The correlation between structure and magnetism of Ni-implanted TiO2 annealed at different temperatures

Journal of Magnetism and Magnetic Materials 324 (2012) 33–36

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The correlation between structure and magnetism of Ni-implanted TiO2 annealed at different temperatures Binfeng Ding a,b, Fengfeng Cheng a, Feng Pan a,c, Tao Fa a, Shude Yao a,n, Kay Potzger d, Shengqiang Zhou a,d a

State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China Department of Physics and Electronic Information, Langfang Teachers College, Langfang 065000, China c Department of Physics, Shanxi University of Technology, Hanzhong 723001, China d Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), P.O.Box 510119, Dresden 01314, Germany b

a r t i c l e i n f o

abstract

Article history: Received 22 March 2011 Available online 23 July 2011

In this paper, the structural and magnetic properties of Ni metal implanted TiO2 single crystals are discussed. Ni nanocrystals (NCs) have been formed in TiO2 after ion implantation. Their crystallite sizes increased with increasing post-annealing temperature. Metallic Ni nanocrystals inside the TiO2 matrix are stable up to an annealing temperature of 1073 K. The Ni NCs formed inside TiO2 make the major contribution to the measured ferromagnetism. & 2011 Elsevier B.V. All rights reserved.

Keywords: Diluted magnetic oxide Ion implantation TiO2

1. Introduction Currently there is a great deal of interest in diluted magnetic oxides (DMO), because the ferromagnetism is robust well above room temperature. These kinds of materials are fabricated by doping oxide semiconductors, such as ZnO, TiO2, SnO2, and In2O3 with transition metals. Many groups have reported the observation of ferromagnetism in transition-metal-doped rutile and anatase TiO2 [1–4]. However, the origin of the measured ferromagnetism is questionable. It was also found that the measured ferromagnetic properties can originate from nanoscale precipitates [5–10], or defects inside TiO2 [11–13]. Akdogan et al. identified two ferromagnetic contributions in Co implanted TiO2 from diluted Co2 þ and metallic Co nanocrystals [14]. Recently several review articles have addressed the complexity of transition-metal-doped oxides concerning the origin of ferromagnetism [15–17]. At the same time, there are some reports about Fe-doped TiO2, Co-doped TiO2, and La or Sr-doped TiO2 [7,18,19]. In those papers, the authors observed ferromagnetic nanocrystals (NCs) and pronounced magnetoelectric effects. To optimize these magnetoelectric effects, a systematic investigation of structural, and magnetic properties and temperature stability of these NCs is needed. In this article, a correlation between structure and magnetism in Ni-implanted TiO2 is presented. We try to answer two questions: (i) what is the origin of ferromagnetism in Ni-implanted

n

Corresponding author. Tel.: þ86 10 62757534 E-mail address: [email protected] (S. Yao).

0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.07.031

TiO2 and (ii) what is the effect of post-annealing on the magnetic and structural properties of Ni-implanted TiO2.

2. Experiments Commercial TiO2 bulk crystals were implanted with Ni ions at 623 K and the fluence of 4  1016 cm  2. The implantation energy was 180 keV, which resulted in a projected range of RP ¼847 29 nm and a maximum atomic concentration of about 5% (TRIM code). Annealing was performed in a high-vacuum (base pressure r10  6 mbar) furnace at temperature ranging from 823 to 1073 K for 15 min. Four samples are investigated and listed in Table 1. The lattice damage recovered after annealing was evaluated by Rutherford backscattering/channeling spectrometry (RBS/C). The RBS/C spectra were collected with a collimated 1.7 MeV He þ beam at a backscattering angle of 1701. The sample was mounted on a three-axis goniometer with a precision of 0.011. The channeling spectra were collected by aligning the sample to make the impinging He þ beam parallel to the TiO2[1 1 0] axis. Structural analysis was also performed by synchrotron radiation X-ray diffraction (SRXRD). SRXRD was performed at the Rossendorf beamline (BM20) of ESRF with an X-ray wavelength of 0.154 nm; y–2y scan was used to identify crystalline precipitates. The magnetic properties were measured with a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 5–350 K. The samples were measured with the field along the in-plane direction. To measure the temperature-dependent magnetization after zero field cooling and field cooling (ZFC/FC), the sample was cooled in zero field from above room temperature to 5 K. Then a 50 Oe field was applied. The ZFC

B. Ding et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 33–36

curve was measured with increasing temperature from 5 to 300(or 350) K, after which the FC curve was measured in the same field from 300(or 350) to 5 K with decreasing temperature.

3. Results and discussion 3.1. Lattice damage and recovery Fig. 1(a) shows representative RBS/C spectra. The arrows labeled Ni and Ti indicate the energy for He þ backscattering from surface Ni and Ti atoms, respectively. In the channeling spectra there are two peaks, which mainly originate from the lattice disorder due to implantation, namely, the bulk (the broad hump) and the surface damage regions (the sharp peak) [20]. The surface damage region is often a sink for ion-implantation-induced point defects, while the bulk region is where implanted Ni is deposited. However implanted Ni ions, if they do not substitute Ti sites, also contribute significantly to the surface peak; wmin is the channeling minimum yield in RBS/C, which is the ratio of the backscattering yields at channeling condition to that for a random beam incidence [21]. Therefore wmin labels the degree of lattice disorder upon implantation, i.e., an amorphous sample shows a wmin value of 100%, while a perfect single crystal corresponds to a wmin value of 1–2%; wmin was calculated for the bulk damage region and is listed in Table 1 and Fig. 1(b). With increasing annealing temperature up to 923 K, the damage peak is reduced. Finally after annealing at 1073 K the lattice damage was almost mainly removed, as demonstrated by comparison with virgin TiO2. Although the backscattering signal from Ni partially overlaps with that of Ti one can obtain a qualitative evaluation of the depth

distribution of Ni by RUMP (The Rutherford Universal Manipulation Program) [22], a simulation program for RBS spectra. Fig. 2 shows the distribution of Ni simulated by RUMP together with the simulation of the implantation profile by TRIM. Already in the as-implanted state, Ni diffuses toward the surface. The profile does change significantly upon increasing the annealing temperature to 923 K. After annealing at 1073 K we can see that the distribution of Ni is more concentrated, which indicates that the Ni grain is growing and located in a narrow depth range. The diffusion behavior of Ni in TiO2 is different from that of Fe [23]. Fe diffuses towards the surface with increasing annealing temperature. 3.2. Structural properties SRXRD was used to identify the secondary phase in Ni-implanted TiO2. As shown in Fig. 3(a) in addition to the sharp peaks arising from the TiO2 substrate one sees the appearance of Ni(1 1 1), which is growing and becoming narrow with increasing annealing temperature. The average crystallite size shown in Fig. 3(b) was calculated using the Scherrer formula [23] d ¼ 0:9l=ðb cos yÞ

wmin by RBS/C (%) Crystallite size (XRD) of fcc-Ni (nm) Coercivity (Oe) Ni NC radius r (nm) TB by ZFC curves (K) TB by Eq. (2) (K)

As-implanted

823 K annealed

923 K annealed

53 0.9

53 3

37 6.8

130 0.45 13 0.05

150 1.5 15 2

300 3.4 17 23

1200

As-imp. random

simulated (by TRIM) As-imp. 923 K ann. 1073 K ann.

6

Table 1 Structural and magnetic properties for Ni-implanted TiO2 at different annealing temperatures. Sample

1073 K annealed

5 4 3 2

0 200

150

100 50 Depth (nm)

0

-50

Fig. 2. Ni-implantation profile simulated by TRIM (solid curve) and Ni distribution after annealing at different temperatures obtained from RBS spectra.

80

Ni: TiO2

Ni:TiO2

Ti at surface

1000

Ni:TiO2

1

13 19 800 9.5 – 494

ð1Þ

7

Ni concentration (%)

34

60

600

As-imp. aligned 923K aligned

400 200

χmin (%)

Yield (Counts)

800

Ni at surface

40

As-implanted

20

1073K aligned

Damage recovering Virgin aligned

0 550

600

650 Channel #

700

750

0

600

700 800 900 1000 1100 Annealing temperature (K)

Fig. 1. (a) Representative RBS/channeling spectra of Ni-implanted TiO2 after annealing at different temperatures. A channeling spectrum for virgin TiO2 is shown as the solid blue line for comparison and (b) the calculated wmin for samples annealed at different temperatures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

B. Ding et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 33–36

where l is the wavelength of X-ray, y the Bragg angle, and b the full width at half maximum (FWHM) of 2y in radians. During annealing at 823 and 923 K, Ni NCs grew gradually in grain size [Fig. 3(a)] while the annealing at higher temperatures (1073 K) resulted in a much larger crystallite for Ni NCs. Note that the peaks of Ni(1 1 1), Ni(2 0 0) and Ni(2 2 0) (Fig. 3(a)) appear simultaneously. It indicates that Ni NCs are not crystallographically oriented inside the TiO2 matrix [24]. Upon thermal treatment, Ni NCs in TiO2 are much more stable than Fe in TiO2 [23]. With increasing annealing temperature, Fe NCs were oxidized into FeTiO3. Finally after annealing at 1073 K, only FeTiO3 was observed. In contrast, Ni NCs were stable up to 1073 K or higher.

35

gets much larger. For a dc magnetization measurement in a small magnetic field by SQUID, the blocking temperature TB is given by TB ¼

Keff V 30kB

ð2Þ

where Keff(V) is the anisotropy energy density, V is the particle volume, and kB is the Boltzmann constant. With this equation, one can estimate the blocking temperature TB. Table 1 shows the value estimated using Eq. (2). In the calculation we assume Keff as 5.7  104 J m  3, the anisotropy energy density for bulk Ni [23]. One notes a large deviation in the blocking temperature for samples with smaller Ni nanocrystals. This might be due to the large error in the estimation of the Ni crystallite size using SRXRD. As shown in

3.3. Magnetic properties

Ni:TiO2, M-H at 5 K

Moment (μB/Ni)

M-H at 300 K 0.2 0.0

-3000

50 60 2θ (deg.)

70

-2000

-1000

0

1000

2000

3000

Field (Oe) Fig. 4. (a) Hysteresis loops measured at 5 K for Ni-implanted TiO2 at different annealing temperatures and (b) hysteresis loops measured at 300 K for Ni-implanted TiO2 at different annealing temperatures.

30 fcc-Ni in Ni:TiO2

As-imp. 623K

100

As-imp. 823K ann. 923K ann. 1073K ann.

-0.2 -0.4

823K

101

4000

Ni:TiO2

0.4

Crystalline size (nm)

102

2000

0

-2000

Field (Oe)

923K

40

-0.4

-4000

1073K

103

30

0.0

TiO2(220)

104

As-imp. 823K ann. 923K ann. 1073K ann.

0.4

-0.8

Ni(220)

Intensity

105

Ni(200)

TiO2(110)

106

Ni(111)

107

Moment (μB/Ni)

0.8

Employing SRXRD and RBS/C, we have systematically investigated the formation of Ni NCs. Using SQUID, we present the corresponding magnetic properties of Ni-implanted TiO2 annealed at different temperatures. We measured the hysteresis loops for four samples, at 5 and 300 K. Fig. 4(a) shows the magnetization versus field reversal (M–H) of samples annealed at different temperatures. At 5 K, hysteresis loops were observed for all samples. The saturation magnetization and coercivity increased with increasing annealing temperatures (see Table 1 and Fig. 4(a)). This is due to the fact that the size of the nanoparticles is increased with increasing annealing temperature [25,26] (see Fig. 3(b) and Table 1). As a comparison, Fig. 4(b) shows the M–H curves at 300 K. As expected for a magnetic nanoparticle system, above the blocking temperature, both remanence and coercivity drop to zero [27,28]; only one hysteresis loop was observed for the sample annealed at 1073 K, because its TB is above 300 K as shown later in Fig. 5. Fig. 5(a) shows the ZFC/FC magnetization curves [29,30] in a 50 Oe field. Superparamagnetism is present in four samples. ZFC curves show a gradual increase (deblocking) at low temperatures, and reach a broad peak with a maximum, while FC curves continue to increase with decreasing temperature. The broad peak in the ZFC curves is due to the size distribution of Ni NCs. In this article, the temperature at the maximum of the ZFC curve is taken as the average blocking temperature (referred to as TB in Table 1). The ZFC/FC curves are general characteristics of magnetic nanoparticle systems with a broad size distribution [31]. Fig. 5(b) shows the blocking temperature versus annealing temperature. The blocking temperature is less variable when the annealing is below 923 K, and when annealing temperature is 1073 K TB

25 20 15 As-implanted 10 5 0 600 700 800 900 1000 1100 Annealing temperature (K)

Fig. 3. (a) SRXRD symmetric y–2y scan of Ni-implanted TiO2 after annealing at different temperatures and (b) the crystalline size of Ni NCs calculated using the Scherrer formula after implantation and annealing.

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B. Ding et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 33–36

(2) The structure and magnetic properties of Ni NCs embedded inside TiO2 can be tuned by post-annealing. With increasing annealing temperature the Ni NCs grow in size and are stable up to 1073 K, the highest annealing temperature applied. (3) Compared with Fe ions in the TiO2 matrix, Ni ions are less mobile. No significant out-diffusion has been observed for Ni.

0.3

Moment (μB/Ni)

Ni:TiO2

0.2 ZFC/FC at H=50 Oe As-imp. 823K ann. 923K ann. 1073K ann.

0.1

Acknowledgment The research at Peking University was supported by the National Natural Science Foundation of China under Grant nos. 10875004 and 11005005, and National Basic Research Program of China under Grant no. 2010CB832904.

0.0 50

0

100

150 200 Temperature (K)

250

300

350 References

400 350

ZFC curve at H=50 Oe

300 Ni:TiO2

TB(K)

250 200 18 16 14 12 10 600

700

900 800 1000 Annealing temperature

1100

Fig. 5. (a) Magnetization curves with an applied field of 50 Oe after ZFC/FC for Ni-implanted TiO2 with different annealing temperatures and non-annealing and (b) the blocking temperature versus the annealing temperature for Ni-implanted TiO2.

Fig. 1a, the Ni peaks for the as-implanted and 823 K annealed samples are very faint. 3.4. Discussion on the origin of ferromagnetism By correlating the structure and magnetic properties, we can conclude that the major contribution for the measured ferromagnetism is from metallic Ni nanocrystals. However, there is one observation that cannot be explained by only considering Ni nanocrystals. There is a large paramagnetic component at 5 K [see Fig. 4(a)], which probably comes from single Ni ions or very small Ni NCs. However the saturation magnetization is larger than 0.61 mB/Ni, the saturation magnetization for ferromagnetic bulk Ni. This feature indicates other contributions for the observed magnetic properties. Irradiation induced defects [12] or substituted Ni ions mediated by defects [1] are the possible candidates.

4. Summary and conclusions By correlating the structural and magnetic properties of all investigated Ni-implanted TiO2 samples, we can draw the following conclusions. (1) Ni NCs have been formed in TiO2 after ion implantation and annealing. The Ni NCs are the origin of the measured ferromagnetism.

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