Structural and magnetic properties in the powder form of Sn1−xCrxO2 solid solution

Physica B 407 (2012) 624–628

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Structural and magnetic properties in the powder form of Sn1  xCrxO2 solid solution Kun Xu a,b,c,n, Zhe Li a,b, Xiaofeng Zhou a,b, Mitsuru Izumi c a

Department of Physics and Electronic Engineering, Qujing Normal University, Sanjiang Road, Qujing 655000, People’s Republic of China Key Laboratory of Yunnan Provincial Universities for Advanced Functional and Low Dimensional Materials, Qujing Normal University, Sanjiang Road, Qujing 655000, People’s Republic of China c Laboratory of Applied Physics, Tokyo University of Marine Science and Technology, 2-1-6, Etchu-jima, Koto-ku, Tokyo 135-8533, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 January 2011 Received in revised form 21 November 2011 Accepted 26 November 2011 Available online 2 December 2011

Structural and magnetic properties were studied in powder form of Sn1  xCrxO2 with x ¼ 0.01, 0.02, 0.03, 0.04 and 0.05 in nominal composition. The structural parameters were obtained at room temperature by the Rietveld refinement of the x-ray powder diffraction profiles. Samples of x ¼ 0 to 0.04 are tetragonal phase with a space group P42/mnm. The lattice parameters indicate three-step changes with increasing Cr content. The distortion of the metal-oxygen octahedral unit occurs. The substitution of Cr ions on the Sn sites shortens the lattice parameters and the octahedral unit becomes elongated with a displacement of an apical oxygen from x ¼0 to x ¼ 0.02. The incorporation of Cr over x ¼ 0.02 leads to the recovery of the length of lattice parameters together with a relaxation of the octahedral unit. This result indicates a possible interstitial occupation of Cr ions from x ¼ 0.03 to x ¼ 0.04. The Cr doping reaches a saturation limit at x ¼ 0.05 with a trace of the excess Cr oxides in the x-ray study. A room temperature ferromagnetism appears in the sample with x ¼0.01 and becomes remarkable in one with x ¼ 0.02. The magnetization decreases with increasing the Cr doping with the amount x 40.02. Thus, the appearance of ferromagnetism highly correlated with the oxygen displacements at the apical position of the octahedral in the Sn1  xCrxO2 system at room temperature. The critical oxygen displacement in the elongated octahedral at around x ¼0.02 may encourage the vacancy of the apical oxygen and eventually leads to appearance of a ferromagnetism based on an F-center exchange with a micro- and/or nanostructural transition. The observed ferromagnetism is highly correlated with the averaged structural change appeared in the x-ray powder diffraction. & 2011 Elsevier B.V. All rights reserved.

Keywords: Room temperature ferromagnetism

1. Introduction Recently, oxide-diluted magnetic semiconductors (DMS) have attracted considerable interests from the viewpoint of potential application to the spintronics [1]. SnO2 is considered to be one of the candidates for its applications as transparent conducting layers [2] and gas sensors [3]. The room-temperature ferromagnetism (RTFM) of transition-metal doped SnO2 has been studied by first-principles calculations [4,5] and experiments [6–9]. Among them, it has been noted that the oxygen vacancies around the doped metal ions contribute to form a long-range ferromagnetic order according to the spin-split impurity band induced exchange interaction. The experimental results approved that the RTFM originates from the transition-metal doping and n Corresponding author at: Department of Physics and Electronic Engineering, Qujing Normal University, Sanjiang Road, Qujing 655000, People’s Republic of China. E-mail address: [email protected] (K. Xu).

0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.11.045

consequent oxygen vacancies instead of metallic clusters or any secondary phases. However, the microscopic mechanism is still an open question at present. Ghosh et al. [10] have studied both nano and bulk states of Sn1  xCoxO2 and suspected the possible origin, which is associated with a nano-structure included into the bulk phase, although they have observed a corresponding contraction of the lattice as a proof of ionic substitution. They have stated the origin of ferromagnetism may come from the structure induced by oxygen vacancies on the surface, since the sintering process suppressed the ferromagnetism. As a local probe, according to the magnetic resonance techniques, how the transition-metal ions are incorporated into SnO2 lattice (ionic state and local environment) can be understood. And the ferromagnetic order was revealed to origin from the oxygen vacancies and transition-metal ions incorporated into the SnO2 lattice, which consist the main elements in the F-center exchange mechanism [11]. If such F-center exchange mechanism is a motive origin of the ferromagnetism of Cr-doped SnO2, the oxygen vacancy and its related structural change has to be

K. Xu et al. / Physica B 407 (2012) 624–628

2. Experimental The Sn1  xCrxO2 samples in powder form were prepared by the standard solid state reaction method. The starting materials of Cr2O3 (99.99%) and SnO2 (99.9%) were mixed with a Cr concentration x varying from 0.01 to 0.05. The mixture was first ball milled using zirconia balls for 15 min, and then annealed at 1000 1C for 12 h. Both the ball milling and annealing processes were done in the air. In order to prevent contamination, all of the processes were managed without using any kind of metallic tools. Powder x-ray diffraction measurement was carried out using Bruker MXP18 with the rotating anode generator operated at ˚ was used. All of 40 kV and 200 mA. Cu Ka radiation (l ¼1.5418 A) the samples were scanned in the steps of 0.021 (2y), in the 2y range from 101 to 1101 with a fixed counting time 6 s /step. Resulting x-ray diffraction profiles were indexed and the structural parameters were refined using Rietveld method with a program RIETAN-2000 for single phase powder [12]. High resolution transmission electron microscopy (HRTEM) observation was carried out to identify the morphology of the sample. The magnetic hysteresis loops of the samples were measured using a Quantum Design SQUID magnetometer (MPMS-XL-A). Each sample was weighted to be 50 mg. And the samples were mounted in the straw holder without using kapton tapes or cotton. All these processes were carefully done in order to minimize the background signals.

the observed peaks were successfully indexed with a tetragonal structure SnO2. There is no trace of a mixture of orthorhombic phase as has been prepared by a sol–gel method to our accuracy [7]. As doping level x is increased from x¼0, a small trace of the (012) Cr2O3 peak was identified only in the sample with x¼0.05. There is no remarkable trace of the existence of Cr2O3 between x ¼0 and x ¼0.04. This result indicates that the Cr2O3 is successfully introduced into SnO2 associated with the formation of the solid solution of Sn1  xCrxO2 between x ¼0.01 and 0.04 since they both have a hexagonal packing of oxygen anions and the cations positioned in the octahedral holes [13], and reaches a saturation limit of the mixed solution including the substitution of Sn ions with Cr ions in the sample with x ¼0.05. For the x-ray diffraction patterns of SnO2 (110) profiles, no apparent change was observed as a function of x. Meanwhile, according to the full width at half maximum of SnO2 (110) peak, the average crystallite sizes of the samples were estimated using the Scherrer formula. The estimated size consists well with the HRTEM observation as shown in the inset of Fig. 2. As plotted in Fig. 2, the average crystallite size the pure SnO2 is a little larger, and becomes slightly smaller with increasing Cr concentrations x. It approves that the incorporation of Cr ions leads to crystallite

36

Average crystallite size (nm)

observed in association with the doping of transition-metal ions as the averaged crystal structure. In the present study, we clarify the relationship between the structural modification, the creation of the oxygen vacancies and the appearance of the ferromagnetism in the powder forms of Sn1  xCrxO2 (x ¼0.01, 0.02, 0.03, 0.04 and 0.05). The powdered samples were synthesized from the mixture of SnO2 and Cr2O3 powders. The refinement of the structural parameters was performed using a x-ray profile analysis with the Rietveld method to clarify the structural change as a function of doped Cr content in Sn1  xCrxO2 (x¼ 0.01, 0.02, 0.03, 0.04 and 0.05). Accordingly, magnetic properties were studied in a series of powdered samples with the above compositions. A strong correlation between the structural change and magnetic properties was observed.

625

34

32

30

0.00

0.01

0.02 0.03 0.04 Cr doping concentration x

0.05

Fig. 2. Variation of average crystallite sizes of samples for different Cr doping concentrations. Inset: HRTEM image of the Sn0.98Cr0.02O2 powder.

3. Results and discussion Fig. 1 shows the x-ray diffraction profiles of Sn1  xCrxO2 (x¼0.01, 0.02, 0.03, 0.04 and 0.05) solid solution. For each sample,

15000

(321) (400) (222) (330) (312)

(202)

(310) (112) (301)

(211) (220) (002)

(101) x=0

(200) (111) (210)

Intensity (a. u.)

40000

20000

Intensity (a. u.)

(110)

50000

30000

Calculated Observed Obs.-Cal. reflection position

x = 0.01

10000

x = 0.01

5000

x = 0.02

10000 0

0

x = 0.03 x = 0.04 x = 0.05

0.2

0.2

0.4

0.6 1/d

0.8

1.0

Fig. 1. Room temperature x-ray diffraction profiles in powder form of Sn1  xCrxO2 (x¼ 0.01, 0.02, 0.03, 0.04 and 0.05) in nominal composition.

0.4

0.6 1/ d (1/Å)

0.8

1.0

Fig. 3. Rietveld refinement patterns of the Sn0.99Cr0.01O2 powder. Dots represent the observed intensities, and the solid line is calculated ones. A difference (obs.– cal.) plot is shown beneath. Vertical bars are the reflection position markers.

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K. Xu et al. / Physica B 407 (2012) 624–628

Table 1 4þ Refined crystal parameters by Rietveld refinement for ball milled Sn1  xCrxO2 (x ¼0–0.05) systems with the cassiterite structure: space group D14 4h P42 =mnm, Sn occupying the 2a sites (0, 0, 0; 1/2, 1/2, 1/2) and O2  occupying the 4f sites 7(u, u, 0; 1/2þ u, 1/2 u, 1/2).

Cr content

a

b

c

u

V

Rwp

Rp

S

0 0.01 0.02 0.03 0.04 0.05

4.7394(3) 4.7386(3) 4.7383(3) 4.7398(3) 4.7398(3) 4.7394(3)

4.7394(3) 4.7386(3) 4.7383(3) 4.7398(3) 4.7398(3) 4.7394(3)

3.1877(2) 3.1867(2) 3.1869(2) 3.1873(2) 3.1873(2) 3.1872(2)

0.309(8) 0.310(2) 0.311(5) 0.309(0) 0.309(0) 0.309(4)

71.604(8) 71.555(8) 71.551(8) 71.605(9) 71.606(8) 71.592(8)

14.17 13.01 13.14 12.46 12.71 12.05

10.97 9.97 9.78 9.49 9.67 9.12

2.628 2.367 2.434 2.164 2.299 2.115

3.1880 4.7400

3.1875 c (Å)

a (Å)

4.7395

4.7390 3.1870 4.7385

0.05

Fig. 4. Lattice parameters a and c as a function of x obtained from the Rietveld refinement.

defects, such as oxygen vacancies, interstitial Cr3 þ ions, that prevent the nucleation and growth of SnO2 particles. There is no strong evidence of the existence of the phase separation and/or intergrowth of Cr2O3 itself among SnO2 between x ¼0.01 and x¼ 0.04 as far as we study both x-ray diffraction and HRTEM. In order to obtain the detailed crystal structure, all of the six x-ray powder diffraction profiles including pure SnO2 were analyzed by the Rietveld method with a program RIETAN. The structure model is of cassiterite with tetragonal symmetry: space 4þ group D14 occupying the 2a sites (0, 0, 0; 1/2, 1/ 4h P42 =mnm, Sn 2 2, 1/2) and O occupying the 4f sites 7(u, u, 0; 1/2þu, 1/2 u, 1/ 2) with u ¼0.307 [5]. The observed and calculated profiles for the sample x¼0.01 are representatively shown in Fig. 3. The unit-cell parameters (a and c) and oxygen positional parameter u were obtained as shown in Table 1. And variation of a and c is plotted in Fig. 4. From the results, no significant alteration was observed in the tetragonality factor (c/a), which is around 0.673, thus the unit cell almost changes isotropically. In this structure, the six oxygen atoms around Sn4 þ ion form a SnO6 octahedral as shown in Fig. 5(a). Inside the octahedral, the lengths of six Sn–O bonds are not equivalent within the crystal symmetry. We define the interatomic distance between Sn (1/2, 1/2, 1/2) and O (u, u, 0) as d1, and distance between Sn (1/2, 1/2, 1/2) and O (1/2þu, 1/ 2 u, 1/2) as d2. The d2 are slightly longer than the other four d1. Besides, the bond angle between O–Sn–O is defined as y. The variation of interatomic distances d1, d2 and bond angle y, which were calculated from the refined parameters (a, c, u), are also plotted in Fig. 5(b) and (c). For low Cr doping concentrations, the lattice shrinks in three dimensions with a decreasing remarkably from 4.7394 A˚ (pure SnO2) to a minimum 4.7383 A˚ (x ¼0.02). This is highly correlated with the smaller size of Cr ions (Cr3 þ is ˚ for which the ionic substitution may occur at the Sn4 þ 0.76 A) ˚ sites. This kind of substitution leads to the decrease of the (0.83 A) lattice parameter [7]. Besides of the change of the lattice

2.044

77.4 77.2

2.040

 (°)

0.01 0.02 0.03 0.04 Cr doping concentration x

Interatomic distance d1 (Å)

0.00

77.6

77.0 2.036 76.8 2.090

Interatomic distance d2 (Å)

3.1865

4.7380

2.085 2.080 2.075 2.070 0.00

0.01 0.02 0.03 0.04 Cr doping concentration x

0.05

Fig. 5. (a) Unit cell of the rutile lattice. (b) and (c), variation of interatomic distance d1 (Sn–O), d2 (Sn–O) and the angle y of O–Sn–O bond calculated from the parameters from Fig. 4.

dimension, the shorter bond d1 is shortened, and the longer bond d2 is elongated. That is, the oxygen octahedral distortion increases thanks to the substitution with Cr for Sn site. With further increasing Cr doping concentration, the lattice expands drastically to a ¼4.7398 A˚ at x¼0.04. Apparently, a totally different doping mechanism dominates in the samples with x40.02. The interstitial incorporation of Cr and corresponding structural disorder might be the main reason for the expansion of the lattice in the samples from x¼0.03 to 0.05 [7,14]. Correspondingly, the increase of bond length d1 as well as decrease of d2, causes the recovery of the distortion of the octahedral unit as indicated in Fig. 5. Accordingly, magnetic hysteresis loop measurements on the powdered samples were carried out between 710 kOe at the

K. Xu et al. / Physica B 407 (2012) 624–628

room temperature. The hysteresis loops measurements of Sn1  xCrxO2 (x ¼0.01, 0.02, 0.03, 0.04 and 0.05) were performed as shown in Fig. 6. The results show a behavior of the mixture of paramagnetic and ferromagnetic signals. In order to investigate the ferromagnetism separately, the paramagnetic (linear) component was subtracted from the measured hysteresis loops. As shown in Fig. 6, the signal of ferromagnetic component is quite obvious in the sample x ¼0.02 compared with the other samples. The variation of saturation magnetization (MS) with x is shown in the inset of Fig. 6. Up to x¼0.02, the doping of Cr ions causes the ferromagnetic magnetization to increase quickly. Samples x ¼0.01 and 0.02 illustrate clear ferromagnetic behaviors with a large coercivity, in spite of the diamagnetic and antiferromagnetic characters of SnO2 and Cr2O3, respectively. Furthermore, the value of M S ¼ 0:31 emu=gCr2 O3 for x ¼0.02 consists well with the result in ball milled Co3O4 and anatase TiO2 mixtures ð  0:35 emu=gCr3 O4 Þ [15]. In order to keep charge neutrality caused by the substitutional Cr ions, oxygen vacancies (VO) are prefer to form near the Cr ions instead of Sn4 þ sites. The consequently trapped spin-polarized electrons at VO overlap the d-orbital of the neighboring Cr ions. The formed F-center exchange coupling (FCE) would be crucial for the appearance of ferromagnetism [11]. While for x increasing from 0.02 to 0.04, the sharp decrease of MS might be induced by the increasing amount of Cr ions on the interstitial sites. The FCE coupling weakens quickly since the interstitial site

0.4

x = 0.01 x = 0.02 x = 0.03 x = 0.04 x = 0.05

0.2

dopant will not produce the oxygen vacancies. The paramagnetic components are dominant in the hysteresis loops of the x Z0.03 samples as indicated in Fig. 6. As has been indicated in Ref. [16], there is another aspect in which the charge carrier type changes from n-type to p-type around the critical transition region with x¼0.02. In FMR study by Misra et al. [17], there appears a ferromagnetic resonance line due to oxygen defects in the sample with Cr3 þ concentration of less than x¼0.025. The schematic diagrams of Sn1 xCrxO2 (x¼0, 0.02 and 0.03) crystal structure are shown in Fig. 7. The variation of the SnO6 octahedral distortion seems to have a close relationship with the oxygen defect formation process as indicated in previous works [17]. This may be related to the formation of the orthorhombic phase, which is difficult to detect with the present x-ray diffraction because it observed an averaged crystal structure. As indicated by arrows in Fig. 7, the distortion of SnO6 octahedral associated with the apical oxygen displacement and MS maximum coincide with each other at x¼0.02. This indicates that the distorted octahedral together with the induced oxygen deficiency contributes to the room temperature ferromagnetism in our Sn1 xCrxO2 powder systems. In this structure, an apical oxygen of the elongated octahedral is also one of the planar oxygen of the neighbor octahedral unit. The each oxygen may have an equivalent probability to be deficient according to the increase of internal pressure induced by the ionic substitution of Sn4 þ ions with Cr3 þ ions. The similar phenomenon was also observed in the Sn1 xCoxO2 thin films [18]. The disappearance of ferromagnetism in xZ0.03 may be related to the reduction of SnO6 octahedral distortion. Meanwhile, as illustrated in Fig. 7, for xr0.02, the reduction of interatomic distance d1, which is shorter than d2 gives a chance to the enhancement of the exchange coupling between Cr ions and potentially oxygen vacancies, since the magnetic exchange interaction is extremely sensitive to the distance between the interacting spins.

0.1 4. Conclusion

0.0

Magnetization (emu/Cr2O3)

Magnetization (emu/gCr2O3)

0.3

627

-0.1 -0.2 -0.3 -0.4

0.3 0.2 0.1 0.0 0.01 0.02 0.03 0.04 0.05 x

-10000

-5000

0 5000 Magnetic Field (Oe)

10000

Fig. 6. Room temperature hysteresis loops for the ball milled Sn1  xCrxO2 (x¼ 0.01–0.05) powders. Inset: variation of ferromagnetic magnetization as a function of Cr concentration x.

In summary, we have investigated the structural and magnetic properties Sn1  xCrxO2 (x ¼0.01, 0.02, 0.03, 0.04 and 0.05) powders focus on the x-ray diffraction and magnetic hysteresis loops. The structural property was studied by the Rietveld refinement of the diffraction profiles. The substitution/interstitial Cr ions, which clearly induce an elongation of octahedral unit with oxygen displacements, dominate for different ranges of Cr doping concentrations. It has a crucial effect on the appearance of ferromagnetism with x ¼0.02. The observed oxygen displacement in the SnO6 octahedral unit motivates an internal pressure to create an oxygen vacancy around the substituted Cr ions. This leads to exchange magnetic interaction for the ferromagnetism according

b c

a

x=0

x = 0.02

x = 0.03

Fig. 7. Schematic diagram of atoms in a crystal structure of Sn1  xCrxO2 (x ¼0, 0.02 and 0.03) from (001) direction. The gray balls represent Sn atoms, and red (black in printed version) balls represent O atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

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K. Xu et al. / Physica B 407 (2012) 624–628

to the spin-split impurity band induced exchange interaction proposed in the previous works. Present study submits the evidence of the critical structural change of the octahedral unit associated with the appearance of the room-temperature ferromagnetism and indicates the key existence of a deep correlation between appearance of ferromagnetism and the elongation of the octahedral unit, which may affect to the possible oxygen vacancy.

Acknowledgments The present work was performed using a SQUID magnetometer of the Institute for Solid State Physics, Materials Design and Characterization Laboratory, the University of Tokyo. The authors are grateful to Takashi Yamauchi for technical assistance. The work was financially supported by the Sasakawa Scientific Research Grant from The Japan Science Society. References [1] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Science 291 (2001) 854.

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