITO sample with the ion fluence of 1.5 1017 ions/cm2.
The hysteresis loops and temperature dependence of the magnetization have been measured. Hysteresis loops at various temperatures in range of 10–300 K are given in Fig. 6. The central parts of the hysteresis loops at three representative temperature points are given in Fig. 7. Temperature dependence of magnetization has been measured for the samples both in ZFC (zero-field cooled) and FC (field cooled) conditions at static magnetic field H ¼100 Oe (Fig. 8).
Fig. 2. EDX results the Ni:ITO sample with the ion fluence of 1.5 1017 ions/cm2 for the rectangular area (a), and the spot (b) (see Fig. 1 for the regions probed).
F. Ay et al. / Journal of Magnetism and Magnetic Materials 375 (2015) 129–135
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3.4. FMR measurements
4. Discussion
FMR measurements have been made for various orientations by rotating direction of static magnetic field in the in-plane and out-of-plane geometries. In the out-of-plane geometry the static magnetic field is rotated in the plane perpendicular to the film plane, while in the in-plane geometry the field is rotated parallel to the film plane. No signal have been recorded for the Ni:ITO sample with the lowest implantation fluence. FMR spectra are presented in Figs. 9a and 10a, while the out-of-plane angular dependence of resonance fields are given in Figs. 9b and 10b. No in-plane anisotropy has been observed in the FMR measurements. However, a large out-of-plane anisotropy and strong resonance signal observed in our measurements reveal behavior, which is typical for a thin ferromagnetic film. It is remarkable also that the anisotropy increases with the implantation fluence.
Three phases are clearly observed in ZFC and FC temperature dependences of the magnetization as well in the hysteresis loops for the samples with the fluence of 1.0 1017 ions/cm2 and the fluence of 1.5 1017 ions/cm2 (see Figs. 6–8), which are ferromagnetic, superparamagnetic and paramagnetic phases. The ferromagnetic phase is observed in the hysteresis loops and in the high temperature range of M(T) curves, superparamagnetic phase is obvious through the divergence of FC and ZFC curves at TB, and the paramagnetic phase is responsible for divergence of the magnetization curves at temperatures below the blocking temperatures TB. Appearance of the paramagnetic phase is related to the large concentration paramagnetic defects that persists in the quartz substrate and, probably, in the virgin ITO films. At least a part of these defects is related to Fe2 þ ions in quartz substrate, as obvious from the Electron Paramagnetic Resonance (EPR) signal with g E4 observed in our magnetic resonance measurements at the DC magnetic field value of 1800 Oe (see Figs. 9 and 10). As regards the superparamagnetic phase, the size of Ni nanoparticles could be calculated from the observed blocking temperatures. According to Neel relaxation theory [22] the magnetization direction of small ferromagnetic nanoparticles randomly flip due to thermal fluctuations. The mean time between two flips is called the Néel relaxation time τN and given by the following Néel– Arrhenius equation [22]:
Table 1 Arbitrary concentrations of O, Ni, In, Sn atoms in the ITO thin films implanted with Ni þ ions to the fluence of 1.5 1017 ions/cm2. Spot/area
Area Spot
At% O
Ni
In
Sn
Ni/ITO (%)
44.46 46.44
2.28 5.93
0.25 0.25
0.48 0.42
5.0 12.5
⎛ KV ⎞ ǀ N = ǀ 0exp⎜ ⎟ ⎝ kBT ⎠
Fig. 3. XRR curve (at the left) and thickness calculation (at the right) of non-implanted (virgin) ITO sample.
Fig. 4. XRR curve (at the left) and thickness calculation (at the right) of the ITO sample implanted with Ni þ ions to the fluence of 0.5 1017 ions/cm2.
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Fig. 5. XRR curve (at the left) and thickness calculation (at the right) of non-implanted ITO sample of the ITO sample implanted with Ni þ ions to the fluence of 1.5 1017 ions/cm2.
Fig. 6. Hysteresis loops at various temperatures for the Ni:ITO sample with the ion fluence of 1.0 1017 ions/cm2 (a) and for the Ni:ITO sample with the ion fluence of 1.5 1017 ions/cm2 (b).
Fig. 7. The central part of the hysteresis loops at temperatures of 10 K, 50 K and 300 K for the Ni:ITO sample with the ion fluence of 1.0 1017 ions/cm2 (a) and for the Ni:ITO sample with the ion fluence of 1.5 1017 ions/cm2. (b) The paramagnetic or diamagnetic contributions have been subtracted.
Here K is the magnetic anisotropy energy density, V is volume of a ferromagnetic nanoparticle, kB is the Boltzmann constant, T is the temperature, and τ0 is the characteristic time (“attempt time”) of the material, with typical value between 10 9 and 10 10 s. The blocking temperature is determined as the temperature for which the time τN is equal to the measurement time τm (the time to
probe the magnetization of a superparamagnetic nanoparticle):
TB =
KV kB ln ǀ m/ǀ 0
Assuming TB ¼ 30 K, τm ¼1 s (a typical time to measure one data point in our VSM measurements), τ0 ¼ 10 9, K ¼0.5 105 erg/cm3
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Consequently, the mean diameter is:
D=
3
3V = 1.1789 ¬ 10ℬ6 cm ↠ 11.8 nm . ư
Applying the same procedure to the Ni nanoparticles in the sample with the ion fluence of 1.5 1017 ions/cm2, we have: D = 3 3V /ư = 1.635 ¬ 10ℬ6 cm ↠ 16.35 nm . Thus, it is clear that implantation of the ITO thin films by Ni þ ions to the high fluences results in formation of superparamagnetic Ni nanoparticles dispersed in the subsurface layer of ITO matrix. However, it is remarkable that for the Ni:ITO sample with the highest implantation dose rounding of M(T) curve with lowering temperature is also observed (see Fig. 8b, temperature range between TB and 300 K). On the other hand, the central parts of the VSM hysteresis loops (Fig. 7) as well as the room temperature magnetic resonance experiments show the behavior which is typical for the ferromagnetic phase. The resonance signal reveals the strong out-of-plane anisotropy and its magnitude is increasing with the implantation fluence. Besides the obtained g-factor value is very close to that of the bulk metal Ni (g E2.21) [23]. All these experimental observations indicate that an essential part of the implanted Ni ions forms a magnetically percolated granular layer consisting of agglomerated ferromagnetic metal Ni particles. In the case of quasicontinuous ferromagnetic layer consisting of nanoparticles, so called effective magnetization approach could be applied [24–26]. The effective magnetization Meff and the gfactor values have been obtained by theoretical modeling of the angular dependences of FMR resonance fields (Figs. 9 and 10) and given in the Table 2 below. Nickel ferromagnetic phase filling factor f for such granular ferromagnetic layer have been calculated for both Ni:ITO samples with fluences of 1.0 1017 ions/cm2 and 1.5 1017 ions/cm2: For the sample with the fluence of 1.0 1017 ions/cm2: f ¼Meff /M¼ 30/485¼ 0.062. For the sample with the fluence of 1.5 1017 ions/cm2: f ¼Meff /M¼ 100/485¼ 0.206.
Fig. 8. Temperature dependence of magnetic moment in ZFC and FC VSM measurements for the virgin ITO (a), for the Ni:ITO samples implanted with the ion fluence of 1.0 1017 ions/cm2 (b) and 1.5 1017 ions/cm2 (c).
and the spherical shape of Ni nanoparticles, the volume of metal Ni nanoparticles in the sample with the ion fluence of 1.0 1017 ions/cm2 is obtained as follows:
V=
TBkB ln 109 V = 1.716 ¬ 10ℬ18 cm3 . K
where M is the magnetization of bulk nickel. The filling ratios obtained above are compatible with those obtained for other oxides implanted with Co, Ni, Fe and other ions. For example, the filling ratio of 23% has been observed in the TiO2 rutile single crystal substrates implanted with 40 keV Co þ ions to the fluence of 1.5 1017 ions/cm2 [27]. Thus, the bulk of experimental results indicate that the implantation by Ni þ ions results in formation of the bilayer ferromagnetic–superparamagnetic system, consisting of the nanosized Ni nanoparticles. Because the peak of Ni concentration in oxide materials implanted with 40 keV transition ions is usually located very close to the surface ( 15–20 nm) [21], the ferromagnetic phase is expected to be concentrated mainly in the ITO matrix forming a quasicontinuous granular layer of strongly-interacting Ni nanoparticles. As regards the superparamagnetic phase it is presumably attributed to the non-agglomerated and weakly interacting nanoparticles located deeper either at the interface between ITO and quartz substrate or inside the quartz substrate.
5. Conclusions Commercially available ITO thin films on fused silica substrates have been implanted at room temperature with 40 keV Ni þ ions to the fluences of 0.5 1017, 1.0 1017 and 1.5 1017 ions/cm2. XRR measurements provides the thickness of the non-implanted and
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Fig. 9. Angular dependences of the FMR spectra (a) and the FMR resonance field (b) for the sample with the ion fluence of 1.0 1017 ions/cm2.
Fig. 10. Angular dependences of the FMR spectra (a) and the FMR resonance field and (b) for the sample with the ion fluence of 1.5 1017 ions/cm2.
Table 2 Effective magnetism and g-factors obtained by theoretical modeling of the FMR resonance fields. Sample
g-factor
Effective magnetization (emu/cm3)
Ni:ITO sample with fluence of 1.0 1017 ions/cm2 Ni:ITO sample with fluence of 1.5 1017 ions/cm2
2.17 2.24
30 100
Ni-implanted ITO thin films to be about 28.5 nm, and it does not change noticeably with the implantation fluence. In contrast the surface roughness increases an order of magnitude from the virgin film to the film implanted with fluence 1.5 1017 ions/cm2. SEM and EDX studies prove a highly non-uniform distribution of Ni atoms in the lateral plane of the implanted ITO films. Room temperature ferromagnetism has been observed by both FMR technique and VSM hysteresis measurements in the nickel-implanted ITO samples with fluences of 1.0 1017 and 1.5 1017 ions/cm2. The middle part of the hysteresis curves in VSM as well as the out-of-plane anisotropy of FMR signal points to the formation of a ferromagnetic layer in the volume of ITO film. This layer is attributed to the agglomerates of closely-spaced metal Ni nanoparticles, which form a quasi-continuous magnetic layer responding as a whole system with the magnetic point of view. Superparamagnetic and paramagnetic phases have been also
observed in the temperature dependence of magnetization by VSM measurements. Paramagnetic phase dominant at lowest temperatures is attributed to the presence of paramagnetic impurities which persist in the quartz substrate and virgin ITO thin film. The EPR signal observed in all samples at g 4 indicates that Fe2 þ impurity centers in the quartz substrate contribute to the paramagnetic phase observed. Superparamagnetic phase is related to the Ni nanoparticles located in deeper regions probably near the film/substrate interface or within the quartz substrate. For the Niimplanted ITO samples with fluences of 1.0 1017 and 1.5 1017 ions/cm2 average sizes of the superparamagnetic nanoparticles have been also calculated from the blocking temperatures TB observed in temperature-dependent magnetization measurements. Finally we did not find the experimental evidences for the formation of an diluted magnetic oxide phase in the Ni-implanted ITO thin films.
F. Ay et al. / Journal of Magnetism and Magnetic Materials 375 (2015) 129–135
Acknowledgments The work was partially supported by the TÜBITAK / RFBR joint project program, grant No. 213M524 / 14-02-91374_ст-а. The authors would like to thank Dr. N.Doğan and A.Nazım for their assistance in the VSM-PPMS and SEM/EDX measurements, correspondingly.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of Ni-implanted ITO thin films F. Ay a,n, B. Aktaş a, R.I. Khaibullin b, V.I. Nuzhdin b, B.Z. Rameev a,b a b
Gebze Institute of Technology, Department of Physics, 41400 Kocaeli, Turkey Kazan Physical-Technical Institute, 420029 Kazan, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2014 Received in revised form 22 September 2014 Accepted 22 September 2014 Available online 8 October 2014
The magnetic properties of Ni-implanted ITO thin films have been investigated by ferromagnetic resonance (FMR) technique and vibrating sample magnetometry (VSM) techniques. Commercially available ITO thin films on fused silica substrates have been implanted with different fluences of Ni þ ions with energy of 40 keV and ion current density of 8 mA/cm2 at room temperature. The samples with three doses of 0.5 1017, 1.0 1017 and 1.5 1017 ions/cm2 have been studied. Room temperature ferromagnetism has been observed in the nickel-implanted ITO samples with fluences of 1.0 1017 and 1.5 1017 ions/cm2. The magnetic properties of the samples have been explained by the formation of Ninanoparticles in the implanted surface layer of the ITO films. Although the formation of a diluted magnetic oxide phase cannot be ruled out entirely, the analysis of our FMR and VSM data reveals that the metallic Ni nanoparticles, formed during high-dose implantation process, have major contribution to the magnetic properties of the Ni-implanted ITO thin films. The sizes of the Ni-nanoparticles have been calculated from the blocking temperatures obtained by the VSM measurements. The filling factor of the Ni ferromagnetic phase in the granular magnetic layer has also been estimated by the effective magnetization approach applied to the FMR results. & Elsevier B.V. All rights reserved.
1. Introduction Room temperature ferromagnetism in the oxide semiconductors, such as ZnO, SnO2 or TiO2 doped by transition-metal (TM) ions (Fe, Co, Ni etc.), is very intriguing topic of solid state research due to their potential applications in the spintronics [1–3]. However, the studies of diluted magnetic semiconductors showing ferromagnetic features have revealed a huge number of controversies in both theoretical and experimental understanding of their properties [4]. One of the possible candidates for realization of the oxide-based ferromagnetic semiconductor is Indium Tin Oxide (ITO, or Sn-doped In2O3) [5], which is widely used in the industry as a transparent conductor [6–8]. A number of publications on the magnetic properties of the transition metal doped ITO have been published. It has been shown that doping of ITO by transition-metal (TM) ions could be a route to make this semiconductor oxide magnetic [9–20]. In this respect it would be interesting to apply the ion implantation technique in attempt to prepare diluted magnetic oxide material. In this paper, ITO films implanted with 40 keV Ni þ ions to the fluences of 0.5 1017, 1.0 1017 and 1.5 1017 ions/cm2 have been studied. The Ni-implanted ITO films on quartz substrates have been characterized by scanning electron microscope (SEM) and the energyn
Corresponding author. Tel.: þ 90 262 6051351; fax: þ90 262 6051305. E-mail address: ayfi[email protected] (F. Ay).
http://dx.doi.org/10.1016/j.jmmm.2014.09.075 0304-8853/& Elsevier B.V. All rights reserved.
dispersive X-ray (EDX) analysis. The thickness of the virgin and implanted films has been controlled by X-Ray Reflectivity (XRR) technique. The magnetic properties of the Ni-implanted ITO films have been studied by ferromagnetic resonance (FMR) and Vibrating Sample Magnetometer (VSM) techniques. Room-temperature ferromagnetism in the Ni-implanted ITO films has been observed with the magnetization increasing with the implanted ion fluence. It has been shown that the ferromagnetism in the ITO system implanted by Ni ions to high fluences is related to appearance of nano-sized clusters of nickel dispersed in the ITO matrix.
2. Sample preparation and experimental techniques Ni:ITO thin flms have been prepared by ion implantation technique at room temperature using ion-beam accelerator ILU-3 under vacuum at a residual pressure of 10 5 Torr. Commercially available ITO thin films on fused silica substrates have been implanted with 40 keV Ni þ ions to fluences of 0.5 1017, 1.0 1017 and 1.5 1017 ions/cm2 and ion current density J¼ 8 mA/cm2 at room temperature. The ITO films have the nominal thickness of about 30 nm. Surface morphology has been investigated by the scanning electron microscope Philips XL 30 SFEG and the chemical composition of the samples has been characterized by the energy-dispersiveX-ray (EDX) spectrometer. The thicknesses of the deposited
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F. Ay et al. / Journal of Magnetism and Magnetic Materials 375 (2015) 129–135
layers have been determined by X-Ray Reflectivity (XRR) technique using Rigaku Smartlab X-ray Diffractometer. Magnetic properties of the thin films have been investigated by ferromagnetic resonance (FMR) and Vibrating Sample Magnetometer (VSM) techniques. Bruker EMX X-band electrons spin resonance spectrometer has been used in FMR studies. The measurements have been performed at room temperature with static magnetic field up to 22 kOe. For VSM magnetometry measurements Quantum Design Physical Property Measurement System (PPMS) with the 9 T superconducting solenoid equipped with EverCool system has been used.
3. Experimental results 3.1. SEM and EDX measurements SEM image of the ITO film on the quartz substrate implanted with Ni þ to a dose (fluence) of 1.5 1017 ions/cm2 is presented in Fig. 1. As seen in Fig. 1, the high-dose implantation results in formation of the bright spots. Energy dispersive X-ray (EDX)
results at 15 kV, for various areas shown in Fig. 1, are given in Fig. 2a and b, for rectangular area and the spot, respectively. The results of the EDX analysis (Table 1) shows that Ni/ITO ratio for the spot is more than two times higher than for the larger rectangular area. 3.2. XRR Measurements and results The thicknesses of all ITO thin films have been determined by X-Ray Reflectivity (XRR) technique to be about 28.5 nm, as shown in Figs. 3–5. Surprisingly, the XRR data does not reveal a appreciable thinning of the ITO films with the fluence as it has been observed previously in other implanted oxides (see e.g. [21]). In such system as ZnO wurtzite or TiO2 rutile the high-dose implantation results in the noticeable surface sputtering of the implanted host material. On the other hand, the XRR study shows an essential increase of the film surface roughness with the fluence. The surface roughness of the virgin film has been estimated with use of Rigaku-GlobalFit to be about of 0.4 nm. Unfortunately it turned out hardly possible to obtain an accurate value of the surface roughness for the implanted films. However, our rough estimation shows that it increases to 2 nm for the ITO film implanted to the highest fluence (1.5 1017 ions/cm2). 3.3. VSM measurements
Fig. 1. SEM image of the Ni:ITO sample with the ion fluence of 1.5 1017 ions/cm2.
The hysteresis loops and temperature dependence of the magnetization have been measured. Hysteresis loops at various temperatures in range of 10–300 K are given in Fig. 6. The central parts of the hysteresis loops at three representative temperature points are given in Fig. 7. Temperature dependence of magnetization has been measured for the samples both in ZFC (zero-field cooled) and FC (field cooled) conditions at static magnetic field H ¼100 Oe (Fig. 8).
Fig. 2. EDX results the Ni:ITO sample with the ion fluence of 1.5 1017 ions/cm2 for the rectangular area (a), and the spot (b) (see Fig. 1 for the regions probed).
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3.4. FMR measurements
4. Discussion
FMR measurements have been made for various orientations by rotating direction of static magnetic field in the in-plane and out-of-plane geometries. In the out-of-plane geometry the static magnetic field is rotated in the plane perpendicular to the film plane, while in the in-plane geometry the field is rotated parallel to the film plane. No signal have been recorded for the Ni:ITO sample with the lowest implantation fluence. FMR spectra are presented in Figs. 9a and 10a, while the out-of-plane angular dependence of resonance fields are given in Figs. 9b and 10b. No in-plane anisotropy has been observed in the FMR measurements. However, a large out-of-plane anisotropy and strong resonance signal observed in our measurements reveal behavior, which is typical for a thin ferromagnetic film. It is remarkable also that the anisotropy increases with the implantation fluence.
Three phases are clearly observed in ZFC and FC temperature dependences of the magnetization as well in the hysteresis loops for the samples with the fluence of 1.0 1017 ions/cm2 and the fluence of 1.5 1017 ions/cm2 (see Figs. 6–8), which are ferromagnetic, superparamagnetic and paramagnetic phases. The ferromagnetic phase is observed in the hysteresis loops and in the high temperature range of M(T) curves, superparamagnetic phase is obvious through the divergence of FC and ZFC curves at TB, and the paramagnetic phase is responsible for divergence of the magnetization curves at temperatures below the blocking temperatures TB. Appearance of the paramagnetic phase is related to the large concentration paramagnetic defects that persists in the quartz substrate and, probably, in the virgin ITO films. At least a part of these defects is related to Fe2 þ ions in quartz substrate, as obvious from the Electron Paramagnetic Resonance (EPR) signal with g E4 observed in our magnetic resonance measurements at the DC magnetic field value of 1800 Oe (see Figs. 9 and 10). As regards the superparamagnetic phase, the size of Ni nanoparticles could be calculated from the observed blocking temperatures. According to Neel relaxation theory [22] the magnetization direction of small ferromagnetic nanoparticles randomly flip due to thermal fluctuations. The mean time between two flips is called the Néel relaxation time τN and given by the following Néel– Arrhenius equation [22]:
Table 1 Arbitrary concentrations of O, Ni, In, Sn atoms in the ITO thin films implanted with Ni þ ions to the fluence of 1.5 1017 ions/cm2. Spot/area
Area Spot
At% O
Ni
In
Sn
Ni/ITO (%)
44.46 46.44
2.28 5.93
0.25 0.25
0.48 0.42
5.0 12.5
⎛ KV ⎞ ǀ N = ǀ 0exp⎜ ⎟ ⎝ kBT ⎠
Fig. 3. XRR curve (at the left) and thickness calculation (at the right) of non-implanted (virgin) ITO sample.
Fig. 4. XRR curve (at the left) and thickness calculation (at the right) of the ITO sample implanted with Ni þ ions to the fluence of 0.5 1017 ions/cm2.
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Fig. 5. XRR curve (at the left) and thickness calculation (at the right) of non-implanted ITO sample of the ITO sample implanted with Ni þ ions to the fluence of 1.5 1017 ions/cm2.
Fig. 6. Hysteresis loops at various temperatures for the Ni:ITO sample with the ion fluence of 1.0 1017 ions/cm2 (a) and for the Ni:ITO sample with the ion fluence of 1.5 1017 ions/cm2 (b).
Fig. 7. The central part of the hysteresis loops at temperatures of 10 K, 50 K and 300 K for the Ni:ITO sample with the ion fluence of 1.0 1017 ions/cm2 (a) and for the Ni:ITO sample with the ion fluence of 1.5 1017 ions/cm2. (b) The paramagnetic or diamagnetic contributions have been subtracted.
Here K is the magnetic anisotropy energy density, V is volume of a ferromagnetic nanoparticle, kB is the Boltzmann constant, T is the temperature, and τ0 is the characteristic time (“attempt time”) of the material, with typical value between 10 9 and 10 10 s. The blocking temperature is determined as the temperature for which the time τN is equal to the measurement time τm (the time to
probe the magnetization of a superparamagnetic nanoparticle):
TB =
KV kB ln ǀ m/ǀ 0
Assuming TB ¼ 30 K, τm ¼1 s (a typical time to measure one data point in our VSM measurements), τ0 ¼ 10 9, K ¼0.5 105 erg/cm3
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Consequently, the mean diameter is:
D=
3
3V = 1.1789 ¬ 10ℬ6 cm ↠ 11.8 nm . ư
Applying the same procedure to the Ni nanoparticles in the sample with the ion fluence of 1.5 1017 ions/cm2, we have: D = 3 3V /ư = 1.635 ¬ 10ℬ6 cm ↠ 16.35 nm . Thus, it is clear that implantation of the ITO thin films by Ni þ ions to the high fluences results in formation of superparamagnetic Ni nanoparticles dispersed in the subsurface layer of ITO matrix. However, it is remarkable that for the Ni:ITO sample with the highest implantation dose rounding of M(T) curve with lowering temperature is also observed (see Fig. 8b, temperature range between TB and 300 K). On the other hand, the central parts of the VSM hysteresis loops (Fig. 7) as well as the room temperature magnetic resonance experiments show the behavior which is typical for the ferromagnetic phase. The resonance signal reveals the strong out-of-plane anisotropy and its magnitude is increasing with the implantation fluence. Besides the obtained g-factor value is very close to that of the bulk metal Ni (g E2.21) [23]. All these experimental observations indicate that an essential part of the implanted Ni ions forms a magnetically percolated granular layer consisting of agglomerated ferromagnetic metal Ni particles. In the case of quasicontinuous ferromagnetic layer consisting of nanoparticles, so called effective magnetization approach could be applied [24–26]. The effective magnetization Meff and the gfactor values have been obtained by theoretical modeling of the angular dependences of FMR resonance fields (Figs. 9 and 10) and given in the Table 2 below. Nickel ferromagnetic phase filling factor f for such granular ferromagnetic layer have been calculated for both Ni:ITO samples with fluences of 1.0 1017 ions/cm2 and 1.5 1017 ions/cm2: For the sample with the fluence of 1.0 1017 ions/cm2: f ¼Meff /M¼ 30/485¼ 0.062. For the sample with the fluence of 1.5 1017 ions/cm2: f ¼Meff /M¼ 100/485¼ 0.206.
Fig. 8. Temperature dependence of magnetic moment in ZFC and FC VSM measurements for the virgin ITO (a), for the Ni:ITO samples implanted with the ion fluence of 1.0 1017 ions/cm2 (b) and 1.5 1017 ions/cm2 (c).
and the spherical shape of Ni nanoparticles, the volume of metal Ni nanoparticles in the sample with the ion fluence of 1.0 1017 ions/cm2 is obtained as follows:
V=
TBkB ln 109 V = 1.716 ¬ 10ℬ18 cm3 . K
where M is the magnetization of bulk nickel. The filling ratios obtained above are compatible with those obtained for other oxides implanted with Co, Ni, Fe and other ions. For example, the filling ratio of 23% has been observed in the TiO2 rutile single crystal substrates implanted with 40 keV Co þ ions to the fluence of 1.5 1017 ions/cm2 [27]. Thus, the bulk of experimental results indicate that the implantation by Ni þ ions results in formation of the bilayer ferromagnetic–superparamagnetic system, consisting of the nanosized Ni nanoparticles. Because the peak of Ni concentration in oxide materials implanted with 40 keV transition ions is usually located very close to the surface ( 15–20 nm) [21], the ferromagnetic phase is expected to be concentrated mainly in the ITO matrix forming a quasicontinuous granular layer of strongly-interacting Ni nanoparticles. As regards the superparamagnetic phase it is presumably attributed to the non-agglomerated and weakly interacting nanoparticles located deeper either at the interface between ITO and quartz substrate or inside the quartz substrate.
5. Conclusions Commercially available ITO thin films on fused silica substrates have been implanted at room temperature with 40 keV Ni þ ions to the fluences of 0.5 1017, 1.0 1017 and 1.5 1017 ions/cm2. XRR measurements provides the thickness of the non-implanted and
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Fig. 9. Angular dependences of the FMR spectra (a) and the FMR resonance field (b) for the sample with the ion fluence of 1.0 1017 ions/cm2.
Fig. 10. Angular dependences of the FMR spectra (a) and the FMR resonance field and (b) for the sample with the ion fluence of 1.5 1017 ions/cm2.
Table 2 Effective magnetism and g-factors obtained by theoretical modeling of the FMR resonance fields. Sample
g-factor
Effective magnetization (emu/cm3)
Ni:ITO sample with fluence of 1.0 1017 ions/cm2 Ni:ITO sample with fluence of 1.5 1017 ions/cm2
2.17 2.24
30 100
Ni-implanted ITO thin films to be about 28.5 nm, and it does not change noticeably with the implantation fluence. In contrast the surface roughness increases an order of magnitude from the virgin film to the film implanted with fluence 1.5 1017 ions/cm2. SEM and EDX studies prove a highly non-uniform distribution of Ni atoms in the lateral plane of the implanted ITO films. Room temperature ferromagnetism has been observed by both FMR technique and VSM hysteresis measurements in the nickel-implanted ITO samples with fluences of 1.0 1017 and 1.5 1017 ions/cm2. The middle part of the hysteresis curves in VSM as well as the out-of-plane anisotropy of FMR signal points to the formation of a ferromagnetic layer in the volume of ITO film. This layer is attributed to the agglomerates of closely-spaced metal Ni nanoparticles, which form a quasi-continuous magnetic layer responding as a whole system with the magnetic point of view. Superparamagnetic and paramagnetic phases have been also
observed in the temperature dependence of magnetization by VSM measurements. Paramagnetic phase dominant at lowest temperatures is attributed to the presence of paramagnetic impurities which persist in the quartz substrate and virgin ITO thin film. The EPR signal observed in all samples at g 4 indicates that Fe2 þ impurity centers in the quartz substrate contribute to the paramagnetic phase observed. Superparamagnetic phase is related to the Ni nanoparticles located in deeper regions probably near the film/substrate interface or within the quartz substrate. For the Niimplanted ITO samples with fluences of 1.0 1017 and 1.5 1017 ions/cm2 average sizes of the superparamagnetic nanoparticles have been also calculated from the blocking temperatures TB observed in temperature-dependent magnetization measurements. Finally we did not find the experimental evidences for the formation of an diluted magnetic oxide phase in the Ni-implanted ITO thin films.
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Acknowledgments The work was partially supported by the TÜBITAK / RFBR joint project program, grant No. 213M524 / 14-02-91374_ст-а. The authors would like to thank Dr. N.Doğan and A.Nazım for their assistance in the VSM-PPMS and SEM/EDX measurements, correspondingly.
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